The Anterior Transcallosal Interhemispheric Approach

This approach provides excellent exposure of the frontal horns and bodies of the lateral cavities and even gives access to the anterior part of the third ventricle. The standard position of the patient is supine with elevation and flexion of the head. The incision is made behind the hairline, usually in a bicoronal fashion. For the craniotomy, two bur holes are drilled on the contralateral side close to the sagittal sinus. Preoperative imaging studies with MR or CT venography and intraoperative neuronavigation are recommended for planning placement of the bur holes to avoid damage not only to the sinus but also to irregular bridging veins.⁴⁶ In general, a craniotomy diameter of 4 to 5 cm is sufficient for tumor removal; however, it may be necessary to vary the size of the craniotomy based on the extent of the tumor and the depth at which it has to be resected. In the best case, the chosen trajectory preserves the bridging veins. Mobilization of the draining veins is important for gaining space, and their sacrifice should be avoided.

Brain relaxation can be achieved by osmotherapy and, most importantly, by release of CSF, mainly from the sulcal and callosal cisterns. This strategy is preferred over the use of rigid retractors so as to protect neural structures from pressure and tearing damage. Entrance to and advance within the interhemispheric fissure are achieved by blunt dissection with tailed cotton strips and balls.

When the corpus callosum is reached, an entrance of 10 to 15 mm is generally enough for removal of most lesions located in the frontal and middle parts of the lateral ventricles. The smoothest way of opening is to dissect along the plane of the fibers with fine-tipped bipolar forceps and small tailed cotton strips. When the ventricular cavity is entered, drainage of CSF leads to additional relaxation and can be augmented by incision of the septum pellucidum.

For better orientation in the presence of large tumor masses with severely distorted ventricles, the surgeon will find the thalamostriate vein located on the right side of the plexus in the right lateral ventricle and on the left side of the plexus in the left lateral ventricle. The further course of the plexus and vein leads to the internal cerebral vein, which serves as a guide for localization of the fornix and thalamus and thus acts as an important landmark.

Caution needs to be taken regarding the genu of the internal capsule, which reaches the surface of the ventricle lateral to the foramen of Monro where the thalamostriate vein turns medially toward the internal cerebral vein.^47–49 Recent publications show that complications related to this approach include mainly seizures, sensory-motor deficits, and visual and cognitive impairment.^14,23,50,51

The Anterior Transcortical Approach

This route provides excellent access to the frontal horns and bodies of the lateral cavities, and even the anterior part of the third ventricle can be exposed. This approach is most suitable for patients with large ventricles and for pathologies within the lateral frontal horn of the nondominant cerebral hemisphere. The working corridor is directed mainly through the middle frontal gyrus. With the patient in the supine position and the head turned between 10 and 15 degrees to the opposite side, the skin incision and craniotomy are placed over the main part of the middle frontal gyrus. The intraventricular landmarks are the same as for the transcallosal approach.

The advantage of the transcortical approach is that bridging veins are not a concern.⁴⁶ The disadvantage of this approach relative to the interhemispheric transcallosal approach is that not only are the commissural fibers dissected but also parts of the projection fibers and the short and long association fibers.

The Superior Frontal Sulcus Approach

As in the transcortical approach, the patient is placed in the supine position with elevation and flexion of the head, depending on the side in which the tumor is located. Instead of entering toward the lateral ventricle by opening the middle frontal gyrus as described earlier, the superior frontal sulcus is chosen for creation of the corridor. Neuronavigation can be used to plan and verify the exact trajectory. The advantages and disadvantages are the same as described for the transcortical approach. One has to be aware that the superior frontal sulcus can be short in length. Therefore, clear identification of it may not be possible in every case.⁵²

The Posterior Interhemispheric Transcingular Approach

The posterior interhemispheric transcingular approach is suitable for exposure of tumors of the atrium and the posterior horn of the lateral ventricle. The patient can be operated on in the prone or sitting position. The craniotomy is placed superior and inferior to the lambdoid suture, including the midline (as in the anterior interhemispheric approach), and can be varied according to the position of the superficial bridging veins. The posterior interhemispheric fissure is then opened widely to minimize retraction, and the precuneus and isthmus of the cingulate gyrus are opened to give access to the ventricle.⁵³ Because the posterior part of the optic radiation courses in the lateral wall of the atrium and posterior horn, one does not interfere with the fibers of the optic radiation when using this approach.⁵⁴

The Intraparietal Sulcus Approach

The intraparietal sulcus approach has been described for lesions of the medial and lateral portion of the trigone.^29,51,54,55 The ventricle is explored by opening the intraparietal sulcus and then dissecting the parietal white matter toward the ventricle. Although this approach leads to direct access to the trigone, one should be aware that it poses a risk for possible neurological complications, such as visual field defects, apraxia, and acalculia.

The Transsylvian Approach to the Temporal Horn

The proximal transsylvian approach was developed by Yasargil in 1967 for the treatment of vascular and neoplastic lesions in the mesial temporal area. Later, this approach was used for selective amygdalohippocampectomy.⁵⁶ The approach consists of pterional craniotomy and opening of the proximal part of the sylvian fissure with an incision about 3.0 to 5.0 cm long. Through a pial incision lateral to the M1 segment of the middle cerebral artery between the origin of the anterior temporal and temporopolar arteries, one can gain access to the temporal horn. In particular, limbic tumors of the amygdala, hippocampus, and parahippocampal area extending into the temporal horn can be removed with this approach without injuring the adjacent neocortex of the superior, middle, and inferior temporal gyrus and lateral temporo-occipital gyrus.

The Occipitotemporal Sulcus Approach

The occipitotemporal sulcus approach can be used to access lesions in the posterior part of the temporal horn.⁵¹ The occipitotemporal sulcus can be opened with a subtemporal craniotomy to allow access to the temporal horn. Placing a lumbar drain before surgery for release of CSF before opening the dura reduces the need for brain retraction. As is the case with other lateral transcortical approaches, there is a risk for postoperative visual field deficits.

Ependymomas

The first description of ependymomas was made by Bailey and Cushing in 1926.⁵⁷ Ependymomas account for about 2% to 9% of intracranial tumors, and they are more frequent in the pediatric population, with an incidence of 6% to 12% and a peak in the first 3 years of life. They are equally likely to occur in boys and girls.⁵⁸ Ependymomas arise from ependymal cells, and thus they are found in the entire ventricular system, especially in the infratentorial region.^59–63 These tumors usually tend to grow slowly and show perivascular pseudorosettes and ependymal rosettes as key features on histologic examination. Most ependymomas are classified as grade II tumors according to the World Health Organization (WHO) classification of 2007, but there also exists an anaplastic variant, and progression to it from lower grades has been described.^58,63,64 Therefore, histopathologic examination is recommended in any case. CT images of ependymomas appear isodense, with heterogeneous contrast enhancement. On MRI, they are well circumscribed, isointense to hypointense, and enhance heterogeneously.

Clinical symptoms arise from obstructive hydrocephalus, which leads to intracranial hypertension. In addition, cognitive changes are seen frequently, especially memory and attention deficits.⁶⁵

Surgery plays a key role in the treatment of ependymomas because gross-total resection (Fig. 138-4) is associated with improved prognosis.⁶⁶ If pathologic examination confirms malignant features and MRI studies show tumor remnants, radiotherapy should be discussed (Fig. 138-5).⁶⁷

FIGURE 138-4 A and B, Preoperative coronal magnetic resonance images (fluid-attenuated inversion recovery [FLAIR] sequence) of a 32-year-old man with symptoms of raised intracranial pressure caused by a large ependymoma that filled the left lateral ventricle. C-F, Postoperative computed tomography scans demonstrating total tumor removal. There were no postoperative neurological deficits; however, the patient needed placement of a ventriculoperitoneal shunt because of hydrocephalus.

FIGURE 138-5 Preoperative axial (A and B), coronal (C), and sagittal (D) magnetic resonance images (MRIs) of a 3-year-old girl with disturbed consciousness. Initially, an external ventricular drain was placed to treat the occlusive hydrocephalus. E, The child was operated on in the supine position with the aid of intraoperative neuronavigation. F-H, Gross-total removal of an anaplastic ependymoma was achieved as demonstrated on postoperative MRIs. The postoperative course was uneventful, and the young patient underwent chemotherapy.

Subependymomas

Subependymomas were first described by Scheinker in 1945.⁶⁸ Subependymomas are rare and account for just 0.2% to 0.7% of intracerebral tumors.^69,70 They are benign, slow-growing tumors, often attached to the ventricular wall, but they do not invade surrounding structures. Therefore, this pathology is classified as WHO grade I. Although they most commonly arise within the fourth ventricle, they are located within the lateral ventricles in up to 40% of patients.^58,71,72 Subependymomas show a male preponderance, with a male-to-female ratio of 2.3 : 1. Histologically, this tumor shows isomorphic nuclei in clustered patterns, a fibrillary matrix, and small cysts.⁵⁸ On CT these lesions appear as hypodense to isodense lobular masses with cystic components that do not usually show contrast enhancement. On MRI, this pathology appears as a solid to cystic tumor.

Depending on location and progression, subependymomas become clinically evident by obstruction of CSF pathways, with signs of increased intracranial pressure typically causing headache, nausea and vomiting, gait disturbances, or vertigo (Fig. 138-6). Patients most commonly become symptomatic between the ages of 40 and 60. Because of the benign natural history of these tumors, they remain clinically silent in many cases and are often detected only during autopsy studies.^69,72 At present, with easy access to cerebral imaging studies, a growing number of subependymomas are diagnosed incidentally.^70,72–74

FIGURE 138-6 Preoperative axial (A and B), coronal (C and D), and sagittal (E and F) magnetic resonance images (MRIs) of a 32-year-old woman (the patient shown in Fig. 138-3). The patient had severe headache, vomiting, and memory deficit. The solid, highly vascularized tumor, a subependymoma, was completely removed as shown on postoperative coronal (G) and sagittal (H) MRIs. There were no neurological or other deficits after the microsurgical procedure.

Management of patients in whom an incidental finding is diagnosed is still under debate. If clinical signs are absent, routine follow-up with imaging studies is recommended. If dynamic growth during follow-up or the development of clinical symptoms occurs, microsurgical removal of the mass is the treatment of choice. Because of the benign origin of this entity, radical extirpation defines cure. If imaging studies reveal residual tumor, further treatment options depend on the patient’s condition. If no further deficits have occurred, surveillance with MRI is justified. In patients with existing neurological deficits caused by the remnant mass, operative resection (or eventually radiotherapy) should be discussed.^70,72,74

Central Neurocytomas

These rare tumors were first described by Hassoun and colleagues in 1982.⁷⁵ This emphasizes the fact that this lesion was just recently determined to be distinct from other intraventricular pathologies and therefore has often been mistaken for another entity.⁷⁵ Overall, central neurocytomas represent only 0.1% to 0.5% of all intracranial tumors.^76,77 They generally occur in the lateral ventricles, with a predilection for the left frontal horn. Typically, they are detected between the second and fourth decades of life, with equal distribution between the sexes.^77–79 Histologic examination of central neurocytomas shows uniform round cells, perivascular pseudorosettes, and a honeycomb appearance, thus indicating their tendency to mimic the architectural patterns of intracerebral gliomas.^77,80 They are classified as WHO grade II tumors.⁵⁸

On CT and MRI, central neurocytomas appear hyperdense and hyperintense (with moderate contrast enhancement), respectively. The presence of necrosis, cysts, and calcifications in these tumors sometimes causes a heterogeneous appearance.^77,81 Differential diagnosis includes oligodendroglioma, ependymoma, and papilloma arising from the choroid plexus.^77,82

In most cases, central neurocytomas are slow growing and become clinically evident as a result of increased intracranial pressure from obstructive hydrocephalus that causes symptoms of headache, nausea, vomiting, gait disturbance, and cognitive deficits. Surgical removal remains the treatment of choice and defines cure (Fig. 138-7). Nevertheless, although central neurocytomas are generally considered to be benign, malignant courses with progression of remnant tumor and cerebral and spinal dissemination have been reported and associated with unfavorable clinical outcomes.^83–86 If radical removal cannot be achieved, adjuvant therapies such as radiation therapy and chemotherapy are options, but their benefits are still under debate. Reduction of tumor mass has been reported with both therapies.^{58,84,85,87,88}

FIGURE 138-7 Preoperative axial (A-C), coronal (D and E), and sagittal (F and G) magnetic resonance images of a 26-year-old woman with symptoms of raised intracranial pressure. The tumor (a central neurocytoma) was totally removed via the interhemispheric transcallosal approach. Postoperatively, mutism was observed for several days but completely resolved within the first week after surgery. No neurological or cognitive deficits were present thereafter. The patient has now been monitored for more than 12 years continuously and has shown an uneventful course.

Low-Grade Gliomas

Fibrillary astrocytomas and subependymal giant cell astrocytomas (SEGAs) can occur frequently within the lateral ventricles. The fibrillary component is the most frequent variant of common astrocytomas. On histopathologic examination, nuclear atypia is present, including enlarged, irregular hyperchromatic nuclei. Additionally, mucoid cysts can occur within the matrix. This tumor shows diffuse and infiltrative growth; giant cell astrocytomas are found in patients suffering from tuberous sclerosis. This coexistence is well documented in the literature, and the neoplasm is typically located in the vicinity of the foramen of Monro and can extend to the cavity of the lateral ventricle.^89–92 Unfortunately, astrocytomas are closely associated with morbidity and mortality in this particular setting.^91,93,94 SEGA is considered a benign and thus slow-growing tumor originating from the wall of the lateral ventricles. The lesion is structurally based on spindle and gemistocyte-like cells. In some cases, additional ganglion-like cells can be found. Clusters, perivascular pseudopalisades, and calcification are typical histologic characteristics. Consequently, SEGA was categorized as WHO grade I in 2007.^58,89 The tumor typically occurs in patients younger than 20 years.

On CT, this lesion appears hypodense and well circumscribed and shows contrast enhancement. On T1- and T2-weighted MRI, the tumor appears as a hypointense mass but in some cases can have high signal intensity on T2-weighted images (Fig. 138-8A). In all cases the tumor shows enhancement when contrast material is administered (Fig. 138-8B), with a somewhat heterogeneous appearance caused by the irregular occurrence of calcification.^91,92

FIGURE 138-8 A, This 33-year-old pregnant woman had severe headache, gait ataxia, and a slight memory deficit. Preoperative nonenhanced magnetic resonance images (MRIs) taken in the axial (top left, bottom left), coronal (top right), and sagittal (bottom right) planes demonstrate a large cystic tumor that is occupying the posterior portion of both lateral ventricles. Cyst evacuation and tumor biopsy were performed immediately to release the intracranial pressure. Histologic examination revealed a pilocytic astrocytoma. The patient quickly recovered from this procedure, and no further therapy was necessary during the following period of pregnancy. B, Contrast-enhanced coronal (top left), axial (bottom left), and sagittal (top middle) T2-weighted MRIs of the patient shown in A taken after uneventful delivery of the child. The patient then underwent elective surgery via the posterior interhemispheric subcallosal approach, and the tumor was removed completely, with preservation of the involved fornices on both sides. Postoperatively, the patient complained of a mild memory deficit for a few weeks; thereafter, no neurological or cognitive deficits were present, and follow-up MRI controls (top right, bottom middle and right) excluded any tumor recurrence.

Neurological symptoms are caused mainly by obstruction of CSF pathways, which leads to typical clinical symptoms related to the increased intracranial pressure; however, astrocytomas can bleed, which can result in intraventricular hemorrhage, depending on their location, and lead to sudden deterioration. If symptomatic lesions are diagnosed, surgical resection is the treatment of choice. With regard to incidental findings of ventricular gliomas, treatment is still controversial. In patients with small nonsymptomatic tumors, clinical follow-up with regular outpatient consultations and imaging studies is justified. If the lesion displays dynamic growth and can potentially displace CSF pathways, surgical removal in the absence of actual neurological deficits should be considered.^{89,91,92,95,96}

High-Grade Gliomas

The tendency toward malignant change in astrocytomas correlates with the patient’s age. High-grade gliomas of the lateral ventricle may frequently arise from the corpus callosum, the septum pellucidum, or the thalamus and show a predilection for the anterior horn of the lateral ventricle.⁹⁷

Anaplastic astrocytomas are malignant lesions (WHO grade III) with typical diffuse and infiltrating growth patterns (Fig. 138-9). The mean age at diagnosis peaks between 45 and 51 years. There is a slight male preponderance, with a male-to-female ratio that varies from 1.1 : 1 to 1.6 : 1.^98,99 Histologic features include increased cellularity, nuclear atypia, and mitotic activity. When anaplastic transformation is in progress, the nuclei vary in number, size, and shape. Additionally, abnormal mitoses can be detected.

FIGURE 138-9 Axial (A), sagittal (B), and coronal (C) contrast-enhanced T1-weighted magnetic resonance images of a 24-year-old man revealing a highly vascular (D) irregular tumor of the left atrium that infiltrated the surrounding structures. The patient was suffering from progressive headache and gait ataxia. E and F, He underwent microsurgical resection of the tumor via the intraparietal sulcus approach with the aid of the BrainLab neuronavigation system. Histologic examination confirmed the presence of an anaplastic glioma. G and H, Gross-total removal of the lesion was documented on postoperative computed tomography scans. In addition to surgery, the patient underwent subsequent radiochemotherapy.

Glioblastoma is the most malignant tumor known in the central nervous system. This tumor accounts for 12% to 15% of all intracranial tumors and shows astrocytic differentiation in 60% to 75% of cases. The mean age at diagnosis is 45 to 75 years, with a male-to-female ratio of 1.28 : 1. Histopathologic study reveals the typical malignant characteristics, such as nuclear atypia, cellular pleomorphism, a high mitotic rate, and necrosis. These are WHO grade IV tumors.^58,100

MRI may show the typical aspect of high-grade glioma, a central necrotic area surrounded by ring contrast enhancement. Whether to surgically resect a glioblastoma and to what extent depends on the patient’s neurological and neuropsychological status and the size, location, and extension of the tumor within the adjacent anatomic structures. In particular, if the fornices have been invaded, biopsy is preferred instead of extensive surgery.

Choroid Plexus Papillomas

Choroid plexus papillomas account for 0.5% to 0.6% of intracranial lesions in adults and for 2% to 5% of intracranial neoplastic lesions in children. They are among the most common tumors in children younger than 2 years. When these lesions are located in the lateral ventricle, the median age at diagnosis is 18 months, and there is no predilection to occur in either sex.⁵⁸ In around 20% of pediatric cases, these tumors show malignant transformation; approximately half of them are located within the atrium. They are similar in aspect to the glomus of the choroid plexus, being formed by a single cell layer of cuboid or cylindrical cells surrounded by a thin fibrovascular structure. In the latest WHO classification, choroid plexus papillomas are classified as grade I. In imaging studies, these tumors frequently contain calcifications, hemorrhagic elements, or cysts. In some cases, choroid plexus papillomas may fill the ventricular cavity, whereas in others, the ventricle may be markedly dilated ipsilaterally.

Surgical removal is the treatment of choice (Fig. 138-10). It is helpful to expose and interrupt the proximal arterial supply from the choroidal artery to devascularize the tumor at an early stage of the procedure. Debulking is then continued with the aid of an ultrasonic aspirator. At the end of the procedure, the remaining portions of the apparently healthy choroid plexus are resected as well to ensure complete tumor removal.^101–104

FIGURE 138-10 A and B, Preoperative axial contrast-enhanced T1-weighted magnetic resonance image of a 65-year-old man with cognitive deficits. Initially, a metastatic tumor was suspected because the patient had previously suffered from renal carcinoma. C, The tumor was approached via the right intraparietal sulcus. Surprisingly, histologic examination furnished evidence of a benign plexus papilloma that was completely removed, as seen on postoperative computed tomography (CT) (D). Similar to the previous patient, a large plexus papilloma within the right lateral ventricle is seen on this preoperative CT scan of a 48-year-old woman. E and F, The tumor was completely removed via a left parietal transsulcal/transcortical approach. Before surgery, the patient suffered from symptoms of raised intracranial pressure. The surgical procedure and postoperative course were uneventful.

Meningiomas

Meningiomas of the lateral ventricle originate from the stroma of the choroid plexus and arise at the tela choroidea. Ventricular meningiomas account only for 1% to 5% of all ventricular tumors and are located within the atrium in most cases.^105,106 They may show calcifications and, occasionally, cystic degeneration.¹⁰⁷ Usually, these tumors are highly vascularized (Fig. 138-11). The arterial supply stems from branches of the choroidal arteries, and they drain into the deep ventricular veins. Their histopathologic features and natural behavior are the same as those in any other location in the neural axis. Therefore, most intraventricular meningiomas are benign, slow-growing lesions (WHO grade I). Reports exist of a malignant course in some meningiomas, with recurrence and progression to atypical (WHO grade II) or anaplastic (WHO grade III) variants with the occurrence of metastases.^108–110

FIGURE 138-11 Preoperative axial (A and B), coronal (C), and sagittal (D and E) contrast-enhanced T1-weighted magnetic resonance images of a 58-year-old man revealing a highly vascular tumor (meningioma) of the left atrium that was severely compressing and distorting the surrounding structures. The patient had progressive headache, vertigo, and visual field disturbance. F, After being placed in the right park bench position, he underwent surgery with the aid of the BrainLab neuronavigation system. G and H, Note the significant vascularization as shown by digital subtraction angiography.

On radiologic images, they typically appear as a solid enhancing mass with cystic components. Meningiomas usually become clinically evident as a result of obstruction of CSF pathways, which consequently leads to obstructive hydrocephalus. In most cases, typical symptoms of elevated intracranial pressure do not occur, but rather, cognitive deficits affecting attention and memory dominate the clinical findings. Patients with these symptoms have a good prognosis overall after removal of the cause.⁶⁵ As a result of their location, these tumors can gain mass volume before causing neurological symptoms.^106,111

Because of their typically benign nature, radical microsurgical removal of these tumors defines cure, but postoperative follow-up with regular imaging studies is recommended. Asymptomatic lesions can be monitored by imaging studies, whereas symptomatic lesions and meningiomas with documented growth should be resected microsurgically before they can cause obstructive hydrocephalus and result in rapid neurological deterioration.¹¹²

Epidermoid Cysts

Epidermoid cysts only occur rarely within the ventricular system, and if so, they show a predilection for the fourth ventricle, from which they may extend into the third ventricle. In our follow-up series, we have encountered two patients who harbored an epidermoid cyst within the lateral cavity: one was a huge paraventricular lesion that extended widely into the lateral ventricle, and the other was a primary lateral ventricle epidermoid cyst (Fig. 138-12).

FIGURE 138-12 Preoperative axial (A and B) and coronal (C and D) T1-weighted magnetic resonance images taken in a 44-year-old woman who suffered from recurrent epileptic seizures. A large tumor mass compatible with an epidermoid cyst is extending within the right lateral ventricle, the largest amount of tumor being located within the atrium and inferior horn. E and F, After placing the patient in the supine position with the head rotated to the left side, a curved skin incision was made in the right temporoparietal region. G-I, Gross total removal of this epidermoid cyst was confirmed on postoperative computed tomography.

The Transnasal Transsphenoidal Endoscopic Approach to the Third Ventricle

During the past decade, use of the endoscope for transsphenoidal surgery has gained increased popularity. Although it was an adjunct in the beginning, pure endoscopic removal of pituitary adenomas has now become routine at many centers.^113–115 The endoscopic transsphenoidal approach to the third ventricle is used primarily for tumors that originate in the sellar area and for those showing suprasellar extension, such as pituitary adenomas, Rathke’s cleft cysts, or cystic craniopharyngiomas. The main limitation of this approach is difficulty in performing fine dissection if the tumor has a firm consistency and is adherent to the optic pathway and perforators.

The Transfrontal Transsphenoidal Subchiasmatic Approach

This approach through the frontal base along the extended transsphenoidal route below the optic chiasm and between the optic nerves allows access to the floor of the third ventricle. Because of the limited working space and depth of field, this approach is suitable only in combination with endoscopic technique. A purely transnasal transsphenoidal approach with drilling of the tuberculum sellae and the posterior floor of the planum sphenoidale will provide the same exposure.

The Subfrontal Trans–Lamina Terminalis Approach

The subfrontal interhemispheric trans–lamina terminalis approach is most suitable for lesions located in the anterior part of the third ventricle, especially for those that develop anterior to a line joining the anterior ridge of the foramen of Monro and the cerebral aqueduct.¹¹⁶ Before surgery, a lumbar drain is inserted for early CSF release and relaxation of the brain. The patient is placed in the supine position with slight extension of the head. A bicoronal incision is made with preservation of a large pericranial flap for closure of the frontal sinus. A unilateral or bilateral craniotomy is performed, depending on the precise location and extension of the tumor. The dura is opened parallel to the orbital roof. The olfactory bulbs are sharply dissected from the olfactory sulcus. All basal arachnoid membranes are opened to allow the brain to fall backward and thus increase the working space. The optic nerves, the chiasm, and the vascular complex of the anterior cerebral arteries have to be well identified. The lamina terminalis is then opened for further tumor resection.

The advantage of this approach is good visualization of the optic nerves and chiasm (behind or above it), the anterior communicating artery, the lamina terminalis, both A2 segments (and below the chiasm), both internal carotid arteries, the posterior communicating arteries, their perforating branches, and the pituitary stalk. With careful dissection, the cerebral cortex can be kept intact.^117–119

The Anterior Transcortical Approach

As described in the section on removal of tumors in the lateral ventricle, this route provides excellent access to the frontal horns and bodies of the lateral cavities, and the anterior part of the third ventricle can be exposed in a manner similar to that of the anterior interhemispheric transcallosal approach. The main disadvantage of this approach relative to the interhemispheric transcallosal route is that not only are commissural fibers disrupted but also parts of the projection fibers and short and long association fibers.

The Anterior Interhemispheric Transcallosal Approach

This approach was also described earlier in the discussion on lateral ventricular tumors. Lesions that extend from the third ventricle within the lateral ventricles with a wide foramen of Monro are especially suitable for this approach. Depending on the venous anatomy, the foramen of Monro can be enlarged by opening the choroidal fissure posteriorly as far as the junction of the anterior septal vein with the internal cerebral vein.¹² The exact anatomic variations can be studied preoperatively with CT or MR venography.

The Interforniceal Approach

After a standard parasagittal craniotomy, as performed for the anterior transcallosal approach, the interhemispheric fissure is opened along the falx toward the corpus callosum. Instead of using rigid brain retractors, the brain can be held apart with soft cotton strips and balls. When advancing near the area of the corpus callosum, the key is to identify the callosal and pericallosal arteries.

The corpus callosum should be entered by blunt dissection technique. The size of the opening depends on the size of the lesion. The vascular ependyma tends to bleed in a diffuse way, and hemostasis must be established before further advances to prevent visual compromise.

At this stage, it is crucial to identify the interforniceal raphe, which is not consistent and should be evaluated by MRI before surgery for each patient individually. In some cases, the septum possesses a cavum that can be chosen as an entry point. However, this cavum is not usually present and the septal leaves are fused, which means that they have to be sharply dissected to create a raphe. This corridor generally guides the surgeon to the tumor, which becomes obvious in the surgical field at this stage of the procedure. The point of entry should be approximately 2 cm posterior to the foramen of Monro to preserve the hippocampal commissure. Damage to this structure results in temporary or permanent problems with memory function. The foramen and raphe provide sufficient space to inspect the entire cavity and remove the tumor masses.

Complications of this approach are related to disruptions of forniceal projections and include cognitive impairment and temporary or permanent memory loss. Therefore, retractor force and traction on midline, ventricular, and paraventricular structures should be minimized.

The Posterior Transcallosal Transcortical Approach

This approach is suitable for lesions in the region of the posterior third ventricle and the area of the quadrigeminal plate. It also provides excellent exposure of pathologies involving the vein of Galen and the straight sinus and provides access to the pineal region.

An external ventricular drain may be placed before surgery for release of CSF to provide additional brain relaxation. The patient is placed in either the prone or semisitting position. As in the anterior approach, the interhemispheric fissure is dissected carefully. Special attention has to be paid to the cortical draining veins, which may have to be mobilized to gain more space. The falx, corpus callosum, posterior incisura, and splenium serve as anatomic landmarks. At this stage of the procedure, neuronavigation may be of help to verify the planned trajectory. Extension of the surgical field can be achieved by partial incision of the falx, tentorium, and splenium. When the corpus callosum is identified, a midline incision and smooth retraction (with tailed cotton strips and balls) are performed to enter the cavity of the third ventricle. The ventricular drain should be kept in place during the postoperative course to serve as both a diagnostic and therapeutic means in the event of temporary obstructive hydrocephalus caused by swelling or blood within the ventricular system.

Morbidity related to this approach includes seizures, hemiparesis, and visual field deficits. Damage to the corpus callosum and splenium can result in dyslexia, possible mutism, auditory deficits, and memory loss. As mentioned already, there is a risk for venous infarction from partial sacrifice of the cortical veins. Damage to the quadrigeminal plate located in the depth of the surgical field can cause visual deficits or disorders of eye movement, including those as severe as a complete Parinaud syndrome.

The Occipital Transtentorial Approach

This approach offers excellent exposure of the pineal region and posterior third ventricle. The patient is typically placed in the prone “Concorde” position. The advantage here is that surgeons can place themselves in a more comfortable body position and have a straight view to the surgical field, thus making anatomic orientation easier than, for example, the so-called park bench position, where the patient is turned 90 degrees. The disadvantage of the Concorde position is elevated venous pressure in the patient, which renders control of bleeding more challenging.

By inserting an external drain into the posterior part of the lateral ventricle, CSF can be drained, which leads to brain relaxation, offers additional space, and reduces the impact of brain retraction. The tentorium is best incised parallel and approximately 1 cm lateral to the straight sinus to expose the surface of the cerebellum. To advance toward the ventricle, the splenium has to be dissected, and by opening the tela choroidea, the cavity of the third ventricle can be entered for removal of tumor. It is useful to leave an external drain in place after surgery to allow CSF drainage. Postoperative swelling can lead to stenosis of the aqueduct and result in obstructive hydrocephalus with the need for temporary external drainage.

This approach carries the risk of damaging the occipital lobe, splenium, and corpus callosum, as well as the venous drainage to the straight sinus, which can lead to visual and cognitive impairment and possible split brain syndrome.

The Infratentorial Supracerebellar Approach

This approach offers excellent access to the third ventricle with the least damage to neuronal and surrounding structures and complete preservation of the corpus callosum.

Placement of the patient in the sitting position is ideal when a patent foramen ovale is excluded. The advantage of this position is lower venous pressure than occurs with the alternative Concorde and park bench positions. In addition, gravity drags the cerebellum down and the tentorium holds the supratentorial contents in place, thereby providing a route that needs less or, in the best case, no retraction force. The head is fixed in the Mayfield holder in the flexed position. The craniotomy is performed in such a way that the transverse sinuses are exposed, as well as the torcular, and it should be large enough for the cerebellum to fall back. The dura is opened in a typical horseshoe fashion, with the base at the transverse sinus. After reflection of the dura, the upper cerebellar hemispheres are visible. Extensive release of CSF from the cisterna magna improves brain relaxation. Blunt dissection is performed with bipolar forceps and tailed cotton strips to advance to the third ventricle. The vein of Galen, the pineal body, and the quadrigeminal plate serve as important landmarks. To access the third ventricle, a trajectory is chosen lateral to the pineal body through the velum interpositum and above the suprapineal recess.

Caution must be taken to preserve the vein of Galen, habenula, posterior commissure, and quadrigeminal plate. Damage to these structures can lead to devastating morbidity, cognitive impairment, or permanent mutism.

The Interhemispheric Subfrontal Trans–Lamina Terminalis Approach

This approach provides straight-line access to the anterior portion of the third ventricle, which is most suitable, for instance, for craniopharyngiomas (Fig. 138-13A and B). The patient is placed in the supine position. A bicoronal skin incision is made, and a midline frontobasal craniotomy is performed. After opening the dura mater on both sides of the frontal poles, the initial portion of the superior sagittal sinus is ligated, and the insertion of the falx is completely resected from the crista galli. To gently elevate both frontal lobes, the olfactory nerves are freed from their arachnoid sleeve on both sides. Arachnoid dissection is carried out interhemispherically, and the lamina terminalis is exposed by gently mobilizing the anterior cerebral arteries. Opening the lamina terminalis up to the genu of the corpus callosum provides wide access into the third ventricle (see Video 138-1).

FIGURE 138-13 A, Preoperative contrast-enhanced T1-weighted magnetic resonance images (MRIs) in axial (a), coronal (b), and sagittal sections (c) and T2-weighted axial MRIs (d and e) showing a highly vascular tumor (a craniopharyngioma) within the third ventricle that is expanding the mesencephalic tegmentum and extending up to the level of the foramen of Monro. This 46-year-old man suffered from headache and visual disturbance and had severely disturbed consciousness. Surgery was performed via the anterior subfrontal interhemispheric approach. Postoperatively, the patient’s condition and cognitive function gradually improved, and there was no additional morbidity. B, Postoperative contrast-enhanced T1-weighted MRIs of the patient shown in A taken in the axial (a-c), coronal (d), and sagittal (e) planes.

The Lateral Subfrontal Trans–Lamina Terminalis Approach

This approach requires a combined pterional cranio-orbital craniotomy. Extradural resection of the anterior clinoid process and incision of the falciform ligament allow mobilization of the optic nerve. The patient is placed in the supine position with the head rotated 45 degrees to the left side and extended dorsally. A typical pterional craniotomy is performed that includes the right orbital rim and removal of the orbital roof. In some instances, a cranio-orbitozygomatic approach can be performed to allow even more retraction and provide a good inferior to superior viewing trajectory (Fig. 138-14).

FIGURE 138-14 Axial (A and B), sagittal (C), and coronal (D) contrast-enhanced T1-weighted magnetic resonance images (MRIs) of a 34-year-old man suffering from severe headache. A vascular tumor entirely confined to the third ventricle is apparent. E, Exposure of the tumor was achieved by a right-sided combined pterional-orbitozygomatic craniotomy through a curved skin incision. F and G, A one-piece orbitozygomatic bone flap was used. H and I, Total tumor removal was achieved, as seen on postoperative MRIs, without producing additional neurological deficits.

After removal of the orbital roof, the meningo-orbital band is incised along the lateral rim of the anterior clinoid process. This bony structure is separated from the dura with a sharp dissector. Drilling is carried out extradurally in the lateral portion of the anterior clinoid process and gradually extended medially to expose the optic canal. Further drilling toward the optic strut and carotid artery completely detaches the anterior clinoid process, which can then be removed. If bleeding from the cavernous sinus is noted, extradural hemostasis is achieved by injecting fibrin glue into the cavernous sinus. The dura mater is incised and reflected. Opening of the sylvian fissure is followed by arachnoid dissection and opening of both optic cisterns. After gentle elevation of the frontal lobe, the course of the A1 portion of the anterior cerebral artery is exposed up to the midline, and the lamina terminalis is opened widely.

In contrast to the subfrontal midline approach, a slightly oblique view into the third ventricle is obtained. Nonetheless, this approach allows good control of the oculomotor nerve on both sides and the interpeduncular cistern and pituitary stalk through the opticocarotid space. With this approach, the left thalamus can easily be visualized; however, the right superior wall of the third ventricle escapes direct visualization. This shortcoming can be compensated for by using an endoscope with a 30- or 70-degree angled optic tip, which offers good visualization of the superior portion of the third ventricle. We have used this approach to remove gliomas and craniopharyngiomas of the third ventricle (see Video 138-2).

The anterior transcortical approach can be used to first enter the right lateral ventricle via a transsulcal transfrontal parenchymal approach. Use of neuronavigation is helpful for establishing the appropriate trajectory. After opening the ventricular ependyma, CSF is released completely from the ventricle, and the foramen of Monro is visualized. With this approach, contrary to the midline interhemispheric transcallosal approach, a slightly oblique view is obtained. Incision of the tela choroidea lateral to the fornix and dissection of the internal cerebral veins and the medial posterior choroidal artery provide access to the third ventricle. This approach has been used, for instance, in patients with cavernous malformations, as shown in Figure 138-15 and Video 138-3.

FIGURE 138-15 A-C, Preoperative magnetic resonance images (MRIs) of a 34-year-old woman demonstrating two separate cavernous malformations: one located in the right frontal periventricular area (arrow) and the other within the third ventricle at the level of the foramen of Monro. The patient had symptoms of raised intracranial pressure. D, Both lesions were removed via a right frontal transsulcal, transcortical access route with the patient in the supine position; a coronal skin incision behind the hairline was used. Both vascular malformations were completely resected, as seen on the postoperative computed tomography scan (E) and MRIs (F-H). The transfrontal access route can be seen on postoperative T2-weighted sagittal (G) and coronal MRIs (H). The patient experienced no additional neurological or cognitive disturbances.

The Anterior Interhemispheric Transcallosal Paraforniceal Approach

After adequate exposure and reflection of the dura mater, the interhemispheric space on the right side is dissected down to the anterior portion of the corpus callosum. The corpus callosum is incised medial to the pericallosal artery, and the right lateral ventricle is opened. In lesions that have already enlarged the foramen of Monro, no further action may be necessary. The tumor can be debulked and gradually removed. However, in many instances a paraforniceal transchoroidal approach is necessary. This approach is suitable for gaining access to the anterior two thirds of the third ventricle from the roof to the bottom (Fig. 138-16). Dissection starts lateral to the right fornix at the posterior margin of the foramen of Monro, and the tela choroidea is gradually incised. The plexus choroideus and the right internal vein are left laterally. Dissection is carried out between the two internal cerebral veins while being careful to avoid the medial posterior choroidal artery. After incising the inferior membrane of the tela choroidea and mobilizing the choroid plexus of the third ventricle, the right fornix is gently mobilized toward the opposite side, and the right thalamus (including the thalamostriate vein) can be visualized posteriorly up to the posterior commissure and superiorly to the splenium of the corpus callosum (see Video 138-4).

FIGURE 138-16 Preoperative axial (A), coronal (B), and sagittal (C) T1-weighted magnetic resonance images (MRIs) demonstrating a contrast-enhancing irregular tumor confined to the third ventricle. This 23-year-old man suffered from severe headache. Initially, endoscopic ventriculocisternostomy was performed at another institution. D, Subsequently, the tumor was removed via the interhemispheric transcallosal paraforniceal approach with the patient placed in the supine position. E-H, Complete tumor excision was confirmed on postoperative MRIs. There were no additional neurological or cognitive deficits postoperatively.

The Occipital Transtentorial Approach to the Pineal Region

This approach is most suitable for patients with a tight infratentorial space or large tumors of the pineal region with significant caudal extension that requires an adequate superior to inferior viewing trajectory (Fig. 138-17).

FIGURE 138-17 Axial (A and B), coronal (C), and sagittal (D) contrast-enhanced T1-weighted magnetic resonance images (MRIs) taken in a 56-year-old man showing a homogeneous tumor located in the tectal region and posterior third ventricle. Gross-total removal of the tumor was achieved via the combined supracerebellar infratentorial and right occipital transtentorial access route. This was confirmed on postoperative MRIs (axial, E; sagittal, F). Histologic examination revealed a solitary fibrous tumor. Preoperatively, the patient suffered from diffuse headache, vertigo, urinary disturbance, and hiccups. All symptoms gradually improved after surgery, and there was no additional morbidity.

The difference between our technique and the standard approach (see earlier) lies in the fact that we prefer to combine the supratentorial craniotomy with an infratentorial craniotomy extending bilaterally to the midline. Surgery is begun by opening the cerebellar dura below the transverse sinus on both sides. This allows inspection of the supracerebellar bridging veins before exposing the supratentorial region (see Video 138-5). Incision of the tentorium just lateral to the straight sinus can be done more safely with precise knowledge of the infratentorial venous pattern. Moreover, dissection of the firm arachnoid surrounding the tectal plate and the vein of Galen can easily be performed before incising the tentorium from the infratentorial direction at the beginning of the procedure. This facilitates orientation and subsequent dissection from the supratentorial direction.

The Infratentorial Supracerebellar Approach

We use this approach for accessing small tumors of the pineal region, such as pineocytomas (or fibrillary astrocytomas) (Fig. 138-18) or lesions involving the midbrain tectum or posterior thalamus (e.g., cavernous malformations, various gliomas). Great attention is paid to the superficial cerebellar veins, which are always preserved. Usually, these lesions are located more or less in the midline; however, they can be accessed unilaterally via the infratentorial supracerebellar space. The side of access depends on the local venous pattern. In all cases, the arachnoid that surrounds the tectal plate and the veins that drain into the vein of Galen are dissected meticulously to clearly identify all anatomic structures in the region.

FIGURE 138-18 Preoperative contrast-enhanced T1-weighted magnetic resonance images (MRIs) taken in the axial (A and B) and sagittal (C) planes showing a small tumor within the posterior part of the third ventricle. D, The patient, a 16-year-old girl, was operated on in the sitting position. E, Intraoperative photograph of the supracerebellar infratentorial surgical approach that was used. The cerebellar culmen has been gently mobilized inferiorly, and the tumor is visible in the vicinity of the vein of Galen. F, Macroscopic aspect and histologic specimen of the lesion confirmed as a fibrillary astrocytoma. The tumor was completely removed as seen using postoperative computed tomography (H) and T2-weighted MRI (I). There was no postoperative morbidity.

Colloid Cysts

Colloid cysts are benign tumors with an incidence of 0.5% to 1% that are most commonly located in the roof of the third ventricle.¹²⁰ Most contain gelatinous viscous material. This pathology was first described by Wallmann in 1858 as an autopsy finding, and Walter Dandy was the first to successfully remove this kind of tumor in 1921. The tumor can remain clinically silent for a long time and is therefore often detected only at autopsy.¹²¹ Colloid cysts can be found in patients of any age, with a peak between the second and fourth decades and no differences in distribution according to sex.

Colloid cysts can attain a large volume that potentially occludes the foramen of Monro and leads to acute, obstructive hydrocephalus. Mechanisms of occlusion include shifting as a result of changes in CSF flow patterns after lumbar puncture, shunt dysfunction, or hemorrhage. In 75% of patients, the initial symptom is headache with changing intensity. The episodic appearance of symptoms with irregular symptom-free intervals makes the diagnosis somewhat challenging based purely on clinical examination. Other symptoms arising from elevated intracranial pressure include nausea, vomiting, dizziness, and fatigue. A sudden increase in intracranial pressure from rapid-onset hydrocephalus can lead to sudden death.^122,123 By compression of surrounding anatomic structures (e.g., thalamus, pituitary gland, and optic chiasm), the cyst can cause thermal deregulation, electrolyte and hormone imbalances, endocrine dysfunction, altered personality, memory loss, or visual changes.

Differential diagnosis includes neurocysticercosis, intracranial aneurysm of the posterior circulation, and vertebrobasilar dolichoectasia. Normally, a colloid cyst does not grow significantly. More common is an increase in volume secondary to intracystic hemorrhage.

On plain CT, the lesion appears as a hyperdense mass but also has isodense and hypodense parts. This lesion does not usually show contrast enhancement and rarely demonstrates rim enhancement. On MRI, the tumor is either isointense to hyperintense on T1-weighted images or hypointense to hyperintense on T2 sequences. The radiologic appearance depends on the amount of gelatinous material within the tumor (Fig. 138-19).

FIGURE 138-19 This 51-year-old man was admitted on an emergency basis because of obstructive hydrocephalus, as seen on preoperative computed tomography scans (A and B). Initially, an external ventricular drain was placed to release the raised intraventricular pressure. Preoperative T2-weighted magnetic resonance images (MRIs) in the axial (C) and sagittal (D) planes show a large colloid cyst within the third ventricle obstructing the foramen of Monro. Complete removal of the colloid cyst was achieved microsurgically via the right frontal interhemispheric transcallosal approach and confirmed on postoperative T1-weighted (E and F) and T2-weighted (G and H) MRIs. The postoperative course was entirely uneventful.

Treatment options include conservative, microsurgical, or endoscopic therapy. An asymptomatic patient may be managed with follow-up imaging studies. If neurological changes are seen, prompt imaging studies should be obtained. The only definite treatment of this pathology is resection of the cyst by microsurgical or endoscopic techniques. Neuronavigation serves as an important tool to further optimize the surgical trajectory and improves the safety and efficiency of operative interventions. In any case, rupture of the cyst must be avoided because this will lead to ventriculitis and meningitis.^{13,120,124–136}

Ependymomas

As described earlier in the section “Common Tumors of the Lateral Ventricles,” ependymomas are generally considered to be slow-growing, noninvasive tumors that arise from the walls of the ventricles. Clinical evidence of third ventricular ependymomas results mainly from obstructive hydrocephalus with a consequent increase in intracranial pressure and cognitive deficits. On MRI and CT, ependymomas appear as well-circumscribed lesions with heterogeneous contrast enhancement (see Fig. 138-14). Removal is still in the domain of microsurgery, but there are reports of successful removal purely by endoscopic technique.^{58,65,67,137–139}

Craniopharyngiomas

Craniopharyngiomas account for between 1.2% and 4.6% of all intracranial tumors and are the most non-neuroepithelial intracranial pathologies seen in the pediatric population, with a frequency of occurrence of 5% to 10%. They have two incidence peaks, between 5 and 10 years and between 45 and 60 years of age. They tend to occur equally in males and females.

Craniopharyngiomas originate from layers of Rathke’s pouch within the sellar region. They are considered benign and slow growing. With increasing volume, they tend to displace surrounding neuronal and vascular structures and can potentially invade the third ventricle.¹⁴⁰ The most common histologic pattern is that of an adamantinomatous craniopharyngioma. Its microscopic features include squamous epithelium, lobules, palisades, and cysts filled with debris. Structures consisting of keratin and cholesterol can be detected. This entity is classified as WHO grade I.⁵⁸

CT reveals a mixed-appearing solid mass with calcification and heterogeneous contrast enhancement caused by the presence of nodules and capsules. Cysts appear hyperintense on T1-weighted MRI, whereas nodules and solid portions of the tumor are isointense. When contrast material is administered, these components show enhancement.¹⁴¹

Clinical symptoms are caused by displacement or compression of surrounding structures and include visual field deficits, endocrine imbalance, and signs of intracranial hypertension caused by obstructive hydrocephalus.

Microsurgical resection is the treatment of choice. The lesion can be accessed either by the anterior interhemispheric subfrontal approach (see Fig. 138-13A and B) or by the combined pterional cranio-orbital transsylvian approach (Fig. 138-20). In recent years, successful treatment by endoscopic technique has also been reported; cystic intrasellar tumors are especially suitable for this treatment. Neuroendoscopy can be used for both diagnosis and treatment of these tumors. Moreover, tumor resection and additional resolution of tumor-related hydrocephalus can be achieved at the same time.^135,142 If radical resection is not accomplished, tumor recurrence is expected.^{58,116,143–145}

FIGURE 138-20 Preoperative contrast-enhanced coronal (A), sagittal (B), and axial (C) T1-weighted magnetic resonance images of an 11-year-old girl with severe visual disturbance. A solid vascularized tumor is occupying the third ventricle. D, Resection was performed via a right-sided combined pterional-orbitozygomatic craniotomy. Histologic examination revealed a craniopharyngioma. E and F, Early postoperative computed tomography scans confirmed total removal. Clinically, the patient recovered fully after an initial slight hormonal imbalance.

Gliomas

Glial tumors include pilocytic (WHO grade I) and anaplastic (WHO grade III) astrocytomas. They originate from the thalamic or hypothalamic regions or from areas of the midbrain, and they expand into the third ventricle. Depending on their nature, they become clinically evident by displacing or infiltrating the surrounding anatomy. By compromising these neuronal structures, symptoms such as endocrine imbalance, intracranial hypertension, and visual and cognitive impairment occur.

On CT and MRI, pilocytic astrocytomas appear well circumscribed and show contrast enhancement. In anaplastic astrocytomas, MRI shows the typical feature of high-grade glioma, a central necrotic area surrounded by ring contrast enhancement.⁵⁸

Therapeutic options depend on the patient’s clinical condition, histologic findings, and tumor size and degree of expansion. Therefore, interventions range from performing radical resection and diagnostic biopsy to the administration of adjuvant therapies, or a combined approach. Although microsurgical removal has been the only operative means for decades, neuroendoscopy has brought a new treatment option to the field.^{13,20,146–149}

Chordoid gliomas originate within the third ventricle (Fig. 138-21). They are noninvasive and show slow progress. Choroid gliomas were defined as an independent entity in 1998, and they correspond histologically to WHO grade II tumors.^58,150 These tumors are rare, and knowledge of their incidence is still based mainly on a few small series and case reports with short follow-up periods.¹⁵¹ Analyses show that this tumor is diagnosed at a mean age of 46 years, with a male-to-female ratio of 1 : 2.

FIGURE 138-21 This 67-year-old man was evaluated for progressive visual disturbance. On preoperative contrast-enhanced magnetic resonance images (MRIs) in axial (A and B), sagittal (C), and coronal (D) sections, a solid tumor is seen within the anterior third ventricle. E, A right-sided combined pterional-orbitozygomatic craniotomy was used to expose the tumor. Histologic examination revealed a grade II chordoid glioma. F-I, On postoperative MRIs, total tumor removal was confirmed. There was no additional postoperative morbidity.

Chordoid gliomas are located in the midline, mostly in the anterior part of the third ventricle. With increasing volume, this tumor displaces surrounding neuronal structures and causes obliteration of the CSF pathway, consequently leading to elevated intracranial pressure. Depending on the location of the anatomic displacements, endocrine disturbances, impaired vision, and cognitive deficits have been reported.¹⁵² In terms of histopathologic findings, this neoplasm consists of epithelioid cells in clusters and row formations.^153,154

On CT, the tumor appears ovoid shaped and well circumscribed, with homogeneous contrast enhancement. T1-weighted MRI reveals the mass as isointense to hypointense, and T2-weighted images display the surrounding edema.¹⁵⁵ The use of dynamic contrast medium has been reported to further help differentiate this lesion radiologically from meningiomas and malignant gliomas.¹⁵⁵ Microsurgical resection is the only treatment option available, and thus far, adjuvant therapies have failed. If radical removal of the tumor is not achieved, it can recur.^156,157

Pineal Region Tumors

In 1913, the first attempts to remove tumors located in the pineal region resulted in significant morbidity and mortality. As neuroanatomic knowledge and technical expertise improved, Oppenheim, Krause, and Dandy were able to report successful removal of pineal tumors. Since then, there has been steady improvement in treatment modalities and surgical approaches for these lesions.¹⁵⁸

In addition to lesions arising primarily from pineal parenchyma, secondary neoplasms are commonly detected in the pineal region, such as astrocytomas, meningiomas, ependymomas, and metastases to the brain.^159–162 Primary lesions originate from pineal and germ cells and range histologically from benign to malignant entities. They include pineocytomas, pineoblastomas, and papillary lesions. Although pineocytomas are considered benign (WHO grade I), papillary tumors vary in their behavior and are graded WHO II to III. Histopathologically, pineoblastomas are malignant lesions and are thus classified as WHO grade IV.

Pineocytomas are considered benign and rare neoplasms that typically show slow progress, but progression toward malignancy has been reported.¹⁶³ They originate from pineal cells and account for less than 1% of intracranial lesions. The mean age of patients at diagnosis is 38 years. Because of their benign behavior, they displace the surrounding anatomy and can expand into the third ventricle. Clinical evidence of pineocytomas results from compromise of the affected neuronal structures and includes Parinaud’s syndrome, endocrine imbalances, cranial nerve palsies, diminished mental state, or symptoms of intracranial hypertension.

Imaging-wise, pineocytomas appear as well-demarcated, hypodense or slightly hyperdense masses that show homogeneous contrast enhancement on CT. On MRI, they look lobulated and isointense on T1-weighted images and hyperintense on T2-weighted images, and they again show homogeneous enhancement when contrast material is administered. Their appearance is well circumscribed, and they displace surrounding structures without invasion. In addition, calcification can be detected.¹⁶⁴

Pineoblastomas are ranked WHO grade IV and arise from the pineal parenchyma. Although their incidence overall is very low, they represent 40% of all primary pineal neoplasms. Extraneural dissemination through an implanted ventriculoperitoneal shunting system has been reported. In this case, radiotherapy was administered.¹⁶⁵ These lesions occur predominantly in children and affect both sexes equally.

The cause and type of clinical symptoms in patients with these tumors are the same as for other tumors harbored in this anatomic region.

CT reveals a mass that is less well demarcated than noted with pineocytomas. This lesion also shows heterogeneous contrast enhancement, and signs of calcification are common. Pineoblastomas appear hypointense to isointense on T1-weighted images. Defined margins with surrounding neuronal structures are poorly discriminated. On T2-weighted sequences these lesions display hyperintensity, and radiologic features of invasion of the surrounding anatomy are seen.^166,167

Papillary tumors are additional primary neoplasms found in the pineal region. They have typical papillary structures and originate from ependymocytes. Because of their rare occurrence, no solid information about their incidence can be presented. Based on limited data, the mean age of patients at diagnosis is 32 years, and both sexes are affected equally.

Clinical symptoms are caused by increased intracranial pressure as a result of obstruction of the CSF pathway. On imaging studies, they appear hypodense with heterogeneous contrast enhancement on CT and hyperintense on contrast-enhanced T1- and T2-weighted MRI sequences. The absence of fat, hemorrhage, and calcification helps differentiate this lesion from other entities found in the pineal region.¹⁶⁸

Radical surgical resection is considered the treatment of choice, followed by radiotherapy. If tumor remnants are present, recurrence is possible. Craniospinal dissemination may occur, and thus imaging studies of the whole neuraxis should be included in staging and follow-up.¹⁶⁹

Germ cell tumors constitute 0.4% to 3.4% of all intracranial lesions. Germinomas, with a frequency of occurrence of up to 65%, are the most common entity in this subgroup. Other significant germ cell tumors and their corresponding incidences include teratomas (18% to 20%) and the so-called mixed germ cell tumors (25%).

Germ cell tumors usually affect young adults, and the diagnosis is therefore generally established between 10 and 21 years of age (in 70% of cases), with the peak incidence at 10 to 12 years of age and a frequency of occurrence of 27%, but no difference in distribution between the sexes is evident. After the third decade, these tumors are not usually diagnosed.

When these lesions occur within the central nervous system, the pineal region and the third ventricle are the designated sites. As with other tumors located within the pineal region, clinical symptoms arise as a result of compression of surrounding structures and consequently include signs of intracranial hypertension, neuro-ophthalmic syndromes, cranial nerve palsies, endocrine disturbances, or cerebellar signs.

Depending on the histopathology of pineal region tumors, the patient’s neurological condition, and characteristics of the tumor (location, size, and expansion rate), the following should be discussed with patients on an individual basis: microsurgical or endoscopic removal, biopsy, and adjuvant treatments.^{166,170–175}

Tectal Tumors Involving the Third Ventricle

Tumors arising from the tectal region are a heterogeneous collection and grossly belong to the group of glial neoplasms.

Low-grade gliomas have their peak incidence in the third decade. They tend to grow in a diffuse and infiltrative fashion. Clinical symptoms include cranial nerve palsy and intracranial hypertension when the third ventricle is invaded.

On MRI sequences, this tumor does not show contrast enhancement, and necrosis is absent. Malignant brainstem gliomas show all aspects of malignancy. Patients are usually older than 40 years. Tumor histology shows nuclear atypia, cellular anaplasia, and mitoses. On imaging studies, ring enhancement and necrosis can be displayed.

Focal tectal gliomas typically cause signs of elevated intracranial pressure as a result of obstructive hydrocephalus (Fig. 138-22). Their histopathologic features are consistent with those of other low-grade gliomas.

FIGURE 138-22 Preoperative T1-weighted (A and B) and T2-weighted (C and D) magnetic resonance images (MRIs) of a 3-year-old boy with Parinaud’s syndrome (E) caused by a partially cystic tectal glioma extending within the posterior portion of the third ventricle. This tumor was completely removed via the supracerebellar infratentorial approach as can be seen on postoperative MRIs (F and G). The child’s eye movement gradually improved over the following months after surgery.

Depending on the patient’s neurological status and the histologic patterns, size, and expansion of the lesion, treatment options include surgical removal by microsurgical or endoscopic means, diagnostic biopsy, radiotherapy, or a combined approach (see Video 138-6).^58,176,177

Cavernous Malformation

The first report of this vascular pathology was by Luschka in 1854, and Cushing and Bailey further described this lesion in 1928.

Cavernomas have an overall incidence of 0.1% to 0.5% and account for about 5% to 20% of intracranial vascular pathologies. The mean age at diagnosis ranges from 27 to 37 years, and both sexes are affected equally. Usually, cavernomas arise within supratentorial, subcortical parenchyma, but when they expand exophytically, they can invade the ventricular cavities. This tumor shows somewhat dynamic behavior, with enlargement and regression in size and microhemorrhages, changes that mostly occur without clinical correlates.

In terms of histopathologic features, intraventricular cavernous malformations do not differ from other similar pathologies detected within the neuronal axis. Thus, sinusoidal vessels can be detected that typically have thin endothelial walls lacking in smooth muscle and elastic fibers.

On CT, these lesions typically appear as lobular, well-circumscribed masses with a round to ovoid shape. They look hyperdense, and in the absence of hemorrhage, no mass effect or compromise of surrounding brain tissue is present. MRI is superior to CT and is considered the gold standard because it can detect hemosiderin rims and deposits as remnants of an occult history of microhemorrhage, even when the appearance on CT is unremarkable. MRI sequences of cavernous malformations display an overall mixed intensity, and contrast enhancement is minimal or absent in most cases.

Even when located within the ventricular system, the tumor usually remains clinically silent but potentially becomes symptomatic by obstruction of CSF pathways, which produces clinical signs of elevated intracranial pressure (see Fig. 138-15). However, when bleeding within the third ventricle occurs, a dynamic course of neurological deterioration is possible.

With incidental findings, annual outpatient-based clinical and imaging follow-up is justified. Clinical evidence is caused mainly by tumor bleeding rather than by increasing tumor size. Therefore, complete microsurgical removal is the treatment of choice.

Depending on the patient’s condition and the location, size, and degree of expansion of this pathology, radiation therapy is regarded as an alternative option.^178–183

The Telovelar or Transcerebellomedullary Fissure Approach

The telovelar or transcerebellomedullary fissure approach consists of a standard median suboccipital craniotomy extending from the transverse sinus to the foramen magnum. It is performed with the patient either in the sitting or in the prone position. Depending on the size and caudal extension of the lesion, the C1 lamina must sometimes also be removed. The dura is opened in a Y-shaped fashion. The arachnoid membranes are then dissected around the tonsils, the uvula, and the medulla oblongata while taking care to protect the PICA and its branches. By gentle retraction of the tonsils laterally and superiorly, the cerebellomedullary fissure (consisting of the tonsillouvular and tonsillomedullary spaces) can then be opened widely. To further enhance exposure of the area of the aqueduct and lateral recess, the inferior medullary velum should be dissected and incised sharply.^190–192

Ependymomas

Ependymomas are the most common tumors of the fourth ventricle in adults. In most instances, they originate from the lower floor of the fourth ventricle and expand in all directions (see Fig. 138-23A and B). Invasion of the neuraxis may be very limited, but occasionally, the tumor may infiltrate deeply into the lower medulla. Such infiltration of the neuraxis can usually be anticipated from preoperative MRI sequences. Further expansion of the tumor may be seen in the superior, inferior, dorsal, and lateral directions, with or without invasion of the surrounding neural structures. In some instances, even large ependymomas may have a rather small site of attachment (vascular pedicle) located at the lower floor of the fourth ventricle, whereas the remaining portion of tumor may not be firmly attached to the neuraxis.^2,58

Large tumors must first be debulked with the aid of an ultrasonic aspirator. After significant volume reduction, careful inspection of the caudal PICA loop reveals the branches that supply both the tumor and the neuraxis. Branches that exclusively supply the tumor are coagulated and cut. This devascularizes the tumor and allows further volume reduction, followed by separation of the tumor from the lobulus biventer and inferior vermis. Gentle dissection between the superior surface of the tumor and the inferior part of the roof of the fourth ventricle provides access to the ventricle and releases CSF. This superior portion of the tumor may completely fill the fourth ventricle without being attached to the superior half of the rhomboid fossa. These portions can readily be resected with the ultrasonic aspirator. In similar fashion, caudal portions of the tumor that extend beyond the level of the obex can be separated from the neuraxis and resected. Thus, even large tumors are gradually transformed into small lesions attached to the lower rhomboid fossa. The last step in tumor resection is meticulous dissection between the tumor and medulla with bipolar forceps and small cottonoids. Depending on the degree of invasion of the neuraxis, this is the most sensitive and dangerous part of the surgery. By alternately separating portions of the tumor from the neuraxis and removing these parts with the Cavitron ultrasonic surgical aspirator, a clear-cut cleavage plane between the tumor and neuraxis appears.

When a tumor extends into the lateral recess or even into the cerebellopontine angle, these lateral portions are dealt with separately. Small supplying arteries from the lower medulla are coagulated at low current intensity and cut sharply with microscissors. Continuous electrophysiologic monitoring during dissection is mandatory. At the end of resection, meticulous hemostasis is obtained at the site of tumor attachment. This not only controls bleeding efficiently but may also eliminate microscopic tumor residue. As a general rule, it is important to remove the tumor completely because total resection is associated with a lower risk for recurrence¹⁹³ (see Video 138-7).

Medulloblastomas

Medulloblastomas are the most common tumors of the fourth ventricle in childhood. In many instances, medulloblastomas do not invade the floor of the fourth ventricle but rather the middle cerebellar peduncles; nevertheless, the fourth ventricle may be completely filled with tumor up to the aqueduct, and the neuraxis and cerebellum may be severely compressed. Moreover, these neoplasms are highly vascularized, and at their site of attachment (usually the roof of the fourth ventricle) there is no clear-cut boundary with the cerebellum. Exposure is obtained with the telovelar approach (Fig. 138-24). It is important to reduce the volume of the mass as rapidly as possible to avoid intralesional bleeding and reduce the duration of surgery and thus the amount of blood loss.

FIGURE 138-24 A large contrast-enhancing tumor is seen on preoperative axial (A), coronal (B), and sagittal (C) magnetic resonance images taken in a 3-year-old girl suffering from headache and gait ataxia. Surgery was performed with the child in the sitting position (D). Intraoperative photographs demonstrate a highly vascular medulloblastoma within the fourth ventricle (E) that could be completely removed (F and G). After surgery, the fourth ventricle is free of tumor, and the aqueduct can be visualized (G). Postoperative computed tomography scans (H-J) showed complete tumor removal and a significant frontal air collection immediately after the procedure. The postoperative course was uneventful.

Because the tumor is located posterior to the neuraxis, the superior floor of the fourth ventricle cannot be seen at the beginning of surgery. To maintain orientation during the initial steps of tumor debulking, the surgeon may rely on anatomic landmarks such as the caudal PICA loop on both sides, the superior portion of the cervical cord up to the obex, and the roof of the fourth ventricle. Dissection is continued laterally (where the tumor usually extends into the lateral recess) to separate the tumor from the cerebellum. After the initial debulking, the superior pole of the tumor should be removed to obtain a straight-line view to the upper part of the fourth ventricle and aqueduct. Once the superior portion of the rhomboid fossa is visualized, the level of the lower rhomboid fossa can easily be estimated, and tumor resection is carried out in a superior to inferior direction. Attention is paid to avoiding contact with the floor of the fourth ventricle by inserting small cottonoid pledgets between the tumor and the floor of the fourth ventricle.

Many tumors will have an insertion point at the level of the brachium pontis and lateral recess but rarely in the midline. The tumor insertion site should be inspected carefully. Occasionally, hemorrhagic tumor residues are found here that can easily be separated from the cerebellum. Concomitantly, hemostasis is obtained by gentle bipolar coagulation. This aids in avoiding significant blood loss, maintaining a bloodless surgical field, and recognizing all pertinent anatomic structures. Finally, the dura is sutured in watertight fashion.^8,58,192,194

Hemangioblastomas

Hemangioblastomas of the fourth ventricle are highly vascularized lesions (Fig. 138-25). Resection of hemangioblastomas requires a special technique that differs completely from removal of an ependymoma or medulloblastoma. These lesions have a significant arterial supply, mainly from branches of the distal PICA and from numerous small arteries emerging from the cerebellum and neuraxis, in addition to one or several large draining veins. Moreover, branches of the choroid plexus may also supply the lesion. Precise study of preoperative digital subtraction angiograms is mandatory to understand where the main feeding arteries and draining veins are located. Hemangioblastomas cannot be debulked because of their typical morphology. If a hemangioblastoma is penetrated before it is completely separated from its arterial supply, hemostasis can be very difficult or impossible to achieve. Therefore, these lesions must be gradually separated from their arterial supply and removed in one piece while preserving the main draining veins intact until the entire lesion has been isolated from the cerebellum and neuraxis.

FIGURE 138-25 A highly vascular lesion extending within the fourth ventricle, typical of hemangioblastoma, is seen on preoperative magnetic resonance images (MRIs) (A-C) and digital subtraction angiography (D-F). The patient, a 51-year-old woman, was operated on in the sitting position. Intraoperative photographs show the lesion exposed in the dorsal subvermian region (G) and, during microsurgical dissection, in the vicinity of the large draining vein (H). Complete removal of the hemangioblastoma is seen on postoperative MRIs (I-K). There was no additional postoperative morbidity.

The exposure is similar to that for resection of other tumors of the fourth ventricle. Arachnoid dissection to separate the cerebellar tonsils and caudal PICA loops from the lower vermis can be carried out in a fashion similar to that for ependymomas or other tumors. However, the first important step should be to identify the main arterial supply with the aid of an intraoperative ultrasound probe. The supplying arteries can easily be coagulated and separated from the lesion such that in many instances, an initial view of the interior of the superior fourth ventricle is possible. Dissection and devascularization of the lesion are then performed in a circular fashion. Use of small cottonoids and gentle compression of the lesion allow further dissection between the hemangioblastoma and neuraxis for gradual exposure of the surface of the lesion. Care is taken to maintain patency of the draining vein until the very end of this dissection. One should bear in mind that even if most of the visible supplying arteries have been interrupted, a significant amount of residual arterial supply from small transparenchymal arteries of the neuraxis may still be present. Only after the lesion has been completely separated from the cerebellum and neuraxis can the draining veins also be coagulated and divided. With this technique, the surgical field is continuously kept bloodless so that all anatomic details can be seen clearly throughout the procedure, and even large lesions can be removed safely (see Fig. 138-25). Frequent use of the Doppler microprobe is helpful in verifying local hemodynamics and assessing the direction of flow in lesion-supplying vessels. Furthermore, the Doppler signal obtained from the draining vein provides useful information about the amount of residual arterial supply and progress in devascularizing the lesion. Continuous electrophysiologic monitoring reassures the surgeon that function of the neuraxis is intact during these maneuvers.^{190,191,195–199}

Epidermoid Cysts

Epidermoid cysts can sometimes fill the fourth ventricle completely and expand into the lateral recess (Fig. 138-26). Because the lesion itself is not vascularized, debulking appears readily possible. However, epidermoid cysts may be separated by firm arachnoid membranes that divide the lesion into multiple compartments. Moreover, on the outer surface of the neuraxis, the lesion may involve pial vessels that must be sharply dissected and preserved. With a normal suction tube of large caliber or an ultrasonic aspirator, the portion located within the fourth ventricle can easily be debulked. Gentle use of a dissector also helps in separating remote portions of epidermoid cysts from the cerebellum or neuraxis. Within the lateral recess and in patients in whom the cerebellopontine angle has also been invaded, meticulous dissection around the rootlets of the caudal cranial nerves and the PICA and its branches is necessary. In most instances, complete removal of the avascular lesion is possible, and the arachnoid membranes that form compartments of the epidermoid cysts can also be resected. At the end of resection, we always irrigate the fourth ventricle, the lateral recess, and the perimedullary space with saline solution to eliminate small lesion residues that may serve as foci for regrowth of the cyst.^201–202

FIGURE 138-26 A typical epidermoid cyst is seen on preoperative T1-weighted (A) and T2-weighted (B-D) magnetic resonance images. Tumor removal was performed in the sitting position (E) in this 50-year-old man with severe gait disturbance. Gross-total removal of the lesion was achieved (F) without producing additional neurological deficits.

Pilocytic Astrocytoma

Juvenile pilocytic astrocytomas are well-circumscribed tumors that may grow predominantly within the cerebellum, may have a significant cystic portion, and constitute approximately one third of pediatric glial neoplasias.

Pilocytic astrocytomas of the cerebellum that expand within the fourth ventricle are frequently composed of a solid and a cystic tumor portion. Because the tumor attachment is more often located at the roof of the fourth ventricle, these tumors rarely invade the floor of the fourth ventricle; instead, they expand laterally into the brachium pontis or lateral recess. Exposure of these tumors is similar to the procedure for ependymomas or medulloblastomas. However, large tumors may require additional supracerebellar exposure when they extend up to the tectal plate of the midbrain and infiltrate the superior medullary velum.

The best exposure is the telovelar approach, and tumor debulking is performed with an ultrasonic aspirator. When a cyst is present, it is punctured and the cyst fluid is evacuated. The initially expanded lesion then collapses, and the surrounding wall of solid tumor is gradually resected. Visualization of the roof of the fourth ventricle requires a certain retraction of the uvula and nodulus of the vermis, which can be the site of tumor origin. Great effort should be applied to remove the tumor completely because cure can be achieved by radical resection of a benign glioma. For this purpose, combining the telovelar and supracerebellar approaches allows optimal visualization of all portions of the tumor (Fig. 138-27).

FIGURE 138-27 A typical cerebellar astrocytoma with a small solid and a large cystic component is seen on preoperative contrast-enhanced T1-weighted magnetic resonance images in sagittal (A), axial (B), and coronal sections (C and D). This 15-year-old boy had incomplete right-sided sixth and seventh cranial nerve palsy, as well as progressive gait ataxia. The tumor was removed via a combined supracerebellar and telovelar approach. The tumor is seen as a grayish mass on the intraoperative photograph (E); the intact rhomboid fossa is seen at the end of resection (F). Complete tumor removal was confirmed on early postoperative computed tomography scans (G-I).

When using the supracerebellar approach, care is taken to avoid damage to draining veins on the superior surface of the cerebellum. The thick arachnoid of the tectal cistern must be incised gradually while paying attention to the medial cerebellar draining veins. Once the tectal plate has been exposed, gentle retraction of the anterior lobulus quadrangularis, the lobulus centralis, and the culmen of the cerebellar vermis provides access to the superior medullary velum invaded by the tumor. Meticulous hemostasis in the most superior portion of the fourth ventricle is also achieved with use of the supracerebellar route.^58,203–205

Exophytic Cavernous Malformations

Although cavernous malformations involving the fourth ventricle originate mainly from the neuraxis, intralesional hemorrhage and exophytic growth may lead to significant expansion into the fourth ventricle (Fig. 138-28). As for other lesions, optimal exposure is obtained with the telovelar approach. Similar to resection of ependymomas, dissection is carried out on the floor of the fourth ventricle because most of these lesions are not directly connected to the cerebellum. If a hematoma cavity is present, the cavity is opened and the hematoma is evacuated by aspiration. By using small cottonoid pledgets and bipolar coagulation at low current intensity, the vascular lesion is gently separated from the neuraxis, and the caverns are coagulated to shrink the entire lesion. Dissection is carried out in a circular fashion. Separated portions of the malformation are either removed directly or isolated with additional cottonoid pledgets. Electrophysiologic mapping of the rhomboid fossa and establishment of the exact site of the facial colliculus play a crucial role in preserving the function of the facial nerve (see Video 138-8). With this technique, total removal is achieved without leaving residual portions of cavernoma behind. We always perform meticulous bipolar coagulation of the parenchymal cavernoma bed for local hemostasis and to avoid recurrence at this site.^{179,192,206–209}

FIGURE 138-28 Preoperative T2-weighted magnetic resonance images (MRIs) taken in the axial (A) and sagittal (B) planes showing a typical dorsally exophytic cavernous malformation of the dorsal pons extending into the fourth ventricle. The patient, a 19-year-old man, had slight left-sided facial nerve weakness and gait ataxia. C, Microsurgical excision was performed with the patient placed in the prone (Concorde) position via the telovelar approach. D, Intraoperative photograph demonstrating the vascular malformation within the fourth ventricle. E and F, Complete removal of the lesion was achieved without additional morbidity and was confirmed on postoperative MRIs.

Endoscopic Resection of Colloid Cysts

Endoscopic resection of colloid cysts has proved to be a safe method with good long-term results.^{124,127,131,225} When compared with microsurgical resection, the endoscopic technique showed similar neurological outcomes and the same percentage of recurrence. However, hospitalization and the duration of the surgical procedure are significantly shorter when using the endoscopic approach.^129,237 A good indication for open microsurgical resection is a very firm consistency of the colloid cyst, because with the endoscope, neither piecemeal removal nor suction of its contents is possible. Unfortunately, reliable information concerning the cyst’s consistency is not available from preoperative imaging (Fig. 138-29).

FIGURE 138-29 Images of a 45-year-old woman with symptoms of raised intracranial pressure. A, Preoperative T2-weighted magnetic resonance image (MRI) demonstrating a small colloid cyst within the foramen of Monro. B, On the T2-weighted MRI after the endoscopic procedure, complete removal of the colloid cyst is seen. C, Intraoperative photograph taken through the endoscope demonstrating the colloid cyst (CC), choroid plexus (P), septum pellucidum (SP), and thalamostriate vein (TV). D, Photograph showing positioning of the patient with the incision marked behind the hairline.