Chapter 28 Transcallosal Surgery of Lesions Affecting the Third Ventricle: Basic Principles
Intraventricular tumors represent a relatively small proportion of central nervous system lesions, accounting for approximately 10% of CNS neoplasms.1 Despite this, they represent a diverse range of pathologic entities which pose technical challenges for neurosurgeons. Third ventricular lesions, in particular, are challenging in terms of surgical access and corridor, owing to their central location in the brain. As such, these lesions hold a special place in neurosurgical history and literature.
There is a wide differential diagnosis for third ventricular neoplasms, with the most common entities being colloid cysts, astrocytomas, and craniopharyngiomas. Other lesions include arachnoid cysts, pituitary adenomas, ependymomas, germinomas, metastasis, subependymoma, central neurocytoma, teratoma, dermoid, arteriovenous malformation, meningioma and choroid plexus papilloma (Table 28-1).2,3 The differential can often be refined by location within the third ventricle; for example, astrocytomas and craniopharyngiomas tend to be located in the anterior third ventricle, meningioma and choroid plexus papilloma in the posterior third ventricle, and colloid cysts at the level of the foramen of Monro.4
TABLE 28-1 Frequency of Tumors of the Third Ventricle
| Tumor | Number | Percent |
|---|---|---|
| Astrocytoma | 5 | 21% |
| Craniopharyngioma | 5 | 21% |
| Ependymoma | 4 | 17% |
| Choroid plexus papilloma | 3 | 13% |
| Cystercercosis | 2 | 8.3% |
| Colloid cyst | 1 | 4.2% |
| Epidermoid | 1 | 4.2% |
| Dermoid | 1 | 4.2% |
| Subependymoma | 1 | 4.2% |
| Medulloblastoma | 1 | 4.2% |
| Total | 24 |
Adapted from data in Morrison G, Sobel DF, Kelley WM, Norman D. Intraventricular mass lesions. Radiology. 1984;153:435-442.
Given that the majority of intraventricular lesions are benign, surgery is often the preferred, and potentially curative, option for tumors of this region.5 However, owing to the relative rarity of these lesions, few surgeons have extensive experience with approaches to the ventricular system. Interhemispheric, transcallosal approaches to the third ventricle offer the advantage of potential avoidance of postoperative cortical hemispheric deficits or other long-term complications (e.g., seizures) as compared to transcortical approaches. Partial sectioning of the corpus callosum does not in general lead to significant neurologic impairment. In patients with crossed-dominance, where the hemisphere controlling the dominant hand is contralateral to the hemisphere controlling speech and language, callosal sectioning can lead to severe disability.6–8 This condition can be present in patients with a history a childhood cerebral injury or other dysfunction forcing relocation of language function within the brain. Furthermore, sectioning the splenium of the corpus callosum in patients with a dominant hemisphere hemianopsia may also be contraindicated as this can lead to alexia.8 In modern times, the complications related to transcallosal approaches are generally related to manipulations in the floor of the third ventricle and not the approach.
Historical Notes
Dandy first described the transcallosal approach for accessing the ventricular system in 1922.9 Before that time, localization of tumors of the third ventricle was limited by insufficient diagnostic methods, and the introduction of ventriculography by Dandy in 1918 helped in this regard.10 Dandy accessed the third ventricle primarily through a posterior transcallosal approach. In 1944, Edward Busch described division of the midline forniceal raphe and entrance into the third ventricle by splitting of the plane between the two fornices.11 Several series and reports detailing the use of transcallosal techniques began to appear in the 1950s and 1960s.12–14 In the 1970s and 1980s, series reporting on psychometric and neuropsychological outcomes following transcallosal surgery further elucidated the then growing body of knowledge regarding the physiology of corpus callosum sectioning.15–18
Anatomic Considerations
The third ventricle is a narrow, funnel-shaped structure that lies in the center of the brain. It lies below the corpus callosum and body of the lateral ventricles, between the two thalami and walls of hypothalamus, and above the pituitary and midbrain (Fig. 28-1).19 It is a unilocular cavity and communicates with the lateral ventricles at its anterosuperior margin via the foramen of Monro; posteriorly, it communicates with the fourth ventricle via the sylvian aqueduct.19
The roof of the third ventricle contains five layers and extends from the foramen of Monro anteriorly to the suprapineal recess posteriorly.19,20 The fornices make up the upper, or neural, layer. The axons of the fornices arise in the temporal horns from the hippocampi and extend around the thalami to reach the mammillary bodies.19 The inferior aspects, or fimbria, arise in the floor of the temporal horn and pass posteriorly to become the posterior portions, or crura of the fornices. These unite at the hippocampal commissure, which covers the posterior portion of the third ventricle; the bodies of the fornices cover the middle portion; the columns of the fornices, along with anterior commissure and lamina terminalis, cover the anterior portion of the third ventricle.19 The second and fourth layers of the roof of the third ventricle are comprised of the superior and inferior layers of the tela choroidea. The superior and inferior layers of the tela choroidea form a space called the velum interpositum, which contains the third, and vascular, layer of the roof of the third ventricle.20 In this vascular layer lie the internal cerebral vein (ICV) and posterior choroidal arteries (PChA). The fifth layer is the choroid plexus of the lateral ventricle, which extends from the foramen of Monro posterosuperolaterally toward the pineal gland.
The choroid plexus runs bilaterally in the choroidal fissures, which are incisuras between the lateral edge of the fornix and the superomedial surface of the thalamus.21 The choroid plexus of the lateral ventricle has an ependymal attachment to the fornix and to the thalamus called the teniae fornicis and the teniae thalami, respectively.22 The teniae fornicies contains no arteries or veins; however, tributaries of the internal cerebral vein, the superior and anterior thalamic veins and the thalamostriate vein, as well as the branches of the medial PChA traverse the teniae thalami.23 Surgically, this is an important point, as opening the choroidal fissure along the teniae fornicis avoids these vascular structures.24
The floor of the third ventricle extends anteriorly from the optic chiasm to the sylvian aqueduct posteriorly.19 The anterior half of the floor is formed by diencephalic structures and the posterior half is formed by those of the mesencephalon.19 From anterior to posterior, structures encountered are the optic chiasm, the infundibular recess, the median eminence, the tuber cinereum, the pair of mammillary bodies, and the ventricular side corresponding to the posterior perforated substance, and part of the tegmentum of the midbrain.19,25 The anterior wall of the third ventricle is formed from superior to inferiorly by the columns of the fornix, the foramina of Monro, the anterior commissure, lamina terminalis, optic recess, and optic chiasm. The posterior wall consists of, from superiorly to inferiorly, the suprapineal recess, the habenular commissure, and the sylvian aqueduct. The lateral walls are formed by the thalamus superiorly and hypothalamus interiorly, which are delineated by the hypothalamic sulcus extending from the foramen of Monro to the sylvian aqueduct.19 The massa intermedia connecting the two thalami is present in roughly three quarters of brains approximately 4 ml posterior to the foramen of Monro.26
Anatomy of the lateral ventricle and the relationship to the foramen of Monro is key in these approaches as well as these provide a corridor of access to the third ventricle. In the lateral ventricle, medial and lateral groups of veins converge at the level of the foramen of Monro to form the internal cerebral veins. The main medial vein is the septal vein, which crosses the septum pellucidum and the fornix. The lateral veins include the caudate veins, which run from posterolateral to anteromedial to cross the caudate nucleus and the thalamus, and the thalamostriate vein.21 The thalamostriate vein is located in the body of the lateral ventricle. It is the largest tributary of the ICV and runs in the striothalamic sulcus between the caudate nucleus and the thalamus. At the posterior margin of the foramen of Monro, the thalamostriate vein curves medially around the anterior tubercle of the thalamus to form the ICV.20 This U-shaped junction is termed the venous angle.20,27,28
Surgical Technique
After induction of general anesthesia with endotracheal intubation, we typically place the patient in Mayfield pin fixation with the patient supine and the head elevated 15-20 degrees above horizontal (Fig. 28-2). Some surgeons prefer a lateral head position with the head turned 90 degrees laterally with the vertex 30 to 40 degrees above horizontal. Usually in the lateral position, the nondominant hemisphere is placed inferiorly such that it falls away with gravity and opens up the midline corridor; however, based on anatomy of the offending lesion and the patient’s venous drainage, it may be desirable to place the dominant hemisphere down. Advocates for lateral positioning cite that gravity may be used to allow the inferiorly placed hemisphere to fall away, giving midline access with minimal retraction and that lateral positioning of the head allows the surgeon to operate with hands side-by-side versus on top of one another. We have found in our experience that the main disadvantage of lateral positioning is the distortion of midline by displacement of the brain by gravity. This disadvantage is most significant in interforniceal approaches, as identification of the midline is crucial to the prevention of forniceal injury. Furthermore, we have found that by moving slightly to the patient’s side, it is quite easy to operate with hands side-by-side in the supine position. For these reasons, we primarily utilize supine positioning.
FIGURE 28-2 Patient in supine position with head flexed 15 to 20 degrees in Mayfield head fixation.
(From Apuzzo ML, Amar AP. Transcallosal interforniceal approach. In: Apuzzo ML, ed. Surgery of the Third Ventricle. Baltimore: Williams & Wilkins; 1998:421-452, with permission.)
Consideration is given to placement of preoperative ventriculostomy placement contralateral to the side of dissection. This assists in relaxation of the hemispheres for dissection and should be strongly considered in patients with preoperative ventriculomegaly. Typically, if preoperative ventriculostomy is not performed, we will leave behind a ventricular catheter at the time of closure. Furosemide and mannitol are also administered preoperatively to provide brain relaxation.
We typically use a two limbed curvilinear scalp flap or horseshoe flap, though other incisions can be tailored to achieve satisfactory cosmesis. A right-sided, 6 cm x 4 cm x 3 cm trapezoidal bone flap with the base at approximately 1 centimeter left of midline is most commonly used, with placement of burr holes at the vertices of the flap to obtain control of the sagittal sinus. For optimal access of the foramen of Monro, anatomy dictates that the bone flap should be placed such that it is bisected by the coronal suture.29 However, the bone flap is classically placed two thirds in front of the coronal suture and one third behind it as generally there is a corridor of relative paucity of parasagittal draining veins at this location.20,29
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