INTRODUCTION

Intraventricular meningiomas are relatively rare and represent 1% to 2% of all meningiomas. The first recorded case of intraventricular meningiomas is probably by Shaw in 1854.¹ He identified an encapsulated fibrous tumor in the trigone of the lateral ventricle. MacDowell performed a left trigonal tumor operation in 1881.² In 1938, Cushing and Eisenhardt first described a case of meningioma located within the third ventricle.³ Since then, there have been multiple reports of cases of intraventricular meningiomas in the literature addressing the potential problems and difficulties encountered in management of these formidable surgical lesions. To date, approximately 600 cases of intraventricular meningiomas have been reported. Of these, approximately 80% are found in the lateral ventricle, 15% in the third ventricle, and 5% in the fourth ventricle.^1–48

We base our analysis on a review of our experience with 50 cases of lateral ventricular meningiomas treated in our Institute from 1990 to 2006. The discussion is focused on lateral ventricular meningiomas, and meningiomas in other ventricles are not considered. In our series, lateral ventricular meningiomas represented 2.1% of the total meningiomas seen during the period. Slow progression of these essentially benign tumors and accommodative capacity of the ventricles make these tumors unique in the sense that the symptoms are subtle and long-standing and they are significantly large at the time of diagnosis. Lateral ventricular meningiomas present a surgical challenge because of the deep location, large size, firm consistency, and the frequently observed intense vascularity. Although meningiomas have been rarely recorded in other sites of the lateral ventricular cavity, most of them are located in the trigone or have an extension toward it. The presence of bulkier choroid plexus may be the reason for higher incidence of tumors in the region of atrium of lateral ventricle. The majority of the lateral ventricular meningiomas are benign in nature and resection often leads to cure from the disease. Malignant meningioma in the lateral ventricle with cerebrospinal fluid (CSF) borne distant metastasis in the brain and spinal cord is also rarely reported.^16,29,45

The exact site of origin of these tumors is unclear, but the various reviews have suggested that these tumors probably arise from the meningeal cells that are transported together with the choroid plexus as the ventricular system invaginates during embryogenesis.^3,22,27 This feature is supported by the fact that the vascular supply of most of these tumors is from the choroidal vessels. Due to the relationship with choroid plexus, some authors prefer to label these meningiomas as “choroidal” meningioma.³⁵

CLINICAL FEATURES

Like most other meningiomas, intraventricular meningiomas also have a female preponderance. Although not observed in our series, some authors have identified a relatively higher incidence of intraventricular meningiomas in children.^8,31,41 Some authors have observed left side predominance,^21,22 while others have noted no significant side preference. These meningiomas are sometime seen as a part of neurofibromatosis 2 (NF2) complex and also in association with multiple meningiomas. The patients may not have any significant neurologic symptoms, even when the tumor has reached a massive size. Owing to the nature of growth pattern, some of these tumors may be detected incidentally. In such cases, considering the slow nature of growth and difficulties that can be encountered during surgery, a conservative observation may be a viable treatment option. High vascularity of the tumors has rarely been associated with intratumoral bleeding and acute onset of symptoms.^34,36,42,46

In general, these patients usually have long-standing dull and generalized headaches as the initial presenting symptom, which increases in intensity as the tumor grows in size. Duration of headache may be as long as 10 to 15 years.³⁸ With tumor growth, the additional symptoms of raised intracranial pressure in the form of vomiting and visual symptoms related to papilledema become prominent. Although not seen in this series, episodic severe headaches simulating symptoms generally related to colloid cysts have been observed in cases with small sized lateral ventricular meningiomas.²¹ Visual field defects are relatively frequent. In our series, 5 of the 14 patients with preoperative visual field defect had almost complete homonymous hemianopia. The exact cause of the visual field defect before surgery is unclear. A vascular steal from the posterior cerebral artery supply zone by the “vascular” tumor could be a probable cause of this symptom. Direct stretching and compression of the optic radiation projection fibers is another possible cause. Considering that the visual field defect improved in eight patients (57%) after surgery, involvement of fibers of optic radiation by pressure could be an important factor. Memory disturbances, neuropsychological deficits and personality changes are relatively late and were seen in 12% cases.¹³ Decrease in scholastic performance due to progressive cognitive changes secondary to chronically raised intracranial pressure is a frequent form of presentation in children. Verbal and visual memory is more frequently affected. The short-term or recent memory is relatively spared.

The incidence of seizure is relatively uncommon in patients with lateral ventricular meningiomas. Focal motor or generalized seizures as presenting symptom have been reported and were seen in three cases (6%). Sensory seizures occurred in five patients (10%). Postoperative seizures are reported in 29% to 70% patients operated by transcortical approaches. In our series, 11 (22%) patients had postoperative seizures. Parietal and temporal lobe signs have been recorded in the literature and were seen in 8% of our cases. Preoperative localizing signs such as hemiparesis, aphasia, agnosia, and alexia are rarely present even in large lateral ventricular meningiomas.¹⁷

INVESTIGATIONS

Magnetic resonance imaging scan (MRI) is an important tool for diagnosing lateral ventricular meningiomas.^39,40 It differentiates intraventricular lesions from the paraventricular tumors. It helps in defining the extent of the tumor, its physical nature, and its vascularity and thereby assists the surgeon in planning the surgical approach. The most common site for the lateral ventricular meningiomas is the trigone of the lateral ventricle and irrespective of its size they are attached to the choroid plexus in the region of the trigone. Asymmetric hydrocephalus with ipsilateral dilatation of the temporal and occipital horns is also common and frequently diagnostic of intraventricular location of the lesion and meningioma. The MRI, in general, shows the lesion as iso- to slightly hypointense on T1-weighted images and iso- to mildly hyperintense on T2-weighted images. There is usually homogeneous enhancement on contrast administration. The advent of MRI has limited the role of angiography in lateral ventricular meningiomas. The MR spectroscopy and blood volume-time intensity maps may assist in the preoperative diagnosis. A high alanine-to-creatinine ratio has been reported as specific MR spectroscopic findings for meningiomas. Functional MRI may be useful in preoperative cortical mapping, so as to identify the sensorimotor and language cortices to plan an optimal surgical approach and select the safest area in which to make a cortical incision.

It is important preoperatively to differentiate these benign intraventricular meningiomas from more frequently encountered malignant tumors in the region and in the ventricle. The differential diagnosis of lateral ventricular meningioma includes choroid plexus papilloma, intraventricular ependymomas, central neurocytomas, and subependymomas.²⁷ Each of these tumors has a characteristic nature of clinical presentation and radiologic features. The presence of a large sized and well-defined lobulated trigone centered tumor in a physically healthy and neurologically stable patient favors a preoperative diagnosis of a meningioma. The duration of the presenting symptoms is also relatively longer in intraventricular meningiomas when compared to other malignant intraventricular tumors. Intense and homogeneous tumor enhancement is more frequent in intraventricular meningiomas. The presence of intratumoral necrotic and cystic changes is relatively uncommon in intraventricular meningiomas and was seen in nine (18%) cases. As in other brain tumors, we observed that longer the duration of clinical symptoms the firmer was the consistency of the tumor. Calcification within the tumor was seen in 14% cases. Moderate to severe tumor surrounding parenchymal edema was seen in 64% of patients. Enlargement of the contralateral ventricle, probably suggestive of the chronic nature of the disease process, was seen in all our cases with medium to large sized tumors. Dilatation of the temporal and occipital horns, probably a result of focal obstruction to the CSF pathway, suggests intraventricular location of the tumor.

Other than the fact that there is no bone related or origin site changes, CT scan shows all typical features of meningiomas.⁴⁹ The tumors are hypodense or slightly hyperdense tumor and have intense contrast enhancement. Calcification may be seen. Dense and large calcifications within the tumor may even be visible on plain skull radiographs.

ANGIOGRAPHY

The blood supply to lateral ventricular meningioma is from both anterior and posterior choroidal arteries. They also can receive supply from lenticulostriate and thalamoperforating vessels. The study may provide useful information on the location of the feeding arteries in relationship to the internal dome of the tumor. The carotid angiogram shows enlargement, tortuosity, and displacement of the anterior choroidal artery. Venous drainage is through the vein of Galen, internal cerebral vein, or the basal vein of Rosenthal. Large veins frequently course over the dome of the tumor before joining the main venous channel. In the venous phase, the internal cerebral veins appear stretched and displaced inferiorly and to the opposite side. The vein of Galen may be elongated and straightened. Vertebral angiogram may show enlargement and anterior displacement of the lateral posterior choroidal artery with a reversal of its curve, and marked anterior convexity.²³

SURGICAL STRATEGY

Surgery on intraventricular meningiomas challenges the surgical skills and philosophical understanding of a neurosurgeon. Surgical experience and thorough knowledge of anatomy are crucial in excising these lesions.^50,51 While a successful surgical resection is compatible with normal life, any error in planning the approach or executing the surgery can lead to disabling neurologic deficits or even death. The anatomy of the trigone, the arterial and venous patterns, and the relationship with visual pathways and hemispheric dominance needs to be evaluated, before attempts toward surgery. When planned, surgery has to be directed toward radical or total tumor resection, as incomplete or partial tumor resection can lead to tumor bleeding related issues. The issues that are relevant while planning surgery for intraventricular meningiomas include assessment of the size of the tumor, its consistency, its vascularity, location of the feeding blood vessels, and the direction of the growth. The basic principles that determine the success of the surgery include early access to the blood supply, minimal retraction of the brain for adequate exposure while removal of large masses, and understanding of the function of the surrounding anatomic structures. An interhemispheric approach and exposure of the medial part of the tumor to control the feeding vessels early in the operation appears to be a reasonable surgical option.^28,52