Schwannoma of the third ventricle - a case report.

Summary Schwannomas represent about 8 % of all intracranial tumours, however, ventricular location of schwannomas is extremely rare. If they occur, they are located mostly in the lateral ventricle. To our knowledge there was only one case of third ventricular schwannoma described in the literature. Our case is the second one.

Introduction

Schwannomas represent about 8 % of all intracranial tumours, however, ventricular location of schwannomas is extremely rare. To our knowledge there were 13 cases of intraventricular schwannomas described in the literature. Three of them were described in the fourth ventricle[89], one was found in the posterior part of the third ventricle[6] and the rest were in the lateral ventricle[12357]. Majority of these tumours were benign but two had malignant features[4]. We describe a case report of a patient with a schwannoma of the anterior part of the third ventricle which was mimicking a colloid cyst.

A Case Report:

70 years old female patient presented with a 6 months history of slowering of a psychomotor speed, tremor of her right hand, urinary incontinence and memory and gait disturbances. CT and MRI have shown a 15 mm mass of the third ventricle which was isointense in both T1 and T2 images (fig. 1). Due to obstructive hydrocephalus she was given an external ventricular drainage. The tumour was radically excised via a transcallosal approach. We proceeded via 1.5 cm long callosotomy on the left side of the corpus callosum to the left lateral ventricle. Immediately an oval tumor approximately 1.5 cm in diameter was seen through the left foramen of Monro. The foramen was almost occluded by the tumor which was sitting in the third ventricle. The tumor was very tough, it was not possible to suck out the contents like in the colloid cyst. Even CUSA was not of a big help. The capsule was very vascular. However it was relatively easy to separate the tumor from choroid plexus and from anterior septal and thalamostriate veins. Finally the tumor was split into two peaces and taken out through the enlarged foramen of Monro. The first impression of the operator was meningioma.

Fig.1

Preoperative T1 weighted image MRI, axial plain, showing a round isointense mass in a location typical for a colloid cyst (a), T2 weighted image MRI, sagital (b) and coronal (c) plain.

Postoperatively the patient suffered repeatedly grand mal type paroxysms due to which she was put on arteficial ventilation for the next two days. The external ventricular drainage could be removed on the 7th postoperative day. The next postoperative course was uneventful, the patient was fully mobilised with no neurological deficit and her complaints have gradually disappeared. Follow-up MRI two months postoperatively have shown a radical removal of the tumour and a resolution of hydrocephalus (fig. 2). Histologically intraventricular schwannoma WHO GrI was confirmed (fig. 3).

Fig.2

Postoperative T1 weighted image MRI, sagital plain, showing the total removal of the mass via the transcallosal approach (a), T2 weighted image MRI, axial plain (b)

Fig 3

(a) Hematoxylin-Eosin (HE), 100x. Typical palisading of neoplastic cell nuclei in Verocay body in cellular Antoni A area. (b) HE, 200x. Detail of Verocay body with palisading nuclei. (c) Ki-67, 200x. Proliferative marker Ki-67 is showing a minimal activity (sporadic positive brown nuclei), (d) S-100, 200x: Partial positivity (brown) of neoplastic cells on marker S-100

Discussion

Most of intracranial schwannomas originate from the vestibular portion of the VIII th cranial nerve. Some other very rare schwannomas arise from the sensory fibres of cranial nerves V, VII, IX, X and XI and even less frequent are schwannomas arising from fibres of motor nerves XII, III, IV and VI[11]. Intraparenchymal or intraventricular schwannomas are the less frequent types[3]. This is the second schwannoma to be described in the third ventricle, however the other one was found in the posterior part of the third ventricle[6].

The origin of intraventricular schwannomas is not known. There are several theories trying to explain it. Benedikt in 1874 identified peripheral nerve fibres in the choroid plexus of the fourth ventricle[13]. This observation was confirmed by Stör in 1922, who also suggested that neoplastic transformation of these Schwann cells could lead to an intraventricular schwannoma[12]. Another theory proposes that these tumours may arise from autonomous sympathetic nervous cells. This vascular nervous plexus can be found in the vicinity of medullary vessels, choroid plexus and meningeal vessels[346]. Alternative hypothesis presumes that the tumour originates from ectopic neural cells displaced during embryogenesis[310]

According to all histological and immunophenotype features there is no doubt about this tumour being schwannoma. Also during the operation the surgeon was surprised by the consistence of the tumour which was not cystic but solid and very firm with highly vascularised capsule. Of course this was not a colloid cyst although the localisation of the mass was in a space typical for colloid cysts. Colloid cysts are predominantly located in the anterior part of the third ventricle. MRI examination shows the colloid cysts as smooth, well-delineated lesions, which are filled with viscid gelatinous material or include cholesterol crystals, blood degradation products and mucin. The MR signal is usually variable, in most of the cases it is dependent on cholesterol concentrations. Postcontrast rim enhancement can be found. Schwanomas are usually isointense, sometimes with mixed iso-hypointense signal on T1W images. On T2WI may the lesion be hyperintense or with isointense nodule, postcontrast enhancement is strong. Differentiation between these lesions could be possible after application of contrast medium. However that was not applied in this case, because native image was of typical appearance and localization for coloid cyst. If the contrast medium had been applied and enhancement had been found, we would probably have thought over meningioma primarily, because of intraventricular localization and signal characteristics.

Conclusion

We present a very rare case of the third ventricular schwannoma which was removed via a transcallosal approach. These tumours have to be added to the differential diagnosis of the third ventricular tumours.