Imaging and Clinical Features of an Intra-Axial Brain Stem Schwannoma

Discussion

Intracranial schwannomas account for 8% of brain tumors¹; most of these are acoustic neuromas, with intraparenchymal schwannomas being rare. Intraparenchymal schwannomas not arising from cranial nerves are uncommon and account for only 1%–2% of intracranial schwannomas. The most common sites are the frontal and temporal locations in the cerebral hemispheres.¹ They have also been described in the cerebellar hemispheres and vermis² in relation to the tentorium cerebelli and the lateral and fourth ventricles.³ More than 50 cases have been reported so far.⁴ Our review of literature revealed only 6 cases of intraparenchymal schwannomas located in the brain stem.^1,5–9

Our case has unique and interesting features compared with other cases in the literature: 1) Our patient, at 68 years of age, tied with 1 other patient as the oldest on record (and this other elderly patient had an incidentally discovered lesion in the setting of ischemia⁹). 2) The high level of symptoms in our patient developed during 2 weeks; in the other 6 cases, symptoms developed from 6 weeks to 3 years. 3) Perhaps related to this acute presentation, our case is the second one¹⁰ with recorded T2 signal-intensity abnormality surrounding the lesion (of 3 cases with MR imaging available^1,6,9). 4) Our patient had a multiloculated cystic lesion; all 3 other lesions^1,6,9 with MR imaging data available had unilocular cystic masses. 5) Our case was 1 of only 2 lesions¹ correctly diagnosed at the time of surgery by means of frozen sections. The other 5 cases^5–9 required permanent sections; thus, the diagnosis was made postprocedurally. 6) Unique to our case, continuous motor-evoked potentials recovered intraoperatively in our case. 7) Also, our patient was the first undergoing surgery in an integrated MR imaging operating suite environment, and 8) the tumor in our case was adherent to the brain stem, whereasin 3 cases,^1,6,9 there was a well-developed capsule making the lesions easier to resect.

Our preoperative diagnosis was malignant glioma, a disease with a very poor prognosis, whereas the revised diagnosis of intraparenchymal schwannoma will likely have a considerably better outcome.

Schwannomas are well-circumscribed encapsulated masses usually found in relation to a nerve. Tumors form firm gray masses, and 15%–20% are associated with perilesional or peritumoral cysts.³ The tumors have well-demarcated irregular pushing rather than infiltrative margins.²

It is difficult to explain the origin of intraparenchymal brain schwannomas. Schwann cells are not normally present in the cerebral parenchyma. They have been detected around arteries in the intracranial perivascular nerve plexuses in the subarachnoid space and the brain. Similar tiny myelinated nerves may be present in the tela choroidea.² Feigin and Ogata¹¹ have reported the presence of non-neoplastic Schwann cells in brain tissue in multiple sclerosis plaques (peripheral type myelin) or at the edge of old infarcts. These may undergo neoplastic conversion. Another hypothesis is that pial cells may sometimes undergo conversion to Schwann cells, which can be an origin of some intracerebral schwannomas.³

Characteristic imaging features show a well-circumscribed mass typically with cyst formation, a solid enhancing component, calcification, peritumoral edema, gliosis, and superficial or periventricular location.¹² CT shows an isoattenuated or low-attenuation contrast-enhancing mass associated with a cyst.² The tumors are hypointense and hyperintense on T1- and T2-weighted images, respectively. The solid component enhances strongly post–contrast administration. The cystic portion is a salient radiologic feature.² It is thought that the pathogenesis of cyst formation is that small cysts within solid tumors derive from sites of microhemorrhage, necrosis, and degeneration, which fuse to become a larger cystic component.¹² Such imaging features are also seen in schwannomas arising from cranial nerves.

The cystic degeneration rate of intracerebral schwannomas (54.5%) is much higher than that of acoustic schwannomas (9.6%–20.5%).⁴ Intramural nodules in the cyst wall have been reported rarely; absence of these nodules distinguishes them from cystic astrocytomas.⁷ The presence of calcification¹¹ and frankly hemorrhagic lesions² has also been reported. Calcification, seen on CT, may contribute to the hypoattenuated signal intensity on MR imaging. Diffuse peritumoral edema has been described as another characterstic feature of intraparenchymal schwannomas, especially in the supratentorial compartment (35.7%).^2,10,12 The cause of edema around this benign neoplasm is uncertain.

The outcome of patients with intraparenchymal schwannomas is typically very good. In their series of 9 cases, Casadei et al² reported no evidence of tumor recurrence after complete resection in a follow-up period of 2–24 months with similar histologies, though there were no brain stem lesions in their series and some patients with subtotal resection did become symptomatic again.

Intra-axial schwannoma is difficult to distinguish from the main differential diagnosis of glioma a priori, particularly in the absence of cranial nerve involvement. The prominent neurologic symptoms may have been a clue, but the diagnosis in our case required open biopsy. Intraoperative frozen sections allowed the benign histology to be identified in time for the surgical approach to be modified, and a good functional outcome was obtained for the patient.