Tumours of the posterior fossa extrinsic

TRIGEMINAL NEUROMA/SCHWANNOMA

Rarely neurilemmomas arise from the trigeminal ganglion or nerve root. These lie in the middle fossa or extend into the cerebellopontine angle, compress surrounding structures - cavernous sinus, midbrain and the pons - and erode the apex of the petrous bone.

Clinical features are usually long-standing - facial pain, paresthesia and numbness. Compression of posterior fossa structures results in nystagmus, ataxia and hemiparesis.

CT scan or MRI with contrast demonstrates an enhancing lesion eroding the petrous apex and extending into the middle and/or posterior fossa.

Management: Operative removal, even if subtotal, should provide long-lasting benefit. The tumour is approached through either the middle fossa or through a suboccipital craniectomy depending on the predominant site.

MENINGIOMA

Approximately 8% of all intracranial meningiomas arise in the posterior fossa. Clinical features

These depend on the exact tumour site. Those arising over the cerebellar convexity may not present until the mass obstructs CSF drainage. Meningiomas arising in the cerebellopontine angle may involve any cranial nerve from V to XII. A clivus meningioma may cause bilateral VI nerve palsies before pontine pressure causes long tract signs.

Tumours growing at the foramen magnum, compressing the cervico-medullary junction, produce characteristic effects - pyramidal weakness initially affecting the ipsilateral arm, followed by the ipsilateral leg, spreading to the contralateral limbs with further tumour growth.

Investigations

CT scan with intravenous contrast will identify the tumour site, but MRI with gadolinium enhancement shows more anatomical detail.

Tl weighted MRI with gadolinium showing a large tumour arising from the tentorium and straight sinus, compressing the cerebellar vermis and the 4th ventricle.

Management

As with supratentorial meningiomas, treatment aims at complete tumour removal. In the posterior fossa, cranial nerve involvement makes this difficult and exacting; excision of the tumour origin is seldom possible.

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