Tumours of the cerebral hemispheres extrinsic

CT scan: shows a low density (CSF density) well demarcated lesion, occasionally producing expansion of the over-lying bone.

ARACHNOID CYSTS

These cystic collections of CSF-like fluid lie in the Sylvian fissure, the chiasmatic cistern, the cisterna magna or over the hemisphere convexity. Some are related to a previous infective meningitis with subsequent adhesions but most are probably congenital in origin. Those occurring in the Sylvian fissure may be associated with temporal lobe agenesis.

Occasionally arachnoid cysts present with mass effects, due to a one-way valve effect and progressive enlargement, but more often they are found by chance on CT scan.

Treatment: only required if the mass effect becomes symptomatic - marsupialisatian or cystoperitoneal shunt.

EPIDERMOID/DERMOID CYSTS

These cysts, more commonly found in the posterior fossa (page 325), occasionally develop in the Sylvian or interhemispheric fissure. They are either of congenital or acquired origin due to implantation and sequestration of ectoderm. They may present with epilepsy, features of raised intracranial pressure or with focal neurological signs. Rupture into the subarachnoid space causes a chemical meningitis.

On CT scan, the extreme low attenuation of the cyst contents is characteristic. Symptoms may necessitate operative evacuation of the cyst contents. Removal of the cyst wall is difficult and reaccumulation may occur.

Lipomas

Lipomas are usually found incidentally on imaging or at autopsy and are often associated with other developmental anomalies such as agenesis of the corpus callosum. They are located in midline structures e.g. corpus callosum, dorsal midbrain and cerebellar vermis. They require no treatment.

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