These rare cysts of embryological origin develop from cells predestined to become either epidermis or dermis. They most commonly arise in the cerebellopontine angle but may also occur around the suprasellar cisterns, in the lateral ventricles and in the Sylvian fissures, often extending deeply into brain tissue.
Pathology: Depends on cell of origin:
Epidermoid (epidermis) - a thin transparent cyst wall often adheres firmly to surrounding tissues; the contents - keratinised debris and cholesterol crystals - produce a 'pearly' white appearance.
Dermoid (dermis) - as above, but thicker walled and, in addition, containing hair follicles and glandular tissue. Midline dermoid cysts lying in the posterior fossa often connect to the skin surface through a bony defect. This presents a potential route for infection.
When lying in the cerebellopontine angle, epidermoid/dermoid cysts often cause trigeminal neuralgia (see page 159). Neurological findings may range from a depressed corneal reflex to multiple cranial nerve palsies. Rupture and release of cholesterol into the subarachnoid space produces a severe and occasionally fatal chemical meningitis. The presence of a suboccipital dimple combined with an attack of infective meningitis should raise the possibility of a posterior fossa dermoid cyst with a cutaneous fistula.
CT scan shows a characteristic low density (often 'fat' densi' lesion, unchanged after contrast enhancement or showing or slight peripheral enhancement. Calcification may be evident
T2 weighted MRI appears more sensitive than CT in detecting an abnormality, but the hyperintense signal does not differentiate an arachnoid cyst from an epidermoid
T2 weighted MRI showing lower cranial nerves traversing the lesion.
Adherence of the cyst wall to important structures often prevents complete removal, but evacuation of the contents provides symptomatic relief. Aseptic meningitis in the postoperative period requires prompt treatment with steroids. Even when removal is incomplete, recollection of the keratinised debris is uncommon and may take many years.
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