Epidermoid

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Definition

An epidermoid is characterized as a congenital lesion comprised of desquamated debris lined by a layer epithelium. They are caused by ectodermal cells which are retained during embryologic closure of the neural tube. They frequently involve the skull but when seen intracranially, they tend to occur at the cerebellopontine angle, suprasellar cistern, prepontine cistern and pineal region. They are generally intradural in location and grow in an insinuating manner around the adjacent structures including cranial nerves and vessels.

Patients present with different symptoms based on the location of the epidermoid. A cerebelloropontine angle epidermoid can cause cranial neuropathies including hearing loss, tinnitus, headache, hemifacial spasm or trigeminal neuralgia. Suprasellar and pineal region lesions may cause hydrocephalus. Rarely, the epidermoid cyst can rupture and result in an aseptic meningitis. Another uncommon complication is malignant transformation into a squamous cell carcinoma.

Diagnosis can be suggested based on imaging characteristics described below.

On imaging epidermoids can be seen as water density lesions on CT and can be recognized when they cause displacement of adjacent structures, a similar appearance to an arachnoid cyst. They are much more easily identified by MRI where they are distinguished from arachnoid cysts by characteristic high signal on diffusion and higher signal on FLAIR compared to CSF. Generally there is no associated enhancement of epidermoids unless they become superinfected.

Treatment for epidermoids is surgical. Total resection is often not possible given the insinuating nature of these lesions, and can result in aseptic meningitis from cyst spillage.

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