Epidermoid Cyst (Cerebellopontine Angle)

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Diagnosis Definition

An epidermoid cyst (EC) is bound by a thin layer of squamous epithelial cells and contains desquamated keratin and/or cholesterol; it is analogous to cholesteatomas of the middle ear or petrous temporal bone
They arise from congenital rests of ectodermal cells, from introduction of squamous cells into the CSF space during trauma, or iatrogenically from surgery or lumbar puncture
ECs may become symptomatic due to mass effect and nerve impingement

Imaging Findings

The findings on CT vary, depending on the component parts; often, they have attenuation characteristics similar to cerebrospinal fluid (CSF) and can mimic an arachnoid cyst
On MRI, ECs are hyperintense on T2 images, similar to CSF
On FLAIR, ECs have a “dirty CSF” signal due to incomplete suppression of fluid signal
On T1, ECs are typically isointense to CSF; hyperintensity on T1 is due to proteinaceous contents (referred to as a “white epidermoid”)
Rim enhancement may be seen on T1 images post contrast, however, no central enhancement should be seen
DWI/ADC images show central restricted diffusion, a diagnostic feature of ECs

Pearls

Differential considerations include arachnoid cysts (will not restrict diffusion), dermoids (will not restrict diffusion, are bright on T1 and often midline), and abscess (thicker rim of enhancement with local inflammatory changes)
Common intracranial locations for ECs include the cerebellopontine angle, the suprasellar cistern, and the fourth ventricle

References

Smirniotopoulos JG, Chiechi MV. Teratomas, dermoids, and epidermoids of the head and neck. Radiographics 1995; 15(6):1437-1455
Osborn AG, Preece MT. Intracranial cysts: radiologic-pathologic correlation and imaging approach. Radiology 2006; 239(3):650-664

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