Specific Imaging Findings

Epidermoid (epidermoid cyst, cholesteatoma) is typically a non-enhancing extra-axial intradural mass. Epidermoids are lobulated space-occupying lesions that insinuate through subarachnoid spaces displacing and engulfing adjacent structures, primarily cranial nerves and arteries. They are typically isodense to slightly hyperdense to the CSF on CT. Rare variants may show calcifications and/or hyperdense contents (“dense epidermoid”). Epidermoids are iso- to slightly hyperintense to CSF on T1WI and with CSF-like hyperintensity on T2WI. These neoplasms do not enhance with contrast. FLAIR and especially DWI are the imaging techniques of choice. FLAIR images show internal “dirty” heterogeneity with signal intensity generally similar to the brain parenchyma. DWI reveals their striking hyperintensity, while the ADC values are similar to the brain. A small number of epidermoids may show high T1 and even low T2 signal, due to proteinaceous contents. This T1 hyperintensity is not suppressed on images with fat saturation pulse. Intra-axial and intradiploic skull lesions are much less frequent. The most common locations are cerebellopontine angle, pineal region, parasellar areas, and middle cranial fossa.

Pertinent Clinical Information

Epidermoids are usually silent until the third or fourth decade of life. Clinical presentation depends on the location and includes cranial nerve deficits, hydrocephalus, neuroendocrine symptoms, and seizures (with middle cranial fossa tumors). Surgical resection is curative. Chemical meningitis caused by cyst rupture is a possible complication, which very rarely follows dissemination of the cyst contents.