Case of the Month - March

Left facial weakness in a 33 year old male.

Investigations

Left image: An unenhanced CT head demonstrated subtle distortion of the left middle cerebral peduncle and pons.

Centre image: A T2 weighted sequence again demonstrated the mass effect exerted on the left middle cerebellar peduncle and pons. The lesion was hyperintense on T2 imaging.

Right image: The diffusion weighted (DWI) sequence and apparent diffusion coefficient map (not shown) showed evidence of diffusion within the cystic lesion.

Not shown: High resolution steady state imaging showed that the cystic lesion had an irregular margin and no enhancement was evident on post contrast images.

The imaging findings are those of an epidermoid cyst.

Discussion

Epidermoid cysts are the third most common cerebellar pontine angle masses, following schwannomas (acoustic neuroma) and meningiomas. Other less common cystic lesions in this region include arachnoid cysts, dermoid cysts, inflammatory or infective cysts and other neoplastic cystic lesions.

Epidermoid cysts have an outer wall composed of a thin fibrous capsule and are lined by a simple stratified cuboidal squamous epithelium. The contents of the cyst are composed of keratin and cholesterol crystals. Unlike a dermoid cyst, no dermal appendages are present.

They typically present in the 3^rd to 5^th decades.

They usually present with headache, cranial nerve symptoms (commonly 5^th, 7^th and 8^th nerves). Less commonly they may produce cerebellar dysfunction, elevated intracranial pressure, pituitary dysfunction or a chemical meningitis following rupture. Neoplastic degeneration into a squamous cell carcinoma is rare.

On CT imaging the lesions are typically as dense as fluid and only detectable indirectly through mass effect.

On MR imaging the lesions typically have signal intensity of fluid on T1 and T2 imaging, may be slightly hyperintense on FLAIR imaging and do not enhance. Their characteristic feature is intense diffusion restriction on DWI sequences. This differentiates them from arachnoid cysts.

 

References

Diagnostic imaging. Brain. Osborn A. AMIRSYS 2004.