Case 8 :  

A young male presents with vertigo. 
What is the diagnosis?

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Imaging Findings:
MR scan shows a cerebello-pontine angle extra-axial mass lesion, hypointense on T-1 weighted images, and hyper intense on T2 weighted images, with heterogenous signals on FLAIR.

Epidermoid Cyst at the left CP angle

Epidermoid cysts are slightly more common than dermoids, especially in the cranial cavity. They are most frequently detected in mid-adulthood, owing to their very slow growth and limited symptomatology. Although they may appear anywhere in the neural axis, the majority involve the basal surface of the brain. The most common cranial sites include the cerebellopontine angle and parasellar regions. Following these sites are the rhomboid fossa (ventral to the brainstem), ventricles and choroidal fissures, and subfrontal and interhemispheric regions.

Epidermoids grow locally and insinuate themselves into the subarachnoid cisterns and sulci. Symptoms related to supratentorial epidermoids include seizures and/or headaches. Patients with posterior fossa epidermoids typically present with symptoms of cranial nerve involvement and/or vertigo.

On CT, epidermoids are lobulated, extra-axial masses, which show low density, often similar to CSF. Their density may be slightly different than CSF, and rarely, they may be dense lesions. Intratumoral matrix is occasionally seen. Enhancement is typically not seen following intravenous or intrathecal administration of iodinated contrast medium. In contradistinction to arachnoid cysts, following intrathecal injection, contrast typically insinuates itself into the interstices of the epidermoid.

MR imaging has made the distinction between epidermoids and dermoids clear. Epidermoids commonly exhibit signal characteristics slightly different from CSF, and occasionally intralesional matrix may be identified on any type of MR sequence. Particular attention should be paid to the signal intensity of the lesion on proton-density-weighted images. Rarely, hemorrhagic epidermoids may be seen. In such cases, chemical shift artifact and/or fat suppressed images will help to distinguish the fatty nature of the dermoid from the similar intensities of epidermoid.

An additional diagnostic problem relates to epidermoids with signal characteristics that approximate CSF. Prior to MR imaging, such lesions were differentiated from arachnoid cysts first on the basis of morphology. Failing that, CT-cisternography would be performed, as noted above. On MRI, epidermoids that mimic arachnoid cysts are not easily discriminated. Several techniques have shown promise including gradient-echo imaging, steady-state imaging, and diffusion imaging. In the former, it is not uncommon for three-dimensional radiofrequency-spoiled gradient-recalled images to show different contrast resolution of epidermoids than spin-echo T1-weighted images. In particular, intralesional matrix, often inapparent on spin-echo images, may be revealed with gradient-echo imaging. Steady-state imaging can reveal intralesional motion, characteristic of fluid within the arachnoid cyst. Failing that, diffusion imaging may be employed, if available. Preliminary reports have shown that the diffusion characteristics of CSF within arachnoid cysts differ considerably from the solid and semi-solid pearly matrix within epidermoids.

Dr. Ashok Raghavan, Manipal Hospital, Bangalore