Intracranial epidermoid cyst
Headache and convulsions.
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Large extra-axial posterior fossa mass lesion epicentered upon the left cerebellopontine angle with an irregular outline. It elicits a low signal on T1WI, a high signal on T2WI and a heterogeneous/dirty signal on FLAIR, high signal on diffusion-weighted sequence (due to a combination of true restricted diffusion and T2 shine through) and low signal on ADC map.
It exerts marked mass effect displacing the brainstem to the right and compressing the fourth ventricle with consequent moderate supratentorial hydrocephalic changes, which are seen as moderately dilated third and lateral ventricles with CSF permeation around both lateral ventricles. The lesion also compresses the left cerebellar hemisphere.
This case illustrates typical MRI features of an intracranial epidermoid cyst, with dirty signal/partial suppression of the T2 signal on FLAIR and diffusion restriction. Epidermoids are the third most common cerebellopontine angle mass, after acoustic schwannomas and meningiomas. The lesion causes important mass effect over the posterior fossa structures and consequent obstructive hydrocephalus.