Epidermoid cyst of the Cerebello-Pontine Angle (CPA)

Intracranial epidermoid cysts are uncommon congenital slow growing lesions which becomes symptomatic due to the development of mass effect on adjacent structures in the 2nd - 4th decade of life. They are most commonly located in the cerebello-pontine angle in almost 50% of the cases. It is the third most common CP angle mass after acoustic schwannomas and meningiomas.

In MRI these lesions appears almost isointense to CSF but shows a characteristic restricted diffusion. These appear as lobulated lesions that typically insinuates between the adjacent structures and encases the cranial nerves and vessels. Calcification is seen in 10%-25% cases.

Another characteristic feature of epidermoid and MRI is partial suppression sometimes called the dirty appearance in T2 FLAIR images. In rare cases when they are T1 hyperintense they are called as white epidermoid.

The following images are of a 25 year old male patient.

CSF intensity appearance of the epidermoid cyst  is noted in the T1 and T2 weighted images, with a dirty appearance (partial suppression ) in the T2 FLAIR images. Lesion is predominantly located in right CP angle cistern, with extensions into the right cervico-medullary cistern, pre-pontine cistern, sellar-parasellar areas (details mentioned below). 

Mass effect on the right lateral aspect of pons, middle cerebellar peduncle is noted with  displacement to left. Partial effacement of the fourth ventricle is present. No hydrocephalus was seen.

In the SWI image above no blooming artifacts are seen within the lesion. DWI and ADC images show characteristic restricted diffusion, which helps to differentiate the lesion from an arachnoid cyst.

T1 sagittal and T2 coronal images show the mass effect of the lesion on the adjacent brainstem and the right middle cerebellar peduncle. The 3D T2 CISS images show characteristic lobulated appearance of the epidermoid cyst with insinuations in the CSF spaces and encasement of the cranial nerves and vessels.

Lesion shows no post-contrast enhancement.

The video above shows the right CP angle epidermoid cyst in much more detail in the 3D CISS images, which is helpful in determining the extent of the lesion. Diffusion weighted images also shows the extent of the lesion better than the conventional images due to the CSF intensity of the major portion of the lesion in rest of the sequences.

Significant mass effect and marked lateral displacement with stretching of right trigeminal nerve cisternal segment is noted. The lesion is encasing the right VII-VIII nerve complex, cisternal segment of right oculomotor nerve, and partly encasing the cisternal segment right trigeminal nerve. Lateral displacement of the right oculomotor nerve is also noted by the lesion. Superiorly the lesion is extending into the anterior aspect of midbrain, sellar and suprasellar region, partly encasing the right ICA bifurcation and the pituitary infundibulum. It is noted in contact with the right uncus and floor of third ventricle, tuber cinereum and mammillary body. Partial encasement of the basilar artery and its bifurcation is also present.

References:

https://radiopaedia.org/articles/intracranial-epidermoid-cyst