The cerebellopontine angle (CPA) is formed by the lateral pons and the cerebellum around the level of the middle cerebellar peduncle. At the CPA, there is a triangular extraaxial subarachnoid space known as the cerebellopontine angle cistern. The normal contents of the cerebellopontine angle cistern include the facial and vestibulocochlear cranial nerves, the anterior inferior cerebellar artery, and the flocculus of the cerebellum. CPA masses are relatively common with most related to congenital lesions or tumors arising from the normal contents of the CPA cistern.
Normal anatomic structures can occasionally be confused with a CPA mass. The flocculus of the cerebellum is one of the normal components of the CPA cistern. The foramen of Luschka connects the fourth ventricle with the CPA cistern. Choroid plexus from the fourth ventricle can extend into the CPA cistern and occasional calcify, mimicking a mass lesion. The jugular bulb is the initial segment of the jugular vein, which receives blood from the inferior petrosal and sigmoid venous sinuses. The right jugular bulb is larger than the left jugular bulb in around two-thirds of patients and this asymmetry can cause it to be mistaken for an asymmetric mass lesion. The jugular bulb is typically inferior to the CPA cistern but rarely can be more superior in the CPA cistern.
Vertebrobasilar artery ectasia indicates enlargement of the normal vertebral or basilar arterial diameter, often with a tortuous, atypical course. It can be asymptomatic, associated with brainstem and cerebellar ischemia, or can cause direct compression on the cranial nerves. An aneurysm or arteriovenous malformation is suggested by marked internal T1 and T2 hypointensities (flow voids) ( Fig. 119.1 ) and enhancement with contrast. When uncertainty is present, magnetic resonance angiography should be performed to confirm the visualized structure is vascular. A dural arteriovenous fistula can be suggested by dilated subarachnoid pial vessels on magnetic resonance imaging (MRI); catheter angiography is required to confirm the diagnosis.
Meningiomas ( Fig. 119.2 ) are the second most common extraaxial CPA mass. On MRI, they typically appear iso- to slightly hypointense on T1 sequences and hyperintense on T2 sequences, and there is robust homogenous enhancement. Internal calcifications may be present and this can be particularly helpful in differentiating a meningioma from a schwannoma, which does not typically calcify. A broad “dural tail,” or an adjacent tapering dural thickening, can often be seen.