This young female patient presented to the ophthalmologist due to a third nerve palsy (diplopia/ptosis/dilated pupil) with a history of acute progressive ataxia and vertigo. These symptoms are due to the mass effect of the large trigeminal schwannoma on the midbrain, pons and medulla and additionally, due to mass effect within Meckel's cave affecting the right third cranial nerve.

The lesion has a waist at the porus trigeminus, giving rise to the typical dumb-bell shape of a bi-lobed lesion straddling the posterior fossa (CP angle) and middle cranial fossa (Meckel's cave/temporal lobe). In this instance, the lesion has a third lobule within the right infratemporal fossa (masticator space), and consequent additional waisting at the foramen ovale. This gives it a trilobular shape due to extension and spread along the course of the mandibular division (V3) of the right trigeminal nerve. This is well identified on the coronal and sagittal sequences.

Trigeminus means "thrice twinned" referring to the dual right and left nerve having 3 major branches (ophthalmic-V1, maxillary-V2 and mandibular-V3).

Multiple surgical attempts to excise/debulk the schwannoma are planned. The initial procedure was performed through a right posterior fossa craniotomy and there is a reduced volume of the cisternal component and significantly improved midbrain mass effect on the post-surgical follow up scan. There are expected post-surgical changes of intralesional and surgical tract hemorrhage. MRI brain is otherwise normal.