Isolated cisternal third cranial nerve schwannoma: an insight into a clinico-radiological features

A young adult female in her 20s presented with gradually progressive exodeviation of the left eyeball from the last 3 years. She had mild ptosis and proptosis in her left eye with 3 mm of anisocoria. On examination, she was found to have 60 prism dioptres exotropia with minimal motility limitation (-1 adduction, elevation and depression) in the left eye. With a provisional diagnosis of acquired third nerve palsy, she underwent MRI brain and orbit with contrast, which showed enhancing nodular lesion along the course of the left oculomotor nerve in the prepontine cistern which was hyperintense on T1 post-contrast and hypointense on T2/3D Fast Imaging Employing Steady-state Acquisition-weighted images at interpeduncular cistern suggestive of a schwannoma. This led to the final diagnosis of acquired third nerve palsy in the setting of oculomotor schwannoma. The patient opted for observation in view of the small size, isolated and slowly progressive nature of the tumour.