Pupil-sparing Isolated Fascicular Third Cranial Nerve Palsy due to Infarction: Report of a Case and Literature Review

Because of their anatomic proximity, pupillary and inferior rectus functions are linked in most cases of fascicular oculomotor palsy, with either sparing or involvement of both. A 40-year-old woman presented with painless binocular diplopia and left ptosis. Examination additionally showed limitations of the adduction and depression in the presence of normal supraduction and intact pupillary function in the left eye. Findings of other neurological neuro-ophthalmologic examination were normal. A magnetic resonance imaging revealed a diffusion restriction in the left dorsomedial midbrain. An ischemic lesion restricted to the oculomotor fascicles can cause inferior rectus palsy in the presence of pupillary sparing, which should be differentiated from a microvasculopathic lesion involving the subarachnoid portion of the oculomotor nerve.