Pupil-sparing partial oculomotor palsy with ipsilateral medial longitudinal fasciculus involvement: A novel midbrain infarct phenotype-a case report.

Pupil-sparing third-nerve palsy is often attributed to diabetic microvascular ischemia; however, brainstem infarction can present with a similar pattern. We report the case of a patient with acute painless horizontal diplopia and partial right oculomotor palsy characterized by impaired adduction, mild vertical limitation, partial ptosis, and mildly impaired convergence, with equally reactive pupils. On left gaze, the right eye failed to adduct and the contralateral eye did not demonstrate abducting nystagmus. Brain magnetic resonance imaging revealed a small dorsal paramedian midbrain infarction on diffusion-weighted imaging, and diffusion-tensor tractography demonstrated reduced integrity of the ipsilateral medial longitudinal fasciculus. The patient received dual antiplatelet therapy for 21 days, followed by aspirin, high-intensity statin therapy, and intensive glycemic control. Diplopia resolved within 4 weeks and remained absent at the 3-month follow-up. This case broadens the clinico-radiological spectrum of midbrain infarction and underscores that pupil-sparing third-nerve palsy does not exclude a central lesion. The combination of adduction deficit, impaired convergence, and absent contralateral abducting nystagmus should lower the threshold for urgent brain magnetic resonance imaging with diffusion-weighted imaging; diffusion-tensor tractography may be helpful where available.