Isolated Medial Longitudinal Fasciculus Midbrain Infarction Mimicking Medial Rectus Paralysis.

INTRODUCTION Medial longitudinal fasciculus infarction is rare in clinical practice and generally accompanied by brain tissue damage around the medial longitudinal fasciculus. Isolated medial longitudinal fasciculus midbrain infarction was seldom reported. CASE REPORT An 81-year-old man with hypertension was admitted to our hospital because of sudden onset diplopia. A neurological examination revealed right adduction paresis without abducting nystagmus in the left eye, whereas the convergence reflex was normal. Diffusion-weighted magnetic resonance imaging demonstrated a small acute lacunar medial longitudinal fasciculus infarction in the right midbrain at the level of the inferior colliculus. Diffusion-tensor imaging showed a reduction of the right medial longitudinal fasciculus. Medial longitudinal fasciculus infarction is rare and occurs most commonly in the pons. The authors report on a case of isolated medial longitudinal fasciculus infarction that was diagnosed because of sudden diplopia and manifested as simple internal rectus paralysis with no abducting nystagmus on the contralateral side of the lesion. CONCLUSION Isolated midbrain-medial longitudinal fasciculus infarction without contralateral abducting nystagmus is a rare occurrence. It can be differentiated from partial oculomotor palsy by assessing the convergence reflex, primary gaze, and diffusion-tensor imaging.