Bilateral ptosis with pupil sparing because of a discrete midbrain lesion: magnetic resonance imaging evidence of topographic arrangement within the oculomotor nerve.

The topographic arrangement within the midbrain oculomotor nerve is not adequately elucidated in humans. Two patients with a partial oculomotor palsy because of a localized infarction or hematoma were treated. Both patients had bilateral ptosis, impaired adduction, and supraduction. One patient had impaired infraduction and pupillary involvement on one side. Results of computed tomography and magnetic resonance imaging revealed discrete lesions at the dorsal midbrain tegmentum that spared the rostral midbrain. The authors' cases elucidate that pupillary components take the most rostral course. This report provides indirect magnetic resonance imaging evidence to prove the course of pupillary fibers. Based on the different neuro-ophthalmologic findings in the authors' cases (sparing or affecting pupillary component and infraduction), the nerves of the inferior rectus and inferior oblique for infraduction pass more rostrally than those of medial rectus, superior rectus, and levator palpebrae. The nuclear and fascicular arrangement within the midbrain oculomotor nerve is speculated to be pupillary, extraocular, and eyelid elevation in the rostro-caudal order, based on the neuro-ophthalmologic impairment and magnetic resonance imaging findings in the authors' patients and in previous animal experiments. Knowing the fascicular and nuclear arrangement within the midbrain in detail will offer diagnostic clues for differentiation of causes for partial oculomotor palsy.