Unilateral external ophthalmoplegia caused by ipsilateral oculomotor nuclear lesion: analysis with diffusion weighted MRI

A 49-year-old man who had been suffering from poorly controlled diabetes mellitus for 20 years was admitted to our hospital with the chief complaints of diplopia and right ptosis. The onset of his symptoms was sudden. On admission, he had right palpebral ptosis, upward and left lateral gaze paresis of the right eye. Hess screen chart examination revealed paresis of the right superior rectus, inferior oblique and medial rectus muscles. T2 weighted magnetic resonance imaging (MRI) revealed high intensity area in the medial part of the right midbrain tegmentum where the oculomotor nucleus located, and diffusion weighted MRI demonstrated high intensity area which corresponded to the oculomotor fascicles of the midbrain. The latter represented Wallerian degeneration of the fascicles. The diagnosis of small infarction of the right oculomotor nucleus presenting ipsilateral external ophthalmoplegia was made. The superior rectus muscle is innervated by the neurons in the contralateral oculomotor nucleus, and the other external ocular muscles innervated by the third cranial nerve are conducted by the neurons in the ipsilateral oculomotor nucleus; therefore, unilateral oculomotor nuclear lesion generally causes ipsilateral ophthalmoplegia plus contralateral superior rectus palsy. Our case presented here shows that there may be such a case with oculomotor nuclear disturbance which masquerades with oculomotor fascicular or infranuclear disturbance; therefore, it is important to include these lesions in differential diagnosis.