Diabetic 3rd nerve palsy: evidence for a mesencephalic lesion.

Eleven consecutive adult chronic diabetic patients presented with an isolated 3rd nerve palsy (8 with pupillary sparing) of which 10 had abnormal ipsilateral or bilateral masseter reflexes (MassR). Three patients had an MRI lesion in the ipsilateral oculomotor fasciculus and 3 had subsequent mild brainstem signs. An additional 13 patients with Weber's syndrome had similar ipsilateral or bilateral MassR abnormalities, while 7 patients with 3rd nerve palsies on a known extra-axial basis had none. The findings suggest that an isolated diabetic 3rd nerve palsy, with or without pupillary sparing, is much more likely on the basis of a focal mesencephalic infarct than a peripheral nerve lesion.