Isolated cranial nerve palsies due to brainstem lesions

Isolated cranial nerve palsies are often attributed to lesions of the respective nerves along their extraaxial courses. There are a significant number of reports of individual patients with cranial nerve palsies, mostly of the 3rd and 6th nerves, as the sole manifestation of brainstem lesions proven by magnetic resonance imaging (MRI) or computer-assisted tomography (CT). An intraaxial basis may still be underestimated if based on MRI only, as electrophysiological abnormalities indicating brainstem lesions (masseter reflex, blink reflex, DC electrooculography) may be independent from MRI-documented morphological lesions. This article reviews the evidence that ischemic and demyelinating brainstem lesions are an important and underestimated cause of clinically isolated cranial nerve palsies. Especially in middle-aged and elderly people with 3rd and 6th nerve palsies, small pontine and mesencephalic infarctions seem to be more frequent than small-vessel ischemic infarctions of the extraaxial nerves.