Isolated trochlear nerve palsy caused by a left caudal paramedian midbrain infarction

A 66-year-old man experienced difficulty in eye opening, which improved quickly, and diplopia in the morning. A brain MRI indicated a small infarction in the left paramedian tegmentum of the midbrain at the inferior colliculus level, similar to the anatomical location of the trochlear nucleus. The Hess test and Parks 3-Step test indicated right trochlear nerve paresis, and the Bielschwsky head tilt test was positive. The patient also showed Horner' s syndrome ipsilateral to the lesion, but this was confirmed to be coincidental from a past photograph of the patient. Three months later the patient' s diplopia was resolved. Subjective eye opening difficulty may have been due to involvement of most caudal part of the oculomotor nerve nucleus, which innervates the levator palpebrae superioris muscle. Most reported cases of intramedullary trochlear nerve palsy were caused by head trauma, hemorrhage, or vascular malformation, so the case presented here is extremely rare. Although brain hemorrhage is relatively easy to detect by CT, trochlear nerve paralysis caused by midbrain infarction may be misdiagnosed or underdiagnosed.