Incorrect

Incorrect

Correct! This patient has a left third nerve palsy, but it is not isolated! She also has a mild spastic right hemiparesis. (You might have thought that the left extremities were showing reduced deep tendon reflexes, but actually it is the right extremity reflexes that are accentuated!) The combination of these features localizes to the cerebral peduncle on the left side of the ventral midbrain. It is called a “Weber syndrome.” The peduncular lesion damages the descending corticospinal axons, which later cross to the opposite side, causing a contralateral spastic hemiparesis that variably affects face, arm, and leg. Brain vascular imaging, which is urgent and mandatory in acute isolated third nerve palsy to rule out aneurysm, is not the right approach here. You want high-definition MRI, which will visualize the midbrain. The most common cause is an acute ischemic stroke, so you would be looking especially at a diffusion-weighted sequence.

By the way, ischemic brainstem strokes often cover a small territory and will not be evident even on the best imaging. If it is negative, be sure you have excluded multifocal disease, as might be seen in multiple sclerosis. Once that has been done, you can usually presume “imaging-occult” non-embolic ischemic brainstem stroke, and direct treatment at arteriosclerotic risk factors.