How do unilateral lesions in the midbrain typically present?

Unilateral lesions in the midbrain often affect the oculomotor nerve, which is responsible for eye movements and pupil constriction. When there is a lesion, patients typically experience ipsilateral oculomotor nerve palsy. This means that the neurologic deficits manifest on the same side as the lesion, affecting functions such as eye movements and pupil reflexes.

In addition to the oculomotor nerve involvement, unilateral midbrain lesions can also disrupt descending motor pathways, particularly those originating in the motor cortex. As a result, contralateral hemiparesis may occur due to the disruption of motor signals that travel from the brain to the opposite side of the body. The combination of these two manifestations leads to the distinct clinical presentation of contralateral hemiparesis and ipsilateral oculomotor nerve palsy.

This is consistent with the neurological pathways affected by midbrain lesions, where the crossover of motor fibers means that motor deficits arise on the opposite side of the body from where the lesion is located, while the palsy of the oculomotor control appears on the same side.