Weber Syndrome

Excerpt

Weber's syndrome was described first by the German-born English physician Hermann Weber in a 52-year-old male who had developed left-sided third nerve palsy with right-sided hemiplegia caused by an acute bleed in the left cerebral peduncle.

Weber syndrome, classically described as a midbrain stroke syndrome and superior alternating hemiplegia, involves oculomotor fascicles in the interpeduncular cisterns and cerebral peduncle, thereby causing ipsilateral third nerve palsy with contralateral hemiparesis. It most commonly results from the occlusion of a branch of the posterior cerebral artery.

The oculomotor(third cranial) nerve has two main motor nuclei, the main motor nucleus, and the accessory parasympathetic nucleus. The main motor nucleus is located in the tectum portion of the midbrain at the level of the superior colliculus and supplies all the extraocular muscles except the lateral rectus and the superior oblique muscles, and the levator palpebrae superioris. The accessory parasympathetic nucleus, also known as the Edinger Westphal nucleus, is situated posterior to the main motor nucleus and its postganglionic fibers pass through the short ciliary nerves to supply the constrictor pupillae of the iris and the ciliary muscles.

The nerve fascicles then travel forward and lateral through the red nucleus and converge at the interpeduncular fossa before exiting the midbrain. As the nucleus and fascicles are spread over a large area of the midbrain, lesions in the midbrain can present with either partial or complete third nerve palsy. The lesions in lower midbrain affect the extraocular muscles but spare the pupils, while lesions affecting both upper and middle parts of the midbrain are associated with pupillary dilatation.