Unilateral eyelid retraction

Two muscles are involved in upper eyelid elevation: the tarsal smooth muscle of Muller that has an accessory role limited to the tonic control of eyelid position, and the levator palpabrae, a skeletal muscle innervated by a subdivision of the oculomotor nerve.1 The levator palpabrae shares several features with the superior rectus. Both muscles are often activated in conjunction, allowing for eyelid coordination during vertical eye movements, they have common embryological stages,2 and their motor neurons remain close to each other from the orbit to the mesencephalon. This anatomical parallelism persists at the nuclear level, at least in lateral eyed mammals. In these species, both levator palpabrae and superior rectus nuclei are paired and have crossing axons.1 However, in higher frontal eyed mammals, levator palpabrae innervation is provided by a single medial nucleus, the central caudal nucleus, although the crossed pattern of innervation of the superior rectus remains.3 Anatomical studies have shown that, in most frontal eyed mammals including primates, cell bodies of both levator palpabrae are bilaterally distributed and totally intermixed within the central caudal nucleus.3Furthermore, branching axons—that is, levator motor neurons connected with both levator palpabrae, are absent,3 or extremely rare (2%).4 Very little is known about the premotor network that controls the central caudal nucleus. A recently identified region in the rostral mesencephalon, medial to the rostral interstitial nucleus of medial longitudinal fasciculus, sends pathways to the central caudal nucleus, and could provide an excitatory signal for the upper …