Horner's syndrome is caused by sympathetic paralysis or paresis. The associated ptosis cannot be due to paralysis of the levator palpabrae muscle as this is innervated by the oculomotor nucleus not the sympathetic system. Consequently, another muscle must be involved. It must be a smallish muscle as generally, the ptosis is volitionally correctable by the lavator palpabrae.

In either eyelid, there is a fibrocartilaginous plate called a tarsal. Attached to this plate is a small muscle called Muller's muscle. This muscle is innervated by the sympathetic system through the superior cervical ganglion. The effect of sympathetic stimulation and subsequent contraction of this muscle is to open the eyes wide. Therefore, when the system is paralysed or paretic, the superior eyelid droops.