PathophysiologyAs previously described, Horner syndrome is a consequence of sympathetic disruption. The symptomology depends on the location of the lesion, and severity depends on the degree of denervation. Superior tarsal muscle helps raise the upper eyelid and has a sympathetic nerve supply. Denervation of this muscle causes ptosis which is milder compared to oculomotor (CN III) palsy which supplies the levator palpebrae superioris. Superior tarsal muscle is responsible for keeping upper eyelid in a raised position after levator palpebrae superioris raises it. This explains the partial ptosis seen in Horner syndrome. The lower eyelid may be slightly elevated owing to denervation of lower lid muscle which is analogous to the superior tarsal muscle. Sympathetic nervous supply is responsible for the dilation of the pupil (mydriasis). When disrupted, parasympathetic supply is uninhibited, and constriction of the pupil (miosis) ensues. The reaction of the pupils to light and accommodation is normal as those systems do not depend on sympathetic nerve supply. Ipsilateral anhidrosis, another classic presentation, depends on the level of interruption of sympathetic supply. Anhidrosis with first-order neuron lesions affects the ipsilateral side of the body as the sympathetic supply from its central origin. The ipsilateral face is involved in lesions involving the second-order neurons. Postganglionic third-order neuron lesions occurring after the vasomotor and sudomotor fibers have branched off show very limited involvement of the face (area adjacent to ipsilateral brow). Iris heterochromia (relevant deficiency of pigment in the iris on the affected side) is seen in children younger than 2 years and in the congenital form of Horner syndrome.