Horner syndrome anhidrosis is due to loss of sympathetic innervation for facial sweating.

• A. Loss of sympathetic innervation for facial sweating
• B. Loss of parasympathetic innervation for facial sweating
• C. Loss of sympathetic innervation for pupillary dilation
• D. Loss of parasympathetic innervation to lacrimal glands

Answer: (A)

Horner syndrome stems from disruption of the sympathetic pathway to the face, which includes fibers that control facial sweat glands. When those sympathetic nerves are lost, the facial sweat glands no longer receive signals, leading to anhidrosis on the affected side. Facial sweating is specifically mediated by the sympathetic system, not the parasympathetic, so a loss of parasympathetic innervation wouldn’t cause anhidrosis. While Horner’s also affects pupil size by removing sympathetic input to the dilator pupillae (causing miosis) and can involve other features, the sweating change is best explained by loss of sympathetic innervation to the facial sweat glands.