Horner syndrome miosis is caused by loss of sympathetic innervation of pupillary dilation.

• A. Loss of sympathetic innervation of pupillary dilation
• B. Loss of parasympathetic innervation of pupillary constriction
• C. Loss of sympathetic innervation of eyelid levator
• D. Increased parasympathetic innervation to the iris

Answer: (A)

In Horner syndrome, the pupil becomes small because the sympathetic nerves that normally dilate the pupil are damaged. The dilator pupillae muscle, which widens the pupil in low light or when needed, relies on sympathetic innervation. When that input is lost, the parasympathetic system, which constricts the pupil via the sphincter pupillae, dominates, producing miosis. So the described mechanism—loss of sympathetic innervation of pupillary dilation—best explains the finding. The other possibilities would either cause the opposite effect (loss of parasympathetic input leading to dilation), involve a different structure (ptosis from loss of sympathetic input to the eyelid levator), or imply increased parasympathetic activity, which is not the mechanism in Horner syndrome.