Which combination of signs is most characteristic of Horner's syndrome?

• A. Ptosis and mydriasis
• B. Ptosis and miosis
• C. Ptosis and anisocoria with dilation in darkness
• D. Mydriasis and exophthalmos

Answer: (B)

Horner's syndrome arises from loss of sympathetic input to the eye, so the muscles that normally widen the pupil and lift the upper eyelid are affected. The most telling pairing is a slight ptosis together with a constricted pupil (miosis). Ptosis comes from the loss of Muller's muscle in the upper eyelid, while miosis results from the unopposed parasympathetic action on the pupil due to the missing sympathetic drive to dilator pupillae. This combination is the classic clue that points to disruption of the sympathetic pathway.

In contrast, signs like pupil dilation (mydriasis) reflect preserved or heightened sympathetic activity, which isn’t what Horner’s produces. Exophthalmos isn’t part of Horner’s picture and points to other conditions such as Graves. Anisocoria can be present, but it’s most evident in dim light because the affected pupil cannot dilate properly, making the difference in size with the normal pupil more conspicuous.