What is the classic triad of Horner's syndrome?

Horner's syndrome is characterized by a classic triad of symptoms that results from disruption of the sympathetic nerve pathway to the eye. The correct triad includes ptosis (drooping of the eyelid), miosis (constriction of the pupil), and anhydrosis (lack of sweating).

Ptosis occurs because of weakness in the muscles that elevate the eyelid, which are influenced by sympathetic activity. Miosis happens due to unopposed parasympathetic stimulation of the pupil, as the sympathetic pathway that normally dilates the pupil is interrupted. Anhydrosis arises from the loss of sympathetic innervation to the sweat glands on the affected side of the face, leading to reduced sweating.

This triad is crucial for diagnosing Horner's syndrome, as it helps differentiate the condition from other ocular or systemic diseases. Understanding this set of symptoms is fundamental for identifying the underlying causes of Horner's syndrome, such as a tumor or injury along the sympathetic pathway.