Horner's syndrome is due to a lesion affecting which pathway?

Horner's syndrome arises from disruption of the sympathetic pathway responsible for the innervation of the eye and surrounding structures. This pathway originates in the hypothalamus and descends down through the brainstem and spinal cord, ultimately traveling along the sympathetic trunk to innervate the dilator pupillae muscle, the smooth muscle of the eyelid (Mueller's muscle), and other ocular structures.

When there is a lesion affecting this pathway, several characteristic signs may appear, including ptosis (drooping of the upper eyelid), miosis (constricted pupil), and anhidrosis (lack of sweating) on the affected side of the face. This triad of symptoms is critical in diagnosing Horner's syndrome and helps differentiate it from other conditions affecting the eyes.

The other pathways listed, such as the corticospinal pathway, are involved in motor control, the parasympathetic pathway is primarily associated with rest-and-digest functions and control of glandular secretions, and the somatic pathway is related to voluntary muscle movements. None of these pathways are implicated in the symptoms seen in Horner's syndrome, which specifically relates to sympathetic dysfunction.