In the clinic
Horner’s syndrome is caused by any lesion that leads to a loss of sympathetic function in the head. It is characterized by three typical features: pupillary constriction due to paralysis of the dilator pupillae muscle, partial ptosis (drooping of the upper eyelid) due to paralysis of the superior tarsal muscle, and absence of sweating on the ipsilateral side of the face and the neck due to absence of innervation of the sweat glands.