Horner syndrome refers to a constellation of signs produced when sympathetic innervation to the eye is interrupted. Lesions at any point along the sympathetic pathway may result in Horner syndrome. Depending on the level of the lesion, impaired flushing and sweating may be found ipsilaterally.
Anhydrosis affects the ipsilateral side of the body with central, first-order neuron lesions.
Lesions affecting second-order neurons may cause anhydrosis of the ipsilateral face. With postganglionic lesions occurring after vasomotor and sudomotor fibers have branched off the sympathetic chain, anhydrosis is either absent or limited to an area above the ipsilateral brow. The pupils react normally to light and accommodation. Iris heterochromia also may occur in long-standing Horner syndrome. Second-order preganglionic pupillomotor fibers exit the spinal cord at the level of T1, enter the cervical sympathetic chain, where they are in close proximity to the pulmonary apex and the subclavian artery.