Background
Horner syndrome (Horner’s 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.[1] Signs found in all patients, regardless of the level of interruption include mild-to-moderate ptosis owing to denervation of the sympathetically controlled Müller muscle, slight elevation of the lower lid (upside-down ptosis) due to denervation of the lower lid muscle analogous to the Müller muscle in the upper lid, and miosis and dilation lag, where pupillary dilation after psychosensory stimuli is slower in the affected pupil than the unaffected pupil.
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 (with the affected eye being hypopigmented) is seen in congenital Horner syndrome or Horner syndrome that occurs in children younger than 2 years. Iris heterochromia also may occur in long-standing Horner syndrome.
Pathophysiology
First-order central sympathetic fibers arise from the posterolateral hypothalamus, descend uncrossed through the mid brain and pons, and terminate in the intermediolateral cell column of the spinal cord at the level of C8-T2 (ciliospinal center of Budge). Second-order preganglionic pupillomotor fibers exit the spinal cord at the level of T1, and enter the cervical sympathetic chain, where they are in close proximity to the pulmonary apex and the subclavian artery.
The fibers ascend through the sympathetic chain and synapse in the superior cervical ganglion at the level of the bifurcation of the common carotid artery (C3-C4). Postganglionic pupillomotor fibers exit the superior cervical ganglion and ascend along the internal carotid artery. Shortly after the postganglionic fibers leave the superior cervical ganglion vasomotor and the sudomotor fibers branch off, they travel along the external carotid artery to innervate the blood vessels and sweat glands of the face. The pupillomotor fibers ascending along the internal carotid artery enter the cavernous sinus. Then, the fibers leave the carotid plexus briefly to join the abducens nerve (cranial nerve VI) in the cavernous sinus and enter the orbit through the superior orbital fissure along with the ophthalmic branch of the trigeminal nerve (V1) via the long ciliary nerves. Then, the long ciliary nerves innervate the iris dilator and the Müller muscle.
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