(See also Overview of the Autonomic Nervous System.)
Horner syndrome results when the cervical sympathetic pathway running from the hypothalamus to the eye is disrupted. The causative lesion may be primary (including congenital) or secondary to another disorder.
Lesions are usually divided into the following:
Peripheral lesions may be preganglionic or postganglionic in origin.
Instilling eyedrops can help confirm and characterize Horner syndrome.
First, cocaine (4 to 5%) or apraclonidine (0.5%) drops are put in both eyes:
Cocaine: Cocaine blocks the synaptic reuptake of norepinephrine and causes the pupil of the unaffected eye to dilate. If a postganglionic lesion (peripheral Horner syndrome) is present, the pupil of the affected eye does not dilate because the postganglionic nerve terminals have degenerated; the result is increased anisocoria. If the lesion is above the superior cervical ganglion (preganglionic or central Horner syndrome) and the postganglionic fibers are intact, the pupil of the affected eye also dilates, and anisocoria decreases.
Apraclonidine: Apraclonidine is a weak alpha-adrenergic agonist that minimally dilates the pupil of a normal eye. If a postganglionic lesion is present (peripheral Horner syndrome), the pupil of the affected eye dilates much more than that of the unaffected eye because the iris dilator muscle of the affected eye has lost its sympathetic innervation and has developed adrenergic supersensitivity. As a result, anisocoria decreases. (However, results may be falsely normal if the causative lesion is acute.) If the lesion is preganglionic (or a central Horner syndrome), the pupil of the affected eye does not dilate because the iris dilator muscle does not develop adrenergic supersensitivity; as a result, anisocoria increases.
If results suggest Horner syndrome, hydroxyamphetamine (1%) can be put in both eyes 48 hours later to help locate the lesion. Hydroxyamphetamine works by causing norepinephrine to be released from the presynaptic terminals. It has no effect if postganglionic lesions are present because these lesions cause postganglionic terminals to degenerate. Thus, when hydroxyamphetamine is applied, the following occur:
Postganglionic lesion: The pupil of the affected eye does not dilate, but the pupil of the unaffected eye dilates, resulting in increased anisocoria.
Central or preganglionic lesion: The pupil of the affected eye dilates normally or more than it normally does, and the pupil of the unaffected eye dilates normally, resulting in decreased or unchanged anisocoria. (However, postganglionic lesions sometimes produce the same results.)
Hydroxyamphetamine testing is done less frequently than testing with apraclonidine, partly because hydroxyamphetamine tends to be available less often. For results to be valid, hydroxyamphetamine testing must be delayed until at least 24 hours after apraclonidine instillation.
Patients with Horner syndrome require MRI or CT of the brain, spinal cord, chest, or neck (depending on clinical suspicion) to localize the abnormality.
Horner syndrome causes ptosis, miosis, and anhidrosis.
It results from a central or peripheral lesion (preganglionic or postganglionic) that disrupts the cervical sympathetic pathway, which runs from the hypothalamus to the eye.
Instill cocaine, apraclonidine, and/or hydroxyamphetamine in both eyes to confirm the diagnosis of Horner syndrome and help locate the lesion (preganglionic or postganglionic).
Do MRI or CT of the brain, spinal cord, chest, or neck, depending on clinical suspicion.
Treat the cause, if identified; there is no treatment for primary Horner syndrome.