Horner syndrome
Meaning of Horner syndrome: A drooping lid and a small pupil on one side, signaling that the nerve chain running from the chest to the eye has been interrupted.
Definition of Horner syndrome
Horner syndrome is the combination of a drooping upper eyelid, a constricted pupil and reduced sweating on one side of the face, caused by damage anywhere along the sympathetic nerve pathway supplying the eye. It is a sign of an underlying problem rather than a disease in its own right. The affected side may also look flushed and feel warm, since vessel tone is lost too.
Why do those signs appear together?
They share one nerve supply. Sympathetic fibers destined for the eye take a long and oddly indirect route: down from the brainstem into the upper chest, back up the neck alongside the great vessels, then into the skull with the carotid artery. Along that path they carry instructions to widen the pupil, to hold the upper lid up by a small accessory muscle, to make the face sweat, and to keep the skin vessels partly closed. Cut the supply anywhere and all of those instructions fail together on that side. The eye muscles themselves are unaffected, so this is not a paralysis of eye movement, and the lid droop is slight rather than complete.
What interrupts the pathway?
| Level | Typical causes | Sweating affected |
|---|---|---|
| Brainstem and spinal cord | Stroke, tumor, demyelination, cord injury | Whole side of the body |
| Chest and neck | Tumor at the lung apex, neck surgery, injury | The face |
| Carotid artery to the eye | Tearing of the artery wall, cluster headache | Little or none |
Why does the level matter?
Because the causes differ so sharply in urgency. A Pancoast tumor sitting on the apex of the lung presses the fibers as they cross it, and Horner syndrome can be its first outward sign. A sudden Horner syndrome accompanied by neck or face pain raises the possibility of a tear in the carotid artery wall, which can throw clots to the brain. In an infant, the same picture with a lighter colored iris on that side suggests the interruption dates from early development, since sympathetic supply influences iris pigmentation.
Also known as
Used in a sentence
Examination revealed a partial ptosis and miosis consistent with Horner syndrome.
Good to know
Horner syndrome is a localizing sign rather than a diagnosis, and its sudden appearance with neck or facial pain is associated with tearing of the carotid artery wall, while painless onset in an adult raises the question of a lesion at the lung apex.