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About 1 in 6,250 babies are born with Horner syndrome. The incidence of Horner syndrome that appears later is unknown, but it is considered an uncommon disorder. Rarely, congenital Horner syndrome is passed down within a family in a pattern that appears to be autosomal dominant, which means one copy of an altered gene in each cell is sufficient to cause the disorder.

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Migraine or cluster headaches

Stroke, tumor, or other damage to a part of the brain called the brainstem

Tumor in the top of the lung

Injections or surgery done to interrupt the nerve fibers and relieve pain (sympathectomy)

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In rare cases, Horner syndrome is present at birth (congenital). The condition may occur with a lack of color (pigmentation) of the iris (colored part of the eye).

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Sympathetic nerve fiber injuries can result from:

Injury to one of the main arteries to the brain (carotid artery (Carotid artery disease occurs when the major arteries in your neck become narrowed or blocked. These arteries, called the carotid arteries, supply your brain with blood. Your carotid arteries extend from your aorta in your chest to the brain inside your skull.))

Injury to nerves at the base of the neck called the brachial plexus (The brachial plexus is a network of nerves, running from the spine, formed by the anterior rami of the lower four cervical nerves and first thoracic nerve)

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Changes in how the pupil opens or closes

Eyelid drooping

Red eye

Your healthcare provider will do a complete examination to determine the possible effect on other parts of the nervous system. Depending on the suspected cause, you may need:

Blood tests

Blood vessel tests (cerebral angiogram, CT angiogram, or MR angiogram

Chest x-ray or chest CT scan

MRI or CT scan of the brain

Spinal tap (lumbar puncture)

You may need to be referred to a doctor who specializes in vision problems related to the nervous system (neuro-ophthalmologist).

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Horner's syndrome can be caused by any interruption in a set of nerve fibers (threadlike extension of a cell) that start in the part of the brain called the hypothalamus (The hypothalamus  is a portion of the brain that contains a number of small nuclei with a variety of functions.) and travel to the face and eyes.

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Decreased sweat on the affected side of the face.

Drooping eyelid

Sinking of the eyeball into the face

Small pupil

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A disrupted nerve pathway on one side from the brain to the face and eye.