Horner syndrome usually affects only one side of the face. Common signs and symptoms include:

• A persistently small pupil (miosis)
• A notable difference in pupil size between the two eyes (anisocoria)
• Little or delayed opening (dilation) of the affected pupil in dim light
• Drooping of the upper eyelid (ptosis)
• Slight elevation of the lower lid, sometimes called upside-down ptosis
• Sunken appearance of the affected eye
• Little or no sweating (anhidrosis) on the affected side of the face

Signs and symptoms, particularly ptosis and anhidrosis, may be subtle and difficult to detect.

Children

Additional signs and symptoms in children with Horner syndrome may include:

• Lighter iris color in the affected eye of a child under the age of 1
• Change in color on the affected side of the face that would typically appear from heat, physical exertion or emotional reactions

A number of factors, some more serious than others, can cause Horner syndrome. It is important to get a prompt and accurate diagnosis.

Get emergency care if signs or symptoms associated with Horner syndrome appear suddenly, appear after a traumatic injury, or are accompanied by other signs or symptoms, such as:

• Impaired vision
• Dizziness
• Slurred speech
• Difficulty walking
• Muscle weakness or lack of muscle control
• Severe, sudden headache or neck pain

Horner syndrome is caused by damage to a certain pathway in the sympathetic nervous system. The sympathetic nervous system regulates heart rate, pupil size, perspiration, blood pressure and other functions that enable you to respond quickly to changes in your environment.

The nerve pathway affected by Horner syndrome is divided into three groups of nerve cells (neurons).

First-order neurons

This neuron pathway leads from the hypothalamus at the base of the brain, passes through the brainstem and extends into the upper portion of the spinal cord. Problems in this region that can disrupt nerve function related to Horner syndrome include:

• Stroke
• Tumor
• Diseases that cause the loss of the protective sheath on neurons (myelin)
• Neck trauma
• Cyst in the spinal column (syringomyelia)

Second-order neurons

This neuron path extends from the spinal column, across the upper part of the chest and into the side of the neck. Causes related to nerve damage in this region may include:

• Lung cancer
• Tumor of the myelin sheath (schwannoma)
• Damage to the main blood vessel leading from the heart (aorta)
• Surgery in the chest cavity
• Traumatic injury

Third-order neurons

This neuron path extends along the side of the neck and leads to facial skin and muscles of the iris and eyelids. Nerve damage in this region may be associated with the following:

• Damage to the carotid artery along the side of the neck
• Damage to the jugular vein along the side of the neck
• Tumor or infection near the base of the skull
• Migraines
• Cluster headaches, a disorder that results in cyclical patterns of severe headaches

Children

The most common causes of Horner syndrome in children include:

• Injury to the neck or shoulders during delivery
• Defect of the aorta present at birth
• Tumor of the hormonal and nervous systems (neuroblastoma)

Unknown causes

In some cases, the cause of Horner syndrome cannot be identified. This is known as idiopathic Horner syndrome.

In addition to a general medical examination, your doctor will likely conduct tests to determine the nature of your symptoms and identify a possible cause.

Tests to confirm Horner syndrome

Your doctor may be able to diagnose Horner syndrome based on your history and an assessment of your symptoms.

An eye specialist (ophthalmologist) may also confirm a diagnosis by putting a medicated eye drop in both eyes — either a drop that will dilate the pupil of a healthy eye or a drop that will constrict the pupil in a healthy eye. By comparing the reactions in the healthy eye with that of the suspect eye, the doctor can determine whether nerve damage is the cause of problems in the suspect eye.

Tests to identify the site of nerve damage

The nature of your symptoms may help your doctor narrow the search for the cause of Horner syndrome. Your doctor may also conduct additional tests or order imaging tests to locate the lesion or irregularity disrupting the nerve pathway.

Your doctor may administer a type of eye drop that will significantly dilate the healthy eye and little dilation of the affected eye if Horner syndrome is caused by a third-order neuron irregularity — a disruption somewhere in the neck or above.

Your doctor may order one or more of the following imaging tests to locate the site of a probable irregularity causing Horner syndrome:

• Magnetic resonance imaging (MRI), a technology that uses radio waves and a magnetic field to produce detailed images
• Magnetic resonance angiography (MRA), which is used to evaluate blood vessels
• Chest X-ray
• Computerized tomography (CT), a specialized X-ray technology

There's no specific treatment for Horner syndrome. Often, Horner syndrome disappears when an underlying medical condition is effectively treated.

In most nonemergency situations, you typically start by seeing a family doctor or an (ophthalmologist). You may be referred to a doctor who specializes in nervous system disorders (neurologist) or a specialist in both neurological disorders and disorders affecting the eye and visual pathways (neuro-ophthalmologist).

What you can do

Before your appointment, make a list that includes the following:

• Your symptoms, including any changes that cause you concern
• Key personal information, including past and recent illnesses and injuries as well as any stresses in your life
• All medications, vitamins or other supplements you take, including the doses
• Questions to ask your provider

Take a family member or friend along, if possible, to help you remember the information you're given.

Basic questions to ask your provider include:

• What's likely causing my symptoms?
• Other than the most likely cause, what are other possible causes for my symptoms?
• What tests do I need?
• Is my condition likely temporary or chronic?
• What's the best course of action?
• Will I need any follow-up tests or evaluation?

If possible, bring some relatively recent photographs — ones that were taken before the onset of symptoms — to your appointment. These images may help your doctor assess the current condition of your affected eye.

What to expect from your doctor

Your doctor is likely to take a history of your symptoms and conduct a general medical examination. He or she is likely to ask you a number of questions, including:

• When did you begin experiencing symptoms?
• Have the symptoms changed or worsened over time?
• Do you have a history of cancer?
• Have you experienced any recent injury or trauma?
• Have you experienced any head, neck, shoulder or arm pain?
• Do you have a history of migraines or cluster headaches?