Horner syndrome, also known as Horner's syndrome or Bernard-Horner syndrome, is a rare disorder that affects the nerves that control the muscles of the face, eyes, and neck. It is caused by damage to a nerve pathway that runs from the brain to the face and neck.

The typical signs of Horner syndrome include a drooping of the upper eyelid (ptosis), a small pupil (miosis), and a lack of sweating (anhidrosis) on one side of the face. In some cases, there may also be a noticeable difference in the color of the iris (heterochromia), with the affected eye appearing lighter in color than the unaffected eye.

Horner syndrome can be caused by a variety of underlying conditions, including injury or damage to the nerves in the neck or chest, brain tumors or other brain disorders, migraines, or certain types of infections. In some cases, the cause may be unknown.

Treatment for Horner syndrome depends on the underlying cause. In some cases, the condition may resolve on its own without treatment, while in other cases, medications or surgery may be necessary to address the underlying condition.

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 in 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 or cavity 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 the 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 cylical 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.

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