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 to the eye
• Little or no sweating (anhidrosis) either on the entire side of the face or an isolated patch of skin on the affected side

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
• Lack of redness (flushing) on the affected side of the face that would normally appear from heat, physical exertion or emotional reactions

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.