What Is Horner Syndrome? (2024)

Horner syndrome is a rare neurological condition characterized by a drooping upper eyelid, a constricted pupil, and decreased sweating on one side of the face. It is caused by damage to part of the sympathetic nervous system (which responds to dangerous or stressful situations).

Horner syndrome doesn't occur on its own but develops as a result of another medical condition, such as a stroke, brain tumor, or spinal cord injury. The treatment is focused on resolving the underlying cause.

The article describes the symptoms and causes of Horner syndrome, including how this uncommon neurological syndrome is diagnosed and treated.

What Is Horner Syndrome? (1)

What Are the Symptoms of Horner Syndrome?

Classically, Horner syndrome describes a group of three symptoms that occur concomitantly (at the same time):

  • Partially drooping upper eyelid (ptosis)
  • Small pupil size (miosis)
  • Lack of sweating on the face (facial anhidrosis)

The symptoms are unilateral (affecting one side of the face only).

Other possible symptoms include unilateral facial flushing, blurred vision, or eye pain. In some cases, facial anhidrosis may affect the entire left or right side of the face or just a portion of it (such as the forehead).

Additional symptoms may appear depending on the underlying cause. (An example is the loss of bladder control following a stroke,) These are not considered facets of Horner syndrome. Rather, Horner syndrome is regarded as secondary to the underlying cause.

Horner syndrome occurring in newborns may also cause iris heterochromia. This occurs when the iris (the colored part of the eye) appears lighter on the affected side.

What Is Horner Syndrome Caused By?

Horner syndrome is caused by injury or damage to the sympathetic nervous system. The sympathetic nervous system regulates the body’s "fight or flight” response to perceived real threats.

With Horner syndrome, a specific part of the sympathetic nervous system, called the oculosympathetic pathway, is damaged.

Under normal circ*mstances, activation of the oculosympathetic pathway will cause your eyelids to open (to better see the threat), your pupils to widen (to see better in the dark), and your face to sweat (to cool it down as your body is flooded with inflammatory chemicals).

When any part of the oculosympathetic pathway is disrupted—from the hypothalamus and brainstem of the brain to the cervical and thoracic spine of the neck and chest—Horner syndrome can occur.

The reason that Horner syndrome is unilateral is that each side of the body is controlled by nerves originating on the opposite side of the brain. So if the nerve pathway is damaged on the right side of the brain, the left side of your body is affected (or vice versa).

The intensity of symptoms depends on how severely the nerve pathway is damaged.

How the Autonomic Nervous System Works

Underlying Conditions

The oculosympathetic pathway is divided into three parts, each of which can cause Horner syndrome. Different conditions can affect these different parts.

First-order Horner syndrome involves damage to the nerve running from the hypothalamus and brainstem to the spinal cord. Conditions affecting this section include:

  • Stroke
  • Multiple sclerosis
  • Encephalitis
  • Meningitis
  • Brain or spinal cord tumor
  • Spinal cord injury

Second-order Horner syndrome involves damage to the nerve running from your chest to the top of your lungs and along thecarotid arteryof your neck. Conditions affecting this section include:

  • Lung cancer
  • Cervical rib syndrome (an extra rib that can press on nerves)
  • Dental abscess of the lower jaw
  • Aneurysm (bulging) of the subclavian artery of the shoulder blade area
  • Neck or chest cavity trauma
  • Nerve damage from a medical procedure such as thyroidectomy, tonsillectomy, or carotid angiography

Third-order Horner syndrome involves damage to the nerve running from your neck to your middle ear and eye. Conditions affecting this section include:

  • Temporal arteritis (inflammation of an artery running along the side of the head)
  • Aneurysm of the internal carotid artery
  • Shingles (herpes zoster)
  • Otitis media (middle ear infection)

In some cases, the cause of Horner syndrome is idiopathic, meaning of unknown origin.

Horner Syndrome in Children

Horner syndrome can sometimes be congenital (meaning present at birth) or something that occurs during infancy or early childhood.Most cases are caused by:

  • Nerve damage from birth trauma, including cervical disc herniation
  • Neuroblastomas (a childhood cancer affecting nerves in the neck, chest, and spinal cord)
  • Congenital defect of the carotid artery

There is a rare form of Horner syndrome called "congenital Horner syndrome" that can be passed from parents to a child. It is an autosomal dominant disorder, meaning that the condition can develop if a single copy of the gene mutation is passed from one parent.

Congenital Horner syndrome accounts for only 5% of cases among children.

How Horner Syndrome Is Diagnosed

While the triad of "classic" symptoms may provide ample evidence of Horner syndrome, it is important to remember that Horner syndrome is not the cause but rather the effect of some other condition.

To this end, your healthcare provider will want to perform tests to first confirm Horner syndrome and then narrow the list of possible causes.

The diagnosis will start with a detailed medical history and physical exam. As part of the exam, the healthcare provider may use eyedrops containing apraclonidine to see if they dilate (widen) the pupil as they are meant to. Failure to do so is an indication of nerve damage.

Next, the healthcare provider will order tests to pinpoint where along the oculosympathetic pathway the injury has occurred. These may involve imaging tests such as:

  • Head chest computed tomography (CT) or magnetic resonance imaging (MRI) if stroke or other brain problems are suspected
  • Chest X-ray or chest CT if lung cancer is suspected
  • Magnetic resonance angiography (MRA) to detect problems with blood vessels

Additional tests may be ordered, depending on the suspected cause.

Horner Syndrome Treatment

The treatment for Horner syndrome varies based on the underlying cause. The treatment does not "cure" Horner syndrome but rather resolves or minimizes the underlying cause.

Potential treatments may include:

  • Anti-platelet drugs and minimally invasive treatments like angioplasty(for carotid artery problems)
  • Clot-dissolving agents for specific types of stroke
  • Antibiotics or antiviral drugs for infectious causes
  • Immunosuppressant drugs for conditions like multiple sclerosis
  • Surgery, radiation, or chemotherapy for cancers

Different medical specialists may be involved in the treatment, such as pulmonologists, neuro-ophthalmologists, and oncologists.

In many cases, the symptoms of Horner syndrome will go away once the underlying condition is resolved. In other cases, the symptoms may resolve on their own without treatment.


Horner syndrome is a neurological disorder that affects the eye and surrounding tissues on one side of the face. Symptoms include the drooping of the upper eyelid, a constricted pupil, and absent sweating on one side of the face.

Horner syndrome is caused by damage to the oculosympathetic nerve due to conditions like stroke, meningitis, lung cancer, and shingles, among many others. There is no direct treatment for Horner syndrome. Rather, the treatment involves resolving the underlying cause.

3 Sources

Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

  1. Barrea C, Vigouroux T, Karam J, et al. Horner syndrome in children: a clinical condition with serious underlying disease. Neuropediatrics. 2016;47(4):268-72. doi:10.1055/s-0036-1584085

  2. NIH Genetics Home Reference. Horner syndrome.

  3. Horner's syndrome: genetic and rare diseases information center (GARD) – an NCATS Program. National Center for Advancing Translational Sciences (NIH).

Additional Reading

What Is Horner Syndrome? (2)

By Ruth Jessen Hickman, MD
Dr. Hickman is a freelance medical and health writer specializing in physician news and patient education.

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