Horner Syndrome: Symptoms, Causes, and How It Is Diagnosed
Updated: 3 days ago
Medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: October 2026
TL;DR: Horner syndrome is a condition that affects the face and eye on one side of the body. It is caused by disruption of a nerve pathway that runs from the brain to the head and neck [1]. The key signs are a smaller pupil, a drooping upper eyelid, and reduced sweating on the affected side. It is usually a signal of another underlying medical problem, such as a stroke or tumor. There is no specific treatment for Horner syndrome itself, but treating the underlying cause may restore nerve function [2].
Quick Answer
Horner syndrome is a condition affecting one side of the face and eye, caused by damage to the nerve pathway from the brain to the head and neck. Signs include a smaller pupil, drooping eyelid and reduced sweating. It usually signals a problem such as stroke or tumor, and sudden onset after trauma or with severe headache needs emergency care. [1] [2]
What the research says
Eyelid height can sharpen eye-drop testing. In 36 patients tested with 1% apraclonidine drops, a decrease of more than 0.42 mm in the difference in eyelid opening between the two eyes distinguished Horner syndrome from ordinary unequal pupils with 80% sensitivity and 75% specificity (Fierz et al., Frontiers in Ophthalmology, 2022 [3]).
Pupillometry may rule out Horner syndrome without drops. In 44 adults referred for unequal pupils, automated pupillometry measuring the lag in pupil dilation after the lights go off, using a 0.35 mm cutoff, reached 95% sensitivity and 68% specificity compared with apraclonidine testing, which the authors describe as the current gold standard (Disse et al., Journal of Neuro-Ophthalmology, 2025 [4]).
Surgery in the neck is a rare cause. A systematic review of complications after anterior cervical spine surgery found Horner syndrome in 0.06% to 0.45% of operations across the included studies (Katsikas et al., Brain & Spine, 2025 [5]).
What Is Horner Syndrome?
Horner syndrome is a condition that affects the face and eye on one side of the body. It is caused by the disruption of a nerve pathway that runs from the brain to the head and neck [1].
The typical signs and symptoms include decreased pupil size, a drooping eyelid, and decreased sweating on the affected side of the face.
Horner syndrome may be the result of another medical problem, such as a stroke, tumor, or spinal cord injury. In some cases, no underlying cause can be found. There is no specific treatment for Horner syndrome itself, but treatment for the underlying cause may restore nerve function.
The condition is also known as Bernard-Horner syndrome or oculosympathetic palsy.
Symptoms
Horner syndrome usually affects only one side of the face. Common signs and symptoms include [1]:
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
The signs and symptoms, particularly ptosis and anhidrosis, may be subtle and difficult to detect.
Sign or symptom | What it looks like |
Miosis | The affected pupil stays persistently small |
Anisocoria | Noticeable difference in pupil size between the two eyes |
Slow dilation | The affected pupil opens little or slowly in dim light |
Ptosis | The upper eyelid droops on the affected side |
Upside-down ptosis | The lower lid is slightly elevated |
Sunken eye | The affected eye appears sunken |
Anhidrosis | Little or no sweating on the affected side of the face |
Signs in Children
Additional signs and symptoms in children with Horner syndrome may include a lighter iris color in the affected eye of a child under the age of 1. There may also be a change in color on the affected side of the face that would typically appear from heat, physical exertion, or emotional reactions [1].
When to See a Doctor
A number of factors, some more serious than others, can cause Horner syndrome. It is important to get a prompt and accurate diagnosis [1].
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:
Warning sign | Why it matters |
Symptoms appearing suddenly | May signal an acute event requiring immediate care |
Symptoms after traumatic injury | Trauma can damage the nerve pathway or blood vessels |
Impaired vision | Possible involvement of the eye or brain |
Dizziness | May indicate a neurological event |
Slurred speech | A possible sign of stroke |
Difficulty walking | May signal a serious nerve or brain problem |
Muscle weakness or loss of muscle control | Possible neurological emergency |
Severe, sudden headache or neck pain | Can indicate a life-threatening vascular event |
Causes
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 [1].
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, and a 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, a tumor of the myelin sheath (schwannoma), damage to the main blood vessel leading from the heart (aorta), surgery in the chest cavity, and 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 damage to the carotid artery or jugular vein along the side of the neck, a tumor or infection near the base of the skull, migraines, or cluster headaches.
Neuron group | Pathway | Possible causes of damage |
First-order | Hypothalamus → brainstem → upper spinal cord | Stroke, tumor, myelin-destroying diseases, neck trauma, syringomyelia (spinal cyst) |
Second-order | Spinal column → upper chest → side of neck | Lung cancer, schwannoma, aortic damage, chest surgery, trauma |
Third-order | Side of neck → facial skin, iris, and eyelid muscles | Carotid or jugular damage, tumor or infection near skull base, migraines, cluster headaches |
Causes in Children
The most common causes of Horner syndrome in children include injury to the neck or shoulders during delivery, a defect of the aorta present at birth, and a tumor of the hormonal and nervous systems called neuroblastoma [1].
Unknown Causes
In some cases, the cause of Horner syndrome cannot be identified. This is known as idiopathic Horner syndrome [1].
Complications
Horner syndrome itself does not damage the eye or vision. However, it is a signal that something is disrupting the nerve pathway from the brain to the face. The complications that matter are those caused by the underlying condition. Depending on the cause, complications can include vision problems, loss of nerve function, and, in serious cases, life-threatening events such as stroke or untreated tumors. This is why a prompt, accurate diagnosis of the underlying cause is so important.
Prevention
There is no known way to prevent Horner syndrome, since it results from damage to a nerve pathway caused by conditions such as stroke, trauma, or tumors. The best protection is early recognition of the signs and prompt medical evaluation, especially when symptoms appear suddenly or after an injury.
Diagnosis
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 [2].
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 cause 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:
Test | What it does |
Magnetic resonance imaging (MRI) | Uses radio waves and a magnetic field to produce detailed images |
Magnetic resonance angiography (MRA) | Used to evaluate blood vessels |
Chest X-ray | Images the chest to look for causes such as lung tumors |
Computerized tomography (CT) | A specialized X-ray technology that produces detailed cross-sectional images |
Treatment
There is no specific treatment for Horner syndrome. Often, Horner syndrome disappears when an underlying medical condition is effectively treated [2].
Coping and Support
Finding out that Horner syndrome may be linked to a serious underlying condition can be unsettling. It can help to bring a family member or friend to appointments to take notes and help remember the information you are given. Keep a written list of your symptoms, medical history, and questions so you can make the most of each visit.
Preparing for Your Appointment
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) [2].
What You Can Do
Before your appointment, make a list that includes 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; and questions to ask your provider.
Take a family member or friend along, if possible, to help you remember the information you are given.
Basic questions to ask your provider include:
What is 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 is the best course of action?
Will I need any follow-up tests or evaluation?
If possible, bring some relatively recent photographs — ones 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 a number of questions, including when you began experiencing symptoms, whether the symptoms have changed or worsened over time, whether you have a history of cancer, whether you have experienced any recent injury or trauma, whether you have experienced any head, neck, shoulder, or arm pain, and whether you have a history of migraines or cluster headaches.
Conclusion
Horner syndrome is a noticeable change on one side of the face — a smaller pupil, a drooping eyelid, and less sweating. While the signs themselves are not dangerous, they act as a warning light pointing to an underlying problem in the nerve pathway from the brain to the head and neck. Getting a prompt, accurate diagnosis is the key step, and treating the underlying cause may restore nerve function and make the syndrome disappear. If symptoms appear suddenly, after trauma, or alongside warning signs such as severe headache or slurred speech, seek emergency care immediately.
Concerned about changes in your eye or eyelid? Talk to a healthcare provider. A prompt evaluation can identify the cause and guide the right treatment. Start by noting your symptoms and booking an appointment today.
Frequently Asked Questions
What is Horner syndrome?
Horner syndrome is a condition affecting the face and eye on one side of the body, caused by disruption of a nerve pathway from the brain to the head and neck. Typical signs include a smaller pupil, a drooping upper eyelid, and reduced sweating on the affected side. It is also known as Bernard-Horner syndrome or oculosympathetic palsy.
What are the main signs of Horner syndrome?
The main signs are a persistently small pupil (miosis), unequal pupil size (anisocoria), slow dilation of the affected pupil in dim light, a drooping upper eyelid (ptosis), a slightly elevated lower lid, a sunken appearance of the affected eye, and reduced sweating on the affected side of the face. The drooping and reduced sweating can be subtle and hard to detect.
What causes Horner syndrome?
Horner syndrome is caused by damage to a pathway in the sympathetic nervous system, which runs in three neuron groups: first-order (brain to spinal cord, e.g., stroke or tumor), second-order (spinal cord to neck, e.g., lung cancer or chest surgery), and third-order (neck to face, e.g., carotid artery damage or migraines). Sometimes no cause is found, called idiopathic Horner syndrome.
Is Horner syndrome an emergency?
Not always, but get emergency care if symptoms appear suddenly, after traumatic injury, or alongside impaired vision, dizziness, slurred speech, difficulty walking, muscle weakness, or severe sudden headache or neck pain, since these can signal a stroke or other serious event.
Does Horner syndrome affect vision?
The syndrome itself does not damage vision, but it signals an underlying problem in the nerve pathway that needs diagnosis. Some of the underlying causes, if untreated, can lead to vision and neurological complications.
How is Horner syndrome diagnosed?
Doctors use medicated eye drops in both eyes and compare how each pupil reacts to confirm nerve damage. Imaging tests such as MRI, MRA, chest X-ray, and CT scan help locate where along the three-neuron pathway the disruption is occurring.
Is there a treatment for Horner syndrome?
There is no specific treatment for Horner syndrome itself. It often disappears when the underlying medical condition is effectively treated, which is why identifying and treating the cause is the main goal.
Can children get Horner syndrome?
Yes. In children under age 1, the affected eye may have a lighter iris color, and the affected side of the face may not change color with heat, exertion, or emotion. Common causes in children include delivery-related neck or shoulder injury, an aortic defect present at birth, and neuroblastoma.
Can Horner syndrome go away on its own?
Horner syndrome can resolve when the underlying cause is treated effectively. Since Horner syndrome often reflects another condition such as a tumor or vascular problem, treating that condition is the path to restoring normal nerve function.
What should I bring to a Horner syndrome appointment?
Bring a list of symptoms, past and recent illnesses or injuries, all medications and supplements with doses, and your questions. Recent photographs taken before symptoms began can help the doctor compare the affected eye. A family member or friend can help you remember the discussion.
Related Reading
Stellate Ganglion Block: Uses, Procedure and Side Effects explains stellate ganglion blocks.
PERRLA Eye Exam: What It Means and What Pupil Changes Can Show explains the quick pupil check used in eye and neurological exams.
References
Mayo Clinic Staff. Horner syndrome - Symptoms & causes. Mayo Clinic. May 13, 2022. https://www.mayoclinic.org/diseases-conditions/horner-syndrome/symptoms-causes/syc-20373547
Mayo Clinic Staff. Horner syndrome - Diagnosis & treatment. Mayo Clinic. May 13, 2022. https://www.mayoclinic.org/diseases-conditions/horner-syndrome/diagnosis-treatment/drc-20373551
Fierz FC, Disse LR, Bockisch CJ, et al. Apraclonidine - an eye opener. Frontiers in Ophthalmology. 2022;2:902821. PMID 38983513. https://doi.org/10.3389/fopht.2022.902821
Disse LR, Bockisch CJ, Weber KP, et al. Differentiation of Horner syndrome and physiological anisocoria by automated pupillometry. Journal of Neuro-Ophthalmology. 2025;45(3):327-332. PMID 40833793. https://doi.org/10.1097/WNO.0000000000002262
Katsikas A, Georgountzos G, Stamatopoulou A, et al. Complications associated with anterior cervical spine surgery: a systematic review of literature. Brain & Spine. 2025;6:105897. PMID 41488235. https://doi.org/10.1016/j.bas.2025.105897
Further Reading and Credible Sources
This article is for general informational purposes and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare professional about any medical condition.
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