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Horner syndrome



Nervous system


Central nervous system disorders
Central and peripheral nervous system disorders
Peripheral nervous system disorders
Autonomic nervous system disorders
Nervous system pathology review

Horner syndrome


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High Yield Notes
3 pages

Horner syndrome

12 flashcards

USMLE® Step 1 style questions USMLE

2 questions

A 72-year-old man comes to his primary care physician to evaluate right-sided vision changes over six months. The patient also has noted an increase in swelling in his right upper arm.  Past medical history is significant for myocardial infarction and hyperlipidemia. The patient has a 30 pack-year smoking history. He has a droopy right eyelid and intact extraocular eye movements on examination. Pupils are asymmetric in dim light – 2 mm on the right and 4 mm on the left, with both reactive to light. The pupils become more symmetric in bright light. The skin on the right half of his face appears cracked and dry. Which of the following is the most likely etiology of this patient’s right eye droop?

External References

Horner’s syndrome, named after the ophthalmologist Johann Friedrich Horner, is caused by a problem with the sympathetic nerve supply to one side of the face.

This disruption results in miosis, which is constricted pupil; ptosis, a droopy eyelid; and anhidrosis, a failure to sweat.

Broadly speaking, the autonomic nervous system is a part of the nervous system that controls involuntary body functions like the heart rate, blood pressure and digestion.

The autonomic nervous system can be subdivided into the sympathetic and parasympathetic nervous system, which have opposite effects.

The sympathetic nervous system controls functions like increasing heart rate, blood pressure, and slowing digestion. All of this maximizes blood flow to the muscles, and can help you either run away from a threat or fight it which is why it’s also called the fight-or-flight response.

The parasympathetic nervous system has the opposite effect; it slows heart rate, decreases blood pressure, and stimulates digestion - the effects can be summarized as 'rest and digest'.

Now, with regard to the face and eye, there’s an oculosympathetic pathway with three groups of neurons called first-order, second-order and third-order neurons.

The body of the first-order neuron is located in the hypothalamus, and it’s axon extends down into the spinal cord, where it synapses with the second-order neuron.

The body of the second-order neuron is located in the cervical region of the spinal cord, and it’s axon exits the spinal cord and enters the sympathetic chain, which is a structure full of sympathetic ganglions or nerve cell bodies, and it runs along both sides of the spine.

The sympathetic chain looks similar to a string of pearls where the ganglions are the pearls and the nerve fibers make up the string.

The first three ganglia within the sympathetic chain are called the superior, middle, and inferior cervical ganglion.

The axon of the second-order neuron runs through the inferior and middle cervical ganglion, up the superior cervical ganglion where it synapses with the body of the third-order neuron.

Third order neuron axons extend from the superior cervical ganglion and hitch-hike along nerves that travel along the common carotid artery up the neck until it splits into an internal and external carotid artery.

One group of nerve fibers, called internal carotid plexus, follow the internal carotid artery into the skull and then exit through the orbit to innervate the pupillary dilator muscle, which dilates the pupil; the Müller's muscle, which raises the upper eyelid; and sweat glands of the forehead.

The other group of fibers, follow the external carotid and its branches and innervate the rest of the sweat glands of the face.

Horner’s syndrome occurs when there’s damage along the oculosympathetic pathway.

Damage to the first-order neuron is caused by a spinal cord lesions above level T1 - some causes include a stroke, tumors, or syringomyelia.

Syringomyelia is when there’s a cyst or cavity within the spinal cord, which damages the surrounding nerve fiber.


Horner syndrome is a neurological disorder characterized by the combination of three main symptoms: ptosis (drooping eyelid), miosis (constricted pupil), and anhidrosis (lack of sweating on one side of the body).

Most cases of horner syndrome are caused by damage to the sympathetic nervous system, which can be due to a stroke, tumor, or injury. Less common causes include infections, autoimmune disorders, and medications. Some people also develop horner syndrome as a side effect of cancer treatment.

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  2. "Harrison's Principles of Internal Medicine, Twentieth Edition (Vol.1 & Vol.2)" McGraw-Hill Education / Medical (2018)
  3. "Pathophysiology of Disease: An Introduction to Clinical Medicine 8E" McGraw-Hill Education / Medical (2018)
  4. "CURRENT Medical Diagnosis and Treatment 2020" McGraw-Hill Education / Medical (2019)
  5. "Enophthalmos Is Not Present in Horner Syndrome" PLoS Medicine (2005)
  6. "Neuroimaging Strategies for Three Types of Horner Syndrome with Emphasis on Anatomic Location" American Journal of Roentgenology (2007)