Horner syndrome

Definitions & Key takeaways

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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Introduction0:00–0:21

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 nervous system that controls involuntary body functions like the heart rate, blood pressure and digestion.

Physiology0:21–2:48

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.

Pathology2:48–3:29

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. The most common cause of the damage to second-order neuron is from compression from a Pancoast tumor-- a tumor in the apices or tops of the lungs.
Lastly, damage to third-order neurons is usually associated with dissection of the internal carotid artery - which is when there’s damage to the wall of the internal carotid artery.

Symptoms3:29–3:56

Symptoms of Horner’s syndrome include miosis, a constricted pupil; ptosis, a droopy eyelid; and anhidrosis, the inability to sweat.
All of these symptoms occur on the ipsilateral, or same side, as the damaged nerve. These features can be remembered by mnemonic “SPAM”, where “S” stands for sympathetic fibers, “P” for ptosis, ”A” for anhidrosis and ”M” for miosis.

Diagnosis3:56–4:51

Diagnosis of Horner’s syndrome can be done with an eye drop test which contains cocaine or apraclonidine. Normally, cocaine blocks the reuptake of norepinephrine, which is the neurotransmitter released by the sympathetic nervous system.
So, if a drop of cocaine is placed in the eye, norepinephrine builds up and causes the pupil to dilate. If there’s a block to sympathetic innervation, there’s no norepinephrine release so a drop of cocaine doesn’t cause pupil dilation.
Another test is done with apraclonidine, which is like a weaker form of norepinephrine. Normally, apraclonidine is too weak to cause pupil dilation, but when there’s a block to sympathetic innervation, the pupillary dilator muscle becomes so starved for stimulation, that even apraclonidine causes they eye to dilate.
If there’s evidence of Horner’s syndrome, imaging studies like a CT or MRI can help identify the underlying cause of the problem.

Treatment4:51–5:02

The treatment for Horner’s syndrome depends on the underlying cause. Surgical intervention may be indicated for conditions like syringomyelia, tumors, or carotid artery dissection.
All right, as a quick recap. Horner’s syndrome occurs when there’s a disruption of the sympathetic nerve supply to an eye.

Review5:02–5:30

Symptoms include miosis, ptosis, and anhidrosis. Horner’s syndrome can be diagnosed with a drop of cocaine in the eye which should normally cause pupil dilation from norepinephrine buildup, but shouldn’t elicit a change in Horner’s syndrome.
Similarly, a drop of apraclonidine should normally not cause any change, but should cause pupil dilation in Horner’s syndrome.