What is Horner’s Syndrome?
Horner’s syndrome named after the Swiss Ophthalmologist Johann Friedrich Horner happens due to error occurring in sympathetic nerve supply towards one side of the face.
The result is miosis, ptosis and anhidrosis. Miosis means constriction of pupil; Ptosis is drooping of upper eyelid; Anhidrosis is a condition in which one cannot sweat normally.
Autonomic Nervous System
Before going into further details, let’s talk about the autonomic nervous system
Autonomic nervous system controls our involuntary functions of the body like heart rate, blood pressure and digestion and it is subdivided into Sympathetic and Parasympathetic nervous systems.
Sympathetic nervous system increases heart rate and blood pressure and decreases digestion while the parasympathetic nervous system decreases heart rate and blood pressure and increases digestion.
Since the sympathetic nervous system increases blood flow to the muscles and helps to fight or run away from threat, it is called “Fight or Flight response” whereas the Parasympathetic nervous system is called “Rest and Digest” response.
Oculosympathetic Pathway
Regarding eye and face, there is a sympathetic pathway named the Oculosympathetic pathway which has 3 neuron groups: First order, Second order and Third order neurons.
The body of the first-order neurons is situated in the hypothalamus whereas its axon outstretch down to the spinal cord and synapses with second-order neurons.
The body of the second-order neurons is situated in the cervical region of the spinal cord whereas its axon gets off the spinal cord and sets foot in the sympathetic chain which is a structure completely of sympathetic ganglions or nerve cell bodies.
This particular chain runs along both sides of the spine and looks exactly like a string of pearls in which pearls are our ganglions and strings are the nerve fibres. The first three ganglia observed within the sympathetic chain are referred to as superior, middle and inferior cervical ganglia.
The axon goes through this inferior and middle cervical ganglion and ascends the superior cervical ganglion and synapses with the body of the third-order neuron.
Axons of the third-order neuron stick out from superior cervical ganglion, run along the common carotid artery up the neck till it splits into internal and external carotid arteries.
A group of nerve fibres namely Internal Carotid plexus go after the internal carotid artery into skull and then leave through the orbit to innervate pupillary dilator muscle (that dilates pupil), Muller’s muscle (that raises the upper eyelid) and sweat glands of the forehead.
Another group of nerve fibres go behind the external carotid and its branches and innervate the remaining sweat glands of the face.
What are the causes of Horner’s syndrome?
When damage occurs to first, second and third order neurons in the oculosympathetic pathway, Horner’s syndrome arises!
The cause of damage to the first order neuron are Spinal cord lesions above level T1 that occur in conditions like stroke, tumors or syringomyelia. Syringomyelia is a disorder in which a fluid-filled cyst develops within the spinal cord, damaging the surrounding nerve fibers.
The cause of damage to the second-order neuron is compression from a tumor called Pancoast tumor which is a tumor that present in the apex of the lungs
The cause of damage to the third-order neuron is dissection of the internal carotid artery which occurs due to damage to the internal carotid artery wall.
What are the symptoms of Horner’s syndrome?
Symptoms include sympathetic fibre damage, miosis, ptosis and anhidrosis. Remember that these symptoms occur on the ipsilateral, or same side, of the damaged nerve.
How Horner’s syndrome is Diagnosed?
An Eye drop test that makes use of cocaine or apraclonidine can be done to diagnose Horner’s syndrome. Cocaine blocks the norepinephrine reuptake.
Norepinephrine is nothing but a neurotransmitter that is released by the sympathetic nervous system. What happens normally is if a drop of cocaine is placed in an eye, it results in the build up of norepinephrine and causes pupil dilation.
While if there is a block to sympathetic innervation, there won’t be any release of norepinephrine and therefore a cocaine drop in the eye does not cause pupil dilation.
Apraclonidine is otherwise a weaker form of norepinephrine. Since it is a weaker form, it is too weak to cause pupil dilation in normal cases.
However, when there occurs a block to sympathetic innervation, the pupillary dilator muscle becomes so peckish for stimulation that even apraclonidine drop in an eye causes pupil dilation.
So, a drop of cocaine in the eye normally causes pupil dilation due to norepinephrine build up, but does not bring out a change in Horner’s syndrome.
While a drop of Apraclonidine which normally does not cause any change, elicits pupil dilation in Horner’s syndrome.
Also, Imaging studies like CT or MRI can aid in identifying Horner’s syndrome’s underlying cause.
What is the treatment of Horner’s syndrome?
Treatment depends on the underlying cause of Horner’s syndrome. For conditions such as Syringomyelia, tumors or Carotid artery dissection, Surgical intervention might be advisable.
Quick Summary
Horner’s syndrome occurs when there is a disruption of the sympathetic nerve supply to an eye.
Symptoms are miosis, ptosis, and anhidrosis.
Horner’s syndrome can be diagnosed by an eye drop test which contains cocaine or apraclonidine. CT or MRI imaging studies are also indicated to find out the cause.
The treatment of horner’s syndrome depends on the underlying cause. Surgical intervention may be advisable for conditions like syringomyelia, tumors, or carotid artery dissection.
Reference
Encyclopedia of Molecular Mechanisms of Disease by Florian Lang
Encyclopedia of Molecular Biology by Thomas E. Creighton
Rare Diseases: Integrative PPPM Approach as the Medicine of the Future from Springer
