Horner's syndrome is characterized as a rare condition best identified by three main symptoms: miosis (pupil constriction), ptosis (upper eyelid sagging), and anhidrosis (inability to secrete sweat). It may also present with symptoms such as the inability to completely shut or open the eyes, flushing of one half of the face, frequent headaches, and sharp pains. Horner’s occurs as a result of damage of the sympathetic nerves that supply blood to the face. Root causes of Horner's syndrome vary greatly, however most stem from damage to the sympathetic nerves via a tumor, stroke, injury, etc. Most cases of Horner’s are also acquired, but in rare instances, may be congenital. Congenital Horner’s syndrome is associated with a fourth common symptom, heterochromia, or the absence of pigment in the iris.

As previously addressed, Horner syndrome is the result of sympathetic nerve denervation that wraps around the carotid artery. The symptoms that are present depend on the site and severity of the lesion. For example, ptosis occurs as a result of the denervation of the superior tarsal muscle, which raises the upper eyelid and also has a sympathetic nerve supply. This muscle helps to maintain the upper eyelid in a raised position after another muscle, the levator palpebrae superioris, initially raises it. Miosis occurs as a result of damage to the iris dilator muscle, which is supplied by the sympathetic nervous chain along the carotid and responsible for pupil dilation. When this chain is damaged, the parasympathetic supply is uninhibited so the pupil stays constricted. Since processes such as accommodation and adjusting of the pupil to light do not rely on the sympathetic nerve chain, they are unaffected by sympathetic nerve denervation. Anhidrosis occurs in different areas depending on the site of the lesion. Lesions of first order neurons affect the body ipsilaterally whereas lesions of second-order neurons affect the face ipsilaterally. Typically, lesions occurring after branching of the vasomotor and sudomotor fibers present as anhidrosis of the brow, also ipsilaterally. Iris heterochromia affects the ipsilateral eye of denervation.

There are various potential causes of Horner's syndrome, since the denervation of any sympathetic nerve fibers that supply the face may present symptoms. These fibers originate in the hypothalamus, extend into the upper regions of the spinal cord, around the carotid artery, and to the face. Factors such as trauma, compression, or a disease progression threaten healthy nerve function. Potential causes of Horner’s syndrome include birth trauma or the cervical spine, brainstem stroke, damage to the carotid artery, a tumor of the brainstem or upper spinal cord, migraines, demyelinating diseases, syringomyelia, inflammation of the lymph nodes, or pancoast tumor.

Eye and neurological exams may be necessary after diagnosis with Horner’s to determine if other areas of the nervous system are also affected. Although Horner’s does not typically cause any damage to visual functions or sight, its clinical importance lies in its indication that the oculosympathetic pathway has been disrupted. As such, this disorder, although presenting subtle symptoms, serves as an important warning for potentially life-threatening lesions in the head, neck, and chest. Cranial MRIs, Carotid ultrasounds, CT/XRays of the chest, blood tests, and eye drop tests are commonly used in diagnosis. Three common eye-drop tests used in diagnosis include Hydroxyamphetamine, Apraclonidine, and Cocaine eye drops. Hydroxyamphetamine drops can be used to distinguish third-order neuron lesions from first- and second-order lesions.  Apraclonidine, an adrenergic agonist, can be used to determine if there is an upregulation of postsynaptic receptors for norepinephrine due to greater sensitivity of the iris dilator muscle. This upregulation and increased sensitivity manifests as pupil dilation in patients with Horner’s. Cocaine eye-drops block the reuptake of norepinephrine to effectively induce pupil dilation and the raising of the eyelid in healthy patients. In patients with Horner’s, these two processes would not occur since the absence of norepinephrine in the post-synaptic cleft prevents reuptake. 

Although there are no specific treatments for Horner's syndrome, therapeutics often work to target underlying neurological conditions such as stroke, tumor, or trauma to the spinal cord to restore nerve function. In rare instances, Horner’s cannot be attributed to an underlying cause, so treatment primarily relies on remediating symptoms. Neurosurgical intervention is recommended for aneurysm-related Horner syndrome whereas vascular surgical care is recommended for causes such as carotid artery dissection or aneurysm. Another type of surgical intervention, the levator resection is used to more specifically correct the superficial ptosis and has a high success rate. Aside from surgery, non-surgical methods such as medication have proven to treat symptoms. As mentioned, apraclonidine eye drops used in diagnosing Horner’s, also work to reverse ptosis. Likewise, naphazoline, commonly found in over-the-counter eye drops, is used to constrict blood vessels to counteract red eyes, and can have similar effects to that of Apraclonidine. All in all, although there are few therapeutics that specifically target Horner’s syndrome, novel therapeutics and diagnostic measures may not only provide insight into treating Horner’s but also more life-threatening illnesses that show comorbidity. 

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Ruth Jessen Hickman, MD. “Shrunken Pupil and Drooping Eyelid: Might Be Horner Syndrome.” Verywell Health, Verywell Health, 29 Jan. 2020, https://www.verywellhealth.com/horner-syndrome-overview-4176967. 

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