Horner Syndrome

Etiology

Horner syndrome is primarily an acquired condition secondary to systemic/local diseases or iatrogenic causes but maybe congenital and purely hereditary in some rare cases.[8][9] Sympathetic fibers have an extensive course and can be interrupted during extracranial, intracranial, or intra-orbital traversal.[10][11][12] The sympathetic fibers may be interrupted centrally, between the hypothalamus and the exit point of fibers from the spinal cord (C8 to T2), or the discontinuation could be peripheral, in the cervical sympathetic chain, at the level of superior cervical ganglion, or along the course of the carotid artery.[10] Preganglionic Horner syndrome can be an ominous sign due to its association with pulmonary malignancies. Overall, the causes of Horner syndrome can be divided according to the anatomical location of disruption.

First-order neurons are mostly affected by intracranial conditions and include the following:[13][14][15]

• Cerebral vascular accidents (CVA)
• Multiple sclerosis
• Arnold-Chiari malformation
• Encephalitis
• Meningitis
• Lateral medullary syndrome 
• Syringomyelia
• Intracranial tumors (pituitary or basal skull)
• Spinal trauma above the T2-T3 level
• Spinal cord tumors

Second-order neurons traverse the thoracic region and are affected by the following:

• Malignancies involving the apex of the lungs (Pancoast tumor)
• Cervical rib (tractional injury)
• Lesions of the subclavian artery (an aneurysm)
• Mediastinal lymphadenopathy
• Trauma to brachial plexus[11]
• Neuroblastoma of the paravertebral sympathetic chain[15]
• A dental abscess involving the mandibular region
• Iatrogenic (including thyroidectomy, radical neck dissection, tonsillectomy, coronary artery bypass grafting, or carotid angiography)[12][16][13][17][13]

Third-order neurons are in close proximity to the internal carotid artery and cavernous sinus and are affected by the following:

• Carotid cavernous fistula
• Internal carotid artery dissection or an aneurysm[18]
• Cluster headaches or migraines
• Raeder paratrigeminal syndrome (unilateral facial pain, headache, and Horner syndrome)
• Herpes zoster infection
• Temporal arteritis

Epidemiology

Horner syndrome is uncommon, occurring with a frequency of approximately 1 per 6,000. It may occur at any age or with any ethnic group.[15][19]

Pathophysiology

As previously described, Horner syndrome is a consequence of sympathetic disruption. The symptomology depends on the location of the lesion, and severity depends on the degree of denervation.[9]

Superior tarsal muscle helps raise the upper eyelid and has a sympathetic nerve supply. Denervation of this muscle causes ptosis, which is milder than oculomotor (CN III) palsy which supplies the levator palpebrae superioris. The superior tarsal muscle is responsible for keeping the upper eyelid in a raised position after levator palpebrae superioris raises it. This explains the partial ptosis seen in Horner syndrome. The lower eyelid may be slightly elevated owing to denervation of the lower lid muscle, which is analogous to the superior tarsal muscle.[20]

The sympathetic nervous supply is responsible for pupil dilationupil (mydriasis). When disrupted, parasympathetic supply is uninhibited, and constriction of the pupil (miosis) ensues. The reaction of the pupils to light and accommodation is normal as those systems do not depend on sympathetic nerve supply.[21]

Ipsilateral anhidrosis, another classic presentation, depends on the level of interruption of the sympathetic supply. Anhidrosis with first-order neuron lesions affects the ipsilateral side of the body as the sympathetic supply from its central origin.[22] The ipsilateral face is involved in lesions involving the second-order neurons. Postganglionic third-order neuron lesions after the vasomotor and sudomotor fibers have branched off show very limited involvement of the face (area adjacent to the ipsilateral brow).

Iris heterochromia (relevant deficiency of pigment in the iris on the affected side) is seen in children younger than 2 years and is the congenital form of Horner syndrome.[9]

History and Physical

Localization of the lesion in Horner syndrome is crucial in subsequent management. A detailed history and physical examination are, therefore, of vital importance. When evaluating, the following points need to be considered:

• Balance, hearing, sensory, and swallowing problems can point towards a more central process involving the first-order neurons.
• Prior history of trauma or surgical intervention involving the head, face, neck, shoulder, or back points towards the involvement of second-order neurons.[17]
• A detailed past medical and medication history to rule out the use of a miotic or mydriatic agent.
• A headache, double vision, facial numbness, or pain indicate third-order neuron involvement.
• The presence of anhidrosis and its location can help in the localization.
• A detailed history of a headache if present.
• Longstanding symptoms point towards a more benign underlying cause versus recently progressive symptoms such as weight loss, hemoptysis, low-grade fever, and lymphadenopathy.
• Facial flushing suggests a preganglionic lesion.[23]
• Facial or orbital pain in combination with miosis and ptosis should point towards Raeder paratrigeminal syndrome.[24]
• History of skin lesions or previously diagnosed herpes zoster infection.
• Severity and location of the pain.

A detailed examination of the eyes is warranted and should include the following:

• Reactivity of the pupils to light and accommodation
• Measurement of pupillary diameter in dim and bright light
• Examination of the upper eyelid for ptosis and fatigability
• Evaluation of extraocular movements
• Vision, including color vision and visual fields
• Slit-lamp examination for structural details 
• Evaluation of the presence of nystagmus

A detailed exam can reveal a round and constricted pupil. The affected pupil exhibits dilation lag (dilates more slowly), and the unequal pupils are appreciated more in darkness compared to light. In addition, the ciliospinal reflex may be absent.

Furthermore, partial ptosis, iris heterochromia, apparent enophthalmos, contralateral eyelid retraction, injected conjunctivae, and no change or a transient decrease in intraocular pressure may be seen.[25][18][17]

It is important to perform a detailed systemic examination, paying specific attention to neurological, pulmonary, and cardiovascular systems and considering various differentials discussed later.

Evaluation

Labs

• The initial workup should include a complete blood count (CBC), erythrocyte sedimentation rate (ESR), and serum chemistry panel.
• Urine or blood cultures may be ordered if an infectious agent is suspected.
• Testing for suspected neurosyphilis, HIV, thyroid function, vitamin B-12, and folate levels may be ordered if indicated following detailed history and examination.
• Urine testing for elevated metabolic catecholamine by-products is important in the pediatric population with suspected neuroblastoma.
• Purified protein derivative (PPD) is warranted in suspected tuberculosis.

Imaging

• A chest X-ray followed by computed tomography (CT) scan must be ordered in patients when pulmonary malignancy is suspected.
• Head CT and magnetic resonance imaging (MRI) are advised in cases of possible stroke.
• MRI is warranted and preferred over ultrasonography when carotid artery dissection is a possibility (painful Horner syndrome).[26]

Pharmacological Testing

This is the most helpful testing modality for diagnostic localization.[27]

Topical Cocaine Test: Cocaine acts as an indirect sympathomimetic inhibiting the reuptake of norepinephrine from the synaptic cleft. Cocaine solution (ranging from 2% to 10%) is instilled into both eyes. Both eyes are evaluated after at least 30 or more minutes for an optimal response. Denervation in the affected eye causes it to dilate poorly compared to the normal one. Anisocoria of 0.8 mm or more is considered diagnostic. However, the test does not help in identifying the level of the lesion.

The test has other disadvantages such as comparatively high prices of the compound, time-consuming, test yielding ambiguous results, and the metabolites of cocaine can be detected in urine.[28]

Topical Hydroxyamphetamine Test: This test is particularly helpful in the localization of the lesion. Hydroxyamphetamine stimulates the release of stored norepinephrine from the postganglionic terminals into the synapse. Postganglionic third-order lesions can be differentiated from presynaptic second-order or first-order ones.

Two drops of 1% hydroxyamphetamine solution are instilled into both eyes. As a result, the affected eye (third-order lesion) will not dilate as well as the normal eye. While in the case of intact postganglionic fibers (first and second-order lesions), the affected pupil dilates to an equal or greater extent.

Disadvantages involving this test include it not being able to be performed on the same day as the cocaine test and false-negative results.[28] 

Topical Apraclonidine Test: This test is considered the test of choice due to its good sensitivity and overall practicality. Apraclonidine acts as a weak alpha1-agonist and strong alpha2-agonist. It is categorized as an ocular hypotensive agent. The upregulation of alpha1-receptors in Horner syndrome translates into an exaggerated response of the iris dilators (denervation supersensitivity) to an agonist agent like apraclonidine.[29]

A 0.5-1% solution is instilled in both eyes. The affected eye will show mydriasis, while the normal eye is predominantly insensitive. Consequent instillation of the solution results in an evident reversal of anisocoria (the affected pupil dilates and the normal pupil constricts). This is because of the stronger alpha 2 agonist activity compared to the weaker alpha 1 agonist activity of apraclonidine.

Some disadvantages of this form of testing include false-negative results in acute cases, systemic side effects in the pediatric population, inability to localize the lesion, and relatively long half-life of the drug.

Other agents have been proposed but fall short of clinical relevance due to several reasons, such as:

• Pholedrine, an N-methyl derivative of hydroxyamphetamine, is roughly half as potent as hydroxyamphetamine and has shown comparable results, but due to limited availability, it has been restricted.
• Adrenaline, a direct-acting sympathomimetic, also has been proposed but has poor penetration through the cornea, and sensitivity to the drug is variable.
• Phenylephrine, a selective alpha1 agonist, also has been proposed but suffers from poor penetration through the cornea and varying results, depending on the degree of denervation.[30]

Treatment / Management

Treatment options are based on the diagnosis and management of the underlying cause. Timely diagnosis is of critical importance, followed by referral to an appropriate specialist. Healthcare professionals are advised to incorporate the importance of eye examinations into their acumen.

Whether surgical intervention is indicated and what type is appropriate largely depends on the cause of the disease. Surgical treatment options include the following:

• Neurosurgical attention for aneurysm-related Horner syndrome 
• Vascular surgical care for causes such as carotid artery dissection or aneurysm

Differential Diagnosis

As previously described, pain accompanied by Horner syndrome points towards a more insidious underlying cause and should be evaluated thoroughly. Systemic symptoms such as weight loss and progressive fatigue can indicate an underlying malignancy. Furthermore, Horner syndrome can be an early manifestation of neuroblastoma in the pediatric population. 

Carotid artery dissection can present with a unilateral headache and facial or neck pain. If suspected, urgent appropriate workup and treatment are warranted.

Raeder paratrigeminal syndrome also can present with headaches but is accompanied by trigeminal nerve (CN V) impairment.

Anisocoria and/or ptosis can be due to myriad diseases and conditions such as Holmes-Adie syndrome, neurosyphilis (Argyll Robertson pupil), third nerve palsy, or optic neuritis.

Prognosis

The long-term prognosis of idiopathic Horner syndrome appears to be benign. Clinically, patients do not generally become worse, and in fact, half of the patients no longer experience anisocoria. Almost a third of patients with ptosis notice spontaneous improvement or complete resolution in a mean duration of 7.9 years after Horner syndrome is diagnosed. In unilateral cases, the oculo-sympathetic defect does not progress to become bilateral. It is possible that new medical disorders later develop between 3 and 20 years after the diagnosis of Horner syndrome; none are usually related to Horner syndrome.[31]

Complications

Horner syndrome is almost always secondary to an underlying cause, such as tumor, trauma, infection, etc, and is rarely idiopathic. The complications that occur as a result of Horner syndrome are essentially related to the cause of Horner syndrome. For instance, if Horner syndrome is secondary to multiple sclerosis, it could lead to muscle stiffness or spasms, impaired bladder, bowel, or sexual function; and seizures. Similarly, Horner syndrome-related eye problems can lead to problems with vision that could be temporary or permanent. Visual involvement can lead to partial or complete blindness.