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The authors provide an updated clinical review of Horner syndrome. The most current recommendations regarding pharmacologic diagnosis and radiographic evaluation are highlighted. Important issues regarding the evaluation of Horner syndrome in children are also reviewed.
• Horner syndrome is caused by an interruption of the oculosympathetic pathway that extends from the hypothalamus through the brainstem, upper spinal cord, paraspinal region, chest, neck, and skull base orbit.
• Ipsilateral ptosis and miosis are the principal clinical manifestations, but both abnormalities are minimal and often overlooked.
• When the cause is not obvious, pharmacologic testing with topical apraclonidine 0.5% should be performed to confirm the diagnosis of Horner syndrome, as there are other causes of ptosis and miosis.
• Topical cocaine 10% is used as a diagnostic test in preference to apraclonidine in children aged 2 years or younger because apraclonidine may cause cardiopulmonary side effects, but the diagnostic endpoint is not as clear as for apraclonidine.
• Topical hydroxyamphetamine, formerly used to localize Horner syndrome, is no longer recommended.
• Common causes of Horner syndrome are neck, chest, and upper spinal procedures, trauma, and tumors, as well as carotid dissection and cluster headache.
• In isolated Horner syndrome without preceding trauma or procedures, the cause will remain indeterminate in up to 70% of cases even after adequate imaging evaluation.
• When the cause of Horner syndrome is clinically obvious—after internal jugular vein cannulation, neck, chest, or spinal surgery, or a known cavernous sinus lesion—imaging is not necessary.
• When there are localizing clues to a Horner syndrome, imaging can be targeted to that location.
• When there are no localizing clues, MRI/MRA or CT/CTA should scan from the skull base to the mid-thoracic region.
Horner syndrome consists of the clinical triad of ptosis, miosis, and anhidrosis and is caused by interruption of the ipsilateral oculosympathetic pathway. That pathway courses from the hypothalamus through the brainstem, upper spinal cord, paraspinal region, chest, neck, and skull base and orbits to reach the Muller muscle elevator of the upper lid and the dilator muscle of the iris.
The syndrome is named after a Swiss ophthalmologist named Johann Friedrich Horner, but he was not the first to describe it. In 1727, Pourfour du Petit reported ptosis, miosis, and enophthalmos after cutting the sympathetic nerve trunk in dogs (46). Edward Selleck Hare reported similar signs in a published letter to the London Medical Gazette in 1838 (22; 16).
Rabbit experiments in 1852 by the French physiologist Claude Bernard led to the connection between the cervical sympathetic fibers and Horner syndrome (44; 47), so that the French typically call it the “Bernard-Horner syndrome.”
In 1864, Mitchell described ipsilateral ptosis, miosis, and reduced facial sweating in an injury of the sympathetic trunk from a gunshot wound to the neck. In 1869, Horner published the more complete syndrome as ptosis, miosis, enophthalmos, increased skin temperature, and dryness of the ipsilateral face in a patient who had poor dilation of the affected pupil following instillation of atropine and preserved pupillary constriction to the parasympathomimetic agent calabar (23).
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