Horner’s syndrome is a neurological sign caused by a disruption in the sympathetic nerve pathway that travels from the brain to the eye and face. This disruption affects involuntary bodily functions such as pupil dilation and facial perspiration on one side of the body. The condition is not a disease itself, but rather a collection of symptoms indicating an underlying medical issue. The signs appear on the same side of the face as the nerve damage.
Recognizing the Hallmark Symptoms
The presence of Horner’s syndrome is identified by a characteristic group of three symptoms, known as the classic triad, all occurring on the affected side of the face.
Partial ptosis is a mild drooping of the upper eyelid. This occurs because the Müller’s muscle, which keeps the eyelid slightly elevated, loses its sympathetic nerve supply and becomes weakened.
Miosis is a constricted or abnormally small pupil. Since sympathetic nerves widen the pupil (dilation), their interruption leaves the opposing parasympathetic system unopposed, causing constriction. This difference in pupil size between the two eyes, called anisocoria, is often more noticeable in dim lighting when the normal pupil attempts to dilate but the affected one cannot.
Anhidrosis is the decrease or absence of sweating on the involved side of the face. The sympathetic nervous system controls the sweat glands, and the loss of this function results in drier skin on that half of the face.
The Sympathetic Pathway and Causes
The sympathetic nerve supply to the eye and face is structured as a three-neuron chain, and damage at any point along this path results in Horner’s syndrome. The underlying causes are categorized by which of the three neurons is affected.
The first-order neurons, or central neurons, begin in the hypothalamus and travel down through the brainstem and spinal cord to the upper chest. Lesions affecting these neurons are related to central nervous system issues, such as a stroke or tumor in the brainstem, or lesions within the spinal cord. Because the fibers travel through the brain and spinal cord, first-order Horner’s syndrome often appears alongside other neurological signs, such as difficulty with coordination or vertigo.
The second-order neurons, or preganglionic neurons, begin in the upper spinal cord (C8 to T2 level) and ascend through the chest cavity and over the apex of the lung. Damage in this segment is often caused by conditions in the neck and chest. The most well-known cause is a Pancoast tumor, a type of lung cancer located at the top of the lung. Trauma to the neck or chest, such as from surgery or injury, can also interrupt this pathway segment.
The third-order neurons, or postganglionic neurons, start high in the neck and travel along the wall of the internal carotid artery as they ascend toward the eye. Causes include internal carotid artery dissection, which is a tear in the artery wall that should be suspected if the syndrome is accompanied by sudden, severe head or neck pain. Other causes are cluster headaches, lesions at the base of the skull, or middle ear infections. The location of anhidrosis can sometimes help pinpoint the affected neuron, as third-order lesions may only cause sweating loss on a small area of the face.
Diagnostic Confirmation and Localization
The diagnostic process involves two main steps: pharmacologically confirming the sympathetic nerve damage and then using imaging to locate the underlying cause. Specialized eye drops confirm the syndrome by exploiting how the affected pupil reacts to certain chemicals.
One common test uses apraclonidine eye drops, which cause the constricted pupil to dilate, often reversing the size difference between the two eyes. Apraclonidine works because the damaged sympathetic nerves cause the pupil muscle to become hypersensitive to adrenaline-like substances, a process known as denervation supersensitivity. The older cocaine test is also sometimes used; it forces the normal pupil to dilate by preventing norepinephrine reuptake, but fails to dilate the affected pupil due to the lack of available norepinephrine.
Once the syndrome is confirmed, the second step is localization, which requires imaging to find the site of nerve damage. Because the sympathetic pathway spans from the brain to the chest, imaging studies like Magnetic Resonance Imaging (MRI) or Computed Tomography (CT) scans are needed. The specific area scanned (brain, neck, or chest) depends on the suspected location of the lesion based on clinical symptoms and history.
Treatment Focus
Treatment for Horner’s syndrome focuses on managing the underlying medical condition, as there is no direct cure for the nerve damage itself. The priority is to identify and address the cause, especially since conditions like a carotid artery dissection, tumor, or stroke can be life-threatening. Prompt diagnosis and intervention for these serious conditions can prevent severe complications.
If the cause is a tumor, treatment involves surgical removal, chemotherapy, or radiation. If a carotid dissection is identified, management may involve antiplatelet or anticoagulant therapy to prevent stroke. If the syndrome is caused by a benign or temporary issue, such as minor trauma or infection, the symptoms may lessen or remain stable over time. In rare cases where residual ptosis is cosmetically bothersome, eye drops like apraclonidine can temporarily elevate the eyelid, or cosmetic surgery can be considered.