The medical term for having pupils of unequal size is anisocoria, which occurs in up to 20% of the population. Although this difference in pupil size can be startling, it is most often a harmless variation requiring no treatment. However, anisocoria can sometimes signal a serious underlying medical issue, so it requires proper medical evaluation to determine the cause. The primary goal of assessment is distinguishing between a benign, lifelong difference and a sudden, serious neurological event.
Understanding Pupil Function
The size of the pupil, the black center of the eye, is controlled by two opposing muscles within the iris, the colored part of the eye. The iris sphincter muscle is arranged circularly and constricts the pupil, making it smaller (miosis). Conversely, the iris dilator muscle is arranged radially and contracts to widen the pupil (mydriasis).
These muscles are regulated by the autonomic nervous system, which manages involuntary body functions. The parasympathetic nervous system controls constriction via the oculomotor nerve (cranial nerve III), while the sympathetic nervous system controls dilation. Anisocoria occurs when this balance is disrupted, affecting the nerve pathway responsible for either constriction or dilation on one side.
Benign Causes of Unequal Pupil Size
The most common reason for unequal pupils is physiological anisocoria, a normal state affecting approximately one in five individuals. This type is characterized by a small difference, typically less than 1 millimeter, that remains constant regardless of light conditions. Physiological anisocoria is present constantly or intermittently and is not associated with other symptoms, making it a harmless variation.
Certain medications can induce temporary pharmacological anisocoria. Eye drops used to treat glaucoma, or inadvertent contact with medicines like scopolamine patches or certain cold medicines, can cause one pupil to dilate. This happens because the medication contacts only one eye, temporarily paralyzing the muscles controlling pupil size.
Physical damage to the iris can cause mechanical anisocoria, resulting in a pupil unable to change size properly. This occurs following eye trauma, eye surgery, or inflammation within the eye, such as uveitis. In these cases, the size difference is due to structural damage to the muscle tissue rather than a problem with the controlling nerve signals.
When Unequal Pupils Signal a Serious Medical Issue
If the anisocoria is new, severe, or accompanied by other symptoms, it may signal a pathological process requiring immediate attention. A pupil larger in bright light suggests a problem with constriction, often indicating a failure of the parasympathetic pathway. The most concerning cause of a persistently dilated pupil is a third cranial nerve palsy, potentially caused by pressure from a brain aneurysm. This is a neurosurgical emergency, especially if the large pupil is accompanied by a drooping eyelid (ptosis) and problems with eye movement.
Conversely, anisocoria more pronounced in dim light suggests the smaller pupil is abnormal, indicating a failure of the sympathetic system to dilate. This presentation is characteristic of Horner syndrome, which results from damage to the sympathetic nerve pathway traveling from the brain down the neck into the eye. Associated symptoms include a mild drooping of the upper eyelid, a slightly raised lower eyelid, and a lack of sweating on the affected side of the face.
Horner syndrome can be caused by serious underlying conditions like a carotid artery dissection, a tumor in the chest or neck, or a stroke, warranting urgent investigation. When anisocoria occurs alongside symptoms such as a severe, sudden headache, double vision, dizziness, or impaired speech, it suggests a neurological emergency. Increased intracranial pressure from a mass or hemorrhage can also cause the pupil to dilate due to compression of the oculomotor nerve.
Diagnosis and Necessary Medical Evaluation
A professional evaluation begins with a detailed patient history, including the onset of unequal pupils and any associated symptoms like headache, neck pain, or vision changes. The doctor performs a physical examination to measure the pupils in both bright and dim light. This measurement determines which pupil is abnormal and which nervous system pathway is affected: greater difference in light means the larger pupil fails to constrict; greater difference in dark means the smaller pupil fails to dilate.
Specialized eye drops, known as pharmacological testing, may be used to confirm the diagnosis and pinpoint the problem’s location. For example, an apraclonidine drop helps identify Horner syndrome, while a weak pilocarpine drop confirms Adie’s tonic pupil, a non-emergent cause of a pathologically large pupil. If a serious neurological cause is suspected, especially with new-onset anisocoria and concerning symptoms, the next step involves immediate neuroimaging. Imaging studies like a Computed Tomography (CT) scan or Magnetic Resonance Imaging (MRI) of the brain, neck, or chest rule out life-threatening conditions such as aneurysms, tumors, or carotid artery dissection.