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Anisocoria: Causes, Symptoms & Treatment

Anisocoria is a condition in which the two pupils of the eyes are unequal in size. It can be harmless or indicate a serious neurological condition.

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Things worth knowing about "Anisocoria"

Anisocoria is a condition in which the two pupils of the eyes are unequal in size. It can be harmless or indicate a serious neurological condition.

What is Anisocoria?

Anisocoria is a medical term describing unequal pupil sizes in the two eyes. Under normal circumstances, both pupils are the same size and react symmetrically to changes in light. When anisocoria is present, a visible difference exists between the diameter of the left and right pupil. A difference of up to one millimeter can occur as a normal physiological variant in healthy individuals and typically requires no treatment.

Causes

The causes of anisocoria range from benign to potentially life-threatening:

Physiological anisocoria: Approximately 20% of the healthy population have a slight, non-pathological pupil size difference that requires no intervention.
Neurological causes: Damage to the third cranial nerve (oculomotor nerve), for example due to increased intracranial pressure, an aneurysm, or a brain tumor, can cause a unilaterally dilated, non-reactive pupil.
Horner syndrome: Disruption of the sympathetic nervous system leads to a constricted pupil on the affected side, accompanied by drooping eyelid and a sunken eyeball.
Traumatic causes: Injuries to the eye or skull can directly affect pupillary function.
Pharmacological effects: Certain eye drops, drugs, or medications can selectively dilate or constrict one pupil.
Inflammatory conditions: Iritis (inflammation of the iris) can lead to asymmetric pupillary response.
Adie syndrome: A usually benign condition in which one pupil is enlarged and reacts sluggishly to light.

Symptoms

The most noticeable feature of anisocoria is the visible difference in pupil size. Associated symptoms can vary depending on the underlying cause:

Light sensitivity (photophobia)
Altered or blurred vision
Drooping eyelid (ptosis)
Double vision (diplopia)
Headache or neck pain
Altered consciousness (in severe neurological conditions)

Diagnosis

Diagnosis of anisocoria is primarily based on a thorough clinical examination:

Inspection: Assessment of pupil size in bright and dark conditions, and evaluation of the light reflex in both eyes.
Slit-lamp examination: Detailed assessment of the anterior segment of the eye by an ophthalmologist.
Pharmacological tests: Eye drop tests using cocaine, apraclonidine, or pilocarpine help differentiate between Horner syndrome and other causes.
Imaging: In cases where intracranial causes are suspected, CT (computed tomography) or MRI (magnetic resonance imaging) may be used.
Neurological examination: Assessment of other cranial nerves and neurological functions.

Treatment

Treatment of anisocoria is directed entirely at the underlying cause:

Physiological anisocoria: No treatment required.
Drug-induced anisocoria: Discontinuation or adjustment of the causative medication.
Neurological emergencies: Acute intracranial pressure or aneurysm requires immediate emergency treatment, which may include surgical intervention or intensive care.
Horner syndrome: Treatment of the underlying disease, such as a tumor or vascular condition.
Adie syndrome: Generally observation only, with pilocarpine eye drops to relieve associated symptoms.
Iritis: Anti-inflammatory eye drops (e.g., corticosteroids) and pupil-dilating agents.

When to See a Doctor?

Newly occurring anisocoria should always be evaluated by a physician, especially if it appears suddenly or is accompanied by additional symptoms such as severe headache, altered consciousness, double vision, or a drooping eyelid. In such cases, it may represent a neurological emergency requiring immediate medical attention.

References

Tintinalli, J. E. et al. - Tintinalli's Emergency Medicine: A Comprehensive Study Guide, 9th Edition. McGraw-Hill Education, 2020.
Leigh, R. J. & Zee, D. S. - The Neurology of Eye Movements, 5th Edition. Oxford University Press, 2015.
World Health Organization (WHO) - International Classification of Diseases (ICD-11). Available at: https://icd.who.int

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