Unequal Pupil Size (Anisocoria)

Anisocoria (an-eye-so-KOR-ee-uh) is the medical term for unequal pupil size. While slight differences are normal in many people, significant or new anisocoria can indicate important neurological conditions requiring evaluation.

Understanding Pupils

Pupils control how much light enters the eye. They:

• Constrict (get smaller) in bright light
• Dilate (get larger) in dim light
• Both should react equally and together

Pupil size is controlled by:

• Parasympathetic nerves—constrict the pupil (via third cranial nerve)
• Sympathetic nerves—dilate the pupil

Types of Anisocoria

Physiologic Anisocoria

• Normal variation present in ~20% of population
• Usually less than 1mm difference
• Same in light and dark
• No other symptoms
• Doesn't change over time
• Old photos often show it's longstanding

Larger Pupil Abnormal

Third cranial nerve palsy

• Pupil dilated and poorly reactive
• Usually with ptosis (droopy eyelid)
• Eye deviated outward/downward
• If acute, rule out aneurysm—emergency

Pharmacologic dilation

• Exposure to dilating agents (atropine, scopolamine patches)
• Pupil very large and doesn't react
• May be occupational or accidental

Adie's tonic pupil

• Usually in young women
• Pupil larger, reacts sluggishly
• Better near reaction than light reaction
• Often benign

Trauma

• Iris damage from eye injury
• May be irregular pupil

Smaller Pupil Abnormal

Horner syndrome

• Pupil smaller (miosis)
• Mild ptosis (1-2mm)
• May have decreased sweating on face
• Indicates sympathetic pathway problem
• Requires workup to find cause

Pharmacologic constriction

• Exposure to constricting agents (pilocarpine, opiates)
• Very small pupil

Inflammation (uveitis)

• Pupil may be small and irregular
• Pain, redness, light sensitivity

Warning Signs

For comprehensive information about understanding and evaluating pupil abnormalities, see our complete guide: Understanding Pupil Abnormalities - When Unequal Pupils Need Attention.

When to Be Reassured

Anisocoria is less concerning when:

• It has been present for years (check old photos)
• Difference is small (less than 1mm) and stable
• Both pupils react briskly to light
• No other symptoms
• No ptosis, no double vision, no headache

How Anisocoria Is Evaluated

Key Questions

1. Which pupil is abnormal?

   • Compare in light vs. dark
   • If difference greater in dark → smaller pupil is abnormal (doesn't dilate)
   • If difference greater in light → larger pupil is abnormal (doesn't constrict)

2. Is it old or new?

   • Look at old photos
   • Driver's license photos helpful

3. Are there other symptoms?

   • Ptosis, double vision, pain, headache

Clinical Tests

• Light reaction testing—each pupil should constrict equally
• Near reaction testing—constriction with focusing close
• Pupil reactivity comparison—direct and consensual responses
• Examination in light and dark—to identify abnormal pupil

Pharmacologic Testing

If needed to diagnose:

• Apraclonidine drops—confirms Horner syndrome
• Pilocarpine drops (dilute)—confirms Adie's pupil
• Pilocarpine drops (regular)—tests for pharmacologic dilation

Imaging

• MRI brain/orbits—for third nerve palsy
• MRA or CTA—urgently if aneurysm suspected
• MRI/CT of neck and chest—for Horner syndrome workup

Specific Conditions

Horner Syndrome

Caused by interruption of sympathetic nerve pathway. Features:

• Mild ptosis
• Smaller pupil (miosis)
• May have anhidrosis (decreased sweating)

Requires workup to find underlying cause (can range from benign to serious).

See Horner syndrome

Adie's Tonic Pupil

Usually benign condition. Features:

• Larger pupil
• Sluggish or absent light reaction
• Better near reaction (light-near dissociation)
• Often in young women
• May have reduced tendon reflexes (Holmes-Adie syndrome)

See Adie's tonic pupil

Third Nerve Palsy with Pupil Involvement

Concerning for aneurysm. Features:

• Dilated, unreactive pupil
• Complete ptosis
• Eye deviated outward and down
• Severe headache may be present

Requires emergency imaging.

See Third nerve palsy

Frequently Asked Questions

Is having different pupil sizes dangerous?

Not necessarily. Physiologic anisocoria is normal and harmless. However, new or significant anisocoria can indicate important neurological conditions and should be evaluated.

How do I know if my anisocoria is old?

Look at old photos—driver's license, yearbook, family photos. If the same difference has been present for years, it's likely physiologic (benign).

Why does my doctor care which pupil is abnormal?

Because the cause is completely different. A larger abnormal pupil suggests parasympathetic or third nerve problem. A smaller abnormal pupil suggests sympathetic problem (Horner syndrome). The workup depends on which is abnormal.

Should I go to the ER for unequal pupils?

Yes, if they're new and accompanied by headache, ptosis, double vision, eye pain, or any neurological symptoms. If you've had stable, mild anisocoria for years with no symptoms, it's likely not urgent.

Can medications cause unequal pupils?

Yes. Many medications can affect pupil size—dilating drops, scopolamine patches, certain eye drops, some systemic medications. Accidental exposure (touching scopolamine patch then touching eye) is common.

References