Unequal Pupils (Anisocoria) in Babies

Content reviewed against published AAP, Mayo Clinic, AAP guidelines

Editorial policy

Last reviewed:

Anisocoria (unequal pupil size) occurs when one pupil is larger or smaller than the other. In about 20% of the population, mild anisocoria (less than 1mm difference) is a normal variant called physiological anisocoria. However, new-onset or significant pupil asymmetry, especially after head injury or with other neurological symptoms, requires urgent medical evaluation.

Key takeaways

  • Anisocoria (unequal pupil size) occurs when one pupil is larger or smaller than the other.
  • Most common cause: Physiological anisocoria (normal variant, less than 1mm difference)
  • Emergency: Unequal pupils after head injury with drowsiness or vomiting
  • Home care: Check if the asymmetry is the same in bright light and dim light (use a flashlight)

Possible Causes

Physiological anisocoria (normal variant, less than 1mm difference)common
Viral illness causing temporary pupil asymmetryuncommon
Eye inflammation (uveitis, iritis)uncommon
Head injury with increased intracranial pressurerare
Horner syndrome (damage to sympathetic nerves)rare
Third nerve palsyrare
Pharmacological dilation (accidental exposure to medication)rare

When to Seek Help

Emergency — Call 911 or go to ER:

  • Unequal pupils after head injury with drowsiness or vomiting
  • One large fixed (non-reactive) pupil with altered consciousness
  • Unequal pupils with seizure or sudden neurological changes
  • Rapidly progressive pupil dilation with headache signs (inconsolable crying)

Urgent — See doctor today:

  • New onset anisocoria not present before
  • Unequal pupils with drooping eyelid (possible Horner syndrome)
  • Unequal pupils with eye pain or light sensitivity
  • Anisocoria noted after any head injury even without other symptoms

Same-day appointment:

  • Unequal pupils noticed by parent without other symptoms
  • Mild anisocoria with a cold or viral illness
  • Asymmetry noticed in photos but baby seems well

Monitor at home:

  • Known physiological anisocoria previously documented by doctor
  • Very mild (less than 1mm) long-standing difference with normal exam
  • Transient asymmetry that resolves within minutes

By Age

0-3 months

Normal: Pupils should be equal and reactive to light from birth. Mild difference (less than 1mm) that is consistent in all lighting may be physiological.

Worry if: Significant pupil asymmetry at birth (possible Horner syndrome from birth trauma), one pupil that does not react to light, drooping eyelid with small pupil (Horner triad), or large unreactive pupil.

3-12 months

Normal: Pupils should be round, equal, and reactive. A consistently small difference (less than 1mm) in both light and dark conditions is usually physiological.

Worry if: New onset unequal pupils, pupil asymmetry after head bump or fall, one pupil dilated and not reacting, associated eye movement problems, or drooping eyelid.

1-3 years

Normal: Same as older infants. A longstanding mild difference that has been noted before and does not change is usually benign.

Worry if: New asymmetry after head injury, asymmetry with headache/vomiting/lethargy, large dilated pupil after contact with plants or medications, or associated vision changes.

Home Care

  • Check if the asymmetry is the same in bright light and dim light (use a flashlight)
  • Look at old photos to see if the asymmetry has always been present
  • Check if both pupils constrict when you shine a light in them
  • Note if there is a drooping eyelid on the side of the smaller pupil
  • Consider whether baby had contact with any eye drops, medication patches, or plants
  • Do not delay seeking medical evaluation if asymmetry is new or concerning
  • There are no home remedies for anisocoria—it requires professional evaluation to determine the cause

Frequently asked questions

What causes unequal pupils (anisocoria)?
Physiological anisocoria (normal variant, less than 1mm difference) (common); Viral illness causing temporary pupil asymmetry (uncommon); Eye inflammation (uveitis, iritis) (uncommon); Head injury with increased intracranial pressure (rare); Horner syndrome (damage to sympathetic nerves) (rare); Third nerve palsy (rare); Pharmacological dilation (accidental exposure to medication) (rare)
When is this an emergency?
Unequal pupils after head injury with drowsiness or vomiting. One large fixed (non-reactive) pupil with altered consciousness. Unequal pupils with seizure or sudden neurological changes. Rapidly progressive pupil dilation with headache signs (inconsolable crying)
What can I do at home?
Check if the asymmetry is the same in bright light and dim light (use a flashlight). Look at old photos to see if the asymmetry has always been present. Check if both pupils constrict when you shine a light in them. Note if there is a drooping eyelid on the side of the smaller pupil. Consider whether baby had contact with any eye drops, medication patches, or plants. Do not delay seeking medical evaluation if asymmetry is new or concerning. There are no home remedies for anisocoria—it requires professional evaluation to determine the cause
When should I call the doctor?
Unequal pupils noticed by parent without other symptoms. Mild anisocoria with a cold or viral illness. Asymmetry noticed in photos but baby seems well

Sources

AAPMAYO CLINICAAP

All content follows our editorial policy and is reviewed against published clinical guidelines.

2,705 evidence-based guides6 authoritative medical sources

Related Symptoms

head injuryeye not trackingdrooping eyelid

Bottom line

Most cases of unequal pupils (anisocoria) in babies are not emergencies. However, seek immediate care if unequal pupils after head injury with drowsiness or vomiting.

Trust your instincts. If something feels wrong, reach out to your pediatrician. Worrying about your baby means you care — that is a good thing.