Anisocoria (Unequal Pupils)

Anisocoria means the two pupils are different sizes. While sometimes normal (physiologic anisocoria), it can indicate important conditions affecting pupil control.

For comprehensive information about understanding and evaluating pupil abnormalities, including when to be concerned and what to expect during evaluation, see our guide: Understanding Pupil Abnormalities - When Unequal Pupils Need Attention.

Is It Normal or Abnormal?

Physiologic Anisocoria

• Up to 1mm difference is common
• Equal difference in light and dark
• No other symptoms
• Pupils both react normally
• Present in ~20% of population

Pathologic Anisocoria

• Greater than 1mm typically
• Difference changes in light vs dark
• May have other symptoms
• Abnormal pupil reactions

Finding the Abnormal Pupil

Difference Greater in Light

• Larger pupil is abnormal
• Can't constrict properly
• Causes: third nerve palsy, pharmacologic dilation, Adie's pupil

Difference Greater in Dark

• Smaller pupil is abnormal
• Can't dilate properly
• Causes: Horner syndrome, pharmacologic constriction, old iritis

Important Causes

Larger Pupil Abnormal

• Third nerve palsy—with ptosis, diplopia
• Adie's tonic pupil—benign
• Pharmacologic mydriasis
• Trauma to iris

Smaller Pupil Abnormal

• Horner syndrome—with mild ptosis
• Inflammation (iritis)
• Pharmacologic miosis
• Argyll Robertson pupils

Red Flags