---
title: What causes Adie’s tonic pupil?
description: "Adie's tonic pupil is a condition where one pupil becomes larger and reacts slowly to light but better to near focus. ## Underlying mechanism It is usually due to damage to the parasympathetic nerve supply to the iris. The cause…"
url: "https://bluefinvision.com/knowledge-base/topics/symptoms-signs/what-causes-adies-tonic-pupil/"
updated: "2025-12-31"
category: "Symptoms & Signs"
---

# What causes Adie’s tonic pupil?

Adie's tonic pupil is a condition where one pupil becomes larger and reacts slowly to light but better to near focus.

## Underlying mechanism

It is usually due to damage to the parasympathetic nerve supply to the iris. The cause is often unknown but may follow viral infections or inflammation.

- Common in young to middle-aged adults, more often women
- Typically affects one eye, though the other may be involved later
- Pupil constricts slowly when focusing on near objects
- Light reaction is weak or absent

## Associated features and outlook

Adie's pupil is usually benign but can cause glare and difficulty focusing. Some people also develop reduced reflexes in the legs.

- May be part of Adie's syndrome with absent ankle reflexes
- Reading glasses or dilute pilocarpine drops can improve near vision
- Condition tends to be stable or slowly progressive
- Important to distinguish from serious causes of anisocoria