Dr Moïse Tourabaly · Key points

Two muscles, one opening

The pupil is the opening of the iris. Two opposing muscles set its diameter: a ring that constricts it, and radial fibres that widen it.

sphincter muscle, a ring contracts: the pupil constricts miosis dilator muscle, relaxed dilator muscle, radial fibres contracts: the pupil widens mydriasis sphincter muscle, relaxed
  1. Light, or near vision
  2. Parasympathetic system (oculomotor nerve, III)
  3. Sphincter
  4. Constricted pupil: miosis
  1. Darkness, emotion, effort
  2. Sympathetic system
  3. Dilator
  4. Dilated pupil: mydriasis

Unequal pupils (anisocoria): light is the key. A small difference, the same in light and in the dark, is common. Otherwise, we look for the condition in which the difference becomes more marked.

More marked in bright light: the abnormal pupil is the larger one; it does not constrict properly.
More marked in the dark: the abnormal pupil is the smaller one; it does not dilate properly.
Seek care without delay: a dilated pupil with a red, painful eye, blurred vision and haloes; a dilated pupil with a drooping eyelid and double vision; a constricted pupil with a sudden drooping eyelid and neck pain; any abnormality after an injury.

Educational diagram · Dr Moïse Tourabaly, ophthalmologist. It does not replace a consultation.