The pupil, more commonly known as the "apple of the eye", is an apparently black hole in the centre of the iris, the part that gives the eye its colour. It is a dilatable and contractile opening whose function is to regulate the amount of light entering the eyeball, acting as if it were the diaphragm of a camera.

Generally speaking, the pupil dilates when we are exposed to low lighting or darkness to increase the amount of light entering the eye.  The size of the pupil is normally the same in each eye and the two pupils contract and dilate at the same time.


The term anisocoria refers to the difference in the size of the pupils when they dilate or contract and it may be normal (it is in 20% of the population, which is known as physiological anisocoria) or a sign of a disorder or illness, including:

  • Eye lesions or disorders: when the iris becomes damaged in such a way that the pupil does not contract properly in light conditions: eye trauma, inflammation of the iris (iritis or uveitis), glaucoma, etc.
  • Neurological anomalies: migraine, a stroke or ictus, brain haemorrhage, tumours or infections.
  • Third nerve palsy (the oculomotor nerve that goes from the brain to the eye and controls the movements of the eyeball and the size of the pupil): in these cases, pupil dilation is usually associated with ptosis (droopy eyelid) in the same eye, double vision and/or visibly misaligned eyes.
  • Some medications, both general and topical (eyedrops).
  • Alcohol or drug consumption, like cannabis or cocaine, which may cause significant agitation of the muscular fibres that form the iris, making both pupils dilate.

Therefore, if you have any pupil issues, it is important to consult your eye doctor, so that a thorough examination can be undertaken and any associated causes can be ruled out.

Dra. Mª José Capella Elizalde