What are abnormalities of the pupil?
The pupil (the circular black area in the middle of the coloured part of the eye) is usually the same size as the pupil on the other side, and the two pupils usually react together (for example to light). The pupils have muscles to constrict them (make them smaller) and muscles to dilate them (make them larger) and these are controlled by a network of nerves from the brain which works automatically. We all know that bright light makes the pupils smaller and dim light makes them larger. This makes a 20-times difference to the amount of light entering the eye. The pupils also constrict when we look at near objects (for example when we read), which makes focusing easier.
Normally this system works well, without our being aware of it, but there are many ways in which control can be lost. In a fifth of people, the pupils are usually slightly different in size (‘physiological anisocoria’). This is not a problem.
In 1 in 500 people, the nerves that control pupil constriction in one eye do not work properly (Tonic or Adie’s pupil). Usually a cause cannot be found, but rarely this can affect people following an infection or surgery to the eye socket.
In another condition, there is a small pupil on one side, a drooping of the upper lid, and no sweating on the same side of the face. This is known as Horner’s syndrome. It can occur in babies who are injured during birth, and in people with disease of the blood vessels, cancer or following some types of operation.
In Argyll Robertson pupil, both pupils are small and there is no reaction to light, but constriction for looking at near objects still happens. This condition is usually seen at a late stage of the sexually-transmitted infection syphilis and shows that the disease has affected the nervous system.
How is abnormalities of the pupil managed?
The optometrist who finds one of these conditions will either take no action, having explained the findings to the patient, or refer the patient to the ophthalmologist or neurologist if tests are needed.