The pupil is the central opening of the iris that allows light into the eye. The size of the pupil is determined by two muscle systems in the iris. One is a circumferential muscle called the ‘sphincter pupillae’. When it constricts it makes the pupil smaller. The other is a radial muscle, called the ‘dilator pupillae’. When it constricts it makes the pupil larger. The dilator muscles are controlled by the sympathetic nervous system and the constrictor muscles are controlled by the parasympathetic nervous system.
Normally people have pupils that are the same size and react equally to light. In a small proportion of individuals the pupils can have different sizes. This is known as anisocoria. This is particularly common in young women, and may be normal. There are also a number of pathological conditions that can cause pupil sizes to be different. Two examples of this are Horners pupil and Aides pupil.
A Horners pupil is the result of damage to the sympathetic nervous system of the eye. Horners is a small pupil that does not dilate well. It can be associated with a droopy upper eyelid if the innervation is damaged further behind the eye, or to loss of sweating on one half of the face if the innervation is damaged closer to the spine.
An Aides pupil is a pupil that has damage to the parasympathetic nervous system. This can result from a viral infection or due to poor blood supply. The pupil is often larger, with rhythmic irregular contractile movements, and constricts poorly.
The cause of an abnormal pupil size or response ought to be investigated to determine where the nerve is damaged along its course. Once this is determined a cause for the damage can be found. This is necessary to rule out any serious underlying disease.