Definition – What is an anisocoria?

The anisocoria (anisos = unequal, koros = pupil) describes a lateral difference in pupil size. The pupils can be set at different distances depending on the strength of the incident light. In bright light, the pupil is set very narrowly so that we are not dazzled.

In low-light conditions, we need a wide pupil so that enough light falls on our retina and we can recognize an image. Even if our eyes are illuminated with different levels of brightness, the body positions the pupils evenly and orientates itself towards the brighter illuminated pupil. The width of the pupils is also influenced by the sphincter muscle (Musculus sphincter pupillae) and the dilating muscle (Musculus dilatator pupillae). If there is a disturbance in the muscles, the nerve tracts responsible for the muscles or the perception of brightness, anisocoria may occur. Our pupils are then of different sizes.

What are possible causes of anisocoria?

The causes for anisocoria are manifold. First of all, the unequal size of the pupils can be completely normal, because many people do not have completely equal pupils in a normal state. One can assume that about 20% of the healthy population has anisocoria.

A pathological anisocoria on the other hand is usually caused by a malfunction of the pupil muscles. The disturbance is either in the brain, in the conducting nerve tracts or in the muscles themselves. Typically, anisocoria indicates an increase in pressure in the skull.

This damages the conduction pathways and the brain, resulting in a malfunction of the pupil control. Anisocoria occurs in the case of a unilateral malfunction. If both sides are no longer controlled, the pupils remain dilated even when light enters the eye.

Typical causes for such an increase in pressure in the skull can be

  • Bleeding in the head or brain,
  • A stroke,
  • A craniocerebral trauma or
  • Could be a brain tumor. A brain tumour is basically a mass in the brain. It can be both benign and malignant and in both cases is often manifested by increased intracranial pressure.

Due to the mass, the brain requires more space, but its expansion is severely limited by the skull bones, which instead increases the pressure. This is expressed, for example, by the nerve tracts in the skull no longer functioning reliably. A first sign of this can be pupils of different sizes (i.e. anisocoria).

In the event of a stroke, a circulatory disorder of the brain vessels results in a deficiency in the supply of oxygen and other nutrients to the nerve tissue. On the one hand, this can be caused by an injury to the vessels and a resulting cerebral haemorrhage, on the other hand, a blockage of the vessels, for example by a blood clot (thrombus) or calcium deposits, can also lead to a stroke. Typically, a stroke results in a suspension of brain function in the affected area. The classic symptoms are

  • Suddenly occurring speech disorders,
  • A one-sided paralyzed facial expression or
  • Including paralysis of an arm or leg. – The pupil function can be disturbed unilaterally, resulting in anisocoria.