Pupillary Reflex

The adaptation of the pupil to the prevailing light situation is achieved by the so-called pupillary reflex. A distinction is made between the part that receives the information about the exposure and transmits it to the central nervous system (afference) and the part that, after processing this information, leads to the activation of the appropriate muscle (efference). The illumination of an eye leads to a constriction of the pupil, this happens via the following structures:

What is the pupil distance?

The pupil distance is the distance between the two pupils. It is divided into the total pupil distance, as well as the right and left pupil distance. The right and left pupil distance is the distance between the center of the right or left pupil and the center of the bridge of the nose.

If the right and left pupil distance are added, the total pupil distance is obtained. Thus, the total pupil distance corresponds to the colloquial eye distance. Usually the pupil distance is given in millimeters.

It is important for the fitting of glasses and is therefore often listed in the glasses passport. When determining the pupil distance, it is crucial to look straight ahead. If you look to the right or left, the distance between the center of the pupil and the bridge of the nose changes, of course, and so does the pupil distance.

What are the causes of pupils of different sizes?

The width of the pupils is controlled by the autonomic nervous system. Therefore, it can hardly be influenced intentionally. Normally, both eyes are controlled in exactly the same way, so that both pupils are about the same size.

This happens depending on the light irradiation. Slight lateral differences of up to one millimeter are still considered normal. Several sections of the brain are involved in controlling the pupil width.

The midbrain is particularly important. From here, signals travel over several levels to the pupils.If damage occurs in this area, pupils of different sizes can be the result. This can be caused by injuries, strokes or cerebral hemorrhages, for example.

The signal from the midbrain is sent to the pupils via several circuits. Interference can also occur on this route. In the so-called Horner syndrome, a part of the autonomous nervous system in the head area fails.

A part of this is normally also involved in pupil control. Since the Horner syndrome often occurs unilaterally, the pupil can be disturbed in this way, resulting in unequal pupils. In addition, the upper eyelid hangs down on the affected side and the eyeball appears sunken. Other causes of differently sized pupils can be, for example, disturbances of the muscles that adjust the pupil width or increased intracranial pressure.