Night blindness

Synonyms in a broader sense

Medical: Hemeralopia

Definition

Night blindness is a disturbed adaptability of the eyes to darkness. For those affected, only outlines can be seen. The adaptation of the eyes to light is very fast, while adaptation to darkness takes a very long time, 30 to 50 minutes.

Summary

Night-blind are people whose eyes cannot adapt well to the dark. Mostly such a real night blindness, which occurs rather rarely, is congenital. Due to a vitamin A deficiency, it can also be acquired.

In those affected, the rods (sensory cells of the retina, which are responsible for black and white vision) are impaired in their function. The patients see very little at night and also at twilight. They only recognize contours. At the ophthalmologist (specialist in ophthalmology), night blindness is measured and recognized by means of devices. There is no therapy.

Causes

Night blindness can be acquired or in most cases congenital. Acquired night blindness is due to the poor functioning of certain sensory cells in the retina of the eye. The human retina consists of many different types of cells.

Two of them are the rods and cones. Both have the task of converting incident light into electrical signals and conducting them to the brain. Cones are responsible for color vision, while rods are responsible for black and white vision, i.e. for light and dark – especially at night.

In the case of the night-blind, these rods are only weakly functional and this is where blindness comes from. In darkness only the colors of the rods do not work in the dark (“At night all cats are gray. “) If these colors fail, the patients are almost blind.

The night blindness can also be acquired, which is very rarely the case today, because there must be a vitamin A deficiency. A vitamin A deficiency can be caused either by a too low intake or a non-usability. Night blindness can also occur with various underlying diseases.

For example, retinal changes caused by the decay of the receptors. In the so-called “retinopathia pigmentosa”, it is mainly the rods that are destroyed. Patients notice night blindness at an early stage.

As a rule, the ability to see colors is not particularly limited in these patients. The patients can also see well in daylight. As soon as the light becomes weaker and mainly the receptors for light-dark or black-and-white have to work, the disease becomes noticeable.

Night blindness is noticeable in that the eyes find it very difficult to adapt to the surroundings at dusk and in the dark. The patient’s vision is very limited. However, visual acuity in bright light is not affected. In retinopathy pigmentosa, the rods responsible for night vision are destroyed. In this case, complete blindness exists if the light is too low.