Cataract – When vision becomes blurred

Cataract – When vision becomes blurred

The eye’s lens is an extraordinary organ: it is the only tissue in our body (excluding the thin cornea) that is truly crystal-clear. In order to achieve this, the cells of the lens go through true torture in the course of their development. They stretch to a length of up to one centimetre – with a thickness of only several thousands of a millimetre. If a person’s index finger were to be „stretched“ like this, it would be approximately 40 metres long. Thousands of these ribbon-like lens cells arrange themselves in concentric layers similar to the layers of an onion. In addition to this, all of the organelles – the cell’s „organs“ – within the lens cells, including the nucleus, have to be broken down to make the lens transparent.

This extreme specialisation has its price: without organelles the cells of the lens can no longer constantly renew themselves like other cells of the body do. Moreover, old or damaged cells cannot be removed from the lens and replaced by new ones.

This results in an accumulation of slight amounts of damage to the lens – which at first are not even noticeable. But over the course of a whole lifetime, they finally impair the function of the lens so much that they influence visual perception. So first, with age, every lens becomes less elastic and is therefore no longer able to focus close-up. Nowadays this „old age long-distance vision“ can be easily corrected with glasses, but before the invention of glasses, presented a problem.

Age also gradually brings with it two further changes in the lens which can have serious effects on the eyesight. Firstly, the lens – like yellowing paper – gets more and more yellow; secondly it clouds over. A yellow lens acts like a yellow filter held in front of the eyes. It absorbs light, particularly the short-wave length light, i.e. the „cold colours“ of the spectrum: violet, blue, and with increasing yellowing, green as well. On the other hand, red, yellow and brown light still enter the eye unhindered, making the world appear in increasingly warm tones.

Irrespective of the yellowing, with increasing age certain areas of the lens cloud over. This so-called cataract does not necessarily impair vision. Often the clouding lies on the edge of the lens through which light does not pass when the pupil is normally dilated. Other clouded areas might be so small that one can „look past it“. But as the years pass by, the centre of the lens, the nucleus, also clouds over, and obstructs vision. A first sign of a nuclear cataract is that visual acuity deteriorates – as if a cloudy film is lying across the eye. An increase in the size of the cataract causes blinding in bright light and lessens the perception of contrast, colour and detail. In the late stages it becomes possible to differentiate only between light and dark.

Until now it is still impossible to slow down or reverse the development of a cataract. If the lens is so strongly clouded that vision is severely impaired, the lens is removed and replaced with an artificial one. Today cataract surgery is a routine procedure. But in the first half of the 20th century an operation was often fraught with problems – and if it was actually successful, patients had to wear spectacles with thick lenses to replace the refractive power of the missing lens. Frequently a cataract signified the end of an artist’s career – as, for example, in the case of American painter Mary Cassatt and the French artist HonorĂ© Daumier.

More information about the transparency of the eye lens.