Eye Care Blog

In order to maintain its transparency the lens, a living structure, requires nourishment and metabolic activity. Any agency which disturbs the normal metabolism of the lens will cause a greater or lesser opacification, which is by definition a cataract. Nourishment is provided by the aqueous humour in which the lens lies, the necessary substances passing through the outer capsular membrane to reach the cells within. There are no blood vessels in the lens. Most of it consists of a form of protein, rather like egg-white, which does not occur elsewhere in the human body. Lens protein in different animals is exactly the same. This curiosity, which is called organ specificity rather than species specificity, means that if someone becomes allergic to any animal’s lens, he will also become allergic to his own.

With age the capsule becomes thicker and less permeable, and the general metabolic activity of the lens, which even in youth is of a low order, slows down still further. Eventually degenerative changes, which have a variety of causes, start within the lens and inevitably provoke a loss of transparency — a cataract.

The earliest stage of a cataract is the accumulation of fluid between the lens fibres. Although at this point there may be no true loss of transparency, as the process continues the protein of the lens fibres coagulates, a development which in time leads to complete opacification. It is rather as though the lens had changed from white of egg into a hard-boiled egg.

These changes, which are a normal part of the ageing process in the lens, may also be initiated by a very wide variety of factors, such as injury, certain drugs, infections, and metabolic disturbances elsewhere in the body. Cataract may be present at birth (congenital) or may be acquired later in life.

Congenital Cataract

As previously stated the lens starts to develop at about the sixth week of embryonic life. If at that time the mother has an attack of rubella (German measles) there is a very significant risk that the lens development will be abnormal and that the baby will be born with a congenital cataract. Furthermore, there is a strong possibility that other organs which are developing at much the same time will also be affected — for instance, the inner ear, the heart, and the brain. It may be that these defects are incompatible with life, in which case a miscarriage will occur. However, if the baby is born alive, cataract may be present and may indeed become denser as time passes. Sadly, removal of these cataracts often does not benefit sight, as the retina also may have been affected by the virus.

By no means all congenital cataracts are the result of rubella. All sorts of noxious stimuli during pregnancy, such as injuries or infections, may result in an opacification of part of the lens. If the stimulus is of short duration, then clear lens fibres may be laid down outside the opaque ones, leading to a laminated appearance. These cataracts are called zonular cataracts as only one zone of the lens is involved.

Some congenital cataracts run in families, such as the Coppock cataract named after an Oxfordshire family: 288 descendants of John Coppock, who was born in 1774, were affected over the next seven generations.

Acquired Cataract

Ageing

Most people acquire a cataract by the simple process of getting older. Such cataracts are of two main types, nuclear and cortical. In the nuclear variety the central part of the lens becomes harder and optically denser, and takes on a yellow-brown colour. These cataracts develop very slowly and are usually associated with increasingly short sight, so that reading vision is affected much less than distance vision.

Cortical cataract, on the other hand, affects the outer layers of the lens, and opacities develop which may affect both near and distant vision. Again, progression may be very slow, but if the opacities are mainly in the back part of the lens, even a small cataract may cause reading difficulty, as it is in this region that rays of light entering the eye are being focused more closely.

Diabetes

Cataract is some 10 times more common in people with diabetes mellitus than in the population as a whole. These cataracts may be just the same as those caused by the ageing process, but they appear earlier in life and tend to progress more rapidly. A second type — the true diabetic cataract — occurs in much younger patients. It appears suddenly, usually in juvenile-onset insulin-dependent diabetics. The first variety is common, the latter rare.

Other less common metabolic conditions may also be associated with cataract formation.

Cataract associated with other eye diseases Long-standing intraocular inflammation, retinal detachment, and severe glaucoma may be complicated by cataract, but often the severity of the associated condition will have effectively destroyed the sight and the cataract plays little part in the problem.

Cataract and drugs

The most important drugs in common use that may cause cataract are the corticosteroids. These drugs are of great value in the treatment of asthma and some inflammatory conditions. When steroids are given systemically (by mouth or injection) and over an extended period they may cause cataract. Used as drops or ointment they may lead to a rise in pressure in the eye (glaucoma). People on steroids should not be deterred from using such drugs because of these possibilities. The drugs will be regularly and routinely monitored, as will the condition for which steroids have been prescribed. If cataract does develop, it can always be treated.

Injury

If the capsule of the lens is damaged, either by a penetrating injury or by a foreign body within the eye, cataract rapidly develops. A severe injury may also result in a lens opacity even without rupture of the capsule, but such an opacity is often not progressive. Some foreign bodies in the eye, especially if they are made of copper, brass, or iron, give rise to cataract even if the capsule did not at first suffer damage.

Cataract caused by injury may require urgent treatment at a very early stage.

Cataract as an industrial disease

Occupational cataract is seldom seen nowadays, as it is usual to take appropriate steps to avoid it, but in the past some workers were very much at risk because of the nature of their occupation. For instance, cataract was common among the chain makers of Worcester in the nineteenth century, and even today may be found among glassblowers and steelworkers who have not worn appropriate goggles to protect themselves against the heat and the damaging infra-red light from molten material. Those working with ionising radiation (x-rays, etc.) were also at risk before the dangers of such radiation were recognised.

Acquired lens opacities are usually progressive, although their advance may be extremely slow. On the other hand, congenital opacities other than rubella cataracts tend to be stable and generally do not lead to significant loss of sight. This is thought to be due to the fact that the influence which started the opacity was effective only for a period and did not permanently impair the normal lens metabolism.

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