Eye Care Blog

Symptoms

All the symptoms of cataract are visual, the usual complaint being of a general mistiness of sight, more particularly for distance vision, as reading is often unaffected in the earlier stages. One eye is frequently worse than its fellow, but sooner or later both eyes alter. At this stage dazzle may be a cause of great distress. In normal lighting conditions vision may not be seriously disturbed, but in bright sunlight it becomes obscured by dazzle in the same way that a dirty windscreen becomes almost opaque in the headlights of oncoming cars. In this situation patients may be greatly helped by wearing a brimmed hat or tennis shade. Tinted glasses are of limited assistance because it is the direction of the light as much as its brilliance that causes the trouble.

These difficulties result from the early separation of the lens fibres by fluid, a disruption that may lead to other unexpected symptoms as well — for instance, double vision. Whereas double vision generally results from some muscle imbalance between the two eyes, double vision in one eye only, which is much less common, is usually due to a disturbance in the clear media of the eye. The fibre separation may also have a prismatic effect, breaking up the incoming light into its constituent spectral colours. A patient looking at a street light may therefore notice coloured rings or haloes around it. It is most important that this symptom is distinguished from the very much more serious haloes which are seen by patients suffering from acute glaucoma. In that case the haloes are accompanied by blurring, aching discomfort, or, indeed, severe pain, and their appearance tends to be episodic. On the other hand, the haloes in cataract are present at all times and are not associated with any discomfort. Pain is never a symptom of cataract.

Some patients notice a disturbance in their colour vision. As the cataract develops, blues become subdued and reds enhanced. It is interesting to notice the change in the style of certain painters as their cataracts develop. Turner and Rembrandt are two who are often cited as examples. Their paintings show much more red and orange in later life as their cataracts developed and absorbed more blue and green light, allowing only oranges and reds to reach the retina.

As the cataract becomes more dense, the central (nuclear) part of the lens becomes harder and the effective power of the lens becomes greater. As a result the eye becomes relatively short-sighted. Near vision may paradoxically become clearer than before. When people are suddenly able to do without their reading glasses they should be wary of the implications.

Eventually, when the cataract has become really dense, the proteins within it have become coagulated. Useful vision is seriously affected, but it is never lost altogether. Even the densest cataract will allow some light to reach the retina.

The First Examination

When a patient is first suspected of having cataract he will be examined in the eye clinic to decide the exact cause of visual failure, whether any other eye disease is present and the degree to which the cataract has advanced. A number of tests are carried out to assess the level of visual function, after which the eye itself is examined for any anatomical or physiological abnormality. Both distance and near vision are investigated.

Distance vision

This is measured by the Snellen chart (the optician’s chart with letters diminishing in size) (Figs. 3/1a and 3/lb). The size of the letters on the chart is defined according to a geometrical principle. In former times astronomers found that for the human eye to be able to see as distinct from each other two binary (second-order) stars they had to be separated by 1 minute (the 60th part of 1 degree) of arc. This became accepted as the normal resolving power (the ability to distinguish very small objects) of the eye, and the Snellen visual acuity chart is constructed on this assumption. Each letter on the chart is of such a size that its every detail subtends an angle of 1 minute and the whole letter an angle of 5 minutes at defined distances. The usual charts range from a letter subtending 5 minutes of arc at 60 metres down to 6 or even 5 metres. The patient reads the smallest letters that he can see at a distance of 6 metres and this is then recorded as a fraction, the numerator (the first figure or figure above the line) being the test distance (i.e. 6 metres) and the denominator being the distance at which the letter, correctly read, subtends an angle of 5 minutes. Thus, if only the top letter is read, vision is said to be 6/60. Such a letter would be correctly seen by a normal subject 60 metres away. Normal vision, then, is recorded as 6/6 (in America this becomes 20/20 because feet rather than metres are used).

Near vision

Close vision is usually measured with reading types. These consist of a number of passages which have been printed using different sizes of Times Roman types. Type is measured by points, a point being 1/72 of an inch. Normal close vision is about N (for near) 5 — that is, 5- point type or letters 5/72 of an inch high can be read with ease with the appropriate reading glasses.

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