Myopia

(Date of publication 22 October 2004)

Could there be any truth in the archetypal image of the boffin who wears thick glasses to combat short-sightedness caused by hours of study? It appears there is no definitive answer to this question. According to this article from the New Scientist, a review of 40 studies found that the massive increase in myopia in east Asia (in Singapore the incidence in army recruits has increased from 25% to 80% over the last 30 years) is due solely to changes in lifestyle. The evidence appears compelling; for example, 70% of 18 year old Indian males in Singapore are myopic compared to 10% in India itself, and boys attending Israeli schools which concentrate on reading religious texts have myopia rates of 80%, compared to 30% for state schools. However, other research has found that the most important determining factor is family history.

A knowledge of the anatomy of the eye is necessary in order to understand the basic causes of myopia. Most of the focusing of light rays (70%) is done by the cornea, the transparent membrane covering the pupil and iris, which is normally only about half a millimetre thick. The lens then essentially fine-tunes the process by changing its shape to focus on objects at different distances. This reflex action, known as accommodation, is carried out approximately 100,000 times each day. In the healthy eye a clear image is formed on the back of the retina, which contains light-sensitive cells termed rods and cones.

In myopia , the eye is unable to focus on distant objects, which appear blurred. For readers with perfect vision, you can see how objects appear - close up, in the middle distance and far away to people with the condition, by using the vision simulator . The two main causes are the cornea being too curved, so that incoming light rays are refracted more than they should be, and the eyeball being elongated from front to back. In both cases the effect is to focus images in front of the retina rather than directly upon it, which may lead to symptoms such as headache and tired eyes. The condition tends to develop in childhood or the early teens, is usually relatively mild, and easily corrected with spectacles or contact lenses. As the eyeball grows, myopia tends to worsen, but this deterioration generally stops once adulthood is reached.

However, there is a condition called pathological myopia in which short-sightedness becomes progressively worse. There may be a genetic element to this condition, which affects twice as many women as men, and particularly those of Chinese, Jewish, Arab and Japanese origin. Symptoms may include decreased visual acuity, squint and glaucoma. During the developmental phase the eyeball continues to elongate antero-posteriorly, owing to thinning of the sclera. The subsequent degenerative phase is characterised by vascular changes (choroidal neovascularisation membrane formation or CNVM), which can lead to subretinal haemorrhage and loss of vision.

Surgical treatment for correcting myopia has been around for more than 25 years. The first procedure to be routinely used was radial keratotomy, in which the surgeon makes a series of radial incisions in the peripheral cornea, leaving a central 'optical zone' untouched. The effect is to flatten the cornea and reduce its focusing power, so that images are formed closer to the retina. Some after-surgery photographs that clearly show the fine incisions can be found here . Radial keratotomy is best suited to cases of mild myopia, because fewer incisions then have to be made, and has the advantages of being safe and offering highly predictable results. However, it weakens the cornea and renders it vulnerable to injury long after the surgery. For further information, try the healthAtoZ site .

Photorefractive keratectomy (PRK) is a more recent technique which can be used for patients with all degrees of myopia, and involves re-shaping the external surface of the cornea with a laser. By removing tissue from the centre of the cornea under local anaesthesia, it is made flatter and the refractive power is reduced; see the diagram here . Before surgery, the cornea is measured for shape, size and the correction needed, using an instrument termed a corneal topographer. A computer-controlled ultraviolet laser then re-contours the cornea to the required dimensions, and normal activities can usually be resumed after about three days.

Today, a much more frequent operation is laser-assisted in situ keratomileusis, or LASIK , which has two major advantages; the relative lack of pain afterwards and the fact that good vision is generally achieved almost immediately. In this procedure a thin, circular flap of the cornea is raised using a microkeratome, and folded back (see the photograph ). Tissue is then removed from the under-surface of the flap using a laser, and the flap simply laid back in position. A course of antibiotics is normally prescribed to prevent post-operative infection. For those who would like to know more about this procedure there is a very detailed description, including the treatment of intra-operative and post-operative complications, at the emedicine wesite .

Let us end with some rather bizarre relevant research . A study in the USA has concluded that infants who sleep with a light on up to the age of 2 have a greater chance of developing myopia than those who sleep in the dark, and the effect is dose-dependent. A survey of 479 children found that 10% of those who had slept in darkness became short-sighted, compared to 34% of those who had slept with a night light and 55% of those who had a room light on. Mind you, whether it is worth braving all the advertisements to uncover these results is highly debatable.

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A Tour around Myopia (Short-Sightedness)

A Tour around Myopia (Short-Sightedness)