refractive errors

refractive errors Why, in our forties, does reading become progressively more difficult? People who have always enjoyed clear vision at any distance start to hold the book further away and employ brighter light for comfortable close work. The cornea and lens refract the incoming light rays to focus them on the retina at the back of the eye. In humans the cornea does some two-thirds and the lens one-third of this focusing. The power of the lens can be altered by changes in the shape of the lens — a process called accommodation. This allows rapid changes in focus, according to how near or distant is the object of regard. Better light increases the contrast between print and page, the associated contraction of the pupil increasing the depth of field. Unfortunately, focusing for near objects gradually reduces with age (presbyopia) and has to be helped by converging or convex lenses.

The lens develops from the same layer of embryonic tissue that forms the skin (ectoderm). Both skin and the lens grow throughout our life. Skin is slowly shed or rubbed away and replaced, but the lens is confined within the fixed volume of the eyeball. As it grows it becomes more compact and stiffer, less able to assume greater convexity for near vision. Further compaction causes opacities to develop within the lens — cataract formation. When this causes significant interference with vision it is extracted surgically and replaced by an artificial intra-ocular lens (IOL). However, the IOL cannot accommodate for near vision, and glasses are still needed for reading.

Light from a distant object is brought to a focus on the retina in a normal-sighted, or emmetropic eye. In hypermetropia the eyeball is too short, so that the image falls behind the retina (Fig. 1(a)). Most hypermetropes younger than 35 years can accommodate all the time for clear distance vision but, together with the extra effort still necessary for close work, they may suffer from headaches and other symptoms. Once an ageing hypermetrope has become presbyopic he cannot see clearly at any distance without refractive help (Fig. 1(b)).

Contrast this with myopia, where the eyeball is too long, and distant objects are focused in front of the retina (Fig. 2(a)). A diverging, concave lens can correct this (Fig. 2(c)). Note that diverging light rays from a near object can be accurately focused on the retina in myopia without correcting lenses (Fig. 2(b)). Because the myope can see clearly at short distances without glasses the term ‘short-sighted’ is often used. Hence my hypermetropic wife wearing her glasses can find my concave spectacles for me, but I can read the dials and shower safely without the risks of being scalded or frozen, or wetting my glasses.

Perfectly regular, spherical hypermetropia or myopia are not the commonest refractive errors. Astigmatism may be added to either condition, when the cornea has unequal curvature in different meridians, like the shape of an egg. Light rays from a single point are refracted to two separate focal lines at right angles to each other (Fig. 3). The astigmatic person does not know which focal line to look at, and ‘hunting’ between the two may cause eye-strain. Some people are much more susceptible to this than others and require correction of low levels of astigmatism, but the majority do not. Most astigmatism has its axes close to horizontal and vertical, with the curvature being less along the horizontal axis. Oblique astigmatism is usually more troublesome; even spectacle correction with cylindrical lenses is a compromise, but contact lenses may be much more effective.

Many surgical methods have been tried on the cornea in attempts to correct myopia. Radial cuts with a diamond knife to induce scarring, which changes corneal curvature, have been largely replaced by laser methods, in which the optical zone is re-profiled by removing tissue from the anterior corneal surface. In the latest method (LASIK) a thin corneal flap is cut, some of the underlying substance is removed by laser, and the flap is replaced.

The refractive power of a lens is measured in dioptres (the reciprocal of the focal length in metres). Refractive errors are expressed as the dioptres required for correction (positive or negative); thus hypermetropia requires additional refractive power to focus an image on the retina in the short eyeball (Fig. 1(b)), whilst myopia requires negative correction (Fig. 2(c)).

Measurements of refractive errors of large numbers of individuals, when plotted graphically to show their relative incidence, lie on a sharply-peaked curve, shown in Fig. 4 and compared with a theoretically derived ‘normal’ or ‘Gaussian’ distribution curve. Ninety-eight per cent of all refractions in one series of cases lay between +4 and –4 dioptres, and the number of emmetropes (normals) was greater than anticipated. Furthermore, the mean refractive level was at some +2 dioptres of hypermetropia in infants, moving towards +1 dioptre by 25 years of age, with a similar distribution of relative values at all ages. There appears to be a definite tendency towards normalization of refraction (emmetropization), which implies highly accurate and co-ordinated control of growth of the cornea, the lens, and the whole eyeball. Interestingly, when refractive errors are present they are often closely similar in the two eyes, even down to the degree and axes of any astigmatism. Occasionally there is a large difference between the focusing of the two eyes, making correction difficult and with profound implications for the development of normal binocular vision.

In the Western world some 15–20% of people are myopic and 40–50% hypermetropic, but in Japan and China 50–70% are myopic. The Jewish race has an excess of myopia, but Black African have more hypermetropia than average.

With vision being our principal sensory contact with the environment, it is not surprising that uncorrected focusing can have significant influences, so much so that separate myopic and hypermetropic personalities have been recognized. The myopic child can be introverted, studious, and solitary, with no interest in ball games or outdoor pursuits. ‘Short-sighted’ can be used as a derogatory term, implying an incomplete view, lacking in extent of intellectual outlook. As Disraeli put it, ‘so short-sighted are politicians in power’. The distinctive style of Impressionist painters has been attributed to myopia, and Cézanne, Degas, Pissarro, and Renoir were all known to be myopes. A survey of the teachers and pupils at the École des Beaux-Arts in Paris around the turn of the nineteenth century revealed 48% to be myopes, almost three times as many as in the general population.

Peter Fells

Bibliography

Donder, F. C. (1864). On the anomalies of accommodation and refraction of the eye. The New Syderham Society, London.
Duke-Elder, S. and and Abrams, D. (1970). System of ophthalmology, Vol. 5. Published for Henry Kimpton by C. V. Mosby Company, USA.
Trevor-Roper, P. (1970). The world through blunted sight. Thames and Hudson Ltd., London.

See also accommodation; contact lenses; eyes; optometry; squint.