What is a refractive error?

Rays of light need to be focused sharply on the retina for us to see clearly. We can adjust our focus from near to far by changing the shape of the lens in the eye. The lens gets harder as we get older, making it more difficult to change our focus, which is why we usually need reading glasses later in life. The effect on vision of the lens getting harder is called 'presbyopia.'

The optical power of corrective lenses is measured in 'dioptres.'

Short-sight (myopia) occurs when the eye is longer than usual. Rays of light from distant objects are brought to a focus in front of the retina, making objects in the distance blurred. A 'minus power' lens is needed to move the focus back onto the retina. A spectacle prescription to correct three dioptres of short-sight would be written '-3·00'.

Short Sight

Correction of Short Sight

Long-sight (hyperopia) occurs when the eye is shorter than usual. Rays of light from the distance are brought to a focus behind the retina. Young people may be able to make distance objects clear by a focussing effort, but near objects in particular will become more blurred as age advances. Eventually, even distant objects will be blurred. A 'plus power' lens is used to bring the out of focus image forward onto the retina. A spectacle prescription to correct three dioptres of long-sight would be written '+3·00'.

Long Sight

Correction of Long Sight

Astigmatism (meaning 'without a point' of focus) usually occurs when the surface of the cornea has asymmetric curves, more like a rugby ball than a football. Part of the image will be focussed in one plane, and the other part will be focussed in another plane. For example, the eye might be effectively short-sighted for some rays of light, which will be focussed in front of the retina, but long-sighted for other rays that will be focussed behind the retina. A spectacle prescription to correct three dioptres of short-sight and one dioptre of astigmatism might be written '-3·00/-1·00 x 10°', where 10° is the angle of astigmatism.

The lens inside the eye has to change shape in order to focus on objects close to. As the eye ages, the lens becomes stiffer and the plane of closest focus moves away into the distance. This is called presbyopia. An additional plus power lens is needed to correct the problem. This additional prescription for reading glasses is recorded as an 'add'. So the spectacle prescription for an eye with long-sight and presbyopia might be recorded as +1·00, add +2·00. The 'add' might then be built into a bifocal or progressive multifocal lens thus:

Short-sight, long-sight and astigmatism can all be treated by laser. Presbyopia cannot be reversed by laser but the need for reading glasses can still be removed or reduced by carefully designing the laser treatment. 

What is presbyopia?

Presbyopia is a vision problem that occurs due to aging. It typically begins in the forties and causes nearby items to appear blurry. Presbyopia is unrelated to the shape of the eyeball. Rather, it is believed to stem from a loss of flexibility in the lens of the eye. Because presbyopia is not caused by a refractive error, it cannot be prevented by laser eye surgery. Contact lenses and laser eye surgery can, however, help compensate for the loss of near focus.

What types of contact lenses do you offer at your London practice?

We offer almost every type of contact lenses at our London practice. Disposable contact lenses are soft contacts that can be thrown away either daily, weekly or monthly. For patients with astigmatism, we offer both hard and soft Toric contact lenses. Gas Permeable (GP) contact lenses can be harder to get used to than soft lenses, but often offer London patients a crisper, better quality of vision than other contacts. Bifocal Contact Lenses can improve both distance and near vision for people who also need reading glasses. For patients looking to alter the colour of their eyes, we offer coloured contact lenses. In addition we specialise in fitting complex lenses for medical indications.

What is radial keratotomy (RK)?

Radial keratotomy was an eye surgery method that involved making cuts into the surface of the eye to change its shape. RK was used for the treatment of short sight and astigmatism. Those who have had radial keratotomy can suffer difficulty with nighttime vision, and most have fluctuations in vision. RK was mostly abandoned around 1990, but is still available in one clinic in the London area. We have never recommended or performed RK treatment, which is now widely considered obsolete.

What are thermal techniques for treating vision problems?

Thermal techniques (radio frequency thermokeratoplasty, diode laser thermokeratoplasty, holmium-YAG thermokeratoplasty) have been used to treat low degrees of hyperopia and astigmatism in people over the age of 40. The effects depend on age. The maximum correction that can be expected in a 40 year old is about 1.5 dioptres, rising to about three dioptres for a seventy-five year old. The main disadvantages with all thermal techniques is that they are relatively inaccurate, the eyes can take a year or more to reach reasonable stability, and there may be a tendency for the effects to continue to wear off with time.

C S Bailey BSc FRCS DO CertLRS, 99 Harley Street  London W1G 6AQ. Tel: +44 (0)7973 691727 Fax: +44 (0)20 7084 7729