ASTIGMATISM
How does Astigmatism occur ?
There are three sorts of refractive error. Myopia (or short sight) means the eye cannot focus on distant objects but can focus on near objects. Hypermetropia (or long sight) means the light focuses better on distant objects than near ones. Myopic eyes are usually too long, so the light focuses in front of the retina. Hypermetropic eyes are usually too short, so that it focuses behind. Astigmatism can be present at the same time as short or long sight. It means that the shape of the cornea is not even (rather like a rugby ball), so light is bent more sharply by one direction of the cornea than another. Light cannot then be focused to a point. Normally infants are a little long sighted. Some children are too long sighted and/or astigmatic. This may be noticed early because in some cases a squint is present. Otherwise it may be found by a vision check of the child at 3 to 4 years of age. All children must have the vision of each eye tested separately at this age as part of a developmental check. Short sight usually starts after the age of l0. Children with refractive error need 6 monthly checks of their spectacle correction. Eventually even people with no refractive error require reading glasses. The lens in the eye loses its ability to change shape to focus for close work and hardens as the centre of the lens becomes denser. This is called presbyopia , and reading glasses are often needed by the age of 45. Why does Astigmatism occur ?
Children usually inherit long or short sight from their parents. Other factors also influence refraction but are not well understood. As well as squints being associated with long sight, rarer conditions are associated with refractive error. For instance, individuals with albinism (lack of skin pigment cells) often have astigmatism. Treatment Involved for Astigmatism
SPECTACLES
By far the most important treatment is the prescription of spectacles. For this, children below the age of l0 should ideally have their pupils widened with eye drops before being tested (refracted). This prevents the lens in the eye focusing, giving a more accurate prescription result. Older people may have bifocals (near and distance in the same lens) or reading glasses only. Varifocal lenses are a type of bifocal where the change from distance to near correction is gradual. CONTACT LENSESContact lenses are popular. Of the hard lenses, the gas permeable variety are superior. Soft lenses are larger and initially more comfortable to wear: there are types which can be kept in for a week or longer at a time.INTRA-OCULAR LENS IMPLANTSIntra-ocular lens implants are plastic lenses placed in the eye and are a special form of treatment to avoid refractive error, usually given during a cataract operation. If a cataract operation has been carried out with no implant, it is sometimes possible to put one in later.RADIAL KERATOTOMYThis procedure, developed for the correction of myopia, is controversial. It has proved successful in treatment of higher degrees of myopia but is not without risk. Patients should be fully informed of these risks before undergoing surgery.PHOTO REFRACTIVE SURGERYThe excimer laser has been successfully used to correct myopia, astigmatism and hyperopia. The procedure is continuing to be developed and therefore patients must be fully informed of the current status. A small number of patients continue to be troubled by glare following this procedure.
After Treatment for Astigmatism
SPECTACLESSpectacles can be difficult to get used to. They make things appear a slightly different size, and with astigmatism, can make things appear slanted. Looking towards the edge of the lens gives odd effects of distortion and doubling of the image. Spectacles must be made carefully to suit the face and the position of the eye. This is called dispensing. If done badly, the glasses will never feel comfortable. A common fault is that the lens is not accurately matched for the position of the pupil. Spectacles may need updating after some time, especially in children.CONTACT LENSESSoft contact lenses are easier to get used to at first. There is less of a feeling of something in the eye. Some people find they can't get used to them. They mustn't be worn for too long, and wear time needs to be increased gradually. If the contact lens fitting is too tight on the eye there is a lot of discomfort. If too loose, it flops around, affecting vision. Hard lenses rarely cause problems once settled in. Soft lenses can cause problems. Allergies develop to the lens or to the cleaning solutions. Infections affecting the cornea can start up, especially with weekly wear and extended wear lenses, and can result in permanently damaged vision. Overnight wearing of any contact lens should be avoided.RADIAL KERATOTOMYThis operation is not approved of by most eye surgeons in the UK. It involves making several very deep cuts in the cornea. These never really heal, so there can be serious surgical complications: vision can alter through the day and the eyeball is permanently weakened. After radial keratotomy, glare around objects is sometimes noticed. If the eye is painful, red and sticky or the vision very blurred, the surgeon should be contacted.LASERFor 48 hours after treatment the eye is painful. Then vision takes a couple of months to settle completely. Some patients are troubled by haziness which usually clears up.
If Astigmatism is Left Untreated
Without treatment, a clear focused image cannot be formed in the eye. In children, it is very important to give clear vision to both eyes: there is a danger of developing a lazy eye (amblyopia) if this is not the case, and good vision will not be re-established. Some people believe that avoiding treatment for a refractive error helps train the eye to see better: there is no support for this view. |
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