Myopia or Short-sightedness
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Introduction Myopia, (short-sightedness) is a vision defect resulting from the eye being too long from front to back, or from a steeply curved cornea, so that there is an inadequate correlation between the focusing power of the cornea and lens and the length of the eye. Myopia causes light rays to focus on the front of the retina and so close objects are seen clearly, whilst distant objects appear blurred. The focusing power of a lens is measured in dioptres. Myopia up to 3 dioptres (D) is termed mild degree, 3 to 6D is moderate degree and high degree is 6D and over. (A dioptre is 1 divided by the focal length of the lens in centimetres. So a 1-dioptre lens has a focal length of 1m; a 2-dioptre lens has a focal length of 50cm; a 5D lens has a focal length of 20cm.) Approximately 30% of people in Britain are short-sighted and approximately 5% of those will have high degree myopia.
Symptoms Myopia usually appears around puberty, but may occur at any age from early in childhood to the age of 25. Most commonly, myopia will stabilise when the growth process has been completed and will result in normal sight after correction with glasses or contact lenses. The earlier myopia starts, the more severe it is likely to become. Myopia that starts in early childhood often progresses into adult life, and may become very severe. This high degree myopia (sometimes known as pathological myopia) is a chronic, degenerative condition which can create problems because of its association with degenerative changes at the back of the eye. A person with high degree myopia is more susceptible to eye disorders such as retinal detachment, glaucoma, and cataracts.
Causes There is tendency for myopia to run in families, children have approximately a 30% chance of developing myopia with one myopic parent and a 55% chance with two. High degree or chronic degenerative myopia also runs in families. Myopia is more common in the developed world and its frequency is increasing. Multiple factors probably combine to cause myopia. An individuals genetics, together with their exposure and susceptibility to environmental factors or development events, probably all have an effect on the structure of the eye. Diagnosis Diagnosis of myopia is done using a simple vision test, using a standardised eye chart. Mirrors are used to reflect the chart so it appears to be 20ft. away. Each line of the eye chart is assigned a notation in the form of a fraction that represents the individuals visual ability. The numerator is the distance in feet the patient is from the eye chart. The denominator represents the distance an eye with perfect vision can read the same line. Therefore if a person, for example, can read the 20/40 line, they are able to see at 20 feet what a perfect eye could see at 40.
Treatment Correction of short-sightedness can generally be achieved with glasses (spectacles) or contact lenses. The refraction test determines which are the best corrective lenses to give the best vision. This can be done with computerised equipment, but more usually by an instrument which holds corrective lenses in position in front of the eye. The optician will then adjust the lenses until the eye chart appears the clearest possible for the person whose eyes are being tested. A prescription for suitable lenses can then be prepared. Contact lenses may be hard and float on the surface of the eye or soft and mould to the shape of the eye and allow oxygen to transmit through them.
Surgery for correcting myopia has developed over recent years. Until the 1990s, surgical treatment involved a procedure called radial keratotomy, which entailed making radial cuts in the corneal surface to flatten the curvature to correct myopia. More recently laser surgery (photorefractive keratotomy (PRK) or (laser insitu keratomileusis (LASIK) have become popular. These techniques use the same basic principle, namely the use of an ultraviolet laser to re-profile the surface shape of the cornea. Photo-refractive keratectomy (PRK) involves treating the surface of the cornea with an Excimer laser which results in flattening of the cornea to correct myopia. The treatment is often painful for up to 48 hours after the procedure and is limited to the treatment of lower degrees of myopia. LASIK (laser insitu keratomileusis) involves lifting a small flap of corneal tissue and lasering onto the deeper cornea beneath. Both procedures take less than one minute per eye. LASIK offers quicker rehabilitation, is virtually pain free and offers advantages over PRK for higher degrees of myopia, up to 12 dioptres. These procedures are only available privately at present.
Complications High myopia can be associated with degenerative changes such as, retinal damage or detachment, glaucoma, cataracts and consequently, apart from diabetes, it is the major cause of visual impairment in the working age population. The majority of people who opt for laser procedures do well and are completely satisfied, however problems have been identified arising from these procedures. These include deteriorated night vision, light scatter and haloing, also the necessity to have repeat surgery.
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