High Degree Myopia
This excellent fact-sheet was written by Veronica Bevan, an Information Resource Officer for the Royal National Institute for the Blind (RNIB) in 1988. It has been modified to bring it more up to date.
Dr J.A. Guggenheim
The Family Study of Myopia research group
Myopia and its Causes
Short-sightedness, or myopia, is a vision defect resulting from excessively long growth of the eye, or a steeply curved cornea. Myopia up to 3.00 Dioptres (D) is termed mild degree, 3.00 to 6.00 D is moderate degree and high degree is 6.00 D and over. A Dioptre is a measurement of the focusing power of a lens.
Most commonly, myopia will stabilise when the growth process has been completed and will result in normal sight after correction with glasses, for example. High degree myopia (sometimes known as pathological myopia) is a progressive condition which can create problems because of its association with degenerative changes at the back of the eye.
Effects of the condition
Myopia causes light rays to focus in front of the retina and so close objects are seen clearly whilst distant objects appear blurred. There is a slight danger of sight loss since excessive elongation of the eyeball creates stress on the retina which can become damaged or detached and can also provoke other changes. This is especially the case in high myopia.
Treatment of Myopia
There is no cure for myopia. However the symptoms of blurred vision can be relieved with glasses, contact lenses or corneal laser surgery (see below). Some of the pathological changes associated with high degree myopia can be treated (for example, retinal detachment) whilst others cannot (for example, retinal degeneration).
Corneal Laser Surgery to correct myopia
The cornea is the transparent "window" at the front of the eye. Several different types of corneal surgery have been developed over the years. However, they all have the same aim, which is to "flatten" the curvature of the cornea, so that light rays are brought to a focus on the retina, even though the length of the myopic eye is longer than usual. Before the advent of corneal laser surgey in the 1990s, if myopia was treated it was by a surgical procedure called radial keratotomy. This was popular in parts of Russia, but low success rates and a high rate of complications have meant that it has not been widely performed in the west.
More recently corneal laser surgery (Photorefractive keratectomy or PRK, Laser in-situ keratomileusis or LASIK and Laser assisted subepithelial keratectomy or LASEK) have become popular. These operations take about 15 minutes during which time the laser is only used for about a minute. The best results with these procedures occur in the range up to 6.00 Dioptres. At present, the long term results are still not known, and until long term trials have established its acceptability, laser correction is unlikely to become available on the NHS. Thus, corneal laser surgery is only available privately at present.
For patients with 6.00D of myopia or less, a result within 1.00D of the intended correction is achieved about 80 - 96% of the time. With higher corrections up to -10.00D, about 40% achieve "20/20" vision without glasses. Whilst the majority of people do well and are completely satisfied, others require two or three treatments during the initial six month period before a satisfactory conclusion is reached.
Clinical trials have identified several problems which can arise from the procedure. These include deteriorated night vision, light scatter and haloes, cloudiness, as well as the necessity to have repeat surgery. These usually resolve within 6 - 12 months. However, in very rare cases serious damage occurs, and a corneal transplant is needed.
Risk groups, risk factors and genetic implications
Around 12 million British people are short sighted and some 500,000 of them will have high degree myopia. In a few people, high degree myopia causes retinal damage or detachment. It may also be associated with cataract and glaucoma. Retinal detachment requires rapid treatment, and so people with high degree myopia should be aware of its symptoms, which are flashes of light, increase in floaters, or a "curtain" across the field of view.
High degree or chronic degenerative myopia can run in families and it might be advisable to discuss this with your GP or eye specialist.
RNIB is not aware of a specialist group offering support and information for people with myopia but is happy to discuss problem solving services available from RNIB and other organisations designed to meet individual needs.
RNIB has been offering support to people with severe sight loss for over 125 years. Information on our sixty services is available from 224 Great Portland Street, London W1N 6AA or by telephone on (08457) 669 999.