Eye Care - Myopia (Short sighted)
Short sight occurs when light is focused in front of the retina causing distance vision to become blurred. Near vision, however, is usually clear. Short sight normally develops in childhood or adolescence and is often first noticed at school. Glasses or contact lenses may need to be worn all the time or just for driving, watching TV or sports.
Vision can usually be corrected with glasses or contact lenses, or, in some cases, with laser eye surgery.
The eye and myopia
Light coming into the eye is focused onto the retina - an area on the rear wall of the eyeball. Seventy per cent of light entering the eye is focused by the cornea. The rest is focused by the lens, which sits behind the cornea.
If the light is focused properly on the retina, we see a clear image. If the light focuses in front of the retina, the image is fuzzy.
With myopia, the cornea is too curved or the eyeball too long. This means that images are focused in front of the retina rather than directly on it.
Who gets myopia?
Myopia most commonly develops in childhood or early teens (between 8 and 14). The risk of developing myopia is increased if there is a family history of it. There may also be a link between myopia and prolonged close-up work, such as reading or sitting close to the television, although there is little scientific evidence for this.
Myopia can be associated with certain types of cataracts, where the lens becomes cloudy. It can also be caused by a condition called keratoconus, in which the cornea thins and becomes more cone shaped.
Temporary short-sightedness, called pseudomyopia, can be caused by a number of diseases or certain drugs. For example, myopia may be the first sign of type-2 (non insulin-dependent) diabetes. Symptoms of pseudomyopia usually clear up if the underlying cause is treated.
Effects of myopia
To a short-sighted person, distant objects seem blurred, while near objects can be seen more clearly. Other symptoms can include headache and tired eyes.
Myopia develops as the eyeball grows, so it starts in childhood and may worsen during the teens. It normally stops getting worse in adults.
Children younger than eight may not realise they have blurred vision. Parents or teachers may suspect a child has myopia if they squint or frown a lot, hold books close to their face, or have trouble seeing a blackboard, television or cinema screen.
Related problems
Most people have mild myopia and there are no serious problems.
However, rarely, the eyeball continues to grow and myopia becomes progressively worse. This is called pathological myopia. People with this type of myopia are at an increased risk of developing certain other eye problems, which can sometimes lead to blindness. For example, a few people with severe myopia may get retinal detachment, which is when the retinal lining separates from the back wall of the eye. This requires urgent surgical treatment to prevent permanent loss of vision.
Sight tests
It is important to diagnose myopia as early as possible, because poor sight in young children can be a serious disadvantage. Children may have sight tests at school. If not, parents should book their children in for a test with an optometrist. These are free for children under 16 and for those under 19 in full-time education.
Children under 16 should be tested once every year, if they have no problems. If a problem has been identified, they should have a sight test every six months. Adults should have a test every two years, or more frequently if there are problems.
Myopia is usually diagnosed by reading a standard chart (called a Snellen chart) from a distance of six metres. The chart has large letters at the top and small ones at the bottom. A person with normal eyesight will be able to read the chart at six metres. This is called 6/6 vision. If they have myopia, their vision may be 6/12. This means that at six metres they can read what a person with normal vision can read at 12 metres.
Treatment
Myopia can usually be corrected with glasses or contact lenses. These work by refocusing the image on the retina.
Contact lenses or glasses?
Glasses can be worn by people of all ages, and are less expensive than contact lenses. However, they may get in the way of some types of work, and they fog up in humid or cold weather. Some people find them uncomfortable or unattractive to wear.
Contact lenses give better side vision than glasses, and come in a wide range to meet individual needs. Some people prefer the way they look to glasses. However, contact lenses may need cleaning and disinfecting (if they are not disposable), and they require care in insertion and removal. Also, they can cause infection, dry eyes, and scratches of the cornea.
Corrective surgery
Surgery can correct myopia by altering the shape of the cornea. There are various types of operation available:
LASIK (Laser in-situ keratomileusis)
This is a relatively new treatment for myopia. A small flap is made in the cornea, and a tiny piece of tissue is removed accurately with a laser. It is usually performed as an outpatient, and takes about 15 minutes for each eye. Vision may be hazy or blurry for a few days after surgery. The long-term safety and effectiveness of this procedure has not been proved.
Photorefractive keratectomy (PRK)
This uses a laser to remove tissue from, and reshape the cornea. It can be more painful than LASIK and takes longer to recover.
Radial keratotomy
Several tiny cuts are made in the cornea which flatten it. This is an older procedure and has largely been replaced by the other techniques.
Deciding on surgery
In order to give informed consent, anyone deciding whether or not to have surgery needs to be aware of the possible side-effects and the risk of complications.
Side-effects
Side-effects are the unwanted but usually temporary effects of a successful procedure. These include slight discomfort and watering eyes for a few hours, light sensitivity and possible dryness in the eyes for up to six months afterwards.
Complications
Complications are unexpected problems that can occur during or after the operation. Most people are not affected, but the main possible complications can include:
- clouding of the cornea (this tends to improve quickly)
- glare around bright lights
- over-correction (long-sightedness) or under-correction (under-treated myopia)
- infection of the cornea, which is usually treated with antibiotic eyedrops
- increased pressure in the eye (glaucoma)
- puncture of the cornea or eyeball
- retinal detachment (see above)
Surgery does not correct the normal ageing process of the eyes and some people who are treated for short-sightedness still need glasses after the age of 40.