Myopia and eye disease
There’s more to myopia than being short-sighted.
Myopia usually comes about because the eye grows longer (from front to back) than normal. This means the lens inside the eye can no longer focus light clearly on the retina – it is too far back and the image is out of focus. In this way, myopia – or short-sightedness – is simply a focussing problem corrected by the right pair or glasses or contact lenses.
But unfortunately for many people, the enlargement of the eye causes a lot more than just focussing problems. The delicate tissues of the eye are stretched and thinned as the eye elongates, and this greatly increases your risk of several eye diseases including cataract, glaucoma, retinal detachment and disease of the macula.
NOTE: It is important to understand that laser eye surgery only corrects the focussing of your eye and does not alter the increased risk of all the eye diseases associated with myopia.
Retinal detachment and myopia
This makes sense if you think about it; as the eye grows, everything is stretched and thinned, including the innermost layer – the retina. This light-sensitive layer can split, leading to retinal tears and ultimately to retinal detachment – a potentially sight-threatening condition that requires urgent medical attention. (New symptoms of flashes, floaters, or a veil or curtain blocking out part of your vision need to be investigated immediately to rule out retinal detachment).
Even a small amount of myopia (up to -3 dioptres) is associated with a three-fold risk of retinal detachment compared to the average person. Once you get up to -6 dioptres, your risk is now 20 times that of the normal population, and by -9 dioptres, your risk is over 45 times the normal risk.
What can you do about it? Apart from trying to slow or halt the progression of myopia before it ever gets that high, you should be having regular comprehensive eye examinations with your optometrist, who will dilate your pupil and examine your peripheral retina. Depending on your degree of myopia, this should be repeated every 1-2 years, according to your optometrist’s advice. Not all short-sighted patients will have an abnormal retina, but this will only be apparent at the time of your first retinal exam. Photographic images should be recorded of the structures at the back of your eye for future reference.
Short-sighted eyes are also at 4 times the normal risk for glaucoma – a progressive disease of the optic nerve head. This disease occurs in about 2% of the population – unless you are myopic, when your risk of developing this disease is about 8%. It is thought that the retinal nerve fibres are damaged by the stretching that occurs in myopia.
Regular eye examinations with your family optometrist are your best protection against vision loss. Glaucoma can be well-managed but the problem is that there are no early symptoms and the disease can be missed until permanent damage has occurred.
Cataracts are a worry for short-sighted people. Not only do they have a higher risk of developing cataract, they also have more complications associated with cataract surgery, where the risk of retinal detachment is 5 to 10 times higher than the normal 0.5% risk. This means up 5% of short-sighted patients having cataract surgery could develop an associated retinal detachment.
Your surgeon will carefully examine your eye prior to surgery to assess your individual risk. (It is also harder to calculate the power of the intraocular lens, leading to less accurate results in terms of spectacle independence). It is not understood exactly why cataract occurs more frequently in short-sightedness, but it is thought that the increased length of the eye deprives the crystalline lens of nutrients, or possibly that it results as a by-product of retinal damage.
Cataracts are easily picked up in routine eye examination. Symptoms include blurred vision, glare, and changes in spectacle prescription. If you are short-sighted, be sure to choose an experienced surgeon for your procedure. There is also clinical evidence that femto-cataract surgery (using a laser to replace some of the manual steps of the procedure) can reduce the risk of retinal detachment by reducing surgery time and using less energy in the eye.
The macula is the most sensitive part of the central retina inside your eye. It is responsible for all our central vision, including the details of reading print, facial recognition etc. Damage to the macula is devastating for your eye sights as well as your quality of life.
Even moderate myopia increases the risk of macula disease 10 to 40 times. High errors of short-sightedness are associated with risks hundreds of times higher than normal. As with the other eye disease associated with myopia, regular eye tests with your optometrist are the best defence against vision loss. Anyone developing myopia should discuss myopia control with their optometrist. Strategies such as Atropine eye drops, ortho-k and multifocal contact lenses are all effective in slowing or even halting the progression in your myopia. Remember, there is a world of difference in terms of risk of retinal or macular damage between -3 and -9 dioptres.