Myopia is a condition that usually begins in childhood and lasts throughout a person’s life. It is a condition where the eye has developed a focussing mechanism that is too strong at the front of the eye or if the eyeball itself is too long from front to back for the light to be focussed in the correct position on the retina. Depending on the degree of the myopia, people with this condition may be able to see clearly at varying viewing distances up close but will always have difficulty seeing objects at long distance without blur.
The amount of correction required for myopes depends on the extent to which the focussing mechanism is too strong for the length of the eye. For example, most people require about 60 units of refractive power (called dioptres) in the total eye for them to be able to see objects in the distance with perfect clarity. However if a person has 63 units of power instead of the required 60 units, their vision is excessive by three units and their refractive error is said to be -3.00D (“minus three dioptres”). This person would be able to read well at reasonable close distances but would struggle with watching TV or theatre and would probably not be allowed to drive a car without correction.
A person with 61 units of power instead of the required 60 has a refractive error of -1.00D. This person would usually be able to function quite normally in the visual world and for them a correction would not need to be worn as regularly than if -3.00D is required.
A person with 65 units of power instead of the required 60 has a refractive error of -5.00D.
This person would usually require their correction all the time and would only be able to read at a very close distance without their correction.
There has been much recent research into what actually causes myopia. The exact mechanism for an eye becoming more shortsighted is not known but recently a concept called peripheral defocus seems to play some part. This concept is quite complex but a simplified explanation is that traditional spectacles, and to a lesser extent contact lenses, makes a myopic child’s vision clear by bringing the rays of light coming from long distance to a sharp focus on the macula. This gives the child good central (straight-ahead) vision but not necessarily good peripheral (at the sides) vision. This peripheral defocus seems to be a factor in causing an eye to become more shortsighted as a child gets older.
There are various ways to correct myopia. The traditional forms of correction, spectacles and contact lenses, have recently been complemented by the newer technologies made possible through laser refractive surgery and orthokeratology.
All forms of correction have their advantages and disadvantages, which our optometrist at Vision Excellence at Norwest will discuss with you after carefully measuring the refractive error caused by your myopia. In children (who are obviously too young for surgical correction), the earlier that we are able to measure the degree of myopia, the better chance we have of limiting the progression of further myopia and the need for stronger and stronger corrections as the child matures towards adulthood. If you have a myopic child in your family, we would strongly suggest bringing them to Norwest to see our Vision Excellence optometrist for an assessment of the myopia and the tailoring of an appropriate plan of correction.
In later life it can sometimes actually be an advantage to have some myopia, as it can allow an older person with presbyopia to be able to read or perform other close up tasks without the need for reading glasses. For this reason, an older person who is contemplating a permanent and irreversible change in their vision through refractive or cataract surgery should discuss post-operation outcomes with their surgeon before having the procedure. If possible, it is a good idea to experience the refractive change via safe, reversible means such as orthoK or disposable contact lenses for a short time before the surgery to prepare them for the permanent changes to vision that the operation will bring.