Ok we have now finished the sight test and eye examination, so let’s sit back, relax a little and discuss the results. Today I will explain the types of vision defects which can occur, their symptoms, how they can be corrected and their possible significance.
Myopia, (short-sight)
In its simple form this is the condition where you can see things close to you but distant objects are blurred. The normal cause for this is that the eye-ball is too large so that light comes to a focus in front of the retina, the nerve layer which transmits images to the brain.
Commonly this develops in childhood with a fairly rapid acceleration between the ages of 11 and 16, stabilising somewhere around the 20 mark.
Sometimes, however, it can develop later in life and this must be checked carefully as it can be indicative of diabetes or developing cataracts.
The correction is with a minus (concave) lens either in spectacle or contact lens form, or engraved on the eye with a laser.
Hyperopia, (long sight)
This, as you might guess, is the opposite of short-sight wherein the eye-ball is small so that light has not had chance to focus before reaching the retina.
In low degrees distance vision is good but near vision difficult, although in higher degrees both may be blurred.
Very young children often exhibit hyperopia which will reduce as the eye-ball and its optical system develop, but should be corrected at this early stage to ensure correct binocular development.
Symptoms start to develop later in life, commonly age 30 plus and particularly in people engaged in close work or desk jobs, often initially as tired, red, dry eyes before developing to a point where close objects blur.
The correction is with a plus (convex) lens as before although laser treatment is generally not so easy.
Astigmatism, (mixed vision)
This is the one that confuses most people, so much so that I have had people worried that it is either a disease or a cosmetic defect, but it simply means distorted vision. A simple way to visualise the various optical problems is to imagine myopia as a too large soccer ball, hyperopia as a too small soccer ball and astigmatism as a rugby ball, i.e. different curvatures in different directions.
Symptoms vary from none in young people with low degrees, to completely blurred in higher degrees or older people. As with hyperopia the symptoms increase with age as the optical mechanism of the eye tires often starting again as tired, red, dry eyes and later blurring.
The correction is with a lens of equal and opposite curvature to that of the eye in spectacle, contact lens or laser form.
Presbyopia, (inability
to change focus)
This used to be known as old-age vision but, since I have reached that stage myself, I now prefer to call it ‘40-itis’! As we get older the lens inside the eye which changes its curvature to change focus becomes stiffer and the muscles controlling it become weaker resulting in the change from distance to near vision to become more difficult. Initially you may find you have to look twice at something close to bring it into focus (slow-focussing) but as it develops you will find that you will either need reading glasses for the first time or need to switch to two separate pairs, bifocals or varifocals. At this age, all of a sudden, the myopes who have been wearing glasses all their life get a benefit, they simply remove their glasses and read things for their previously superior sighted friends!
Squints and phorias
We have talked about muscles inside the eye but there are also muscles on the outside of the eye-ball which control its movement, not only to look up, down, left and right, but also when we are looking at something close our eyes converge so that they are both looking inwards at the print or whatever.
Sometimes in high cases of hyperopia the degree of effort required of the internal focussing muscles to bring objects into focus causes the external muscles to react and turn in to produce a squint, which is simply corrected by the appropriate lenses.
In other cases however the problem may be that the muscles were weak from birth, either hereditary or resulting from a difficult delivery, and results in a squint which needs surgical correction in most cases.
If, however, the muscle weakness is only small producing a phoria or potential squint, it may be possible to keep the eyes straight and working together by use of prismatic lenses possibly combined with eye exercises.
Finally, I hope today has not been too heavy reading and that you now know a little more about the focussing problems of your eyes and next time I can tell you a little about what we see at the back of the eye.
By Roy Carpenter
F.B.C.O., F.B.O.A., F.S.M.C.
Optometrist
