The normal sighted eye can be thought of as bringing light rays to a sharp focus on the back of the eye - the retina.
In SHORT SIGHT (myopia), a person cannot see clearly in the distance because light entering the eye focuses in front of the retina. Although the near vision may be good, a short-sighted person usually needs spectacles for clear distance vision.
In LONG SIGHT (hypermetropia), light entering the eye focuses on an imaginary point behind the retina. A long sighted person can sometimes bring their distance and near vision into clear imagery but the focussing effort of doing so often causes headaches and eyestrain. Spectacles are provided not only to provide clear distance and near vision but to lessen the focussing effort by which it is obtained.
In ASTIGMATISM, the circle of vision which the retina receives is not sharp and clear all the way round. If you imagine the circle of a bicycle wheel for example, some of the spokes would seem blurred, whilst other spokes would seem clear. The fault usually derives from a slightly ‘rugby ball’ shaped cornea at the front of the eye. Astigmatism is easily corrected by toric spectacle lenses.
In PRESBYOPIA, the lens inside the eye has become bigger and less able to adjust its focussing power for near vision. This is simply an age related effect. Spectacles are worn for close work.
Some Common Ocular Disorders.
GLAUCOMA: This disease can be loosely described as deriving from abnormally high pressure within the eyeball, as a result of which the eye fails to receive an adequate supply of blood. This comes about insidiously, bringing a slow but progressive loss of visual field. If it is discovered early enough, progression can be halted although the amount of sight already lost cannot be restored.
The possibility of the existence of glaucoma, particularly at middle age and over, makes regular eye examinations a matter of common sense for everyone.
CATARACT: When people become older, the lens inside the eye sometimes tends to become opaque (like frosted glass). Often the opacity is slight so that only spectacles are needed to improve the vision. However in some cases the lens becomes so cloudy that very little light can enter the eye. In this situation an eye surgeon removes the whole lens in an operation usually performed under local anaesthetic. An acrylic lens (an ‘implant’) is inserted into the eye in place of the human lens. Cataract removal has become one of the most frequently performed operations in the U.K.
AGE RELATED MACULAR DEGENERATION: The macula is at the centre of the back of the eye ( the retina) and it is the macula which is responsible for our central vision.
Macular disease tends to occur later in life and is therefore often referred to as age related macular degeneration (ARMD). There are two types of ARMD; ‘dry’ARMD’ or ‘wet’ ARMD.
Dry ARMD: This condition causes a gradual painless deterioration of the macula, usually over many years. Often people carry on as normal for a considerable amount of time. The term ‘dry’ means that there are no haemorrhages to be found at the macula.
Wet ARMD: This condition occurs when abnormal blood vessels grow into the macula. These blood vessels sometimes go on to leak blood onto the macula. This causes a sudden, painless loss of central vision. However nowadays, wet ARMD can often be treated if caught early. Fast referral to a hospital eye specialist is essential.
An early sign of wet ARMD is when the central vision suddenly becomes distorted. For example: if a known straight line (ie; the side of a door) suddenly appears to have a bend or a kink in it.
Of all the defects of the human eye, colour ’blindness’ captures our imagination the most. In fact the truly colour blind are thought to see things not so much in black and white, but rather as shades of grey. However this is an extremely rare condition and afflicts only around one in a million.
In place of ‘blindness’ we should simply substitute the word 'deficiency'. Those with colour deficiency sometimes have problems differentiating between certain colours, especially red and green. The condition is present from birth and usually inherited by males with roughly 8% of men affected by it, whereas it afflicts less than 1% of women. There is no treatment for this condition.