If the shape of the cornea or lens is oval like a rugby ball, it has two different curves, rays of light entering at different angles will therefore be brought to focus in two different positions, producing a distorted image. This condition is known as astigmatism.
However, if the shape of the cornea or crystalline lens is round like a football, it has the same shaped curve all the way round. Rays of light entering the eye from any angle will be brought to a focus at the same point.
Astigmatism is very common and is present in most people. It is corrected with a lens that has 2 different powers running 90% to each other. This is known as a toroidal or toric lens.
kat-uh-rakt: A clouding of the crystaline lens. It has the effect of looking through a dirty window i.e. reducing the quaility of vision.
Other effects include:
dahy-uh-bee-tis: A condition that leads to high blood glucose levels due to defects in either insulin secretion or insulin action.
Estimates suggest that nearly one person in twenty-five in the UK is affected by diabetes mellitus.
There are two different types of diabetes :
Type 1 diabetes (Insulin dependent diabetes mellitus (IDDM).
Commonly occurs before the age of 30 and is the result of the body producing little or no insulin. Type 1 is controlled by insulin injections.
Type 2 diabetes (non-insulin dependent diabetes mellitus (NIDDM).
Commonly occurs after the age of 40. In this type of diabetes the body does produce some insulin, although the amount is either not sufficient or the body is not able to make proper use of it. Type 2 diabetes is generally controlled by diet or tablets, although some people in this group will use insulin injections.
A diabetic person is at a higher risk of developing poor vision, especially if the diabetes is not well controlled.
Most sight loss due to diabetes can be prevented, but it is vital that it is diagnosed early. An annual eye examination is extremely important to allow for the early detection of the effects of diabetes on the eye and can help to prevent sight loss as a result of diabetes.
Effect on the lens: Diabetes causes water retention in the lens of the eye. This causes a deterioration in your distance vision.
Effect on blood vessels: Diabetes affects the smaller blood vessels first. They can start to leak or become blocked. A leaky vessel can cause new blood vessels to grow. Newly formed blood vessels are very fragile. If they leak, blood fills the eye (vitreous haemorrhage) thereby obstructing vision. They can also cause the retina to pull away from the back of they eye causing a detached retina. Blocked vessels cause the retina to swell.
In order to prevent sight loss, it is very important to control diabetes. This can be done by the following means:
Control your diet & ensure medication is taken as directed by your GP
Take regular exercise, avoid smoking and drink alcohol in excess
If you think you may have some symptoms of diabetes, consult your GP immediately. Book an appointment for an eye examination if you feel your eye sight is deteriorating.
glaw-koh-muh: A group of conditions in which the Optic Nerve (which carries visual information from the retinal cells to the visual centre in the brain) is damaged, initially resulting in loss of the peripheral visual field.
It is thought that this damage is caused by an increase in the pressure of the eye and / or a reduction in the blood supply, or other weakness to the optic nerve head.
Hyperopia is caused by an eyeball which is too short or a lens that is weaker than necessary. As a result, the rays of light entering the eye do not focus on the retina but behind it. As your eye has some degree of control (accommodation) it is usually able to focus objects far away, hence you can see clearly in the distance. Closer objects require a lot more effort and if this is not possible, appear blurry.
Hyperopes need a convex or positive powered lens to bring the focal point of the rays of light forward and onto the retina. Positive lenses can be used to correct both hyperopia and presbyopia.
A convex lens is thicker in the middle compared to its edge. For high prescriptions, a flatter, aspheric lens is recommended in order to avoid the lens appearing too bulbous.
mak-yuh-luh:An oval yellow spot near the centre of the retina, responsible for our colour vision & central vision.
AMD is the greatest cause of vision loss in people aged over 60.
THERE ARE 2 TYPES OF MACULA DEGENERATION
DRY AMD: This is the most common form of the condition, where the macula gradually deteriorates due to the ageing process. Degeneration is gradual and slow. Although there is no cure for this type of AMD, recent research suggests minerals such as Zinc, Leutin and anti-Oxidants can help keep the macula healthy. These minerals are available in vegetables such as Spinach, Brocolli and Green Peppers as well as supplement form.
WET AMD: This is a rarer form of AMD where delicate new blood vessels start to grow from under the macula and leak, causing a build up of fluid. Wet AMD can progress quickly and results in sudden vision loss. Early detection and medical treatment is vital in order to offer any chance of controlling the effects of wet AMD.
Effects of AMD AMD results in the loss of central vision. Peripheral vision usually remains unaffected and therefore does not lead to total blindness.
In the beginning with only mild degeneration in one eye, symptoms may go unnoticed as the good eye makes up for the weaker eye.
Symptoms experienced with AMD include:
Reduced central vision
Wavy / distorted straight lines
Problems judging distances
Deterioration in colour vision
Poor vision in poor light
CAUSES OF AMD
The following factors can increase the risk of AMD :
Age : AMD is more prevalent in the older population.
Gender : AMD is more common in Women than Men.
Genetics : Variations & deficiencies in certain genes have been found in many cases of ARMD patients.
Smoking : Smoking can increase the risk of AMD and quitting smoking can reduce the risk of AMD.
Sunlight : Some researchers have shown a link between UV light and AMD.
Nutrition : As mentioned above, some minerals play an important role in maintaining the health of the macula.
If you can’t see distant objects clearly, you are said to be short sighted or Myopic.Myopia occurs when your eye is unable to focus the incoming rays of light on the retina despite the lens being as flat as possible.
Most myopes can see close objects clearly and thats why they may remove their spectacles to read. Myopia can be caused by an eyeball that is too long or a lens with a very high power and often has a genetic link.
The onset of myopia usually occurs in the early years. It tends to stabilise towards 30 – 40 years, after which it commonly reduces so that you become less myopic.
Myopia can be corrected by using concave (minus powered) lenses in front of the eye. These have the ability of bending the light forward until it focuses on the retina.
A concave lens is thicker on the edge compared with its centre. High index (thinner lenses) are therefore recommended for high myopic prescriptions to reduce thickness and weight and to improve appearance.
You may have noticed children holding books very close to their eyes and still being able to read the words without any problem. Throughout life, we tend to hold objects further away from our eyes as this feels more comfortable.
Presbyopia tends to occur in the mid to late forties. The reason for this is that over time the lens becomes harder and therefore less elastic. As a result, its ability to focus close objects reduces (loss of accommodation). A prescription for close work (eg reading, computer etc) is given in order to help relax the eyes and improve close vision.
ret-n-uh: A light sensitive part inside the inner layer of the eye. Some of the cells of the retina receive light and transform it into image-forming signals which are transmitted through the optic nerve to the brain.
A retinal detachment occurs when the retina separates from the actual wall of the eye. The detached area will not work properly and this affects the quality of the picture produced by the brain.
Flashing Lights (short flickers of light)
The feeling of a cobweb in the eye.
A curtain covering some of the visual field – this may cause a shadow or a total black out of part of your vision.
All need urgent examination to rule out the presence of a retinal detachment.
Retinal detachment affects approximately 1 in 10,000 people. It is rarely seen in young adults but more so in short sighted or middle aged people.
There are several different treatment methods for retinal detachment depending on the cause and the state of the retina.