The eye is similar to a camera. Light rays reflected from objects are made into images by the cornea and lens. These images are converted into signals which are passed to the brain by the light sensitive cells in the retina. The retina is the lining inside the eye, which contains a fine network of blood vessels. Light enters the eye through the pupil. The size of the pupil and therefore the amount of light entering the eye is controlled by the iris.
The image on the retina is upside down; it is converted to electrical impulses and transmitted through the optic nerve to the brain, where it is translated and seen as upright.
At the centre of the retina is the macula, which is a small specialized area. It allows us to see fine detail for tasks such as reading and also enables us to recognise colours. The other areas of the retina provide peripheral vision, but not in any great detail.
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The following are definitions of commonly found optical conditions that can be corrected with spectacles or contact lenses:
Hypermetropia (Long or Far Sight)
Long-sight is where the light entering the eye would be brought to a point of focus beyond the retina, causing a blurred image. This can be caused by the eyeball being too small or the lens not having enough focusing power. It is simply corrected with lenses that cause the image to focus on the retina. You will know you have this if the number in the ‘Sphere’ or ‘SPH.’ box on your distance prescription has a + (plus sign) in front of it.
Myopia (Short Sight or Near Sight)
Short-sight occurs when light entering the eye is brought to a point of focus in front of the retina, causing a blurred image. This can be due to the eyeball being too long or the lens having too much focusing power. It is simply corrected with lenses that cause the image to focus on the retina. You will know you have this if the number in the ‘Sphere’ or ‘SPH.’ box on your distance prescription has a – (minus sign) in front of it.
This is a very common condition where the light rays from objects are not brought to a single sharp focus within the eye. This is often caused by the front surface of the cornea having an irregular shape, i.e. being similar in curvature to the side of a rugby ball instead of being spherical like a football. It is very straightforward to correct using lenses with more power in one direction than another. These are called ‘cylinder’ lenses or ‘Cyl.’ and must be prescribed with an ‘axis’ to ensure the power is working in the correct direction for your vision.
Presbyopia is a condition where the eye has difficulty when focusing on near objects, when reading, for example. The process of focusing on near objects is called accommodation. With age the lens, which enables accommodation, becomes less flexible and after we reach our early to mid forties focusing on close objects becomes increasingly difficult. Although it often seems to occur suddenly, the loss of flexibility takes place over a number of years.
Presbyopia is part of the eye’s natural ageing process. It happens to everyone regardless of whether they need spectacles, for short-sight, long-sight or astigmatism, it cannot be prevented. One of the first signs of presbyopia is the need to hold reading material further away to see it clearly – a common complaint is ‘my arms aren’t long enough!’ Other symptoms include blurred vision for reading, or eyestrain and headaches when doing close work.
Early on, simply holding close material further away helps but eventually a near vision correction in the form of spectacles or contact lenses is required. This described on spectacle prescriptions as a ‘reading addition’ or ‘Add.’
Some short-sighted people can delay the need for a near vision correction by removing their spectacles for reading and close work. The most appropriate type of near vision correction is dependent on your lifestyle, occupation and hobbies. If you have good distance vision and only have difficulty seeing close up, a separate pair of near spectacles is often the easiest solution. For others, needing separate prescriptions for distance and near means two sets of spectacles and this can prove frustrating. In these cases, spectacles with bifocal or varifocal may be more appropriate. Varifocal lens technology has advanced considerably over recent years to provide lenses that provide good vision at all distances, including the intermediate range, which is ideal if you use a computer. There are also a wide range of contact lens options for presbyopia.
Strabismus (Squint or Crossed Eyes or Wall Eyes)
This is a visual defect where the eyes are misaligned and point in different directions. There are two different forms of strabismus being referred to:
- Crossed eyes (esotropia) – Where the eye points inwards.
- Wall eyes (exotropia) – Where the eye points outwards
In each case the brain will not be able to use the vision from both eyes because each eye will be receiving light from a different direction. The brain will suppress the image from the weaker eye, called amblyopia, and three dimensional vision and depth perception will be lost.
This can be corrected in children if detected early enough. However, in adults it is corrected with spectacles containing a ‘prism’ designed to bring light from the same object to both eyes.
The following are treatable with surgery or medication or a combination:
The clear lens of the eye, behind the iris, which focuses light to provide sharp images, becomes cloudy and hardens causing a loss of sight. Cataracts are painless and gradually develop over a long period of time. Over 50% of people aged over 65 have some form of cataract development and most cases can be treated successfully with surgery where the lens is replaced by a clear plastic one. It can be caused by age, trauma, diabetes, long term steroid therapy and excessive exposure to UV light or most likely hereditary.
This is due to too much pressure in the front chamber of the eye, which ultimately causes damage to the optic nerve. The increased pressure pushes the contents of the eye back, which eventually leads to damage to the retina. This is sometimes called a retinal detachment and causes the field of vision to be reduced. This measured accurately with a visual field test. Glaucoma affects 2% of the UK population over the age of 40 and there are many different types of glaucoma. All of which can be treated if caught early. Usually drugs are prescribed, but occasionally surgery is needed to reduce the pressure.
Age Related Macular Degeneration (AMD)
Age Related Macular Degeneration is the most common cause of poor sight in people over the age of 60. The macula is the small central part of the retina, which is responsible for what we see when we look straight ahead, allowing us to see fine detail when reading or recognising faces, as well as allowing us to see colour. The cells of the macula are very delicate and can become damaged over time. When this occurs later in life, it is known as age related macular degeneration (AMD). There are 2 main types of AMD:
This accounts for about 90% of cases. It develops slowly over a number of years causing gradual loss of central vision. People often complain of difficulty with detailed tasks such as reading, writing and recognising small objects. Stronger reading glasses can help in the early stages. Low vision aids such as magnifiers can help make the most of the remaining vision. In the advanced stages of the disease, people may notice a blank patch or dark spot in the centre of their vision, making detailed tasks very difficult. Unfortunately, there is no treatment for dry AMD.
This generally develops more quickly, resulting from new blood vessels growing behind the retina leading to bleeding and scarring. It causes a loss of vision which is often more severe than dry AMD.
Wet AMD often causes symptoms of blur or distortion, causing straight lines to appear wavy. Objects may appear an unusual size or shapes. If you develop any of these symptoms suddenly, you should seek advice as soon as you can.
Wet AMD is sometimes treated using lasers to seal leaking vessels. This is an area in which research is ongoing and new treatments are constantly being developed. AMD usually involves both eyes, although one eye can be affected before the other. Using the ‘good’ eye will not wear it out, so don’t be afraid to use it as normal. AMD is not painful, and never leads to total blindness as it only affects the central vision.
What causes AMD?
The exact cause is not known, but a number of risk factors have been identified:
AMD is age-related and therefore is more likely later in life.
Women appear more likely to develop the disease.
Several studies have shown that smoking increases the risk of AMD. Stopping smoking is therefore important for the eyes, as well as your general health.
There are many treatments and vitamin supplements for macular degeneration, but the effectiveness of many of these preparations remains unproven.
From the age of 40 onwards, there is an increased risk of developing eye conditions such as glaucoma. This risk increases over the age of 60, with many people developing cataract and the risk of developing macular degeneration. For this reason, regular eye examinations are advised. Eye examinations are available on the NHS for those over 60.