Clinical features of common causes of Visual Impairment

• Glaucoma
• Cataract
• Macula degeneration
• Diabetic retinopathy
• Retinitis pigmentosa
• Retinal detachment
• Loss of an eye

Glaucoma

This is a condition in which the drainage of the fluid in the anterior part of the eye is blocked while the production of fluid continues. This leads to a build up of pressure inside the eyeball, and the pressure on the optic nerve results in a loss of vision.

Usually this condition occurs in the chronic form, where there is a gradual, painless build up of pressure, and a gradual progression of symptoms, such as reduced peripheral (side) vision, blurring of vision and haloes around lights.

Opticians routinely employ screening tests for glaucoma, and, if detected early enough, prompt treatment by medication (usually eye drops) appears to significantly reduce the progression of the disease. However, the progression can be insidious, because it is painless, and it may not be until a formal testing of visual fields is carried out that damage in the form of defects in the visual fields may show up.

Around 1 in 7 or 8 cases of blindness is due to glaucoma. About 1 in 100 (at around the age of 40) and 1 in 10 people (at around 70 years) have some form of glaucoma.

Cataract

A cataract is a clouding, or formation of opacities in the lens of the eye, which blocks the passage of light to the retina, or disperses it by abnormal refraction. It causes “misty” eyesight, (the same effect as steam on glasses, or frosted glass.) It also causes glare or dazzle from sources such as from car headlights, and a reduction in colour perception, giving a “washed out” look.

• most cataracts are caused by the ageing process but they can be congenital (i.e. you are born with them, for example, in congenital rubella, (German Measles) or
• they can be acquired through damage to the eye, some medications (such as the use of long-term steroids), or from the ageing process, and from certain conditions, such as diabetes.

Macular degeneration

The fovea in the central part of the macula is the area of sharpest vision in the retina, and is of great importance for activities such as reading, drawing sewing, (where fine detail is needed), and for recognising features, and recognising colours. It is a “wear and tear” process.

Age- related Macular Degeneration (MD) is usually connected to ageing, though smoking and diet are now thought to have an impact on its development. It affects older people, and men and women equally. It is more common in fair- skinned people, and it can run in families. There are two types of age- related MD, “wet” and “dry”.

There are also other conditions which affect the macula which are not age- related, but which may result in changes to macular vision, such as in very myopic (short- sighted) patients, and in some systemic diseases.

Age-related macular degeneration accounts for over two thirds of those with vision impairment who are over 65 years of age.

Diabetic retinopathy

The condition of diabetes (in which the body has problems with the usage and storage of glucose) can lead to a condition called diabetic retinopathy. This can lead to a number of problems with vision, including central vision problems, and problems with patchy vision throughout the visual field. 80% of diabetics have retinopathy after 20 years of diabetes.

Over time, the condition of hyperglycaemia (raised blood sugar) has the effect of increasing retinal blood flow, and metabolism. This causes eventual damage to the small blood vessels that nourish the retina, and they begin to leak.

These changes are visible in the retina, and could be manifested as:

• Micro-aneurysms
• Haemorrhages
• Exudates
• Swelling
• Oedema
• The formation of new abnormal blood vessels

Retinitis pigmentosa

This is a hereditary condition in which degeneration of the rods and cones gradually occurs, the Choroidal blood vessels gradually close, and there is laying down of pigment in these areas. This causes the night vision at first to be affected then the field of vision is gradually reduced in both eyes, causing "tunnel vision" of increasing severity. All cases are caused by a genetic problem with the rod and cone cells in the retina.

The initial symptoms are:

• Difficulty in seeing in a dim light ("night blindness")
• Gradually reducing field of vision, which may not be noticeable at first

The problems usually begin at a young age (around 12), and by the late 20’s the visual fields are significantly restricted, with gradual further deterioration to near blindness, which is not usually total, but the residual tiny field of vision is of little use in day to day living.

Retinitis pigmentosa is inherited in different ways; in one form, children of an affected parent have a 1 in 2 chance of developing the disease.

Retinal detachment

A retinal detachment usually occurs in one eye, and occurs when the vitreous (the jelly- like fluid in the posterior segment of the eye), pulls on the retina, causing a tear. Fluid then seeps in behind the retina, causing the retina to separate off from the underlying tissues.

It usually occurs first at the edge (periphery) of the retina, and is noticed as a “shadow” or “curtain” at the edge of vision, which gets worse as it progresses, and, if it extends to the area of the macula, vision can rapidly be affected.

There may be flashes or bright bubbles of light, with distorted wavy vision, with increasing numbers of black spots or "floaters", at the onset.

Retinal detachment usually occurs in middle age, or in those who are very short sighted (myopic). A more complex form can occur in diabetic retinal disease.

Loss of an eye

The loss of an eye should not cause significant disability in a person if the visual acuity and visual fields in the other eye are satisfactory.

After a period of adjustment, the individual would normally have few or no problems with the activities of daily living, self- care, or finding his/ her way about. However, there will be difficulty in the estimation of distance, (although this improves with learning), and the overall visual field is reduced.

He/ she would normally be able to hold a normal (Group 1) driving licence, as long as they have the requisite visual acuity and there is one good normal field of vision.

• Disabling effects of visual impairment
• Visual acuity conversion chart (40KB)
• Visual acuity conversion tool (19KB)

Amended April 2011