A majority of our sensory information is provided by our vision. Human vision is complex. Sometimes our eyes don’t work the way they should because of a common eye conditions. The eye should take in the physical stimuli of light rays. These light rays are focused onto the retina, a thin membrane that lines the inside of the eyeball. The retina converts light into signals which is transmitted along the optic nerve to the brain.
Structure of the eye
Our eyes may be relatively small, but they are incredibly hardworking allowing us to see the world around us.
- Cornea – clear tissue at the front of the eye
- Iris – the coloured part of the eye surrounding the pupil that regulates light
- Pupil – hole in the iris that allows light into the eye
- Lens – a small clear disc that focuses light rays onto the retina
- Retina – a layer of tissue that that lines the back of the inside of the eye eye, senses light, and creates electrical impulses that travel through the optic nerve to the brain
- Macula – small central area in the retina that processes sharp direct vision.
- Optic nerve – connects the eye to the brain and carries the electrical impulses formed by the retina to the visual cortex of the brain
- Vitreous – clear substance that fills the middle of the eye
There are four conditions which are the main causes to vision impairment and blindness in Australia. These are cataracts, diabetic retinopathy, glaucoma and macular degeneration. There are also congenital eye conditions that are inherited and are present at birth.
Eye conditions leading to vision loss include:
Albinism is a genetic condition that reduces the amount of melanin pigment (colour) in the skin, hair and eyes. In ocular albinism, only the eyes are lacking in pigment. In oculocutaneous albinism, the skin, hair and eyes are all lacking in pigment.
People with oculocutaneous albinism will typically have very blonde/white hair, pale skin and low vision. The irises of the eyes range from pale blue to pink and can’t adequately screen out excessive light. It means a child is very sensitive to bright light and glare (photophobia).
Amblyopia is also known as lazy eye. It occurs in just one eye, when it turns and because that one eye is able to see better than the other, the brain begins to ignore the visual impulses from the weaker eye favouring the other eye. If it’s left as it is, the weaker eye will continue to have permanently low vision. Treatment generally consists of patching the stronger eye to strengthen the weaker one.
Aniridia is when the iris or part of the iris of the eye is missing. This means that the muscles that open and close the pupil don’t exist. It makes the eye appear as though it has little or no colour and a larger than normal pupil. Other changes in eye structure are often present (e.g. cataracts and glaucoma). It can be a hereditary condition or develops through other factors. Aniridia reduces the sharpness of vision (visual acuity) can cause other eye disorders such as photophobia (light sensitivity), cataracts, nystagmus and glaucoma.
A cataract is a clouding of the normally clear lens of your eye. For someone who has a cataract it is like looking through a foggy window. It is formed when the proteins that make-up the lens are damaged. Vision becomes poor giving the sense of looking through frosted glass. Cataracts typically occur with advancing age, but can occur at birth.
Effects of cataract include:
- Cloudy or blurry vision
- Colours may seem faded or yellowed
- Glare can cause hazy vision particularly on bright sunny days. The person may see a halo or haze around a light source
- A person with cataracts may have to hold reading material closer than usual.
Cerebral Vision Impairment
Cerebral vision impairment (CV) occurs when the part of the brain (often teh visual cortex) that receives and decodes the message sent from the eye is poorly developed. Usually the eyes appear to be perfectly normal and function correctly under examination.
Sometimes the cerebral blindness is temporary and lasts weeks or months, but the damage can be more permanent.
Diabetic Eye Disease
Diabetic Retinopathy is an eye disease associated with diabetes. Elevated blood sugar damages the blood vessels that nourish the retina: the “seeing” part at the back of the eye. If vessels are damaged, signals are sent to grow new ones which may be fragile or abnormal. If these new vessels leak, blurred vision and blindness can result.
The damage increases with time, causing either vision impairment or blindness. Vision often becomes patchy and can fluctuate with the blood sugar levels
Effects of diabetic eye disease include:
- In the early stages there may be blurring of both central and peripheral (side) vision.
- As it progresses, there may be cloudiness and blind spots or floaters in the vision.
- In advanced stages, scar tissue forms, causing additional distortion and blurred vision. Retinal detachment may then follow.
Glaucoma is an eye disease caused by high pressures in the eye which lead to damage to the optic nerve. This damage reduces the side or peripheral vision, causing “tunnel vision”. In a normal eye, clear fluid flows from the posterior chamber, around the lens and out of the front chamber of the eye draining out through a spongy mesh area where the cornea and iris meet (called the angle). With glaucoma, this does not happen.
f it is not treated, it can cause blindness. Glaucoma has an increased risk with aging, and if there is a family history you have four times the risk. There are different types of glaucoma including open, congenital, closed angle and secondary.
Hemianopia (also known as Hemianopsia) is loss of vision in either the right or left sides of both eyes; a common side effect of stroke or brain injury. This vision loss causes serious problems with mobility, bumping into objects and increased incidence of falls and accidents.
Keratoconus is a degenerative eye condition usually first appearing during adolescence and young adulthood. It causes the clear front surface of the eye (the cornea) to thin and a cone-like bulge develops. The bulging cornea affects the way light hits the retina at the back of the eye, causing distorted vision.
Symptoms begin slowly and can be hard to detect and include:
- Distorted, ghosting or blurred vision
- Glare and light sensitivity.
Glasses can correct early stages of Keratoconus, but hard contact lenses are the best way to correct vision as the condition progresses. Collagen ‘crosslinking’ (the combination of ultra violet light and Vitamin B) can help to ‘harden’ the cornea to stop its progression. Corneal transplants will helpthe most severe cases.
Leber’s Congenital Amaurosis (LCA)
Leber’s Congenital Amaurosis (LCA) is an inherited eye condition, which appears at birth or in the first few months of life. This degenerative disease which progresses over time, is caused by the abnormal development of light-sensitive cells in the retina. It results in very reduced vision or blindness.
Macular Degeneration is a condition affecting the central area of the retina called the macula. The macular is responsible for detailed vision, for uses such as reading.
Macular Degeneration damages the central vision, but side or peripheral vision remains intact. For example, sufferers may be able to see the outline of a clock, but not the numbers or hands. People with Macular Degeneration may experience difficulty reading, writing, seeing colours, watching television and recognising faces. There are both ‘wet’ and ‘dry’ forms of ‘macular degeneration.’
Nystagmus is a fluttering movement of the eyes which a person has no control over. The eyes can move from side to side, but they may also move up and down, around in circles or a combination of these movements. It is usually present in both eyes and can occur alone or together with another vision problem such as congenital cataracts or albinism. Nystagmus causes blurred vision and reduces depth perception.
Optic Nerve Atrophy
Optic nerve atrophy is when damage occurs to part or all of the optic nerve fibres. This results in permanent loss of part or all of the nerve function. The optic nerve carries electrical impulses from the retina to the brain, which translates the message into what we see. Affect on vision ranges from almost normal visual acuity to complete blindness. People with optic nerve atrophy may also develop nystagmus.
Optic Nerve Hypoplasia
Optic nerve hypoplasia occurs when the optic nerve is underdeveloped or even missing. It may occur in combination with other brain and endocrine (system of glands that secrete hormones) abnormalities. Affect on vision ranges from almost normal visual acuity to complete blindness. ONH may affect one or both eyes.
Optic Neuropathy is a disease, generally noninflammatory, of the eye, where ther has been destruction of the optic nerve tissues. Causes include an interruption in the blood supply, compression by a tumor or aneurysm, a nutritional deficiency, and toxic effects of chemical. The disorder, which can lead to blindness, usually affects only one eye.
Retinitis Pigmentosa (RP) is one of the more common genetic eye conditions which causes the retina to slowly and progressively deteriorate. This causes vision to slowly and progressively decrease. One of the first symptoms of RPI is difficulty seeing in dark places or at night. There then follows a loss of peripheral (side) vision, which means that the field of vision gets narrow. People with RP may also become sensitive to light and glare. The symptoms of RP usually increase with time. The condition develops between the ages of 10 and 30 years as this is typically when symptoms begin to develop.
Retinoblastoma is a fast growing childhood cancer of the eye, which develops in the cells of the retina. If it is left untreated, it can spread through the optic nerve to the brain and may be fatal. However the tumour itself is very treatable and recovery rates are good. Treatment can require complete removal of the eye (enucleation) and may need radiation therapy to prevent spread of the tumour.
Retinopathy of Prematurity
Retinopathy of Prematurity (ROP) appears soon after birth, generally in premature infants exposed to high oxygen levels. The combination of high oxygen and immature blood vessels is gives rise to the abnormal development of blood vessels in the retina and can lead to retinal detachment and blindness. Sometimes there is a small area of healthy retina which remains, to give the person some useful vision.
Stargardt’s Macular Dystrophy
Stargardt’s Macular Dystrophy (SMD) is a hereditary condition, which affects the macular of the retina. Central vision is usually affected first with progressive vision loss over a period of years.
You may not notice the condition until after 6 years of age and before 20 years of age. However, in some people it may not cause problems with sight until they are 30 or 40 years old. SMD is an inherited disease, which affects males and females equally.
There are six eye muscles that work in pairs control the movement of each eye. When one eye muscle contracts (tightens), its partner relaxes and the eye moves in the desired direction. If one eyes muscle is out of balance (that is, under-acting or overacting) the affected eye will not move with the unaffected eye. This is strabismus.
When the eyes do not co-ordinate, each eye is seeing a different image to the other. This is confusing to the brain and it shuts out the image received from the ‘turned’ eye. This is known as suppression. A person with strabismus effectively uses only one eye. If the eye isn’t straightened early, before a child’s visual system has stopped developing, it will mean the eye won’t develop normal vision.
Trachoma is present in many aboriginal communities, particularly in remote areas. Australia is the only western country in the world to still have trachoma. It is a bacterium that infects the eyes and initially causes watery eyes and an aversion to light. It is easy to treat, but if not treated the cornea can become severely scarred.