How the eye works
We need light to see what is around us and to see colour. Light bounces off the objects we look at. These reflect different amounts of light which we see as different colours.
Front of the eye
Light rays enter the front of our eye through the clear cornea and lens. It is very important that both the cornea and lens are clear as this allows the light to pass directly through the front of the eye to the retina.
The cornea and lens bends light so that it can focus on the retina at the back of our eye. This gives us a clear, precise image. The cornea focuses the light towards our retina. The lens fine tunes the focussing of this light.
Our tears form a protective layer at the front of the eye and also help to direct the light coming into our eye.
The iris, the coloured circle at the front of our eye, changes the size of the pupil which allows different amounts of light into our eye.
The pupil is the dark hole in the middle of the coloured part of our eye. The pupil gets smaller in bright conditions to let less light in and bigger in dark conditions to let more light in.
Middle of the eye
The middle of our eye is filled with a jelly-like substance called the vitreous. The vitreous is clear and allows light to pass directly from the front to the back of our eye.
Back of the eye
The retina at the back of the eye is a light-sensitive layer which consists of rod and cone cells. These cells collect the light signals directed onto them and send them as electrical signals to the optic nerve at the back of our eye.
Rod cells are concentrated around the edge of the retina. They help us to see things that aren't directly in front of us, giving us a rough idea of what is around us. They help us with our mobility and getting around by stopping us from bumping into things. They also enable us to see things in dim light and to see movement.
Cone cells are concentrated in the centre of our retina where the light is focused by the cornea and lens. This area is called the macula. Cone cells give us our detailed vision which we use when reading, watching TV, sewing and looking at people's faces. They are also responsible for most of our colour vision.
The optic nerve is made up of thousands of nerve fibres. These fibres pass the electrical signals along to our brain where they are processed into the image we are looking at. This connection is known as the visual system and develops through use from birth to around seven years of age, after which it does not develop further. For this system to develop normally all parts of the eye need to be functioning well.
How we see
Seeing can be likened to the process of taking pictures on a film with a camera which you then get developed. The retina is like a camera film which stores an image of what we are looking at. The image directed onto the retina is then sent along to the brain where it is processed, like developing a camera film. Therefore we actually "see" in our brain with the light information sent to it from our eyes. This whole process happens very quickly so that everything we see is in focus.
About coloboma
A coloboma is a gap in part of the structures of the eye. This gap can be large or small and is normally in the bottom part of the eye. A coloboma is caused when a baby's eyes do not develop properly during pregnancy. The eyes develop between the fourth and anything up to the fifteenth week of pregnancy, though development is usually completed around eight weeks. This condition occurs in about 1 in 10,000 births.
Coloboma can affect one eye (unilateral) or both eyes (bilateral). Coloboma can affect a number of different parts of the eye. A coloboma does not mean that there is a hole in the eye, but that certain structures or parts within the eye did not fully form.
How effects the eye
A coloboma can affect the iris which is the coloured part at the front of the eye. It can affect the lens, the part of the eye which helps focus light onto the retina.
Coloboma can also affect the choroid which is a thin network of blood vessels which help to keep the retina healthy. Finally, it can affect the retina at the back of the eye. Very rarely coloboma can also affect the optic disc or the eyelid.
A coloboma forms whilst the baby grows in the womb. The eyes develop early during pregnancy and start off as little buds. Usually the eye folds in on itself as it develops which leaves a small gap called the foetal cleft.
The foetal cleft helps maintain the blood supply to the developing parts of the eye. In the final stage of eye development during pregnancy the cleft seals up from the back of the eye forwards and all the structures of the eye are formed. In an eye with coloboma this gap does not fully close and remains in some of the structures of the eye.
Diagnosis
If the hospital staff suspect a child has a coloboma, which is normally first noticed by the possible keyhole-like shape of the pupil, then an ophthalmologist (eye doctor) will carry out a full eye examination.
The ophthalmologist would normally use an instrument called an ophthalmoscope to examine the inside of a child's eyes. The ophthalmoscope is held close to the eye but will not touch it. A baby may be given a general anaesthetic to allow the ophthalmologist to carry out a eye examination. This allows the doctor to look thoroughly at the baby's eye whilst he or she is still without causing any distress.
This kind of thorough eye examination at the hospital will help them to find out how much of the eye has been affected by the coloboma. It may be hard to tell how much a child's sight has been affected until they are older. This is because small children aren't able to communicate in words how good their vision is.
The hospital may also carry out other examinations to check on your child's general health.
Effect of coloboma on vision
The effect coloboma has on vision depends a lot on which part of the eye is affected and how big the gap is. Normally the gap is at the bottom of the eye (where a 6 is on the face of a clock) and runs from the front to the back of the eye. Coloboma may affect only the front of the eye if most of the cleft has sealed up. It may affect the eye from the front to the centre and back if more of the cleft has failed to close.
Most commonly coloboma only affects the iris. Children with this type of coloboma often have fairly good vision. Their pupil may be shaped a bit like a keyhole. They may have some problems with and dislike bright lights because the iris, which usually limit the amount of light entering the eye by controlling the size of the pupil may not work properly. Too much light entering the eye can cause discomfort and distort the image created. Measures can be taken to help protect a child from sunlight such as tinted glasses, sun hats or sunblinds in cars.
If the coloboma goes further back into the eye then the child may have more visual problems, especially if the retina is involved. If this has happened then central vision may be affected. Central vision is used for detailed activities such as reading, writing and close work or play. If this is the case, specialist support services for children with a visual impairment will be involved.
Support services can help a child and his or her carers learn how to make the most of the vision the child has. Specialist services are mentioned at the end of this leaflet. They can provide information on development, play, education and many other issues.
Coloboma may affect one eye more than the other, so one eye may have better vision than the other.
Effects of coloboma on other parts of the body
Sometimes children with coloboma may have other problems and some 'syndromes' which include coloboma, such as
CHARGE, a rare condition which stands for:
C - coloboma
H - heart defects
A - atresia of the choanae (problems with the nose passages)
R - retarded growth and developments
G - genital hypoplasia (undescended testicles)
E - ear abnormalities.
The hospital and medical staff will tell you if there are any other existing health issues and offer the relevant treatment and support services from a very early age.
Causes of coloboma
Most cases of coloboma appear without any previous family history. In some families coloboma can be inherited. If a child is diagnosed with coloboma it is advisable for the child's parents to have an eye examination. This is because coloboma can be present without causing any visual problems. Therefore a parent might have coloboma but not know it. If an eye examination detected coloboma then a parent could explore inheritance patterns with the hospital or genetic counsellor.
The genetic factor responsible for most cases of coloboma has not yet been identified. Research is on-going to identify genes and any environmental factors that may cause coloboma. Information on this can be found on the MACS website or from the Medical Research Council Edinburgh Eye Malformation Study (MEEMS) website. Their contact details can be found at the end of this fact sheet.
If coloboma has occurred in your family and you would like to talk through the issues of genetics then you could ask your GP to refer you to the local clinical genetics department who could explore this with you.
Treatment
There is no treatment for coloboma at present. A child with coloboma will receive specialist care at hospital during the early years to monitor the effect of the coloboma and their eye health. The frequency of these checks will depend on the child's needs. Children who have coloboma can be more at risk of glaucoma (increased eye pressure) and retinal detachment. There are treatments for both of these conditions which the hospital would explore with you.
If your child's eye health is stable and no further complications appear then they will usually attend an eye test every six months up to the age of seven years and then annually. These eye tests will usually take place with an optician or orthoptist.
Children with coloboma may need glasses. Glasses cannot correct the vision problems caused by the coloboma. However, glasses can correct short-sightedness or long-sightedness which can help to correct the parts of vision that haven't been affected by coloboma.
Sometimes cosmetic contact lenses may be considered at a later stage. These can help to make the pupil look round rather than keyhole shaped.
Prescription sunglasses due to the light-sensitivity may also be suggested, as may some low vision aids and equipment to help a child make the most of their sight.
What next
Talk to someone
The RNIB Helpline is your direct line to the support, advice and products you need. We'll help you to find out what's available in your area and beyond, both from RNIB and other organisations.
Whether you want to know more about your eye condition, buy a product from our shop, join our library, find out about possible benefit entitlements, be put in touch with a trained counsellor, or make a general enquiry, we're only a call away.
RNIB Helpline
Tel: 0303 123 9999
Email: helpline@rnib.org.uk
We are ready to answer your call Monday to Friday 8.45am to 6.00pm and Saturday 9.00am to 4.00pm. Outside these times leave us a message and we'll get back to you as soon as possible.
Unfortunately, RNIB can only answer email enquiries from the UK. But you may find an organisation for people with poor sight in your country through our organisations database.
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Get information
RNIB's guide to Adapting to sight loss offers ideas for people with sight problems on making life easier and ways of getting the most out of life.
You may also find the following useful:
Useful contacts
1. MACS Children Society can provide support and information to parents and carers of children with Microphthalmia, Anophthalmia, Coloboma and related conditions.
22 Lower Park Street
Holyhead
Isle of Anglesey
North Wales
LL65 1DU
Tel: 0800 169 8088 (Freephone)
Email: enquiries@macs.org.uk
2. MRC Edinburgh Eye Malformation Study (MEEMS)
3. LOOK UK is an organisation which helps support families with children aged between 0-16 with vision problems. They have family support officers and help support families at home and in school.
Look National Office
c/o Queen Alexandra College
49, Oak Court Road
Harbourne
Birmingham
B17 9TG
Tel: 0121 428 5038
4. National Blind Children's Society (NBCS) can offer information, support and advice to parents and carers of children with sight loss or an eye condition.
Head Office
Bradbury House
Market Street
Highbridge
Somerset
TA9 3BW
Tel: 01278 764 764
Family Support & Information: 01278 764770
Fax: 01278 764 790
Email: enquiries@nbcs.org.uk
5. Families can use Contact a Family's Freephone Helpline for advice, information and, where possible, links to other families through their web-based linking service Making Contact.org
About this guide
The author of this guide is Anna Mitman, RNIB Eye Health Information Service.
Updated Feb 2008
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