Sub-conjunctival haemorrhage is a leak of blood, from a blood vessel, over the white of the eye (sclera). It looks quite dramatic, but will not do you any serious harm.

The main thing you will notice is a dramatic red patch over the white of one of your eyes. This is limited to the white of the eye. It usually comes on all of a sudden, with no apparent cause, although sometimes it accompanies an obvious injury.

Frequently you will not notice anything until you get a comment from someone else, or you get a surprise when you look in the mirror. Occasionally there is a slight irritation, stinging, or awareness of something not feeling right. It does not affect the vision. If you have a red eye and problems with your vision you should contact a doctor to look for other causes.

The blood, that has leaked from a tiny blood vessel, shows up dramatically on the white of the eye and may just spread across a segment of the white, or cover almost all of it.

The leaked blood is held in place over the white of the eye by a thin, clear membrane (the conjunctiva). The conjunctiva is fixed to the surface of the eye around the outside of the coloured ring on the eye (the iris) and therefore it is impossible for the blood from the haemorrhage to spread across the central part of the eye that we see through. That means that, although a sub-conjunctival haemorrhage looks dramatic, it cannot harm your vision.

The blood will stay there for some days, and slowly go through the same colours that a bruise does as it is absorbed by the body. It takes longer to change colour than a normal bruise, because the membrane is so thin that oxygen from the air can get to the blood to some degree.

Most times it is not clear what has caused the sub-conjunctival haemorrhage. It may be that a tiny blood vessel has burst with coughing or sneezing, or something has caught the surface of the eye.

If you have been banged on the eye or face it is clear what has caused the haemorrhage.

On rare occasions it is associated with a problem with the clotting of the blood, for example in people taking anticoagulants or aspirin, or people with bleeding disorders.

Most times you do not need to do anything about a sub-conjunctival haemorrhage.

If in any doubt, however, it may be worth discussing it with a doctor or nurse. This can usually be during routine hours.

Your doctor will make the diagnosis by a quick glance at the eye, but may want to check your blood pressure, and if there is any question about bruising and bleeding excessively in other parts of your body, may also arrange a blood test.

If the haemorrhage has been caused by an injury, especially something like a squash ball in the eye, your doctor will be taking a closer look at the eye to check for any other damage. In such cases it is well worth seeing a doctor more urgently, especially if the vision is affected. In these circumstances it may be better going to the Accident & Emergency Department at your local hospital.

No treatment is needed for simple sub-conjunctival haemorrhage. It will settle in a few days.