Retinopathy of prematurity (ROP) is, as the name suggests, an eye condition that affects some premature babies. The impact on eyesight varies according to the severity of the condition.
History of ROP
This condition became known in the 1940s when premature babies were found to have a better chance of survival if given high concentrations of oxygen. At that time, ROP was known as retrolental fibroplasia and was thought to be simply a result of prematurity. In affected babies a pale, opaque membrane behind the lens of both eyes was evident, which rendered the child blind. By the early 1950s, the condition accounted for around 50% of children registered blind.
Incidence fell dramatically once the connection with oxygen therapy was established and the condition was renamed. There has since been a small increase in incidence associated with a rise in survival rates of very premature babies, and the condition is more common in countries where oxygen therapy is used without adequate ophthalmic assessment.
Development of the Eye
Although the eye starts forming in the first few weeks of pregnancy, it is still not fully developed at birth. Apart from the macula1, the retina itself is completely developed by about 36 weeks. However, the blood supply to the retina continues to develop until 40 weeks2 with the temporal area the last to be reached. If this process is interrupted, problems can occur.
Cause of ROP
ROP occurs when parts of the retina are not fully vascularised – that is to say, the growth of blood vessels is incomplete. Exposure to high oxygen levels halts the normal progression of blood vessel growth. When the oxygen levels return to normal, the retina becomes relatively hypoxic (short of oxygen), and this stimulates the release of chemicals that influence blood vessel growth. This results in the excessive formation of new blood vessels and scar tissue, which may lead to retinal detachment.
Any of these three factors alone may provoke ROP. A combination of two or more increases the risk and severity of ROP.
Concentration of inspired oxygen. The recommended safe level is 40% oxygen therapy, but this may have to be modified taking the other two factors into consideration.
Duration of oxygen therapy. The longer that oxygen therapy is administered, the greater the risk. A long period of low level oxygen may be just as dangerous as a shorter exposure to a greater concentration.
Prematurity of the baby. The more premature the baby, the less well-developed the blood supply to the retina and the greater risk of ROP even without any oxygen therapy. A baby below 800g birth weight or earlier than 26 weeks is particularly at risk.
Clinical course of ROP
Oxygen therapy-related ROP may appear within hours of treatment. Otherwise signs of ROP appear between 4 to 10 weeks after birth. The condition may halt at any stage.
Stage One. Existing blood vessels constrict and the optic disc3 becomes pale. This is followed within a few weeks by the blood vessels dilating, with some fine new vessel growth at the ends of the blood vessels.
Stage Two. The new vessel growth is moderately abnormal.
Stage Three. Blood vessel growth extends into the vitreous with some peripheral retinal detachment.
Stage Four. Up to half the retina becomes detached.
Stage Five. The entire retina may become detached and there may be a haemorrhage into the vitreous4.
If ROP halts in stage one or two, there is usually spontaneous recovery. Even stage three may regress. As the condition resolves over the next few months, scarring occurs which is graded from 1 (minor changes, minimal impact on vision) to 5 (completely disorganised retina and blindness).
All premature babies need to be screened for ROP. If stage three is reached, laser treatment or cryotherapy (freezing) is used to halt new vessel growth by sealing the vessels. This has halved the incidence of further complications. Retinal detachment in stage four or five may be treated by scleral buckling. This involves a rubber band being wrapped around the eye to relieve traction on the retina and bring it back in contact with underlying layers. This is only done very rarely. Another possible treatment in stage five is vitrectomy5 in which the vitreous and scar tissue are removed.
Premature babies have a greater risk in general of developing strabismus (a 'squint', leading to a risk of a 'lazy eye') being myopic (short-sighted) or amblyopic (with reduced vision). In cases of moderate to severe ROP, glaucoma (high eye pressure) is a common complication. Other possible problems that may develop later are cataracts, corneal opacities and retinal splits or detachment.
1 A small area at the centre of the retina with a high concentration of light-sensitive cells, used for detail in central vision.
2 This is the normal time between conception and birth, and babies born around 40 weeks are considered to be 'term'.
3 This is the area at the back of the eye where nerve fibres exit towards the brain and blood vessels enter and leave the eye.
4 The vitreous is a clear jelly-like substance that fills the rear part of the eye.
5 Vitrectomy is described in the entry on Posterior vitreous detachment.