The internal surface of the back of the eye is lined by a tissue, called the retina. This tissue is equivalent to the film in a camera and is responsible for the initial formation of the visual image. This is then transmitted to the brain. In order to function, the retina requires a blood supply. The normal development of this blood supply for the retina starts at 16 weeks into a pregnancy and is completed by 36 weeks.
If an infant is born prematurely, with the retinal blood vessel development incomplete, problems occur. Abnormal blood vessels may develop which can subsequently lead to bleeding and scar tissue formation. This may then stretch the retina pulling it out of position. Visual loss may result.
The lack of oxygen in areas of retina which have not developed blood vessels, results in the release of chemicals that promote the growth of new blood vessels. These blood vessels often grow in an irregular manner, for reasons that are not completely understood. The presence of large amounts of oxygen (often needed for the baby's survival) is thought to be possibly toxic to the developing blood vessels. As premature babies have underdeveloped lungs they usually require additional oxygen therapy. There are other undetermined factors also contributing.
Currently, treatment relies on the destruction of the areas of the retina that are without blood vessels. This becomes necessary only if the disease has reached a certain, well defined stage, as the milder forms often regress spontaneously. By destroying the retina, the stimulus for the growth of the abnormal blood vessels is removed. The destruction is achieved by cryotherapy (literally freezing the retina, using a small pulse) or photocoagulation with laser energy. Rarely, surgery is undertaken in an attempt to remove the scar tissue. To prevent ROP, Paediatricians attempt to minimise the amount of supplemental oxygen the infant is receiving.
The suggested causes of Retinopathy of Prematurity are those conditions which stop the orderly growth of retinal blood vessels and stimulate their wild overgrowth.
Over 50 conditions have been proposed and thus far, have not been ruled out. The most frequently mentioned conditions are prematurity, excessive oxygen, infection, and/or excessive exposure to light.
It is critical that doctors examine the retinas of premature infants until blood vessels have reached the edge of the retina.
One or more of the following treatments have been suggested to stop the overgrowth of blood vessels or to reattach the retina: cryotherapy (the only treatment proven by rigorous testing), laser treatment, scleral buckling, and vitrectomy.
Retinal blood vessels fail to reach the retinal periphery and multiply abnormally where they end. After a normal birth, the retinal blood vessels fill the anterior periphery of the retina.
Zone 1 is the most posterior retina, that contains the optic nerve and the macula (zone of acute vision). Zone 2 is the intermediate zone where blood vessels often stop in ROP. Zone 3 is the peripheral zone of the retina, where vessels are absent in ROP, but present in normal eyes.