The blood vessels of the retina (in the back of the eye) begin to develop about 3 months into pregnancy. In most cases, they are fully developed at the time of normal birth. The eyes may not develop properly if a baby is born very early. The vessels may stop growing or grow abnormally from the retina into the back of the eye. Because the vessels are fragile, they can leak and cause bleeding in the eye.
Scar tissue may develop and pull the retina loose from the inner surface of the eye (retinal detachment). In severe cases, this can result in vision loss.
In the past, the use of too much oxygen in treating premature babies caused vessels to grow abnormally. Better methods are now available for monitoring oxygen. As a result, the problem has become less common, especially in developed countries. However, there is still uncertainty about the right level of oxygen for premature babies at different ages. Researchers are studying other factors besides oxygen which appear to influence the risk of ROP.
Today, the risk of developing ROP depends on the degree of prematurity. Smaller babies with more medical problems are at higher risk.
Almost all babies who are born before 30 weeks or weigh less than 3 pounds (1500 grams or 1.5 kilograms) at birth are screened for the condition. Some high-risk babies who weigh 3 to 4.5 pounds (1.5 to 2 kilograms) or who are born after 30 weeks should also be screened.
In addition to prematurity, other risk factors may include:
- Brief stop in breathing (apnea)
- Heart disease
- High carbon dioxide (CO2) in the blood
- Low blood acidity (pH)
- Low blood oxygen
- Respiratory distress
- Slow heart rate (bradycardia)
The rate of ROP in most premature infants has gone down greatly in developed countries over the past few decades due to better care in the neonatal intensive care unit (NICU). However, more babies born very early are now able to survive, and these very premature infants are at the highest risk for ROP.