Retinopathy of prematurity (ROP) is a result of abnormal blood vessel growth in the eye. If a premature baby weighs less than 1500 g with a gestational age of below 30 weeks, they are routinely examined for ROP.
The retina of the eye is like the film in a camera. It lines the inside of the eye. At about 16 weeks of pregnancy, the retina begins to develop blood vessels. The blood vessels generally grow from areas of high oxygenation, where the vessels already exist, to areas of low oxygenation, where the vessels have not yet formed. In this way, the retina is gradually and evenly covered in blood vessels.
Fluctuation of oxygen levels in the developing retina is an important factor in the regulation of the growth of blood vessels. Moreover, if the baby is having other complications that affect the regular flow of oxygenated blood, this too can interfere with the normal growth and spreading of the blood vessels. If the blood vessels grow abnormally as a result, this is called retinopathy of prematurity (ROP).
Since development of retinal blood vessels is incomplete until about 35 to 37 weeks gestation, the retina is vulnerable to injury. Abnormal blood vessels can deliver either too much oxygen to the eye or too little. Hypoxemia, an insufficient oxygen supply in the blood, leads to local tissue damage called ischemia, a kind of scarring. Hyperoxemia, which is too much oxygen, reduces the normal stimulus for development of the blood vessels; this will later result in loss of some of the developing blood vessels and ultimately lead to ischemia.
How ROP is diagnosed
All premature babies who fall within a screening protocol defined as birth weight less than 1500 g and gestational age of below 30 weeks are routinely examined for ROP. These premature babies will likely be examined initially at four to six weeks after birth. An eye doctor will use drops to dilate the pupils, which will allow for a better view of the inside of the eye.
Depending on the amount of abnormal blood vessel development, the baby’s condition will be graded and further examinations will be conducted every one to two weeks, depending on a variety of factors. These factors include the severity and location in the eye of ROP, and the rate of progression of blood vessel formation, called vascularization. In the majority of cases, even when ROP develops, it will resolve spontaneously with minimal effect on vision. However, a small percentage of babies screened for ROP, approximately 10%, will progress to the extent that it is no longer safe to wait for spontaneous resolution. For these babies, treatment will be offered to reverse the progression of ROP.
Stages of ROP
The severity of ROP is assessed by the area of retina that is involved, also referred to as the zone of vascularization, and the stage of the disease. The lower the number for the zone, the greater the area of the retina affected. The stages, or extent of the growth of abnormal blood vessels, are defined as follows.
Stage 1 ROP
In Stage 1 ROP, there is a thin line between the area with blood vessels and the area where blood vessels have not grown yet. At this stage, the vessels may grow normally on their own, but the condition must be watched.
Stage 2 ROP
In Stage 2 ROP, the line between the areas with and without blood vessels widens and thickens into a ridge. At this stage, the condition may still resolve, however, it may also progress to stage 3 ROP.
Stage 3 ROP
In Stage 3 ROP, new blood vessels begin to grow along the ridge and extend into the clear gel that fills the eye, called the vitreous body. These blood vessels can bleed and form scar tissue.
Stage 4A ROP
In Stage 4 ROP, the abnormal blood vessels and scar tissue pull on the retina, partially detaching it. In stage 4A, the centre of vision, called the fovea, is not involved.
Stage 4B ROP
In Stage 4B ROP, the retina is still only partially detached, but the fovea is affected usually leaving both the centre and peripheral vision impaired to some degree.
Stage 5 ROP
In Stage 5 ROP, the retina is completely detached, severely affecting vision.