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Retinopathy of Prematurity (ROP)

This note is intended to help people without medical training, primarily parents of premature babies, understand ROP.

First, a little on how the eye is put together. The front of the eye has the small black circular opening called the pupil, which is where the light we see gets into the eye. Right around the pupil is the iris, the part of the eye that can be blue, brown, or whatever one’s "eye color" is. The eyeball is indeed a ball, most of which is hollow and filled with a clear jelly-like substance. The most of the inside surface of the eyeball is the retina. The retina’s job is to take the light, (which has come in the pupil and passed through the clear "jelly"), and change the information in that light into electrical information, much like a television camera does. This electrical information goes back to the brain for processing via the optic nerve. The optic nerve goes out the back of the eyeball and functions as a wire carrying the electrical information.

ROP is a disease of the blood vessels that supply blood to the retina. In a term baby, the vessels start at the back of the eye on the inside surface of the eyeball, where the optic nerve leaves the eye. These blood vessels go along the inside surface of the eyeball until they end, as the retina ends, at the outside edge of the iris. When babies are born prematurely, those blood vessels have not yet grown all the way to the iris and, for reasons not well understood, they grow abnormally after the baby is born.

To help picture these blood vessels, imagine a globe. The pupil is the north pole, and the arctic circle is the iris. The blood vessels start at the south pole, and go to the arctic circle like lines of longitude on the globe. If born prematurely, the vessels have only made it part of the way to their goal, and might end at, say, the equator. Remember, however, that these vessels are on the inside surface, not the outside surface, of the eyeball.

Years ago, when premies as small as the tiny ones that get ROP today did not survive, premies could get ROP and go blind from getting too much oxygen. It is very clear that there is much more to the ROP of today than too much oxygen. (If only it was that simple!) I understand that it was that "old style" ROP that blinded Stevie Wonder, the musician.

The stages of ROP, numbered 1 to 5, describe what’s going on in there, which can only be seen by a fairly sophisticated eye exam. Even milder than stage 1 is "immature vasculature", which means the blood vessels have not reached the iris, but there is nothing visible blocking their path, either. Stage 1 means there is a flat but visible line blocking the forward progress of the vessels toward the iris. In stage 2, the line has become a ridge that grows off the surface of the retina. In stage 3, the ridge has abnormal blood vessels growing off the top of that ridge into the clear jelly that fills the eyeball. These abnormal blood vessels can pull the retina off the inside surface of the eyeball, a retinal detachment. Stage 4 is partial detachment, while stage 5 is total detachment. Once detachment starts, especially if the part of the retina called the macula (the part of the retina that "sees" what we are looking directly at) is involved, the problems are vastly greater, and blindness becomes more likely.

The eye exam will also show how far the blood vessels have progressed in their journey toward the iris. ROP in zone 3 is very far forward in the eye, and ROP there is much less threatening. Zone 1 is way in the back, and ROP there is much more worrisome. Most ROP is in zone 2, which is between zones 1 and 3 in both position and severity. The eye exam may also show what is called "plus" disease, which is when the retinal blood vessels are abnormally dilated. Plus disease is also a sign of more serious disease.

This disease often becomes visible at about 6 weeks of age, most commonly in the smallest premies. The first eye exam is usually done at about 6 weeks of age, because that is the earliest age at which babies have needed treatment for the disease. ROP tends to hit its peak around the due date, and fade thereafter. If that peak gets far enough into stage 3, (I’ll skip the details here), and reaches a point called "threshold", retinal detachment becomes relatively likely if nothing is done, but surgery has a good chance of stopping the disease before the retina detaches. The window of time between hitting threshold and being too late can be short, which is why babies nearing threshold (or "pre-threshold") get eye exams so often, typically every few days. In fact, the time until the next eye exam (decided upon by the eye doctor) is a good indication of how worrisome the exam was. If the next exam is not for two weeks or more, the exam looked reasonably good. If the next exam is in one week, things are beginning to get scary. If the next exam is in just a few days, the need for surgery is a very real possibility. This is a "short cut" way to know how good or bad the exam was, without having to understand all the details of what the eye doctor saw.

There are two forms of surgery, cryo (which kills tissue by freezing it) and laser (which kills tissue by burning it), that basically do the same thing. The idea is to knock out the retina in front of the normal blood vessel’s current progress, because that is where the chemicals that stimulate the growth of the abnormal vessels originate. Cryo was thoroughly tested in a large study in the 1980s, and is the "old" standard therapy. Laser is not as well tested, but presumably works just as well, and has some advantages. Laser is easier to use farther back in the eye, leaves less eyelid swelling in the days after surgery, and can be done in the NICU in some centers. Cryo has more experience behind it, and is easier to use farther forward in the eye. Neither of these surgeries involves any cutting.

Even when the ROP peaks at a mild stage and goes away, longer term problems can still happen. The main worries are the need for glasses (nearsightedness, mostly) and strabismus (where one eye looks at something, but the other eye wanders to the left or right), both of which are relatively treatable. Sometimes glasses will help somewhat with vision, but will not fully correct a child’s vision. In the grand scheme of things, these are fairly minor problems, but they are still a headache.

Nearsightedness and strabismus are more likely in babies that need laser or cryosurgery, and the visual problems can be more severe. Blindness is pretty rare these days, but can still happen. Late retinal detachments can occur years later, with the growth spurt of adolescence and its accompanying eye growth being the time of greatest risk for late detachment. A return visit to the eye doctor at the beginning of the teenage growth spurt is a good idea, especially if surgery was required as a baby. Any eye symptoms in any person who once had ROP should prompt a quick call to the eye doctor. It is not known if ROP will cause any issues as these kids become older adults.

I think the worst thing about ROP is that, if it does get to a scary point, it does so right about the time parents are getting ready to take their baby home, when they’ve already been through so much and are finally starting to relax a little bit.