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Preemies get more retinal irradiance
than safety guidelines allow for adults
This article was published in the peer-reviewed clinical journal Iatrogenics (The Official Journal of the International Society for the Prevention of Iatrogenic Complications--ISPIC) Volume 1, Issue 2, dated April-June 1991: 2: 68-85 but distributed in Summer 1992.
Incidence of the retinal damage
A couple of years after the commercial introduction of fluorescent lamps at the World Fairs of San Francisco in 1938/39 and New York in 1939/40, premature babies in intensive care nurseries across the U.S. began to develop a previously unknown disease which damages and often detaches retinae. This eye disease became first known as retrolental fibroplasia (RLF), "a fibrous mass formed behind the lens"; its current clinical label is retinopathy of prematurity (ROP) which simply means "preemies' disease of the retina".
This new eye disease remained confined to the U.S. until a few years after World War II, when it began to appear in most other industrial countries with such devastating effect that ophthalmologists regarded ROP as the principal cause of blindness in infants. It is estimated that by 1953 ROP had blinded about 10,000 children, 7000 of these in the U.S. (1).
Today, ROP affects the eyes of thousands of preemies each year and severely impairs the vision of many among them. A team of researchers at the Baylor College of Medicine in Houston, Texas, estimated in 1984 that, out of about 20,000 newborns most at risk for contracting the disease -- those weighing less than 1500 grams at birth who survive each year in the U.S. -- ca. 14000, or 7 out of 10, develop some degree of ROP (2). The percentage is lower among the preemies between 1500 and 2500 but since about 220,000 preemies are born into that category each year (3), the number of ROP cases among them could also be high. In sum, every preemie in an intensive care nursery is at risk from ROP. Occasionally, even a baby born at term who is kept at the hospital may develop ROP (4).
For many of these children the disease resolves spontaneously and the damage it causes to their eyes is not called major. However, even temporary ROP often leaves permanent impairments. At the age of 1 yr, the incidence of strong myopia, strabismus, and other vision problems is greater among preemies who had low-grade ROP in the nursery than among those who did not. Other early complications of arrested mild ROP include vitreoretinal adhesions, dragged retinal vessels, displacement of the macula, permanent double vision, and focusing differences between the two eyes. By the time they are 4 yr old, ROP children are also likely to suffer from peripheral retinal degenerations, splitting of the retinal nerve layer, tears and holes in the retina, vitreous detachments, and late retinal detachments (5).
Despite the long history and severity of ROP, there are still no statistical records documenting the damage it does. The most recent estimate, available in the clinical literature was published in 1981 for the number of cases in the U.S. in the base year 1979 (6). Like the Baylor estimate, it covers only preemies with birth weights of less than 1500 g.
Dr. Dale L. Phelps, the author of the 1981 estimate, suggested that vision impairments from ROP were about as prevalent at the time of the study as during the early 1950s, which had previously been regarded as the peak of the ROP epidemic. She extrapolated ROP-blindness rates among premature infants with birth weights up to 1000 g and from 1000 to 1500 g from several individual hospital reports published in the 1970s. Then she compared these extrapolations with extrapolations about the annual number of preemies in these birthweight categories. Her estimating method yielded a yearly total of 36,600 preemies below 1500 g. Comparing her number with the yearly 20,000 preemies below 1500 in the Baylor estimate gives an idea of how vague such estimates can be. Based on her extrapolations, Phelps calculated that, in 1979 in the U.S., anywhere from 397 to 883 low-birthweight infants had been blinded by ROP, with the most probable number being around 546. She further estimated that in the same year about 2100 additional children in the less than 1500 g group may have suffered severe visual impairments such as myopia, strabismus, and possible late retinal detachment from their brush with ROP.
Some hospital reports published after Phelps' estimate suggest a trend towards an increase in the incidence of ROP. For instance, a team at the Vanderbilt University School of Medicine in Nashille, Tennessee, compared the incidence of ROP among babies born in their hospital in 1972 and in 1981/82 and documented greater numbers of children with retinopathy of prematurity (7). A retrospective review at the University of Washington School of Medicine in Seattle showed that ROP had blinded more than three times as many babies a year in the early 1980s than throughout the 1970s (8).
The report attributed this to the higher survival rate of the preemies in the lowest birth-weight categories. It also said that the severity of the individual blindings had increased in the more recent period.
In other countries, the incidence of eye damage from ROP seems to be similar:
In these countries, as in the U.S.A. there is no organized reporting system for the collection of epidemiologic information about ROP. Thus, we lack adequately detailed incidence information and its correlation with possible environmental factors or other agents.
Description of the disease
Retrolental fibroplasia, the outward symptom after which the disease had first been named, appears in the late stages of severe ROP; it is a white-to-grayish opaque membrane that covers the inside of the lens. This membrane is formed by pieces of the retina, flimsy like wet tissue paper, which detach themselves from the inner wall of the eyeball due to an abnormal growth of the retinal blood vessels.
The signs of this abnormal growth typically can be observed in the eyes of the afflicted babies about 5 to 8 wk after birth, depending on the observer and method of observation. The timing of this growth does not depend on the gestational age of the baby but only on age since birth (13). The condition is therefore not due to an abnormality of fetal development, but is clearly related to some factor to which the retinal blood vessels of the preemies are exposed at, or shortly after, birth.
The retinal blood vessels grow relatively late in fetal development and are not fully formed in the eyes of babies who are born before they weigh at least about 2000 g (14), or before they reach a (gestational age of about 8 months (15). Until these vessels are developed, the retina is nourished by the blood flowing through the choroid, the layer right underneath the retina that already has blood vessels.
As the retina gets thicker, oxygen and nutrients can no longer diffuse as easily and the retina needs its own supply of blood. The small capillary vessels destined to fill this need begin their growth at the optic disc where the optic nerve enters the back of the eye. They progress slowly through the retina towards the ora serrata, the zigzag margin at the front of the eye which separates the retina from the lens.
In babies suffering from ROP, the normal formation of these vessels is disturbed. Instead of continuing to grow smoothly inside the retinal layer, the vessels at the forefront of this advance constrict, stop growing, then resume growth in a tortuous fashion and in the wrong direction. They cluster and clump along the edge where they had stopped growing, and then they erupt through the surface of the retina to grow wildly into the vitreous body, the transparent contents of the eyeball.
In most of the cases, the vessels will resume their normal growth at some early stage of this process and leave only the above-listed damage to the retina. But if they crow too wildly, the tortuous vessels create tensions in the delicate retinal layer and are often strong enough to pull it away from the choroid. The resulting retinal detachment can be partial, in which case the baby may retain some limited vision, or it can be total and result in blindness.
Some limited success has been achieved in arresting the proliferation of the tortuous growth with a method called transscleral cryotherapy, which prevents the unwanted vessels from advancing. The surgeon touches the freezing tip of a cryoprobe to the outside wall of the eyeball, the sclera. The cold goes through the wall, destroying that area of the peripheral retina inside the eyeball which the proliferating blood vessels have not yet reached.
The intent of this intervention is to prevent these vessels from growing out of control. The operation is performing when the treating ophthalmologist estimates the chance for damage from further growth to be about the same as the chance for spontaneous self-healing (16).
In 1986, a trial was held in 23 U.S. hospital centers to test this long-established intervention. The results 3 months after the operation showed that cryotherapy had reduced unfavorable outcomes from 43% in untreated eyes to 21.8% in the treated eyes (17). However, the intervention exposes the patients to the risks of an operation under anesthesia (risks which are much higher for preemies than for older people) and destroys a part of their retina which might have developed normally. The operation benefits up to about a quarter of the babies operated on, provided that no complications arise.
Very little follow-up data are available on how this partial destruction of the retina and the freezing of the underlying choroid vessels will affect that eye over the long term. Two groups of Japanese and Israeli investigators evaluated eyes that had been so treated 6 to 8 yr earlier and concluded that the treatment had caused no major problems. However, they had no controls for comparative analysis (18, 19).
Once the natural development of the retina has been disturbed, the risk of later retinal detachment and/or other complications remains high. Dale Phelps, one of the organizers of the multicenter trial, cautions that the treated eyes could deteriorate and lose vision 10 or 20 years later, while many untreated eyes could continue to do better (20).
Aiming to reattach some pieces of the preemies' retinal tissue, pediatric retinal surgeons work with a profusion of techniques: tightening a buckle around the eyeball to push it against the retina (21); surgical reattachment after entering the eyeball with tiny scalpels and needles and injection of air into the vitreous (23). Other techniques use laserbeams or cryoprobes inserted into the opened eye.
The success rates given in the individual reports range from 27% to 58% but come with an important qualifier: even if some part of the retina can be successfully reattached, the patient will usually retain some severe visual impairment. An anatomical success will typically only preserve vision from 6/60 or 6/120 to "ability to count fingers" and "ability to see hand movements" (24).
Even such partial success does not necessarily last. The handbook Retinal Detachment and Allied Diseases by Dr. Charles L. Schepens lists many postoperative complications as being common to all procedures used for reattaching the retina: edema around the eyelid and conjunctival disturbances, heterotropia (or strabismus), lagophthalmos (or inability to shut the eyes completely), corneal damage, ischemia (or bloodlessness) of the anterior segment, claucoma, cataract, choroidal detachment, massive preretinal retraction, delayed hemorrhage inside the eye, postoperative visual loss, sympathetic ophthalmia (or intlanunation of the other eye), and phthisis bulbi (or shrinkage of the eyeball) (25). In sum, the outcome of medical treatments for ROP remains uncertain and often poor.
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