At age 5 1/2 years, the oldest age for which follow-up data are available, children with threshold ROP who were enrolled in the Cryotherapy for Retinopathy of Prematurity (CRYO-ROP) -- Outcome Study showed fewer treated eyes (31.5 percent) than control eyes (48 percent) that were blind (P<0.001). Of those eyes that had a favorable structural outcome, with or without retinal ablation (cryotherapy to destroy the fringe of the retina through freezing), only a small percentage had best corrected visual acuity better than or equal to 20/40 at age 5 1/2 years (13 percent in the treated group; 17 percent in the untreated control group (P=0.19)). Among the 1398 followed from the 5 large natural history centers of the CRYO-ROP follow-up study, children with retinal residua of ROP (structural changes) had measurable visual acuity that was severly affected and tended to worsen with age. The CRYO-ROP Study proved conclusively that peripheral retinal ablation improves the chances of avoiding blindness, but at least 80 percent of eyes are left with acuity less than 20/40.

Two concerns emerged from the CRYO-ROP extensive study on the natural history of ROP and treatment of threshold ROP. The first of these is failure of peripheral retinal ablation to eliminate all, or nearly all cases, of retinal detachment due to ROP. In the CRYO-ROP Study, 26 percent of eyes with threshold disease in zone II and 78 percent of eyes with zone I threshold disease had an unfavorable structural outcome despite treatment. The second concern is that most children who developed threshold ROP disease had visual acuity worse than 20/40 even if the eye had a favorable structural outcome.

Since no other treatment has yet been shown to be effective in preventing blindness from ROP, retinal ablation remains the treatment of choice. The ETROP Study will test whether earlier treatment is more effective than treatment at threshold in improving functional (visual acuity) outcome following ROP, as well as determining whether earlier treatment decreases the probability of an unfavorable structural outcome.

Earlier treatment is defined as retinal ablation administered to the avascular retina when an eye reaches high risk prethreshold retinopathy of prematurity (ROP). Prethreshold indicates any Zone I ROP; or Zone II stage 2 with plus disease, or stage 3; or Zone II with less than 5 contiguous or 8 cumulative clock hours of stage 3 ROP with plus disease. Recognizing that a substantial number of eyes undergo spontaneous resolution of ROP, eyes will be randomized to early treatment only when high risk for an unfavorable visual acuity outcome is identified. High risk will be determined using a risk model analysis program based on longitudinal natural history data obtained from the CRYO-ROP study. This model integrates risk factors to assign a risk of progression to blindness without treatment. These factors include birth weight, gestational age, ethnicity, singleton/multiple status, outborn status, Zone on first exam, severity of ROP and rate of progression of ROP. When an infant develops prethreshold ROP and greater than or equal to 15 percent risk of unfavorable outcome, randomization to early treatment of one eye will occur. Visual acuity outcome will be measured by masked observers after wearing best correction using the Teller Acuity Card Procedure at 9 months corrected age.