variations in blood pressure, sepsis, and acidosis, may injure the endothelia (the cells that line) of the immature retinal blood vessels. The retina then enters a quiescent phase for days to weeks and forms a pathognomonic ridge-like structure of mesenchymal cells between the vascularized and the avascular regions of the retina by 33 to 34 weeks of postmenstrual age. In some infants, this ridge regresses, and the remaining retina is vascularized. In other infants, abnormal blood vessels proliferate from this ridge; and progressive disease can cause exudation, hemorrhage, and fibrosis, with subsequent scarring or retinal detachment (i.e., the retina is pulled off the back of the eye). The presence of plus disease, in which dilated and tortuous blood vessels occur in the posterior pole of the eye, is especially ominous for an adverse visual outcome.

ROP occurs in 16 to 84 percent of infants born with gestational ages of less than 28 weeks, 90 percent of infants with birth weights of less than 500 or 750 grams, and 42 to 47 percent of infants with birth weights of less than 1,000 or 1,500 grams (CRPCG, 1988, 1994; Fledelius and Greisen, 1993; Gibson et al., 1990; Gilbert et al., 1996; Lee et al., 2000; Lefebvre et al., 1996; Lucey et al., 2004; Mikkola et al., 2005; Repka, 2002). Fortunately, severe ROP requiring therapy is less common, occurring in 14 to 40 percent of infants with gestational ages of less than 26 weeks, 10 percent of infants with gestational ages of less than 28 weeks, 16 percent of infants with birth weights of less than 750 grams, and 2 to 11 percent of infants with birth weights of less than 1,000 or 1,500 grams (Coats et al., 2000; Costeloe et al., 2000; Hintz et al., 2005; Ho and Saigal, 2005; Lee et al., 2000; Mikkola et al., 2005; Palmer et al., 1991). ROP resolves without significant visual loss in the majority (80 percent) of infants (CRPCG, 1988; O’Connor et al., 2002). Repka and colleagues (2000) found that involution occurred in 90 percent of infants with ROP by 44 weeks of postmenstrual age.

Treatments have improved the visual outcomes for children with severe ROP (i.e., threshold or plus disease, stages 3 and 4). The ablation of abnormal peripheral vessels with cryotherapy (in earlier studies) and laser therapy (in the last decade) have led to favorable visual outcomes in at least 75 percent of infants with severe ROP (CRPCG, 1988, 1994; Repka, 2002; Shalev et al., 2001; Vander et al., 1997). Continuing improvements in treatments and more timely treatments of severe ROP have served to reduce the proportion of children with severe visual impairment or blindness from 3 to 7 percent down to 1.1 percent in children with birth weights of less than 1,000 or 1,500 grams (Doyle et al., 2005; Hintz et al., 2005; Tudehope et al., 1995; Wilson-Costello et al., 2005) (see Chapter 11). Severe visual impairment or blindness occurs in 0.4 percent of children with gestational ages of 27 to 32 weeks, 1 to 2 percent of children with gestational ages of less than 26 or 27 weeks, 4 percent of children with gestational ages of 24