Advances in perinatal medicine have led to increased survival of extremely low birth weight infants and an increase in the incidence of retinopathy of prematurity (ROP). This is a vascular proliferative disorder of the immature retina in premature infants that can result in impairment of vision and a high (> 47%) chance of blindness if untreated (threshold ROP). Normally the retina has a complete system of blood vessels by 40 weeks. Injury to the developing retinal capillaries occurring before or during birth and in the days following delivery stimulates new vascularization, which is the observable retinopathy. This can result in successful revascularization of the retina (regression of the ROP) or progression to neovascular membranes in the vitreous, subsequent scarring and retinal detachment. The incidence and severity of retinopathy is inversely related to gestational age. Treatment involves removal (ablation) of the part of the retina without blood vessels to preserve central macula vision. Cryoablation and laser techniques are used. The review authors identified two randomised trials involving 291 and 28 premature infants with threshold ROP, reported in the 1980s. Both used cryotherapy for peripheral retinal ablation, reducing the incidence of adverse ophthalmic outcomes. Unfavorable retinal structure at less than 12 months and in early childhood at 5½ years (234 infants) was reduced by some 18 to 20%. Therefore, the number of eyes with threshold ROP needed to treat (NNT) to avoid one unfavorable outcome is around five. The risk of poor visual acuity in early childhood was reduced from 63 to 51% (NNT 8).
The advantage of retinal ablation in these eyes outweighs short term morbidity associated with the therapy. This includes an increase incidence of apnoea and bradycardia both during the procedure and in the following one to three days. The visual fields in sighted eyes were slightly smaller in the group receiving cryoablation as compared to the control group. It is important to note that compared to normal, eyes with ROP that spontaneously recovered also had a reduction in visual field. At this stage, long term outcomes remain unknown.