2.2. ROP classification

Screening for ROP is essential for reducing the blindness rate related to this disease [3]. It should be made by indirect ophthalmoscopy by specially trained ophthalmologists according to criteria which are specific to each country/region.

In classifying ROP, the following criteria are used: zone, stage and the presence/absence of "plus" disease. The retina is divided into three zones and 12 clock hours. Zone 1 corresponds to a circle having the radius equal to the double distance between the optic disc and the fovea. Zone 2 describes a circle with the radius that equals the distance between the optic disc and the nasal ora serrata. Zone 3 is the remaining "crescent" of retina in the temporal area [3, 4].

Stages are defined according to the modifications at the limit between the vascularized and nonvascularized retina. Stage 0 reflects no modification at this level, but the retinal vascularization is not yet completed. In stage 1, a demarcation line (within the retinal plane) is identified at this site. In stage 2, there is a non-vascularized, white ridge (elevated from the retinal plane) at the limit between the two retinal regions, and in stage 3, the ridge is vascularized, red. Stage 4a corresponds to peripheral retinal detachment. In stage 4b, the fovea is also detached, and in stage 5, there is total retinal detachment in an open or closed funnel [3, 4].

"Plus" disease includes retinal vessel dilation and tortuosity, retinal hemorrhages and pupil rigidity [3, 4].

Separately from the features of the "classic" ROP described above, aggressive posterior ROP (AP-ROP) has been described with the following aspects: very severe "plus" disease, posterior location (most commonly zone 1, but also posterior zone 2), ill-defined or no ridge, rapid progression to stage 5 ROP [3, 4].

Also, there are two important terms that have to be defined: threshold and pre-threshold ROP. Threshold ROP includes: stage 3, zone 1 or 2, with "plus" disease, with a ridge extending on more than five contiguous or eight cumulative clock hours. Pre-threshold ROP includes the following situations: less than threshold ROP in zone 1/stage 2, zone 2 with "plus"/stage 3, zone 2, without "plus"/stage 3 with "plus," less extended than threshold ROP [3, 4].
