2.9. Timing of laser treatment for ROP

ROP is a model of acute retinal ischemia that is self-limiting, but if not addressed urgently, it may lead rapidly to retinal detachment and loss of vision. Because the window of opportunity during which laser treatment is likely to have a positive result is very short, timely recognition is crucial for a good outcome [7, 8]. Therefore, active screening should be carried out according to very precise criteria [3].

Basically, treatment should be performed at the point in the natural history of the disease when neovascularization can be reversed because once vitreo-retinal traction is initiated, the disease can no longer be controlled [2]. Therefore, knowing the correlation between the evolution of ROP and time is crucial for a correct screening.

The most vulnerable period, when ROP is at high risk of rapid progression starts at 34–35 weeks PCA up to 36–37 weeks PCA [8]. Usually, ROP does not develop during the first 2 weeks of life. The median age for the detection of stage 1 ROP is 34 weeks PCA, pre-threshold ROP appears around 36 weeks PCA and threshold ROP at about 37 weeks PCA. The vascularization of the retina is completed by 40–44 weeks PCA.

In conclusion, the interval for ROP detection is between 32 and 40 weeks PCA, but the critical phase, during which ROP may progress rapidly and has to be treated, is between 34 and 37 weeks PCA [7].
