Objective: To examine the frequency and timing of progression from type 2 to type 1 retinopathy of prematurity (ROP) in the Early Treatment for Retinopathy of Prematurity Study. Methods: Infants with prethreshold ROP that was no worse than low risk in 1 or both eyes, based on the RMROP2 model, were examined every 2 to 4 days for at least 2 weeks. Using the Early Treatment for Retinopathy of Prematurity Study-defined classification of eyes as having type 1 or type 2 prethreshold ROP, we analyzed the time to conversion from type 2 to type 1. Data were analyzed for 1 randomly selected eye for each child. Results: Of 294 eyes at first diagnosis of type 2 ROP, 65 (22.1%) progressed to type 1 (mean [SD] interval, 9.0 [6.6] days; median, 7.0 days). Of 217 eyes with type 2 ROP that had an examination in less than 7 days, 25 (11.5%) were diagnosed with type 1 ROP in less than 7 days. Of 200 eyes that continued to have type 2 disease at the first follow-up examination and underwent a subsequent examination, 24 (15.7% of the 153 eyes that had an examination in <7 days) developed type 1 ROP in less than 7 days. The risk of progression from type 2 to type 1 in less than 7 days was greatest between 33 and 36 weeks' postmenstrual age, regardless of zone of retinopathy. Conclusions: Type 1 ROP can be identified with weekly examinations in most eyes with initial diagnosis of type 2 ROP; a small subset progresses to type 1 in less than 7 days. Trial Registration: clinicaltrials.gov Identifier: NCT00027222.
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