Objective: To investigate the natural history of retinopathy of prematurity (ROP) in 506 extremely preterm infants born before 27 weeks' gestation in Sweden during a 3-year period.
Methods: A national population-based study was performed in Sweden from April 1, 2004, to March 31, 2007. According to the study protocol, initial eye examinations were to be performed at postnatal week 5, and examinations were repeated until the retina was completely vascularized or until criteria for treatment were met. The examinations were to be performed weekly, enabling study of the course and severity of ROP. In infants without ROP or with mild ROP without progression during the latest examinations, further examinations were performed weekly or every other week from postmenstrual age 35 weeks.
Results: During the study, 368 infants (72.7%) developed ROP. Postmenstrual age at onset of ROP was significantly related to severity of ROP, even when controlling for gestational age (ie, the earlier the onset of ROP, the higher the risk of developing severe ROP). Site of onset of ROP was significantly related to gestational age at birth. The risk of nasal onset was almost doubled for every week of decrease in gestational age at birth. Nasal onset was associated with severe ROP, even after adjusting for gestational age at birth.
Conclusion: This population-based study confirms results of the Cryotherapy for Retinopathy of Prematurity study and shows new correlations regarding time and site of onset of ROP, both of which are associated with disease severity.