What feature defines ROP stage I?

ROP stage I is characterized by the presence of a demarcation line between the vascular and avascular retina, indicating the beginning of retinal zone differentiation in infants at risk for retinopathy of prematurity. This stage represents an early, mild form of ROP where there is no significant abnormality beyond the line that separates the abnormal area from the more normal vascularized area of the retina.

Understanding the significance of this demarcation line is crucial for monitoring the progress of the disease and deciding on potential interventions. It signifies that there is a distinction between areas of the retina that have developed normally and areas that are undergoing abnormal development due to the effects of oxygen exposure and gestational age-related factors.

In contrast, other features like new vessel formation, ridge elevation, or fibrovascular proliferation are associated with more advanced stages of ROP, indicating greater severity and indicating a need for closer management and potential intervention.