There are five stages of ROP from mild (stage I) to severe (stage V).
Stage I: There are very few, abnormal blood vessels. Many babies who develop stage 1 improve with no intervention and ultimately achieve normal vision. At this stage, ROP resolves on its own with no progression.
Stage II: The abnormal growth of blood vessels is more severe as compared to stage I; however, many premature infants who develop stage II still manage to improve without any treatment and are able to develop normal vision without any progression of ROP.
Stage III: The abnormal blood vessel growth can now be classified as severe. These abnormal vessels begin to grow and spread toward the center of the eye instead of following their normal pattern along the surface of the retina. Some babies with stage III ROP improve with no intervention and are able to acquire normal vision. However, if ROP worsens and the retinal vessels become enlarged and distorted, treatment for the condition becomes a consideration, especially since prevention of retinal detachment is a priority.
Stage IV: During this stage, the retina is partly detached due to the expansion of the abnormal leaky blood vessels, and consequent scarring and traction of the retinal surface.
Stage V: This is the most severe stage. At this endpoint in the disease, the retina is completely detached and immediate surgery to avoid permanent blindness becomes a priority.
Many babies do not go past Stage I or Stage II of ROP, but others who progress through Stages III to V require treatment in order to prevent or reduce the growth of abnormal blood vessels, and consequently to reduce the risk of subsequent retinal detachments.