In retinopathy of prematurity, clock hours refer to which aspect of disease description?

• A. Stages
• B. Zones
• C. Levels
• D. Clock hours

Answer: (D)

Clock hours quantify the circumferential extent of retinopathy around the retina. In describing retinopathy of prematurity, clinicians separate location (zones), severity (stages), and extent around the retina (clock hours). The clock hours tell you how much of the retinal circumference is involved—for example, disease spanning several clock hours means a larger arc of retina is affected. This is different from depth or severity (that’s the stage) and different from where the disease sits on the retina (that’s the zone).