There are three parameters to the classification system known as the International Classification of Active Retinopathy of Prematurity (ICROP): location of the disease (zone), extent of the disease (clock hours), and severity of abnormal vascular response (stage). Screening for active retinopathy of prematurity should begin after 4 weeks but before 6 weeks following birth. The critical time for screening is from 32 to 42 weeks postconceptual age. Follow-up examinations vary depending on the clinical findings.
Figure 2.95. The location (zone) and extent (clock hours) of the disease must be drawn on a form similar to this.
Figure 2.96. The volume (stage) of abnormal vascular response (ridge with or without extravascular fibrovas-cular proliferation) must be indicated for each clock hour. The location of this volume and vascular engorgement of the posterior inner retinal vessels determine when surgical intervention must be initiated.
Figure 2.97. This figure depicts active retinopadiy of prematurity: stage 1 (demarcation line).
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