The classification of acute ROP according to the International Classification Scheme [9, 28] is given in Table 5.1 and in Figs. 5.1 and 5.2. The classification comprises three parameters: (1) the location, i.e., zone of the disease in the retina, (2) the extent by clock hours of the developing vasculature involved, and (3) the severity, i.e., stage of abnormal vascular response observed. Zone I is a posterior circle centered on the optic disc, and the radius is twice the distance from the disc to the center of the macula. The zone is defined by the most posterior location of disease. If, therefore, any ROP is found in zone I the eye is a zone I eye. A circle centered on the disc with a radius equal to the distance to the nasal ora serrata defines the boundary between zones II and III. Zone III comprises the remaining temporal crescent. As a general rule, the more posterior the disease, the more aggressive. An example is given in Fig. 5.3. Recently, it has been shown that the border of vasculariza-tion may not lie within a circle centered around the optic disc. In fact, data analysis from wide-angle images indicate that the distance to the nasal periphery may be shorter than that to the temporal periphery [23]. This may have implications for future classification schemes.

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