Retinopathy of Prematurity - International Classification of Retinopathy of Prematurity (ICROP)

International Classification of Retinopathy of Prematurity (ICROP)

The system used for describing the findings of active ROP is entitled The International Classification of Retinopathy of Prematurity (ICROP). ICROP uses a number of parameters to describe the disease. They are location of the disease into zones (1, 2, and 3), the circumferential extent of the disease based on the clock hours (1-12), the severity of the disease (stage 1-5) and the presence or absence of "Plus Disease". Each aspect of the classification has a technical definition. This classification was used for the major clinical trials. It was revised in 2005.

The zones are centered on the optic nerve. Zone 1 is the posterior zone of the retina, defined as the circle with a radius extending from the optic nerve to double the distance to the macula. Zone 2 is an annulus with the inner border defined by zone 1 and the outer border defined by the radius defined as the distance from the optic nerve to the nasal ora serrata. Zone 3 is the residual temporal crescent of the retina.

The circumferential extent of the disease is described in segments as if the top of the eye were 12 on the face of a clock. For example one might report that there is stage 1 disease for 3 clock hours from 4 to 7 o'clock. (The extent is a bit less important since the treatment indications from the Early Treatment for ROP)

The Stages describe the ophthalmoscopic findings at the junction between the vascularized and avascular retina.

• Stage 1 is a faint demarcation line.
• Stage 2 is an elevated ridge.
• Stage 3 is extraretinal fibrovascular tissue.
• Stage 4 is sub-total retinal detachment.
• Stage 5 is total retinal detachment.

In addition, Plus disease may be present at any stage. It describes a significant level of vascular dilation and tortuosity observed at the posterior retinal vessels. This reflects the increase of blood flow through the retina.