Fig. 2.1
ICROP exam form: standard form for documenting Zone and stage as well as presence of Plus disease
ZONE 1: Those children with the most posterior ROP where the normal retinal vessels had very little growth were termed Zone 1. Patients with Zone 1 had their retinal vessels extended from the optic nerve out to the periphery but not beyond the macula. To assist the examining ophthalmologist in determining whether a patient was in Zone 1, the study defined it as a circle centered at the optic nerve that had a radius twice the distance from the optic nerve to the fovea (Fig. 2.2a). To qualify as being in Zone 1, the examiner only needed to identify one clock hour where the retinal vessels stopped within the region. For a quick way to determine Zone 1, an examiner using a 28 diopter lens can place the edge of the lens to just encompass the optic nerve on one side of the image field. The other side of the lens would then be the edge of the Zone 1 (Fig. 2.2b). This technique is an easy way for the examiner to determine Zone 1 as long as the optic nerve is in view with a 28 diopter lens.
Fig. 2.2
a Example of Zone 1 ROP using a RetCam image. The white dotted line marks the edge of Zone 1. The white arrows show areas where the vascularized retina stops short of the Zone 1 border. b Example of Zone 1 ROP using a digital indirect ophthalmoscope and a 28 diopter lens where the optic nerve and the end of the vascularized retina (white arrow) can be seen in the same field of view
ZONE 2: This region is defined as retinal vessels that have extended past Zone 1 but remain within a larger circle that has a radius extending from the optic nerve to the nasal ora serrate (Fig. 2.3). From a practical standpoint, this would be an exam where the patient is not in Zone 1 but the retinal vessels do not extend all the way to the nasal ora serrata. Although the retinal vascular development usually has equal growth from the optic nerve in all directions, there are times that it can be asymmetric with the vessels extending all the way to the nasal ora serrata but falling temporally into Zone 2 in which case the patient would still be classified as Zone 2.
Fig. 2.3
Example of Zone 2 stage 2 ROP using a RetCam image. The vascularized retina extends beyond the border of Zone 1 (white arrow)
ZONE 3: This region is defined as the remaining retina beyond Zone 2 and mostly represents the most peripheral temporal retina. This area can quickly identify when the retinal is fully vascularized nasally and the temporal aspect requires depression to visualize adequately.
Stage
Once the Zone has been identified, the next step is to determine the level of ROP at the interface between the vascular and avascular retina. Fundamentally the stage represents various levels of evolution of the pathologic angiogenesis. The neovascularization typically occurs at the edge of the vascularized retina. This neovascularization grows in a disorganized fashion that starts off as a white flat demarcation line. This is the very early phase of the process. As the pathologic neovascularization progresses, the vessels begin to accumulate at this location leading to an elevation/bump (some refer to the appearance of a “speed bump”). As the process continues the neovascularization begins to spill into the vitreous. This is the tipping point in the disease since the next phase is to recruit scar tissue along with the now vitreous involved vessels that can lead to a tractional detachment and blindness. To describe this, the CRYO-ROP study defined the different stages as follows:
Immature retina: when the retinal vessels stop and there is no visible demarcation line. This can be a challenge to identify because in the early exams there are often no clear landmarks to look for. Some people informally refer to this as stage 0 although this is not a terminology used by ICROP.
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- Stage 1:
The retinal vessels stop and then a linear flat white line is present that usually runs the circumference of the vascular retina.
- Stage 2:
The neovascularization is now accumulating and has developed thickness that manifests as a linear bump (speed bump). Importantly, the neovascularization remains along the surface of the retina and is not beginning to extend off the retina into the cortical vitreous (Fig. 2.3).
- Stage 3:
The neovascularization has now been accumulated at the edge of the vascularized retina so that it is now extending into the vitreous. ICROP has called this extra retinal fibrosis proliferation. In cases of Zone 2 and Zone 3, this will at times be described as a sausage shaped stage 3. Unlike the white/pink color of stage 1 and 2, stage 3 will often have a red appearance consistent with the increased blood being shunted through the accumulating stage 3 (Fig. 2.4a, b). In more posterior Zone 1 disease, the stage 3 can appear as a direct extension of the normal retinal vessels but extending tangentially over the avascular retina. This is in contrast to the typical stage 3, which has the sausage-shaped appearance. Some refer to this as flat neovascularization [3].
