Modeling absolute zone size in retinopathy of prematurity in relation to axial length

Abstract

Treatment outcomes in retinopathy of prematurity (ROP) are closely correlated with the location (i.e. zone) of disease, with more posterior zones having poorer outcomes. The most posterior zone, Zone I, is defined as a circle centered on the optic nerve with radius twice the distance from nerve to fovea, or subtending an angle of 30 degrees. Because the eye enlarges and undergoes refractive changes during the period of ROP screening, the absolute area of Zone I according to these definitions may likewise change. It is possible that these differences may confound accurate assessment of risk in patients with ROP. In this study, we estimated the area of Zone I in relation to different ocular parameters to determine how variability in the size and refractive power of the eye may affect zoning. Using Gaussian optics, a model was constructed to calculate the absolute area of Zone I as a function of corneal power, anterior chamber depth, lens power, lens thickness, and axial length (AL), with Zone I defined as a circle with radius set by a 30-degree visual angle. Our model predicted Zone I area to be most sensitive to changes in AL; for example, an increase of AL from 14.20 to 16.58 mm at postmenstrual age 32 weeks was calculated to expand the area of Zone I by up to 72%. These findings motivate several hypotheses which upon future testing may help optimize treatment decisions for ROP.

Figure 1

Neonatal eye development in ROP may affect zoning. (a) Schematic of a growing eye with the nodal point and nodal angle denoted by n and θ, respectively. As the eye becomes larger and undergoes refractive changes, the area of the region (magenta) encompassed by θ may likewise change. (b) Schematic of the retina in a growing eye. Although both regions in beige are defined as Zone I, the absolute areas corresponding to Zone I might differ substantially.

Figure 2

Zone I area is most sensitive to changes in axial length. (a) Ocular parameters used to estimate the absolute area of ICROP Zone I. After determining the position of the nodal point (n) based on P_cornea, P_lens, ACD, and LT, the radius (magenta) of the circle corresponding to Zone I was calculated as tan(30°) times the distance from the nodal point to the back of the eye. For further details, see Methods. (b–f) Range of estimated Zone I areas at PMA 31 weeks when modifying only corneal power (b), anterior chamber depth (c), lens power (d), lens thickness (e), or axial length (f).

Figure 3

Zone I area increases in relation to axial length. (a) Range of estimated Zone I areas at indicated ages when using the lowest (gray) and highest (magenta) AL values reported by Cook et al.. (b) Depiction of a 72% increase in retina and Zone I areas at PMA 32 weeks based on model predictions following a change in AL from 14.20 (solid lines) to 16.58 (dashed lines) mm.

Figure 4

Postmenstrual age can approximate axial length to model zone area. (a) Range of estimated AL values at different PMAs. (b) Range of estimated ICROP Zone I areas at different PMAs. (c) Depiction of a 60% increase in retina and Zone I areas from PMA 31 (solid lines) to 38 (dashed lines) weeks based on model predictions.

Figure 5

Neonatal eye size may affect zone designations. Schematic illustrating the hypothesis that the same absolute area of ROP involvement could be designated as Zone II in one eye but as Zone I in a larger eye. Zones are indicated by Roman numerals, optic disc in yellow, and macula in gray.