Refractive development II: Modelling normal and myopic eye growth.

PURPOSE: During refractive development, eye growth is controlled by a combination of genetically pre-programmed processes and retinal feedback to minimise the refractive error. This work presents a basic differential model of how this process may take place.

METHODS

The description starts from two bi-exponential descriptions of the axial power Pax (or dioptric distance) and total refractive power Peye, the difference between which corresponds with the spherical refractive error S. This description is rewritten as an ordinary differential equation and supplemented by a retinal feedback function that combines retinal blur (closed loop) with a term describing excessive axial growth (open loop). This model is controlled by a total of 18 parameters that allow for a wide variety of developmental behaviours.

RESULTS

The proposed model reproduces refractive development growth curves found in the literature for both healthy and myopic eyes. An early onset of myopisation, a large growth term and a high minimum for the crystalline lens power all lead to higher degrees of myopia. Assigning more importance to the feedback than to the pre-programmed growth makes the model more sensitive to myopogenic influences. Applying refractive corrections to the model, undercorrection is found to produce more myopia. The model compensates for a low-powered imposed lens and can return to (near) emmetropia if that imposed lens is removed quickly thereafter. Finally, simulating the effect of a diffuser leads to high myopia.

CONCLUSION

Using a series of basic assumptions, the proposed model recreates many well-known experimental and clinical results about refractive development from the literature while placing them in a standardised context. This contributes to a broader understanding of the origins of refractive errors, and future versions may help in the development of solutions for myopia control.