Abstract
Purpose: :
Recent myopia research has shown an increasing interest in the study of refraction in the periphery because peripheral hyperopia might be a risk factor for myopia development and progression. While the retinal shape is an obvious factor in determining peripheral refraction, anterior ocular components could make important contribution also . The purpose of this study is to model the effect on peripheral refraction from the corneal ashericity (Q), pupil size and anterior chamber depth (ACD) of the eye.
Methods: :
Ray-tracing was performed using a self-developed MatLab program on a model eye (Navarro et al. 1985) (with retinal Q of 0.26) to calculate Zernike aberrations up to 5th order at various eccentricities between -40 to 40 deg along the horizontal meridian. The corneal Q was varied from the model-eye values to 0.8, and the pupil size was modeled between 3.0 to 6.0 mm. The ACD was changed from 2.5 to 4.5 mm while axial length was constant. Spherical equivalent error (Rx) was derived from Zernike defocus and astigmatism terms.
Results: :
Relative to central refraction, peripheral Rx becomes more myopic as the corneal Q increases (e.g. the Rx at 40 deg was relatively changed to 3.02D more myopic for corneal Q of 0.8 with a 6.0mm pupil). The relative peripheral Rx was more myopic when ACD was shorter (e.g. 2.66D more myopic for 2.5 mm ACD), and it becomes more hyperopic for longer ACD (e.g. 2.76D more hyperopic for 4.5mm ACD). For 3.0mm pupil, peripheral refraction of the model-eye becomes more hyperopic, but the relative change in peripheral refraction with corneal Q was larger, relative to 6.0mm pupil, so that more myopic change was found for the corneal Q of 0.8. Combination of the change in different parameters does not produce linear sum of the changes in relative peripheral refraction by those from individuals. Both spherical aberration and coma change with corneal Q, but the relative change was small for the spherical aberration.
Conclusions: :
The model suggests that anterior structure of the eye plays important role in determining peripheral refraction. There is a nonlinear contribution from different anterior components to the peripheral refraction. In general, more relative peripheral myopic refraction could be resulted for the eye with higher corneal Q, shorter ACD and smaller pupil size (for high corneal Q only). Increasing the corneal Q (e.g. Orthokeratology) or anterior contact lens Q could provide interesting clinical procedure for slowing myopia progression if the protective role of the peripheral myopic refraction is proved. However, for any individual eye, a comprehensive consideration on all of the contributing factors is required.
Keywords: refraction • myopia • visual fields