Optical modelling of the possible origins of transient refractive changes in diabetic patients.

Purpose: To explore theoretically the ocular components that might be responsible for the transient refractive changes observed in the eyes of diabetic patients, particularly during intensive glycaemic control.

Methods: Paraxial ray tracing with a model eye having a lens with a single equivalent refractive index was used to determine the sensitivity of refraction to change in the values of each biometric component. The changes required to produce a refractive change of 0.50 D were compared with the reliability of current instrumentation to measure each parameter.

Results: If transient shifts of 0.50 D or more were caused by changes in surface curvatures or separations, the latter would easily be detectable by available measurement techniques. The fact that such parameter changes have not been reliably detected in clinical studies supports the concept that changes in refractive index cause the refractive changes. The most probable site of such index changes is the lens, a change of about 0.003 in equivalent index being required to produce a 0.50 change in refractive error.

Conclusion: This analysis supports the concept that transient changes in refraction in diabetics are due to changes in the lens. It is likely that changes in the gradients of refractive index lens within the lens, and in their associated contribution to the overall power of the lens, play the key role, rather than changes in surface powers.