Technical notes on peripheral refraction, peripheral eye length and retinal shape determination.

Purpose: To give an overview of the misconceptions and potential artefacts associated with measuring peripheral refractive error and eye length, the use of these measures to determine the retinal shape and their links to myopia development. Several issues were identified: the relationship between peripheral refractive error and myopia development, inferring the retinal shape from peripheral refraction or eye length patterns, artefacts and accuracy when measuring peripheral eye length using an optical biometer.

Methods: A theory was developed to investigate the influence of artefacts in measuring peripheral eye length and on using peripheral eye length to make inferences about retinal shape.

Results: When determining peripheral axial length, disregarding the need to realign instruments with mounted targets can lead to incorrect field angles and positions of mounted targets by more than 10% for targets placed close to the eye. Peripheral eye length is not a good indicator of the effects of myopia or of treatment for myopia development because eyes of different lengths but with the same retinal shape would be interpreted as having different retinal shapes; the measurement leads to overestimates of changes in retinal curvature as myopia increases. Determining peripheral eye length as a function of estimated retinal height rather than field angle will halve the magnitude of the artefact. The artefact resulting from the peripheral use of biometers with an on-axis calibration is modest and can be ignored.

Conclusion: There are significant issues with peripheral measurements of the refractive error and eye length that must be considered when interpreting these data for myopia research. Some of these issues can be mitigated, while others require further investigation.