A review of peripheral refraction techniques.

The recently discovered link between myopia development and peripheral refraction has triggered a stream of clinical and animal investigations to confirm the theory and to understand the underlying mechanisms. For this, precise peripheral refractometry has now gained importance in myopia research. For more than 70 years, many researchers have measured off-axis refraction of the eye in horizontal and sometimes vertical meridians over a range of angles, using several modified refraction techniques. These techniques varied not only with respect to the instrumentation used, but also the modifications performed to enable off-axis refraction. Modifications included either head or eye turn of the participant with respect to the peripheral angle tested or rotation of the instrument itself around the center of the eye. The main focus of this study is to review and compare all refraction techniques for off-axis measurements including necessary modifications made to equipment or procedures. Because a difference in instrumentation and techniques potentially limits the comparability of reported results, it is of particular importance to understand all the details of the particular refractometric technique chosen and any potential problems. Difficulties relating to all the methods are highlighted to provide information on preference and usefulness of certain peripheral refraction techniques for future technology and research work. All refraction techniques exhibited similar drawbacks, such as off-axis fixation, protraction when many peripheral angles were tested, and difficulties to obtain reliable measurements at large peripheral angles. Yet, from all the methods reviewed, the Shin-Nippon NVision K5001 open field autorefractor and the Hartmann-Shack wavefront sensor technique seem to be the most useful commercially available instruments to measure peripheral refraction.