Revisions to outcome measures

Following advice from the Editorial Team of the Cochrane Eyes and Vision Group, the outcome measures set out in the protocol for this review have been modified post hoc. This was necessary as a result of the diversity of outcome measures reported by the included studies. We defined new primary and secondary outcome measures as listed below. We chose three primary outcome measures that we believe best reflect the effectiveness and safety of these procedures. In this update version of the review we have also included data on higher order aberrations because these have become increasingly used to evaluate outcomes in refractive surgery.

The proportion of eyes with uncorrected visual acuity (UCVA) of 20/20 or better at 12 months post‐treatment and the proportion of eyes within ±0.50 D of target refraction at 12 months post‐treatment were chosen as primary outcome measures of effectiveness. Achievement of these outcomes at 12 months post‐treatment would undoubtedly be considered a success by both surgeon and patient. They reflect the effectiveness in terms of achieving a satisfactory visual result (UCVA of 20/20 or better) and in terms of the accuracy of the procedure (±0.50 D of target refraction). We chose the 12‐month over the six‐month time point because of our concerns that refractive stability may not yet have been achieved and corneal haze not yet resolved in some participants at six months.

Our primary outcome measure of safety was the proportion of eyes that lost 2 or more lines of best‐corrected visual acuity (BCVA) at six months or more post‐treatment. We chose this outcome because it is a commonly employed measure of adverse outcomes in ophthalmology. The presence of haze sufficient to cause loss of 2 or more lines of BCVA at six months post‐treatment was considered to be a significant adverse outcome and will be detected by this outcome. We chose the time point of six months or more because this allows sufficient time for resolution of mild to moderate corneal haze following PRK. It also allows for the detection of any adverse events that may occur after the six‐month time point such as ectasia. A new section concerning quality of vision outcomes has been added to this updated version of the review. Quality of vision will be assessed using outcome data for higher order aberrations and modular transfer function data

A further outcome measure, the assessment of higher order aberrations has been added. This was added as it gives an indication of the quality of vision which is distinct from visual acuity and refractive error. Given that the vast majority of patients undergoing either treatment end up with excellent unaided visual acuity it may take more sophisticated measures of the quality of vision to determine whether there is any difference in the outcomes of these treatments.

We added a new table (Table 6: Higher order aberrations and modulation transfer function data).

Revisions to analyses

Two subgroup analyses were done that were not planned in the original protocol.

1. Comparing results in people with high and low myopia.
2. Comparing results from more recent studies with older studies as surgical techniques have changed.