A prospective study examining the changes in factors that affect visual acuity following trabeculectomy.

Many patients report subjective changes in vision following trabeculectomy. In a prospective study of 16 eyes, we have tried to determine the causes of these visual changes. Ninety-four per cent of eyes showed a change in uncorrected Snellen acuity 1 week following surgery. This change was due to a myopic shift in refraction secondary to changes in the anterior chamber depth. A 1 mm change in anterior chamber depth results in approximately a 2 dioptre change in refractive sphere. Visual acuity starts to return to pre-operative levels by the third post-operative week, from which point no eyes showed a deterioration in corrected acuity of more than one line of Snellen. The vertical corneal radius of curvature is reduced in the early post-operative period and there is a with-the-rule change in corneal astigmatism. This resolves over longer follow-up and the possible causes of this are discussed. No changes were noted in corneal thickness and there was no evidence of macular oedema over the follow-up period. Patients should be warned of these possible visual changes pre-operatively as many will have normal acuity prior to surgery.