Functional outcome of subthreshold versus threshold diode laser photocoagulation in diabetic macular oedema

To compare the efficacy of subthreshold diode laser photocoagulation with conventional threshold diode laser in clinically significant macular oedema (CSME) in diabetics.

Prospective randomised controlled trial. Thirty eyes of 20 patients with CSME having central macular thickness (CMT) >or=250 microm on optical coherence tomography (OCT) and best corrected Snellen visual acuity (BCVA) of 20/80 or better were randomly assigned to receive subthreshold diode laser (group A) or threshold laser (group B). The clinical course of BCVA, CMT on OCT, photo stress recovery time (PSRT), contrast sensitivity (CS) using Pelli Robson chart, and central visual field (10-2 VF, Humphrey Field Analyser II) were monitored regularly for 18 weeks after intervention.

Both groups showed significant (P=0.03) and comparable improvement in mean BCVA at 18 weeks visit. There were also significant and comparable reductions (P<0.01) in mean CMT at all visits compared with baseline in both groups. PSRT and CS (75+/-16 (mean+/-SD) and 0.72+/-0.3 in group A and 81+/-12 and 0.68+/-0.2 in group B pre-laser) were significantly better in group A at the final visit (66+/-17 and 1.04+/-0.3) as compared to group B (78+/-11 and 0.73+/-0.3). One patient in group B showed a paracentral scotoma on 10-2 VF after the laser, which persisted till the end of follow-up.

Subthreshold diode laser photocoagulation is as effective as conventional threshold laser in reducing or eliminating diabetic macular oedema (on OCT) and in improving BCVA. Recovery of macular function as assessed by CS and PSRT maybe better with subthreshold laser.