We read with interest the article ‘Subthreshold diode micropulse panretinal photocoagulation for proliferative diabetic retinopathy’ by Luttrull et al.1We would like to congratulate the authors for their work and cite a clarification.

The authors mention that the treatment parameters were designed to avoid the creation of clinically detectable photocoagulation lesions and that the effectiveness of subthreshold diode pan retinal photocoagulation (PRP) at this low irradiance level along with its efficacy in diabetic macular oedema is an evidence in favour of subthreshold laser in clinical practice. However, the mechanism of action of laser photocoagulation is thought to be different in these two conditions. The decrease in macular oedema is supposed to be mediated through the retinal pigment epithelium2 for which even subthreshold energies may be sufficient. However, in proliferative diabetic retinopathy, destruction of the ischaemic retina thereby decreasing the angiogenic stimulus and improved oxygenation of the remaining retina are among the major hypotheses of the mechanism of action.3 Keeping in mind these factors, the likely mechanism of action of subthreshold PRP stated by the authors needs clarification.

Also, is it justified to treat all the patients with the same energy levels and to titrate it with the pain threshold that has a wide variation independent of the energy required for producing a visible lesion? Titrating the energy for a visible spot and then reducing the power or the time of the laser beam will be a better method for doing subthreshold PRP, as it will provide the required subthreshold energy for a given patient and amount of retinal oedema.