The authors are to be commended for their attempt to look systematically at the outcomes of pan retinal photocoagulation as a function of laser parameters. Authors found that their neovascularization recurrence rates after pan retinal photocoagulation with 20-ms laser pulses were higher than those occurring with 200-ms laser pulses when the same number of lesions and the same beam diameter was applied. We would like to comment on this issue and on mechanisms of shorter-pulse retinal photocoagulation with the PASCAL method.

According to Diabetic Retinopathy Study (DRS) and Early Treatment Diabetic Retinopathy Study (ETDRS) recommendations, lesions are to be applied either as a mild scatter, with 1-lesion diameter spacing, or as a full scatter, with lesions spaced by half a diameter. DRS/ETDRS studies were conducted with pulses of 100 to 200 ms in duration. Lesions produced by shorter pulses are smaller than similar-grade lesions produced by longer pulses. Therefore, if the spacing between the lesions is to be maintained as one (or half) diameter of the lesion, the grid density and the total number of lesions delivered to the same area should increase by the reciprocal of the square of the lesion diameter. This increase simply reflects a requirement to coagulate the same total area of the retina, based on the assumption of a correlation between the size of the coagulated area and reduction in retinal hypoxia or ischemia. Coagulating a smaller area and destroying a lower number of photoreceptors by using an equivalent number of smaller burns, one would be expected to reduce the benefit of decreasing the retinal hypoxia and associated neovascularization, as the authors reported. With the introduction of PASCAL, many physicians started using lighter lesions that naturally occur with shorter pulse durations, which produce less inner retinal damage and reduces scarring. Because lighter lesions have smaller diameters, they therefore require a larger number of spots to coagulate the same total area. We would like to bring to the authors’ attention several publications regarding clinical experience with the PASCAL method in the treatment of proliferative diabetic retinopathy, in which an increased number of spots was applied.

The authors point out: “As pulse durations are decreased below 50 ms, the mechanism of cellular injury transitions from thermal energy at higher pulse durations to mechanical rupture because of transient vapor formed adjacent to melanosomes. ” In the cited paper (by Schuele and associates), which aims at destruction of retinal pigmented epithelium cells by forming microbubbles around melanosomes while sparing photoreceptors, the critical pulse duration at which the mechanism of tissue damage changes from thermal coagulation to vaporization is 50 microseconds. In PASCAL photocoagulation, pulses are much longer (in tens of milliseconds); therefore, heat diffuses into the retina. Therefore, not only the retinal pigment epithelium, but also the photoreceptors and the inner retina are affected. The extent of heat diffusion and the undesirable effect of vaporization in retinal photocoagulation are discussed in multiple publications, such as by Jain and associates.

We thank the authors for highlighting the importance of laser parameters in the management of diabetic retinopathy and suggest that in any treatment—pharmaceutical and laser alike—dosimetry requires careful attention.