Response to Stanga et al.: Structural Changes of the Retina after Laser Photocoagulation in Spectral Domain Optical Coherence Tomography

In the opinion of Stanga et al., we overlooked recently published work that deals with findings in relation to laser-tissue interaction of laser burns. The authors mainly refer to their research with low pulse duration photocoagulation (10–50 ms) from the recently commercially available system “PASCAL,” with which low duration lesions can be applied in predefined patterns. We have several remarks in response to their comments.

The aim of our paper was to compare the OCT appearance of retinal laser effects of clinically standard thermal photocoagulations (which was and still is 100–200 ms pulse duration) with thermo-mechanical-induced SRT lesions, which only and selectively targets the RPE, and proved not to compromise photoreceptors at all. This is clearly stated in the title of the paper.¹

1. It was not the aim of the paper to use OCT to investigate other recently available thermal low power or low pulse duration photocoagulation as micropulsing (provided by IRIDEX) or low pulse duration photocoagulation (10–50 ms), which can be applied with almost all commercially available photocoagulation lasers. To the best of our knowledge, our paper is the first that directly highlights the differences between thermal (100 ms) and thermomechanical (1.7 µs) laser effects in direct comparison in vivo.

2. It is certainly true, that there is excellent research data in the literature regarding the topic of imaging laser burns, especially by the group of Stanga et al. dealing with the PASCAL laser system.^2,3 In this context we are sorry that the first sentence of our summary, “In summary, the recent advent of clinically available high-resolution SD-OCT allows distinct examination of retinal tissue changes after laser treatments for the first time,” in fact, gives the impression that we ourselves did this “first-time” research. We did not interpret it this way but wanted to state, that due to the fact of having these machines clinically available, this kind of research can generally now be performed.

3. We agree with the comments that “full-thickness” damage of the neurosensory retina after conventional laser photocoagulation does not necessarily occur. If coagulations are applied carefully with low laser power and thorough dosimetry just above threshold, as should be done in macular treatments, at least the nerve fiber layer or more of the inner neurosensory layers typically are unaffected. It is widely known that the depth of the coagulations depend on the laser power used. However, in daily clinical practice, often the extent of conventional laser photocoagulation does reach up to the inner retinal layers as also shown in our publication.¹

4. When comparing the SRT lesions in SD-OCT, which present only RPE effects by definitely sparing the photoreceptor layers,¹ with those from the PASCAL system, the extent of neurosensory damage seems to be more enhanced in the SD-OCT images after PASCAL 10 ms–20 ms photocoagulation.² This is very consistent with theoretical calculations, experimental data, and histologic evaluations of Roider and Birngruber, only demonstrating a clear selective effect in a pulsed laser approach using microsecond pulse durations.^4,5 Thus, Stanga et al. might “achieve similar FD-OCT appearances in vivo, as compared to SRT”; however, from the OCT appearance, no selective effect can be derived. Histology of such lesions clearly demonstrates damage to the photoreceptors. While 10 ms pulses might have been “shown to achieve effective and safe laser treatments,” a selective effect in terms of targeting only the RPE can only be achieved with thermo-mechanical effects following pulse durations < 50 µs (Schuele et al.⁶ and Lee et al.⁷). When using longer pulse durations, purely selective thermal effects can hardly be achieved owing to the nature of heat diffusion, except when neighboring cells have different resistance to heat, which to the best of our knowledge is not the case between RPE and photoreceptor cells. The PASCAL system provides an interesting new application strategy for laser spot patterns; however, 10–30 ms pulse durations, as investigated since the 1980’s, by the nature of physics can hardly be used to achieve completely selective RPE effects.

Declaration of interest: The authors report no conflict of interest. The authors alone are responsible for the content and writing of the paper.