Investigations on Retinal Pigment Epithelial Damage at Laser Irradiation in the Lower Microsecond Time Regime

Abstract

Purpose New lasers with a continuous wave power exceeding 15 W are currently investigated for retinal therapies, promising highly localized effects at and close to the Retinal Pigment Epithelium (RPE). The goal of this work is to evaluate mechanisms and thresholds for RPE cell damage by means of pulse durations up to 50 µs. Methods A diode laser with a wavelength of 514 nm, a power of 15 W, and adjustable pulse durations between 2 µs and 50 µs was used. Porcine RPE-choroidal explants (ex vivo) and chinchilla bastard rabbits (in vivo) were irradiated to determine threshold radiant exposures for RPE damage by calcein vitality staining and fluorescence angiography, respectively. Thresholds for microbubble formation (MBF) were evaluated by time-resolved optoacoustics. Exemplary histologies support the findings. Results is significantly higher than at pulse durations ≥ 5 µs ( P < 0.05) ex vivo, while at 2 µs, no statistically significant difference was found. The ratios between and increase with pulse duration from 1.07 to 1.48 ex vivo and 1.1 to 1.6 in vivo, for 5.2 and 50 µs. Conclusions Cellular damage with and without MBF related disintegration are both present and very likely to play a role for pulse durations ≥ 5 µs. With the lower µs pulses, selective RPE disruption might be possible, while higher values allow achieving spatially limited thermal effects without MBF. However, both modi require a very accurate real-time dosing control in order to avoid extended retinal disintegration in this power range.

Dynamic OCT Signal Loss for Determining RPE Radiant Exposure Damage Thresholds in Microsecond Laser Microsurgery

“…However, for longer pulses, up to 20 µs pulse duration, the damage exposure thresholds reported by Seifert et al were significantly higher than those we observed. This is consistent with the theory that for short pulses, particle absorption by single melanosomes dominates, while for longer pulses, the lower bulk absorption by melanosomes plays a major role [40]. In principle, absolute values should always be considered as rough guide values only, as they can vary greatly and the whole concept of a laser system that compensates for differences in melanin concentration across individuals or across species variations argues against the use of an optimum fixed absolute value.…”

Algorithms for optoacoustically controlled selective retina therapy (SRT)

“…This makes any other effect irrelevant, which would happen to the cell otherwise. As it has been reported, at pulse durations shorter than 2 µs, MBF is the initial mechanism for cell damage [5] , [42] , [43] . At the point of MBF, neighboring cells experience an increase in temperature [44] .…”

Fluorescence Lifetime Changes Induced by Laser Irradiation: A Preclinical Study towards the Evaluation of Retinal Metabolic States

“…In this study, we did not examine the laser pulse energy dependence of the FLT by analyzing the spots inside the pattern, nor the FLT change over a wide area around the pattern. Since the current study with FLIO was conducted as a part of the previously reported study [19], the present study had to be limited to such a partial analysis. There are thus some questions left unanswered.…”

“…In order to observe the time-dependent change, the second pattern was applied right next to the first pattern with the same procedure 7-11 days after the first irradiation (Figure 1B left). As mentioned above, these irradiations inside the pattern were performed to investigate the energy threshold of RPE cell death at each pulse length, which was recently reported in another report [19]. Thus, it was difficult to examine the FLT inside the pattern in detail, and it should be noted that this report focuses on the strongly-coagulated marker spots and their surroundings.…”

“…A contact lens was put onto the cornea of the treatment eye with an index matching gel (Methocel 2%), while the other eye was closed to prevent drying. This study was performed as part of the work on our recently published report [19], which aimed to investigate the RPE damage threshold and mechanisms at different laser pulse duration from 5.2 to 50 µs time regimes, using an optoacoustic microbubble detection method. In this study, we focused on the effects of laser irradiation on the FLT, especially on the changes in FLT around the laser spot.…”