Investigation of selective retina treatment (SRT) by means of 8 ns laser pulses in a rabbit model

Abstract

Background: It has been shown that selective retina treatment (SRT) using a train of 1.7 microseconds laser pulses allows selective damage of the retinal pigment epithelium (RPE) while sparing the adjacent photoreceptors and thus avoiding laser scotoma. It was the purpose of this work to investigate SRT laser effects with Q-switched pulses of only 8 nanoseconds in duration by evaluating the angiographic and ophthalmoscopic damage thresholds and the damage range by histology in a rabbit model. Materials and Methods: A flash lamp pumped frequency doubled (532 nm) Nd:YAG laser with 8 nanoseconds pulse duration was used. In total 210 laser lesions, each calculated to be 102 mm in diameter on retina, were applied through a slit lamp onto the fundus of six eyes of Chinchilla Bastard rabbits. The rabbits were irradiated with increasing energies with single pulses and a train of 10 laser pulses at 10 Hz. After treatment fundus photography and angiography were performed to determine the damage thresholds (ED 50 -probability of RPE cell damage and neurosensory retinal damage) as well as the safety range between both thresholds (ratio of angiographic ED 86 vs. ophthalmoscopic ED 14 ). Selected histology was taken for single and repetitive pulse lesions after treatment. Results: Angiographic and ophthalmoscopic ED 50 -thresholds decreased with increasing number of pulses. For single pulse application ophthalmoscopic and angiographic ED 50 were determined to 365 and 144 mJ/cm 2 , respectively. Regarding 10 pulses 266 and 72 mJ/cm 2 were found. No retinal hemorrhages or disruptions were observed for both sets of parameters. The therapeutic window between angiographic and ophthalmoscopic threshold revealed a factor of 3.1 for single pulses and 2.3 for repetitive pulse irradiation. The safety range respectively had a factor of 0.8 (single pulses) and 1.7 (10 pulses). Histologic examination of laser lesions with single and repetitive pulses at radiant exposures within the therapeutic window-292 and 213 mJ/cm 2 respectively-revealed damaged RPE, intact Bruch's membrane and choriocapillaries. Photoreceptors were partly spared but also damaged to various extents. Conclusions: Short laser pulses of 8 nanoseconds pulse duration can damage the RPE without retinal hemorrhage or disruption. Selective damage of the RPE without affecting the photoreceptors can only rarely be achieved due to the small safety range. Thus, so far microsecond laser pulses for SRT seems favorable compared to nanosecond pulses in order to prevent unintentional photoreceptor damage.

Nanosecond pulse lasers for retinal applications

“…The latter study demonstrated no collateral damage either, meaning that in both the latter and the current studies, RPE cells could be specifically targeted. The present data, in addition, are in approximate agreement with the studies of Roider and colleagues, who have used 8‐nanosecond in comparison with microsecond pulse lasers to selectively ablate RPE cells 34, 45, 46. These authors concluded, however, that their nanosecond pulses resulted in a reduced safety range compared with the microsecond lasers in terms of collateral photoreceptor damage; the more favorable outcomes described in the present study may result from the use of shorter 3 nanoseconds pulses, thereby applying reduced energy to the system.…”

Short‐pulse duration retinal lasers: a review

“…Framme et al irradiated chinchilla rabbits using a Nd:YAG 532‐nm laser with 8‐ns pulse lengths. The authors reported that RPE damage with sparing of the photoreceptor layer was not consistently achievable in this model …”

Tissue response of selective retina therapy by means of a feedback‐controlled energy ramping mode

“…Also the choroid was intact, which indicates that the oxygen‐rich choroid can help regeneration of fully functional RPE cells. Within days after laser treatment, the debris of damaged RPE cells was phagocytized by bystander RPE cells sliding in from the neighbourhood or by macrophages originating from the choriocapillaris . In contrast with selective lesions, the full‐thickness structure of the retina was disrupted and disorganized in the conventional laser lesions.…”