Comment and Reply on: Structural Changes of the Retina after Laser Photocoagulation in Spectral Domain Optical Coherence Tomography

We read with interest the article published by Framme and co-workers that described the tissue reactions of laser burns using spectral-domain optical coherence tomography (SD-OCT), and we commend their in-depth evaluation of laser burns.¹ However, we wish to comment on other recent work that has described key findings in relation to laser-tissue interaction in laser burns.

Since 2006 in Manchester, we have conducted research into the Pascal^® (Pattern Scanning Laser; Optimedica, CA, USA) photocoagulator that was introduced in 2005 for retinal photocoagulation.^2,3 It semi-automates the procedure using a brief pulse duration combined with rapid raster scan application of multiple spots. The final size of the Pascal^® 10–30 ms retinal lesion is strongly affected by pulse duration and power.⁴ Blumenkrantz et al.⁴ found a 20-ms laser pulse to be superior to conventional 100 ms laser pulses with regard to the predictability of lesion size, and reduction in collateral injury through reduced axial and lateral thermal spread. Framme and co-workers have not discussed any implications or recent research using this laser system within the article compared to conventional laser photocoagulation.

Framme and co-workers have postulated that retinal pigment epithelium (RPE) proliferation may occur. However, Paulus et al. have recently described the localization of medium-pulse duration laser lesions in outer retina, and proposed that migration of photoreceptors from unaffected areas may fill-in the gap in the photoreceptors layer.⁵ Framme et al.¹ compared the selective retina technique (SRT) to long-pulse 100 ms laser photocoagulation, and found significant differences in SD-OCT laser burn appearances. Current laser parameters use reduced pulse duration, and it would be more relevant to compare the SRT with 50 ms or shorter pulses as conventional laser parameters. This would be more relevant for clinical practice, and readers may see the actual comparison between modern conventional laser and SRT.^3,4,6

In 2009 (epub in 2008), we published a report that used Fourier-domain OCT (FD-OCT; Topcon 3D OCT-1000, Newbury, UK) and fundus autofluorescence (AF; Topcon TRC-50DX, type IA, Newbury, UK) to investigate 10 ms laser lesions in vivo.⁷ FD-OCT images were acquired using 25,000 A-scan/s up to an axial resolution of 5 µm.

We showed that the AF signal changes over time using 10 ms retinal burns, whether ophthalmoscopically visible, barely visible, or invisible.⁷ The FD-OCT features at 1 hr showed that individual 10 ms burns produced a vertical band of moderate and high reflectivity that occupies the outer nuclear layer, with extension from the inner highly reflective layer (HRL) through the outer plexiform layer. Along either side of these vertical bands, small vacuoles of low reflectivity were present. The inner HRL and the apical RPE were disrupted, and inner retinal architecture remained intact. The outer border of the outer HRL showed no signs of disruption. We reported that the vertical bands corresponded spatially with the lack of autofluorescence that most probably resulted from blockage of background signal on AF.

By observing laser tissue interactions via FD-OCT and AF, all burns were initially columnar and displayed equivalent autofluorescence, localized to the junction of the inner and outer segments of the photoreceptors (JI/OSP) and apical RPE.^7,8 At one year, the macula 10 ms burns appeared as square-edged foci of hyporeflectivity within JI/OSP and apical RPE. There was no disruption of the HRL on either side of each laser burn, and the basal aspects of outer HRL remained intact.^7,8

More recently, we have presented findings from a prospective study that compared 20 ms to 100 ms laser burns in diabetic retinopathy.⁹ Framme and co-workers have stated that conventional laser produces “irreversible full-thickness damage,” but we have shown 20 ms and 100 ms burns to both spare the inner retina over time.^2,3 Conventional threshold gray-white ETDRS intensity burns did not produce full-thickness burns on FD-OCT.

The article by Framme and co-workers suggests that SRT may be less damaging to the retina compared to conventional laser.¹ Our work has shown that shorter pulse duration laser can achieve similar FD-OCT appearances in vivo, as compared to SRT. Contemporary argon laser systems available to ophthalmologists, as well as the Pascal^®, have in-built interfaces with medium pulse duration settings that have been shown to achieve effective and safe laser treatments.^5–7 This caveat is important for readers to understand, since ophthalmologists can still achieve effective and minimally-traumatic laser photocoagulation using medium-pulse laser, without a SRT system.

ACKNOWLEDGMENTS

The authors support the NIHR Manchester Biomedical Research Centre.

Declaration of interest: Funding is supported by Optimedica Corporation. Financial disclosures: George R. Marcellino is an employee of Optimedica Corporation. Paulo E. Stanga has received financial support from Optimedica Corporation. Mahiul M. K. Muquit has no conflict of interests.