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To the Editor:
To understand the effect of therapeutic doses of laser application on the neurosensory retina, detailed histologic1-3 and optical coherence tomographic4-10 evaluations have been employed in both animal models and in the human eye. We sought to evaluate photoreceptor structure associated with laser photocoagulation lesions using two high-resolution retinal imaging tools (adaptive optics (AO) and SD-OCT).
Two patients received short duration Pascal™ (532nm, OptiMedica Corp., Santa Clara, CA) laser therapy for clinical indications. Subject 1 was a 57-year-old woman with macular edema from hemicentral retinal vein occlusion. Treatment consisted of 3×3 grid laser applications consisting of 100μm spot diameter and 100μm spacing, with 100mW power and 20ms duration to produce barely visible lesions clinically. Subject 2 was a 43-year-old woman with proliferative diabetic retinopathy, treated with panretinal (PRP) laser photocoagulation, consisting of 4×4 grid arrays with a 200μm spot diameter and 200μm spacing using 425mW power and 20ms duration to produce lesions of moderate intensity. (eFigure 1). For calibration of all retinal images, axial length was measured using an IOL Master (Carl Zeiss Meditec, Inc., Dublin, CA). The Pascal system delivered evenly spaced laser applications with clinical precision, facilitating our investigation in these patients. Institutional IRB approval was obtained.
Volumetric SD-OCT images of the macula were obtained using the Bioptigen SD-OCT (Bioptigen, Inc., Durham, NC) and Cirrus HD-OCT (Carl Zeiss Meditec, Dublin, CA). Volumes were nominally 6 mm x 6 mm and consisted of 128 B scans (512 A-scans/B-scan). Cirrus software (version 5.0) was used to create C-scans (en face reconstructions) from the macular volumes to aid in co-registration with other images.
Images of the photoreceptor mosaic were obtained using an AO flood-illuminated camera and/or11 an AO scanning ophthalmoscope (AOSO).12 Rod and cone densities were estimated using a semi-automated direct counting procedure.13 Lesion size was estimated manually as the edge to edge distance of the disruption of the photoreceptor mosaic (AO) or the disruption of the photoreceptor layers (SD-OCT).
AO imaging was successful after edema regressed. Correlation between B-scan and C-scan SD-OCT images, color fundus photography, and AOSO montage of the photoreceptors in the area of macular grid laser treatment was performed. (Figures 1A-1D, 2A-2D). On AOSO circular zones of hyporeflectivity with uniform absence of photoreceptors corresponded to laser lesions observed by SD-OCT and color photographs. Photoreceptor disturbances appeared to correspond to the area of the laser application and not beyond it.
The average size (± SD) of 20 lesions on AOSO was 92.0 ± 10.9 microns, with substantial variability in their appearance (eFigure 2). In an area between two lesions, we observed an undisturbed photoreceptor mosaic of 82,819 rods/mm2 and 8,658 cones/mm2. Both of these values are consistent with normal values from the same system.12 The areas absent of photoreceptors corresponded in size to the areas of photocoagulation, indicating that photoreceptor cell migration into the laser lesion was limited or absent.
SD-OCT images of representative PRP lesions and surrounding areas are shown in Figure 1E and 1F. AO images of the photoreceptor mosaic and lesion are shown in Figure 3. Lesions consisted of circular areas of central hyperreflectivity surrounded by a ring of hyporeflectivity (Figure 3B), corresponding, respectively, to the central hyperpigmented areas and surrounding concentric rings of hypopigmentation (Figure 3A). Diffusely high reflectivity was observed at the margins of some of the lesions (Figure 3D, 3F). The cone mosaic appeared normal immediately adjacent to the lesion. (Figure 3E, 3F). Cone density at a nearby location was 8,732 cones/mm2, consistent with normal values for this eccentricity (Figure 3C). Approximate mean diameter of the PRP lesions was 306 ± 43.2 microns (5 lesions evaluated), with precise measurements limited by somewhat ill-defined lesion borders. While cones (and sometimes the smaller rods) can be visualized in Figure 3C, 3E, 3F, the hyperreflective spots in Figure 3B are likely not photoreceptors, illustrating a challenge in interpreting AO-derived images of the cone mosaic.
Using high-resolution retinal imaging, we evaluated the tissue response in the human eye to grid and focal laser treatment applied to achieve clinically accepted endpoints using the Pascal™ laser system. We detected no evidence of reduced photoreceptor density around the laser lesions, no apparent size reduction of the lesions relative to the initial application diameters, and, thus no direct evidence of photoreceptor migration or healing. Re-establishment of the photoreceptor layer in areas of retinal photocoagulation has been observed in rabbit eyes subjected to Pascal laser lesions of barely visible to moderate intensity.10 We suspect that observed differences in photoreceptor healing relative to experimental studies may relate to differences among species, degree of pigmentation, cellular maturity, and variability in the grading of lesion intensities.
We are unaware of previously published of laser photocoagulation lesions in the living human eye using AO imaging. Furthermore, the discrimination between rods and cones, each cell type having its own characteristic size and distribution elucidated by confocal AOSO technology,12 is a unique aspect of this study that distinguishes it from other in vivo studies. The ability of AO imaging to directly assess photoreceptor structure with cellular resolution may facilitate new approaches to laser therapy, perhaps with the intent of preserving more photoreceptors.
eFigure 1. Clinical appearance of fundus laser lesions after Pascal ™grid laser photocoagulation for macular edema secondary to branch retinal vein occlusion in Subject 1 (A) and after PRP for proliferative diabetic retinopathy in Subject 2 (B).
eFigure 2. Subject 1. For comparison AOSO images from 2 normal subjects (A,B) along with a series of images from Case 1 (C-I) at comparable retinal locations (~5-10° from the fovea). Scale bar is 100μm. The boundaries of all laser lesions were fairly well-defined. We measured 20 lesions in the AOSO images and the average size (± SD) was 92.0 ± 10.9μm, though we observed variability in their appearance. Most were characterized by a central hyperreflective core surrounded by a hyporeflective ring. In some cases, the hyperreflective structure was cellular in appearance (D,F,H,I), though lacking the same regularity of size and spacing of photoreceptors between lesions. In an area between two lesions (C), we observed an undisturbed photoreceptor mosaic of 82,819 rods/mm2 and 8,658 cones/mm2. Both of these values are consistent with normal values from the same system. AO images show large diameter signals to be distinct from small diameter signals, likely representing cone and rod signals, respectively.
Dennis P. Han, M.D., had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis. We thank Alfredo Dubra, Ph.D., for design of the AO Scanning Ophthalmoscope and Phyllis Summerfelt for technical/administrative assistance in figure preparation. IRB approval and informed consent for imaging studies were obtained.
Support: NIH grants P30EY001931, T32EY014537, & R01EY017607, unrestricted departmental grant from Research to Prevent Blindness, Inc., the Thomas M. Aaberg, Sr., Retina Research Fund, and the Jack A. and Elaine D. Klieger Professorship in Ophthalmology (DPH). JC is the recipient of a Career Development Award from RPB. Temporary use of the Pascal™ Laser was provided by OptiMedica Corp. This investigation was conducted in a facility constructed with support from the Research Facilities Improvement Program, grant number C06 RR-RR016511, from the National Center for Research Resources, National Institutes of Health.