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The Effects of PASCAL Panretinal Photocoagulation on the Peripapillary Retinal Nerve Fiber Layer

Mohla Aditi, Mathur Ranjana, Nongpiur Monisha E, Cheung Carol Y, Milastuti Nia, Foo Valencia and Perera Shamira

Purpose: To investigate the effects of Pascal (Topcon Medical Laser Systems, Inc. Oakland, NJ) pan-retinal photocoagulation (PRP) on the optic nerve head (ONH). Method: This was a prospective case control study comparing 3 groups of patients seen in the diabetic retinopathy (DR) clinics, without any coexistent optic nerve pathology. Group A patients had proliferative or severe non proliferative DR necessitating PRP during the study period. Group B patients had mild or moderate DR and did not require PRP throughout the study period. Group C patients had standard retinal laser (green argon or frequency doubled YAG) PRP at least 2 years ago. All the 3 Groups had retinal oxygenation measurements and retinal vessel caliber readings by Oxymap T1(Oxymap, Reykjavik, Iceland), optic disc stereophotographs, high definition optical coherence tomography (HD-OCT) scans of the ONH by both Cirrus (Carl Zeiss Meditec Inc, Dublin, CA) and Spectralis (Heidelberg Engineering, Heidelberg Germany). These were performed prior to PRP at baseline, then at 3, 6, and 12 months post PRP for group A and at baseline, 3, 6, and 12 months for groups B and C. Paired t-test was used to assess the mean changes in parameters from baseline for each group. Results: A total of 90 patients were recruited including 27 in Group A, 31 in Group B and 32 in Group C. At baseline, the average retinal nerve fibre layer (RNFL) was significantly thicker in group A compared to group B (102.0 ± 16.8 vs. 89.5 ± 11.6 μm, p=0.001) and group C (88.6 ± 11.2 μm, p=0.001) respectively. At 3 months, Group A exhibited a significant increase in the Cirrus average RNFL thickness (5.60 ± 8.54 μ, p=0.003) when compared to baseline. At 6 months, the average RNFL reverted to baseline values (p=0.89), and remained stable at 12 months (p=0.85). There was no significant change noted in the average Cirrus RNFL thickness at 3 and 6 months in Group B. At 12 months, the average RNFL was significantly thinner compared to baseline (-6.68 ± 8.10; p=0.005). In Group C, the average RNFL remained stable from baseline through to month 12 (p>0.05 at the three timepoints). No significant changes were noted in the average Spectralis RNFL thickness at any timepoints in each of the three groups. Oxygen saturation dropped in all 3 groups at 3 months, being significant only in group B (-2.05 ± 4.20%, p=0.03). Conclusion: In eyes treated with Pascal PRP, there was an initial increase in the retinal nerve fibre layer at the optic nerve head at 3 months followed by a thinning back to baseline values at 6 months which then remained stable up to 12 months. Hence the Pascal PRP did not have a significant effect on the nerve fibre layer at 12 months.