according to early treatment of diabetic retinopathy study ( etdrs ) , protocol panretinal photocoagulation ( prp ) can prevent severe visual loss in proliferative diabetic retinopathy ( pdr ) . several studies have shown histopathologic changes in choroid of diabetic patients [ 24 ] . meanwhile , using panretinal photocoagulation ( prp ) , as was suggested beneficially by diabetic retinopathy study for proliferative diabetic retinopathy , has gotten general acceptance . with the advent of enhanced depth imaging optical coherence tomography ( edi - oct ) , it is now possible to visualize and measure choroidal changes easily . to the best of our knowledge , no other study has compared these two laser modalities in changing choroidal or central retinal thickness . in this study , we have evaluated the subfoveal choroidal , central retinal , and peripapillary retinal nerve fiber layer ( rnfl ) thickness changes before and after prp using edi - oct and we compare the green and red laser in this regard . this clinical trial was approved by the institutional review board / ethics committee . written informed consents were obtained from all participants . exclusion criteria were advanced proliferative diabetic retinopathy , a history of any laser treatment ( panretinal or focal laser photocoagulation ) , any history of intravitreal drug injection , ocular surgery , significant media opacity , myopia , or hyperopia with refractive error of more than 3 diopters . primary objective was to determine subfoveal choroidal and retinal thickness and peripapillary nerve fiber layer thickness changes 6 weeks after red versus prp laser . a complete ophthalmic examination including best corrected visual acuity ( bcva ) using snellen chart , goldmann applanation tonometry , and dilated indirect ophthalmoscopy was conducted . enhanced depth imaging optical coherence tomography ( edi - oct ) mapping was performed using sd - oct ( spectralis , heidelberg engineering , heidelberg , germany ) . a total number of 1600 to 2000 spots per eye were applied to ablate retina using ellex integre pro scan laser photocoagulator ( 82 gilbert street , adelaide , sa 5000 , australia ) . the order of treated areas was as follows : nasal , inferior , superior , and then temporal retina . power setting was chosen to have a mild gray - white burn ( grade 2 ) according to etdrs guidelines . laser setting parameters did not differ significantly between two groups ( mean power 303  mwatts in red and 290  mwatts in green laser group , p value : 0.14 ; mean laser spots 1789 spots in red and 1869 spots in green laser group , p value : 0.13 ) . one eye of all patients was randomly assigned to red laser and the other eye to green laser . spectral domain optical coherence tomography images were obtained using spectralis oct ( heidelberg engineering , heidelberg , germany ) to measure central retinal thickness and enhanced depth imaging ( edi ) mode to measure subfoveal choroidal thickness . bcva measurement and an oct scan were performed at baseline and 6 weeks after completion of prp . subfoveal choroidal thickness was measured using apparatus measurement tool at baseline and 6 weeks after prp . choroidal segmentation was done manually , moving the reference lines from the retinal borders to the choroidal borders . the internal limiting membrane line was moved to the base of the retinal pigment epithelium . also retinal nerve fiber layer ( rnfl ) thickness analysis was done at baseline and 6 weeks after treatment . data were analyzed using spss software ( version 16 , spss , inc . ) . a paired sample t - test was used to compare macula and choroid thickness before and after treatment . independent sample t - test was performed to compare macula and choroid thickness changes from baseline between two groups . the mean visual acuity of patients was 0.32  0.28  logmar and 0.27  0.23  logmar in red laser group and green laser group , respectively . the mean subfoveal ct increased significantly in each group at 6 weeks follow - up ( in red laser group , 202.14  24.5 micron at baseline and 211.7  26.6 micron at 6 weeks , p value < 0.00 , showing 4.86% ct increase compared to baseline ) ( in green group , 201.9  21.13 micron at baseline and 208.9  25.0 at 6 weeks , p  value < there was not a significant difference between red and green laser in terms of choroidal thickness changes from baseline ( p value 0.184 ) . in red laser group , the mean central retinal thickness was 271.6  30.33 micron at baseline and 298.5  62.14 micron at week 6 of follow - up ( p value 0.03 ) . in green laser group , the mean central retinal thickness was 267.0  36.9 micron at baseline and 306.4  73.3 micron at week 6 of follow - up , ( p value 0.000 for both groups ) . there was no significant difference between macular thickness increase between two groups ( p value : 0.404 ) ( table 2 ) . there was no significant association between choroidal thickness change and retinal thickness change in each group ( p values : 0.051 and 0.52 , resp . ) . the mean peripapillary rnfl thickness was 100.5  20.1 micron in red laser group at baseline which increased to 108.2  23.1 micron at 6 weeks ( p value 0.000 ) ( 7.8% as compared to baseline ) . in green laser group , it was 103.5  19.8 micron at baseline which increased to 112.7  22.9 in week 6 of follow - up ( p value 0.000 ) ( 9.2% as compared to baseline ) . rnfl change between two groups was not significant ( p value : 0.762 ) ( table 2 ) . in this study , in eyes with proliferative diabetic retinopathy and without significant macular edema , the mean subfoveal choroidal , central retinal , and peripapillary rnfl thickness increased significantly after both red and green argon laser treatments . this is in accordance with some of previously published studies that confirm this finding [ 69 ] . takahashi et al . measured changes in choroidal blood flow in the foveal area one month after prp in patients with severe diabetic retinopathy and no macular edema using a laser - doppler flowmetry . they reported that prp increases both the choroidal blood flow and the choroidal blood volume . changes in subfoveal choroidal thickness did not correlate with changes in macular thickness in their study . first , it has been attributed to redistribution of choroidal blood flow [ 1014 ] . this leads to redistribution of blood in other untreated areas , especially the macula . following blood flow increase , this transient inflammation probably causes release of cytokines and leads to increase of blood flow and choroidal thickness [ 18 , 19 ] . mean ct increase in both groups in this study is similar to what have been reported before in other trials [ 710 ] . it is noteworthy that , in some other trials , the measured subfoveal ct showed decreased thickness after prp in short term follow - up . they have hypothesized that the reason may be due to the destruction of choroidal vasculature after thermal damage of prp [ 20 , 21 ] . also , we found that central macular thickness and rnfl thickness increased significantly in both groups ; this is also similar to what has been found in other studies [ 2024 ] . this finding is similar to what has been reported before about the increase of retinal thickness after prp and it was attributed to the release of proinflammatory cytokines or hypoxia - induced macular edema [ 2024 ] . also , we did not find any correlation between changes of choroidal thickness and central retinal thickness . one of the novel analyses done in this study is the comparison between red and green laser in changing retinal , choroidal , and rnfl thickness . although all three measured variables showed significant increase after treatment in each eye , intereye comparisons were not statistically significant considering central retinal , subfoveal choroidal , and rnfl thickness changes ( p values : 0.404 , 0.184 , and 0.762 , resp . ) that may indicate no considerable difference between red and green laser in this regard . to the best of our knowledge , there is only one report by ghassemi et al .   that compared the difference between red and green laser in pdr patients . they reported significant increase of rnfl thickness after prp without any difference between red and green laser . one of the advantages of this study is selection of both eyes of one patient which can exclude the effect of systemic factors on retinal , choroidal , and rnfl thickness . this study has some limitations including low sample size and short follow - up time . nonetheless , this study showed significant increase of subfoveal choroidal , foveal retinal , and peripapillary rnfl thickness after red and green laser prp and no significant difference between these two lasers in aforementioned measurements .
purpose . to compare subfoveal choroidal , central retinal , and peripapillary nerve fiber layer ( rnfl ) thickness after panretinal photocoagulation ( prp ) with red and green laser in diabetic patients 
 . study design . 
 randomized clinical trial . methods . 
 a total of 50 patients with bilateral proliferative diabetic retinopathy and no diabetic macular edema underwent prp . 
 one eye was randomly assigned to red or green laser . 
 subfoveal choroidal , central retinal , and rnfl thicknesses were evaluated at baseline and 6 weeks after treatment . 
 results . 
 the mean subfoveal choroidal , central retinal , and peripapillary nerve fiber layer ( rnfl ) thickness increased significantly in each eye 6 weeks after prp ( p values in red laser group : < 0.01 , 0.03 , and < 0.01 , resp . , and in green laser group < 0.01 , < 0.01 , and < 0.01 ) 
 . there was no difference between red and green laser considering subfoveal choroidal , central retinal , and peripapillary nerve fiber layer ( rnfl ) thickness increase after prp ( p values : 0.184 , 0.404 , and 0.726 , resp . ) . 
 conclusion . 
 both red and green lasers increased mean subfoveal choroidal , central retinal , and peripapillary nerve fiber layer ( rnfl ) thickness significantly 6 weeks after prp , but there is no difference between these two modalities in this regard .