Mean total RNFL birefringence numbers were statistically significantly lower in diabetic eyes without clinically manifest DR, as well as in eyes with mild to moderate DR stages compared with healthy age-matched controls (mean difference [95% confidence interval {CI}]: –0.0124 [–0.0202; –0.0045],
P = 0.0033; –0.0189 [–0.0266; –0.0111],
P = < 0.0001; respectively;
Fig. 2). Comparison of birefringence values between diabetic subjects with no clinically manifest DR versus mild to moderate DR stages revealed a borderline significant difference (0.0065 [–0.000004; 0.0130],
P = 0.050;
Fig. 2). We further subdivided the peripapillary circular scan into a superior and inferior sector. Similarly to the total birefringence results, separate analysis of either the superior or inferior half ring revealed statistically significant differences between diabetic eyes without DR and nondiabetic healthy age-matched controls (mean difference [95% CI]: –0.0127 [–0.0206; –0.0048],
P = 0.0027; –0.0121 [–0.0207; –0.0036],
P = 0.0069; respectively) and diabetic eyes with mild to moderate DR stages and healthy controls (–0.0174 [–0.0251; –0.0096],
P < 0.0001; –0.0201 [–0.0285; –0.0117],
P < 0.0001; respectively), a weak significance between diabetic eyes with no clinically manifest DR versus mild to moderate DR stages in the inferior half ring (0.0080 [0.0008; 0.0152],
P = 0.0316) but no significant differences between the same groups in the superior area (0.0047 [–0.0021; 0.0115],
P = 0.1668;
Table 2). Across the healthy nondiabetic study subjects mean birefringence values were not significantly different between superior and inferior sectors (paired
t-test: mean difference [95% CI]: –0.0038 [–0.0077; 0.00002;],
P = 0.051). In contrast, diabetic eyes with no clinically detectable DR or mild to moderate DR stages showed significantly lower mean birefringence values in the inferior compared with the superior sectors (mixed model: mean difference [95% CI]: –0.0047 [–0.0068; –0.0026],
P < 0.0001;
Fig. 3). Diabetes duration was not statistically associated with RNFL birefringence values (total, superior, and inferior sectors). Neither sex, HbA1c, and type of diabetes categorized in type 1 or 2 showed any statistically significant associations to birefringence levels. Within the diabetes patients, a statistically significant effect of age on birefringence total and superior could be observed in eyes with subclinical DR (total: estimate [95% CI]: –0.00042 [–0.00079; –0.00005],
P = 0.028; superior: estimate [95% CI]: 0.00041 [–0.00079; –0.00003],
P = 0.033), but not in eyes with mild or moderate DR (total: estimate [95% CI]: –0.00002 [–0.0004; 0.00035],
P = 0.90; superior: estimate 0.00003 [–0.00035; 0.00041],
P = 0.86) For birefringence inferior, the slopes did not differ significantly between the subgroups (subclinical DR, mild to moderate DR) and the effect of age was statistically not significant either (estimate [95% CI] adjusted for DR stage: –0.0026 [–0.0006; 0.00008],
P = 0.13). Within the controls, the effect of age was statistically significant for birefringence total and superior (estimate [95% CI]: –0.0003 [–0.0007; –0.00001],
P = 0.043; –0.0004 [–0.0007; –0.00004],
P = 0.027, respectively) with lower birefringence values with advanced age, but not for birefringence inferior (
P = 0.0950).