Our study found that the mean RNFL thickness was significantly thinner in the AD group than the controls. Likewise, the GCL-IPL thickness was significantly thinner in almost all sectors in the AD group, compared with those in the controls. Overall, there was no correlation between OCT parameters and MoCA scores. However, there was a significant negative correlation between age and OCT parameters.
The loss of RGCs in AD might have resulted from amyloid deposition, which was previously reported in a postmortem mouse model and also in retinal tissue in vivo. Therefore, amyloid accumulation in the retina may result in the degeneration of RGCs in parallel to the neurodegeneration in the brain.12
Many studies have reported significant thinning in the overall RNFL in patients with AD.13 Most studies showed significant thinning in the superior and inferior quadrants of the RNFL in AD,14,15 whereas some studies found thinning only in the superior quadrant, as in our results.16,17 The prominent loss of RNFL in the superior and inferior regions may occur because of a higher number of neurons in those regions. Liu et al. provided a possible explanation for the dominant defect in superior RNFL, in that axons from the superior retina anatomically project to the cuneal gyrus of the primary visual cortex.14 Moreover, Armstrong et al. have documented a higher density of senile plaques and neurofibrillary tangles in the cuneal gyrus that corresponded to the superior RNFL.18
Cheung et al. suggested that the sensitivity of GCL-IPL measurements is higher than that of RNFL measurements for distinguishing patients with AD from controls.11 In our study, the AD group showed significant thinning of the GCL-IPL in almost every sector, compared with only the superior quadrant for the RNFL. Further studies are needed to confirm the concept that the GCL-IPL is a more sensitive biomarker than is the RNFL.
A significant difference was found in BCVA between the two groups in our study. Recent studies have shown that AD patients have significantly more-reduced accuracy in low luminance compared with age-matched controls.19 The increased prevalence of cataracts in AD patients may also affect BCVA.20 We were aware of this confounding factor for OCT parameters between the AD and control groups and, therefore, both the RNFL and GCL-IPL models were adjusted for BCVA.
A previous study that performed OCT analysis of age-related thinning of the RNFL found a significant correlation between age and OCT parameters, consistent with our study.21 However, AD patients had thinner RNFL and GCL measures than did non-AD patients of the same age.8–11 When adjusted for age and BCVA, there was still significant thinning of the RNFL in the AD group compared with controls. Therefore, AD may be an independent factor for RNFL thinning.
A previous study showed a significant correlation between OCT parameters and cognitive function.22 However, we found no correlation between MoCA scores and OCT parameters. Cognitive function is influenced by multiple factors, such as education, economic level, occupation and social activity.23 Thus, the MoCA may not accurately reflect the severity of disease.
This study had some limitations. First, it was conducted in a cross-sectional manner, so there was no follow-up. Second, patients with severely impaired cognition could not be recruited because they had poor attention and did not follow instructions. Conversely, the strengths of our study are as follows. 1) The AD and control groups were age-and-sex matched. 2) The sample size of each group was slightly larger than number required based on the calculation prior to the study. 3) In addition to RNFL thickness, GCL-IPL thickness was evaluated in our study.