In this cross-sectional study, we evaluated the extent and pattern of retinal microvascular alterations, specifically at the capillary network level, in AD and MCI. We compared three retinal OCTA metrics (VD, FD and FAZ area) in two capillary plexuses (superficial and deep) in AD, MCI, and controls. Compared with controls, AD participants showed significantly sparser VD in both plexuses whereas MCI participants only showed reduction at the superficial plexus. In terms of FD, AD and MCI participants exhibited a loss of vessel complexity of the SCP when compared with controls. Our study adds further to the concept that there are possible progressive differences in retinal microvascular alterations between AD and MCI; the use of VD in the SCP (together with DCP) may further distinguish between AD (both SCP and DCP affected) and MCI (only SCP is affected) individuals. Taken together with increasing evidence from other research, our current study demonstrates that differences in retinal microvascular changes using OCTA may potentially be used to identify and screen for AD and earlier cognitive phenotypes (i.e., MCI).
SCP in AD individuals
We showed that AD participants have a sparser VD of the SCP compared with control participants. Our findings support most of the previous OCTA studies in AD participants (Table 4),[12–14, 16] which is in keeping with studies on larger retinal vessels using fundus photographs.[3, 33] It should be noted that two other studies did not observe any differences in the VD of the SCP in AD participants.[11, 15] OCTA quantification metrics may potentially be affected by several confounders. First, OCTA signal strength quality can affect the VD, where the VD decreased linearly with signal strength.[34] In our study, although there was a small difference in the OCTA signal strength quality between controls and AD/MCI (9.6 vs 9.8 out of 10), the scan qualities were extremely high. Second, although studies have excluded participants with uncontrolled hypertension, the BP levels can affect the VD.[25] In the current study, we statistically adjusted the BP levels to remove the BP bias. Last, the physiological variability of FAZ can affect the VD.[35] This is mainly dependent on how much FAZ one includes in the analytical regions. In eyes with a larger FAZ, the FAZ would occupy a larger portion of the analytical area, resulting in a lower VD. In the current study, we mitigated the potential measurement bias by manually delineating the FAZ region and masking it from the calculation.
Table 4
Optical coherence tomography angiography studies on individuals with Alzheimer's disease and mild cognitive impairment.
Author | Sample | Neurocognitive diagnosis | OCTA imaging | Adjusted for confounders | Projection artifact removed | Corrected for FAZ | Superficial capillary plexus (SCP) | Deep capillary plexus (DCP) | FAZ |
Lahme L, 2018 | 36 AD and 38 control | NIA-AA | RTVue XR Avanti | No | No | No | Microvascular densities of SCP were significantly lowered in AD than controls. Negative correlation between flow density and Fazekas scale. | No significant difference. | No significant difference. |
Jiang H, 2018 | 12 AD, 19 MCI, and 21 control | NIA-AA | Zeiss Angioplex | No | Yes | No | Fractal dimensions of SCP were significantly lower in AD than controls. | Fractal dimensions of DCP were significantly lower in AD and MCI than controls. Fractal dimensions were positively related to MMSE in MCI patients. | Not available |
Bulut M, 2018 | 26 AD and 26 control | Clinically (NIA-AA, DSM-IV) | RTVue XR100-2 | No | Not available | No | Microvascular densities of SCP were significantly lower in AD than controls. Correlations were found between MMSE and vascular densities and FAZ. | Not available | FAZ was significantly enlarged in AD than controls. |
Zhang YS, 2019 | 16 AD/MCI and 16 control | NIA-AA | RTVue XR Avanti | Age-matched | Yes | No | Microvascular densities of SCP were significantly lower in early AD or amnestic type MCI than controls. Positive correlations between vascular densities of SCP and MoCA. | No significant difference. | Not available |
Zabel P, 2019 | 27 AD and 27 control | Clinically (NIA-AA, DSM-IV) and radiologically (PET scan) | RTVue XR Avanti | No | Yes | No | No significant difference. | Microvascular densities of DCP were significantly lower in AD than controls. | FAZ was larger in AD than controls. |
Yoon SP, 2019 | 39 AD, 37 MCI, and 133 control | NIA-AA | Cirrus 5000 Angioplex | Yes | Not available | No | Microvascular densities of SCP were significantly lower in AD than controls and AD vs MCI but not between MCI and controls. | Not available | No significant difference. |
Wu J, 2020 | 18 AD, 21 MCI, and 33 control | AD (NINCDS-ADRDA), MCI (Petersen Criteria) | RTVue XR Avanti | No | No | No | No significant difference. | Microvascular densities of DCP were significantly lower in AD and MCI than controls. | FAZ was significantly largest in AD, followed by MCI, and lastly controls. |
Current study | 24 AD, 37 MCI, and 29 control | AD (DSM-IV), MCI (Petersen Criteria) | Cirrus 5000 Angioplex | Yes | Yes | Yes | Microvascular densities of SCP were significantly lower in AD and MCI than controls. | Microvascular densities of DCP were significantly lower in AD than controls. | No significant difference. |
Alzheimer's disease (AD); Diagnostic and Statistical Manual of Mental Disorders (DSM-IV); Foveal avascular zone (FAZ); mild cognitive impairment (MCI); optical coherence tomography angiography (OCTA); Mini-Mental State Examination (MMSE); Montreal Cognitive Assessment (MoCA); National Institute of Aging-Alzheimer’s Association (NIA-AA); National Institute of Neurological and Communicative Disorders and Stroke and Alzheimer’s Disease and Related Disorders Association (NINCDS-ADRDA) |
The SCP is responsible for the metabolic supply of the ganglion cell layer,[17] where reduced number of retinal ganglion cells and axons has also been observed in post-mortem AD retinas.[7, 8] Changes in the SCP seen on OCTA further complement the already established retinal OCT structural markers.[9] Using OCT, several studies have reported the reduction of retinal nerve fibre layer thickness and ganglion cell layer thickness (presumably due to loss of retinal ganglion cells and axonal degeneration) in AD patients.[9] Whether the loss of retinal vessel precedes the loss of retinal neurons is currently unknown.
SCP in MCI individuals
Discordant results have been reported on the OCTA findings in MCI participants (Table 3). The current and one previous study[16] showed a reduced VD of the SCP in those with MCI whereas other studies did not report any differences.[13, 15] Alteration in the retinal vessels in MCI participants is compatible with studies using in vivo Doppler imaging techniques, where a decrease in retinal blood flow has been demonstrated in both AD and MCI participants.[36, 37] The conflicting results between MCI and control participants may lie with the definition of MCI, which represents a continuum of cognitive decline between “normal aging” and dementia. While the person is still able to carry out their activities of daily living with little or no help from others, a wide range of cognitive impairment is possible in MCI.[38] It is plausible that the change in retinal capillaries may occur only at a more severe stage of MCI or when certain cognitive ability is affected.
DCP in AD individuals
Four OCTA studies have investigated DCP VD in AD individuals, but there is generally a lack of agreement between studies (Table 4). Two studies[11, 15] showed a significant reduction in VD in AD individuals whereas the others[14, 16] did not observe any differences. Obtaining accurate OCTA metrics from the DCP layer is particularly challenging as it is affected by the physiologic variability of FAZ[18, 19] and projection artefacts.[20, 21] First, previous OCTA studies did not account for the FAZ in the deeper plexus. This is crucial because the FAZ in the deep plexus is considerably larger than superficial plexus.[39] Second, while Zabel et al. removed the projection artefacts in the DCP,[11] the rest did not.[14–16] In the current study, we quantified the VD of the DCP without the influence of FAZ and projection artefacts, which hopefully reduced measurement bias. We found a sparser VD of the DCP in AD participants but not in MCI individuals, which suggests the possibility of using the VD of the DCP to discriminate between (both SCP and DCP affected) and MCI (only SCP is affected) individuals.
The DCP is important for nutrition of the inner nuclear layer, which comprises of bipolar cells, horizontal cells, and amacrine cells.[17] In transgenic AD mouse models, Aβ deposits have been detected in the inner nuclear layer.[10] Microvascular changes of the DCP are complemented by the structural thinning of the inner nuclear layer thickness in AD individuals.[40] Changes in the DCP in AD individuals may present later in the disease stage, but a longitudinal study will be required to confirm this hypothesis.
Fractal dimension
In addition to capillary loss, we saw a significantly decreased FD of the SCP, which suggests a loss of vessel complexity in the inner retinal macula of those with AD and MCI compared to cognitively normal controls. However, an earlier study[41] observed a significant reduction in the FD in both plexuses in AD and MCI individuals. In contrast, we did not find any difference in the FD of the DCP between the groups. A plausible reason for the disagreement may be related to the vascular arborization pattern of the distinct layers. The SCP is supplied by the central retinal artery and composed of vessels running parallel of the retinal surface, thereby displaying a distinct vascular tree whereas the DCP is supplied by vertical anastomoses from the SCP, presenting as a lobular configuration.[17] Since FD is a measure of vasculature branching pattern complexity, it may be a more relevant biomarker for the SCP than the DCP. Our finding is in keeping with previous publications on larger retinal vessels using fundus photographs, where AD and MCI participants demonstrated a loss of vessel complexity.[30]
Perfusion density of large vessels
It should be noted that although VD in SCP was decreased in participants with cognitive impairment, perfusion density of the large retinal vessels remained unchanged. Previous studies have examined retinal vessels from fundus photographs and reported alterations in the venular caliber in participants with AD.[3] Retinal vessels measured from fundus photos are considerably larger in diameter than those obtained from OCTA.[4] Also, the diameters of retinal vessels measured with OCTA are in good agreement with the ground truth as obtained with adaptive optics ophthalmoscope.[42] Therefore, the lack of large vessel changes despite capillary changes suggests that microvascular alterations precede large vessel changes. The capillaries in the SCP may be particularly susceptible to the deleterious effects of neurodegeneration whereas the large retinal vessels may change later in the pathogenesis of AD. This finding would also suggest that the use of OCTA may be more sensitive in detecting changes in AD and MCI participants than fundus camera.
Foveal avascular zone area
Previous OCTA studies have quantified the FAZ area within the SCP region automatically using the OCTA software and reported conflicting results (Table 4).[11–15] Some reported a significant enlargement of the FAZ in individuals with AD compared to controls,[11, 12, 15] whereas others reported no difference in the FAZ area.[13, 14] In the current study, we did not find any differences in either of the plexuses between groups. Overall, the FAZ area has numerous limitations to serve as a biomarker of cognitive impairment given its physiologic variability, effect of axial length on OCT scan dimensions and segmentation/measurement limitations.[27, 35, 43–45]
Strengths and limitations
Strengths of this study include a well-phenotype cohort of AD and MCI individuals who were diagnosed according to internationally accepted criteria, and a standardized study methodology which further improved the validity of the imaging data. As explained above, we accounted for larger retinal vessels, FAZ dimension and projection artefacts from the analysis. Second, a quarter of our participants were excluded because of poor quality OCTA scans. Such high exclusion is comparable to other OCTA studies (~ 22% OCTA scans were rejected).[13] Since OCTA is based on motion detection, it is particularly sensitive to the patient’s eye movement. The need for good patient fixation can be challenging in elderly patients with cognitive impairment. Incorporating eye tracking during OCTA scanning can lessen the eye movement[46] but may lead to longer image acquisition time, which in turn result in patient fatigue. Nonetheless, we performed rigorous quality control on all scans, where all B-scans were checked for misalignment and segmentation errors. Images with segmentation errors, poor signal strength or eye diseases, which might potentially affect the OCTA measurements, were excluded. This robust data preparation is also evident by the particularly high signal strength among our participants, and further corroborates the validity of our results. Third, we adjusted the possible confounding effects of age, gender, race, and the presence of diabetes, and systemic blood pressure levels during multivariate analysis and excluded possible confounding factors, such as glaucoma, vascular or nonvascular retinopathies, age-related macular degeneration.[24, 25, 32]
Our present study has a few limitations. First, even though there was reduced retinal VD in those with AD and MCI using OCTA, this was only a cross-sectional study. It remains unclear whether the changes of retinal capillary are predictive of cognitive decline. This will be investigated in ongoing follow-up studies. Second, quantification of VD[18] and FAZ[45] can be affected by the effect of OCT magnification. We did not perform ocular biometry and thus were not able to rescale the scan dimensions.[18] Instead, we masked the FAZ from the VD metrics which should have mitigated some of this measurement bias for the VD maps. We acknowledged that the FAZ area is still affected by the individual differences in axial length. Another limitation of this study is the relatively small sample size of those with AD and MCI, limiting the power for evaluating the OCTA metrics in different types of MCI and AD. Lastly, this study was restricted to older adults of Asian ethnicities, therefore generalizability of our results to persons with young-onset dementia or of non-Asian ethnicities may be limited. Also, it remains unclear how the OCTA relates with magnetic resonance imaging (MRI) markers of cerebrovascular disease markers and amyloid PET and CSF biomarkers.
In conclusion, our study shows that compared to cognitively normal controls, eyes of participants with AD demonstrated significantly sparser VD in both plexuses whereas MCI participants only showed reduction of the SCP. Furthermore, we found that AD and MCI participants exhibited a loss of vessel complexity of the SCP when compared with controls. Our findings suggest that the changes at the retinal microvasculature at the capillary level may reflect similar changes in the cerebral vasculature in individuals with neurodegenerative diseases and demonstrates the potential of OCTA for the early screening of cognitively impaired individuals.