Q&A: ERG Biomarkers of Dementia

Which test do you use for contrast sensitivity?

The severity of color vision deficiency was tested using a tablet-based Cone Contrast Test unit (CCT, Provideo CCT Plus System, Innova Systems Inc., Burr Ridge,IL, United States). Further descriptions about this test can be found in Rabin, J., Gooch, J., and Ivan, D. (2011). Rapid quantification of color vision: the cone contrast test. Invest Ophthalmol. Vis. Sci. 52, 816–820.

Is there any explanation on why the contrast sensitivity is increased in Alzheimer Disease Patients?

This question, as pointed out, is not clear at all. I am afraid the question is related to the fact that we need to enhance contrast in the environment where AD people live. So, here is the answer based on this assumption:

Researchers have shown that contrast sensitivity deficits result from damage to the brain rather than damage to the retina in the eye or the optic nerve. Specifically, the neuropathological hallmarks of AD (i.e., the presence of senile amyloid plaques and neurofibrillary tangles) have been identified in higher-order visual areas of the brain, including regions within the occipital lobes (extra-striate visual cortex), parietal lobes (posterior regions), and temporal lobes (inferior areas).

Individuals with AD have more difficulty finding objects in their visual field, especially when they relate to contrast. The smaller the contrast difference (e.g., a scene that changes from black to gray or when pouring black coffee into a black mug), the less likely the individual with AD will be able to find it. This is why it is recommended to enhance contrast in an individual’s everyday environment with AD.

Which tests are included in the model 5 of the ROC Analysis from your paper?

The Model 5 of our ROC analysis includes the multifractal generalized dimensions, the lacunarity parameter, singularity exponents and the implicit time of the ERG.

What is the consequence of the reduced vascularity? Does the reduced vascularity only affect green color vision? Is there any theory on why green color vision (M-cones) are more affected in Alzheimer Disease?

The vascularity does not only change in the eye but in all parts of the body, especially in the brain. This reduced vascularity will lead to a disbalanced oxygen/nutritions supply in all parts and thus leading to decrease of cognitive abilities. In particular, this reduction of blood flow efficiency and impairment in circulatory transport is due to a reduction from optimal vascular network architecture.

In our studies, we found more patients with more green deficiency than red or blue deficiency. However, more research is needed to elucidate whether the reduced vascularity is only affecting green color vision. Interestingly, it has been reported that individuals with cognitive deterioration due to AD struggle discriminating between green and blue stimuli on the Stroop test which relies on a cognitive measure that requires intact color vision (Cohen et al., 1988; Fisher et al., 1990). These results add to the evidence that extrastriate lesions could result in tritanomalous color deficits (Meadows, 1974; Pearlman et al., 1979), and that the extrastriate cortex is severely disturbed neuropathologically in AD (Lewis et al., 1987). Therefore, pathological changes due to cognitive decline observed in the striate area (IVcß) of the brain that receives color information from the lateral geniculate nucleus, suggest additional basis for deficits in color vision in the brain as described in our studies (Beach and McGeer, 1988).

It might be hypothesized that the M-cones are the most demanding cells and thus suffering first from changes. However, most of the findings in Alzheimer’s are still subject to more research and need to be confirmed in different studies.

Also, the differences in the preferential damage of parvocellular pathway (P-cells) versus magnocellular pathway (M-cells) could explain dissimilarities between different neurodegenerative diseases like AD, PD, etc. at the clinical and retinal level. Axons from M-cells (with cell bodies situated in peripheral macula and retina) are in the superior, nasal, and inferior regions around the optic nerve (where the RNFL is measured). On the other side, P-cells predominate in the central macular area (where the macular GCC is measured), their axons project to the temporal portion of the RNFL, and they are highly related to color discrimination, visual acuity, central visual field sensitivity, and contrast sensitivity for high spatial frequencies. The M-cells relay information about achromatic vision, motion detection, peripheral visual field sensitivity, and contrast sensitivity for low spatial frequencies. Interestingly, the involvement of the magnocellular stream of visual processing is significant because the cell loss and predominant deposition of amyloid plaques and neurofibrillary tangles occur in the primary visual cortex of AD individuals with a prevalence in the M-pathway. A study from Sadun et al., 1990 showed that the optic nerve showed predominant loss of the largest class of retinal ganglion cells (M-cells). Sartucci et al. 2010 also hypothesized that AD involves a deficit in the M-pathway. However, the hypothesis of a preferential injury of P-cells or M-cells in AD is still unproven, but it may be related to predominant damage of optic nerve axons in AD (like those from M-cells). Of note, the M pathway is considered to have a greater influence in the peripheral retina, where the drusen-like features or hyperintense spots have been mostly observed in AD. Also, the M system is very sensitive to low spatial frequency stimuli, and people with Alzheimer’s have difficulty perceiving low spatial frequencies.

Does the FD change with the age, or is it determined in the beginning and then considered as a risk factor?

The FD will capture the morphology of the structure as we age. There is a decreasing trend of the fractal dimension associated with aging, and this is one of the reasons why we used age-matched subjects in our pilot study,  see this link for more info: https://pubmed.ncbi.nlm.nih.gov/20472327/).

A longitudinal study with a baseline measurement will benefit by tracking changes over the years to also obtain risk measures.

What type of Drusen are related with Alzheimer and how to differentiate them from AMD?

We referred to drusen-like features or hyperintense spots in the peripheral retina as observed in our cSLO images from patients without/no history of AMD. Some of them were smaller with well distinct margins and located in the superior quadrant, and scattered along the blood vessels.

Prior studies have identified the Alzheimer’s Aβ protein as a potential activator of the complement cascade in the context of drusen formation, so AMD and AD may share common pathogenic mechanisms. People with Alzheimer’s disease are more likely to have drusen-like spots in their peripheral retinas. However, most patients with AMD do not have Alzheimer’s disease. This is a very controversial area of research, and prospective studies are needed to establish whether drusen and AMD are independent markers of AD/dementia and whether drusen and AMD have an added value in the diagnosis of AD/dementia. A good review on the connections of AMD and AD could be found here: https://pubmed.ncbi.nlm.nih.gov/27814598/

Have you been able to use OCT and electrophysiology to determine dementia (such as loss/thinning of certain layers in the retina)? 

OCT could be one exciting technology to detect AD/dementia. Many groups have used the OCT modality and observed a reduction of the nerve fiber layer thickness in patients with Alzheimer’s. Also, the OCT-A could be a helpful addition to the analysis as it facilitates the study of retinal vascularity and quantification of vascular perfusion. I am mainly not using OCT because I desire a test option that can be more accessible and ubiquitous. But in general, OCT is a potentially good solution in diagnosing Alzheimer’s once we could standardize the methodology of analysis and robustly verify OCT retinal signatures and differentiate them from signatures observed in other neurodegenerative diseases with similar retinal alterations.

Have you compared your results to amyloid PET scan?

No, we have not used amyloid PET scan data in our pilot studies. Although some study subjects had PET results in their EMRs within two years of recruitment, we have not used amyloid PET data because we designed the studies to develop the multimodal methodology with a minimal research budget, and not all subjects recruited had these data in their EHRs. Amyloid PET imaging is a very invasive, time-consuming, and expensive technology. However, it is one of the current confirmatory biomarkers that need to be used to test the predictive models using retinal features, and our current/future studies include this brain imaging data.

How confident are you about the observations in a larger scale study?

I am highly confident that our preliminary results will benefit from a longitudinal study with larger sample size, and we are already working on it.