We demonstrate differential relationships between WMH load and voxel-wise and regional GM atrophy in the DMN and ECN with low WMH being positively associated with GMV and high WMH being negatively associated with GMV. Such a differential relationship was only observed in the MCI stage with cognitively normal individuals showing primarily reduced GMV associated with WMH. Specifically, in MCI, low WMH load was related to higher GMV in the ECN and lower voxel-wise frontoparietal GMV, however as the load of WMH increased, only a negative relationship was seen. At high WMH load, frontoparietal voxel-wise GMV and within the DMN and ECN ROIs demonstrated a negative relationship with increasing WMH burden. We also demonstrate GMV to be a key moderator in the relationship between WMH and cognition in MCI. Specifically, higher WMH was related to worse memory and executive function moderated by GMV in the DMN and ECN. While at low WMH load, only memory performance was affected, at high WMH load, both memory and executive function were affected. Our results highlight variable relationships between WMH and GMV, dependent on the amount of cerebral WMH burden and their subsequent association with cognition in participants with MCI, indicating that such associations cannot be assumed to be linear in nature. This variable relationship may have important implications in the clinical management of MCI patients with varying loads of WMH.
Cerebral WMH has been associated with several mechanisms including small vessel cerebrovascular disease, GM atrophy and neuroinflammation (15, 21, 26, 51, 52). It has been widely assumed that irrespective of the underlying mechanism, increasing WMH load will result in progressive increase in GMV loss. Our study demonstrates that this assumption is dependent on the baseline WMH load. While we demonstrate GMV decline in both the DMN and ECN ROI with high WMH volume, at low levels of WMH, this relationship is reversed such that increasing WMH is associated with higher GMV, primarily in the ECN. We additionally show, that at an intermediate load of WMH, the relationship between WMH load and GMV is stable and a negative relationship between increasing WMH load and ROI GMV is only observed when a certain threshold of WMH is reached. Our results are thus one of the first to shed light on a differential relationship between increasing WMH load and GMV, especially in the ECN, a network affected by the presence of cerebrovascular disease. In support of such findings, a few prior studies have illustrated increased cortical thickness related to the presence of WMH in older subjects without dementia (21, 53). Such findings suggest that mechanisms that have not been clearly elucidated may occur in response to low loads of WMH in frontoparietal regions especially in the absence of dementia. Increased cortical thickness has also been found in ageing studies likely indicative of local plasticity (54, 55). In patients with early stage cerebrovascular disease, inflammatory responses related to blood-brain barrier disruption may lead to the build-up of WMH accompanied by an increase in brain structural measures(52, 56). Such processes may underlie the relative sparing of GMV at low and intermediate WMH loads in our study in early stages of disease. However, the functional implications of these associations still remain under conjecture.
The finding of widespread negative association between WMH load and GMV at both the voxel-wise and ROI level at high WMH load evidenced in our study is supported by numerous prior studies (11, 15–22, 26). However, the mechanisms underlying this relationship need to be examined further. One possible mechanism includes anatomical disruptions due to the presence of subcortical WMH lesions subsequently leading to structural alterations of the cortex because of anterograde degeneration (57) as well as damage to specific white matter tracts connecting these regions (25, 58). Additionally, changes in cortical structures themselves can lead to axonal loss and demyelination due to Wallerian degeneration (59). Furthermore, the presence of WMH likely reflects microvascular damage, hypoperfusion and ischaemia within the cortex which may also underlie reduced GMV in overlapping regions and those connecting tracts affected by WMH (60, 61). Notably, in the present study, the relationships between WMH and GMV remain unchanged after controlling for both hypertension status and systolic blood pressure, thus suggesting other independent WMH-related factors might be involved. Thus, in line with and in addition to previous findings, our results lend evidence to differential load-dependent relationships between WMH and brain structure.
The influence of WMH on cognition is predominantly thought to result in poor outcomes with prior studies showing reduction in executive function, memory and global cognition including perceptual speed (15, 27). However, in our study there were no pairwise associations between WMH and cognition, nor between GMV and cognition. Instead, the negative association between WMH and cognition was significant only when GMV in the ECN and DMN were taken into account. Specifically, at tercile 1, memory decline was associated with increasing WMH moderated by lower GMV in the DMN and ECN. At tercile 3, more widespread associations were observed. Memory function and executive function were negatively associated with increasing WMH moderated by GMV in the DMN and ECN. Our results thus support the notion that increasing WMH burden is related to more widespread cognitive decline, with this relationship being strengthened by GMV loss in frontoparietal regions. These findings are in line with prior studies showing cortical atrophy mediating the relationship between WMH and cognition including memory (11, 21, 22, 30). Since GMV moderated the relationship between WMH and cognition at both low and high WMH loads, it is likely that GMV is an important moderator regardless of WMH load. Additionally, increasing load of WMH, specifically periventricular WMH may result in damage to cholinergic neurotransmitter systems, and result in cognitive decline (62). Thus, further studies assessing the effect or varying WMH load on cognition and the role of regional GMV in this association at various stages of disease are required.
The clinical relevance of our study may be that specifically in patients with MCI, presence of low burden of WMH may be indicative of an early stage of cortical neurodegeneration (especially in regions critical for memory and executive function) wherein there is no grey matter loss, but instead there is compensatory grey matter increase (21, 53). Detection of this stage (MCI with low burden of WMH) may provide a window of opportunity to institute interventions to retard the neurodegenerative process. However, when WMH load crosses a certain threshold, irreversible GMV loss begins and disease modifying interventions to slow GMV loss may be less beneficial.
Limitations and future directions
Our study has several limitations. Since our analyses are based on cross-sectional data, our findings need to be further validated using a longitudinal dataset. Some of our findings did not pass multiple comparisons correction due to our moderate sample size though we used normalised data and data-driven methods to classify our levels of WMH load. In addition, although our MCI group was comprised of largely the amnestic sub-type, the presence of non-amnestic MCI subjects might confound the relationship between WMH, GMV and cognition. Notably, our cohort is representative of urban populations in Asia and worldwide. The generalizability of our findings to older populations and patients with lower education attainment will need to be studied further in future studies. We also do not have a sufficient sample size to study this effect on patients with AD dementia. Moreover, the amyloid and tau status of our subjects was not known, thus we were unable to assess the influence of these AD biomarkers on the relationship between WMH and brain structure. Thus, future work will need to involve understanding the interaction between AD risk factors, WMH and GMV as well as the effect of white matter disruption on cognition across the AD spectrum.