In order to characterise the cognitive deficits associated with SIVD, a large sample of elderly stroke patients and healthy controls were investigated using a detailed neuropsychological battery and MRI. The patients with SIVD tended to be older and had less education than the OS patients. After controlling for these demographic factors, the results revealed both quantitative and qualitative differences in cognitive functioning between the study groups. Firstly, as compared to the NC group, the neuropsychological test performance of patients with SIVD was clearly inferior throughout cognitive domains. Secondly, despite comparable levels of global cognitive functioning in the Mini‐Mental State Examination, the patients with SIVD had significantly lower scores in executive functioning and delayed memory recall in contrast to the OS patients with larger infarct volume. However, the performance of these two patient groups did not differ in mental speed, short term memory, immediate memory recall, or verbal and visuospatial functions. Of the individual neuropsychological tests, the measures of mental flexibility, response inhibition, verbal fluency, and delayed recall of learned material significantly differentiated the patients with SIVD from the OS patients.
In previous studies, the cognitive performance of patients with variously defined subcortical vascular disease has been compared to healthy elderly subjects6,32
or to patients with mild cognitive impairment33,34
or Alzheimer's disease.7,8,32
It has been suggested that the pattern of cognitive deficits in patients with subcortical ischaemic lesions is distinctly different from that of Alzheimer's disease,7,8
and resembles Parkinson's disease with disproportionate impairment in executive functioning.35
However, studies investigating patients with mild cognitive impairment and subcortical vascular disease detected on computed tomography scan have reported diverse results.33,34
To our knowledge, the present study is the first to describe in detail the cognitive features of SIVD in comparison with elderly stroke patients. Our results indicate that executive deficits, comprising deficiency in mental flexibility, set shifting, response inhibition, and fluency, are salient cognitive characteristics in SIVD. It is notable that verbal fluency can distinguish patients with SIVD not only from those with Alzheimer's disease as suggested earlier,8
but also from other elderly stroke patients having large vessel disease and major strokes.
As reported in our previous study,5
the patients with SIVD exhibited more depressive symptoms than patients with OS. Subcortical vascular pathology has also been observed to contribute to late life depression in other studies.36
According to the vascular depression hypothesis, depressive syndrome in elderly patients can be predisposed, precipitated, or perpetuated by cerebrovascular disease.37
In our sample, however, depression scores did not explain the cognitive deficits in patients with SIVD. Although cognitive and mood changes are assumed to be parallel consequences of frontal subcortical disconnections,9,37
our results indicate that they are nevertheless independent processes associated with SIVD.
Another important finding was that controlling for MTA had no effect on the group differences in executive functioning. Instead, the patient groups no longer differed from each other in delayed memory recall after adjusting for MTA. Kramer et al32
have found a heterogeneous pattern of forgetting in patients with SIVD, and they suggest that rapid forgetting in SIVD may imply concomitant Alzheimer's disease. The patients with good retention had worse executive performance and thus were assumed to suffer from pure vascular pathology. Similarly, in our study, it is possible that the delayed memory deficits explained by MTA reflect co‐existing Alzheimer pathology, and that only the executive deficits are vascular in origin. Unfortunately, to date there are no means available to unequivocally exclude Alzheimer's disease in vivo and therefore Alzheimer pathology may also contribute to a fraction of our sample in both patient groups. On the other hand, it is not clear whether MTA explicitly indicates Alzheimer's disease or whether it equally belongs to the pathological processes in “pure” SIVD. In a post mortem study, the pattern and the degree of hippocampal neuronal loss were similar in patients with Alzheimer's disease and subcortical vascular dementia.38
It has been proposed that hippocampal atrophy in patients with SIVD is a result of a mixture of ischaemic and degenerative processes.39
Moreover, in our sample the memory deficits associated with white matter hyperintensities have earlier proven to be mediated by executive deficits and, therefore, they may reflect secondary impairment due to inefficient encoding and retrieval strategies.28
It should be noted that the effect sizes of the differences between the two patient groups were rather small. Despite specific MRI findings, there is substantial overlap in the cognitive features of the patients with SIVD and other consecutive stroke patients. The small effect sizes can also reflect high variability in cognitive performance among elderly stroke patients. Notwithstanding this heterogeneity, subtle but significant differences were found between the patient groups. Furthermore, when the patients with SIVD were compared to healthy control subjects, the group differences in cognitive functioning were moderate in magnitude. Another important point concerns the generalisability of the results. They accurately represent the clinical stroke population, but may not be entirely applicable to the general elderly population. Future studies should augment the clinical picture of MRI defined SIVD in population based settings.
The grouping of the neuropsychological test variables into cognitive domains was based on clinical experience and judgment instead of statistical factor modelling. These domains are multidimensional and also, to some extent, overlapping constructs that reflect general, but clinically relevant, aspects of cognitive functioning. In particular, executive functions are here considered as a global concept encompassing a wide array of subfunctions related to attention, volition, planning, and effective performance.21
The reliability coefficients indicated that the individual test variables in each of the composite scores had sufficient internal coherence. In addition, the results of the composite and individual scores were consistent with each other.
In conclusion, cognitive deficits in SIVD 3 months post stroke are notable in severity and can be qualitatively distinguished from those of OS patients. The results support the view that SIVD is a separate clinical entity.