The present study investigated the characteristics of cognitive profiles of patients with DLB and attempted to differentiate DLB from AD and PDD by using comprehensive neuropsychological assessments, which covered a wide range of cognitive domains. It is very important to be able to differentiate between the cognitive profiles of DLB sufferers and those with AD and PDD. The distinctive patterns of neuropsychological dysfunction observed in these dementias probably represent a different distribution of pathological changes. The neuropathological substrate of AD affects predominantly the medial temporal cortex and the neocortical association areas, which explains the predominant dysfunction of episodic memory function. On the other hand, the neuropathological basis of DLB includes neuronal loss and the presence of Lewy bodies in the subcortical nucleus and in the frontal and parietal lobes, which explains the predominantly attentional, executive, and visuospatial dysfunctions.21
Recent clinical pathological data have demonstrated that DLB and PDD patients have the same underlying pathology of the Lewy body spectrum.22,23
Most PDD patients may indeed show clinical features and neuropsychological characteristics that are similar to those exhibited by DLB patients.
Our results showed that verbal memory function in DLB is comparable with that in AD. Unlike previous studies,9,10,24
the degree of impairment in the episodic memory tests was the same in our DLB and AD patients. The discrepancies between the results of our study and those of prior studies9,10,24
may be explained by several factors. First, the K-MMSE scores were lower in our patients than in those included in previous studies.9,10
Thus, the floor effect, in items of memory function, may have occurred in both DLB and AD patients. Second, both DLB and AD are often linked to degeneration in the medial temporal lobes, including the hippocampus and amygdala. Hamilton et al.11,25
suggested that the poor memory function in DLB patients is attributable to an encoding impairment rather than the consolidation defect. Third, despite our attempt to exclude mixed pathologies of DLB and AD, some patients with concomitant DLB and AD were included in our study sample.
Our findings are consistent with those of previous studies in showing that the frontal executive function is much worse in DLB than in AD.9-12
The findings of the current study reflect a different distribution of neuropathological changes in DLB, which include widespread neuronal loss with the presence of Lewy bodies in the subcortical nuclei, and the frontal subcortical association and dorsolateral areas of the frontal lobe. The Stroop test is widely used for evaluating executive function and is considered to measure impairments in the dorsolateral prefrontal system. The results of the Stroop test in the present study suggest that the circuit associated with color reading is frequently disrupted in patients with DLB, with these interruptions occurring in the dorsolateral prefrontal lobe.
Unlike the results of previous studies,9,10,26,27
our data revealed that visuospatial function on RCFT did not differ significantly between the DLB and PDD patients and the AD patients. The current results show that the patients with DLB and PDD had lower scores for K-MMSE and lower educational levels than the participants in previous studies,9,10
suggesting that these factors influenced their performance on the RCFT. The scores for visuospatial function on the RCFT tended to be lower scores in the DLB and PDD groups than in the AD group.
In the present study we found differences between DLB and PDD patients, which contrasts with the findings of previous studies.9,10,13,28
The deficits in the cognitive domains of attention, memory, and executive function were greater in our DLB patients than our PDD patients. However, visual recognition scores did not differ between these two groups.
In a comparison of the cognitive profiles of patients with PDD and those with DLB, the MMSE scores of whom indicated a mild stage of dementia, Aarsland et al.9
and Downes et al.28
demonstrated that executive function was impaired more in patients with DLB than in those with PDD. The K-MMSE scores of our patients indicated a mild stage of dementia, and the performance for attention, executive function, and verbal memory was worse in those with DLB than in those with PDD. These data suggest that although PDD and DLB share similar global cognitive patterns, the cognitive deficits in the frontal and medial temporal-related cognitive functions during the mild stages of dementia are greater in patients with DLB than in PDD patients. It is suggested that the distribution of Lewy body pathology differs between PDD and DLB patients in the early disease stage. There is also evidence that the presence of amyloid pathology differs between DLB and PDD.29,30
Demonstration of a significant amyloid pathology in [11C] Pittsburgh compound B positron-emission tomography might explain why the level of deterioration in memory and executive functions is higher in DLB than in PDD. A very recent study found that a subgroup of PDD patients not experiencing cognitive fluctuations showed significantly less attentional, executive, and memory deficits compared to those with DLB and PDD experiencing cognitive fluctuations.31
The differences in cognitive profiles between DLB and PDD in our data may be influenced by the discrepancy of distribution in the two subgroups of PDD.
There present study was subject to some shortcomings. First, selection bias may have been present in our sample because the patients were recruited either through a memory clinic or through a PD clinic. In addition, the results of our study are limited by the disproportionate numbers of AD patients and DLB and PDD patients. Second, we cannot be certain about the accuracy of our clinical diagnoses because of the lack of neuropathological confirmation. Although we attempted to exclude cases with suspected mixed pathology, some of the patients with DLB may have had concomitant AD. Third, the neuropsychological assessment tool utilized herein is not fully adequate to ascertain the detailed characteristics of several cognitive domains.