Dementia with Lewy bodies (DLB) is the second most common type of cognitive degeneration after Alzheimer's disease (AD) (169). Clinically, DLB is characterized by spontaneous parkinsonism and progressive dementia associated with fluctuating cognitive functions, and hallucination (169). Parkinsonism and dementia tend to co-occur. A history of Parkinsonism predating dementia by more than 1 yr might be better designated "Parkinson's disease with dementia." Since publication of the clinical and pathological diagnosis criteria (169), several studies have tried to delineate the neuropsychological features that distinguish DLB disease from AD. A number of them have demonstrated that, compared with AD, visuospatial and visuoconstructive abilities are disproportionately impaired in patients with DLB disease (170-173). Compared with AD patients matched for age, sex, education, and Mini Mental State Examination (MMSE) score, DLB patients perform worse on the Raven Colored Progressive Matrices test and on the picture arrangement, block design, object assembly, and digit symbol substitution subtests of the Wechsler Adult Intelligence Scale-Revised (173). They also perform worse on size discrimination, form discrimination, visual counting, and overlapping figure identification (174).
Mori et al. (174) also demonstrated that, in the DLB group, patients with visual hallucinations scored significantly lower on overlapping figure identification than those without, whereas patients with television misidentifications gave significantly lower scores on the size discrimination, form discrimination, and visual counting tasks than those without. The underlying assumption is that specific brain regions are involved in the performance of these tests: the occipital visual association cortex for size discrimination, the occipito-temporal visual association cortex for size discrimination, and the occipito-parietal cortex for visual counting.
Similar results were obtained by Simard, Rikum, and Myran (175). These authors compared the performance on the Benton Judgement Line Orientation Test of patients with DLB and predominant parkinsonism, with DLB and predominant psychosis, and with AD. For this purpose they analyzed errors as resulting from visual attention and visuospatial perception failures. The study did not find significant differences on the total score of the Benton Judgement Line Orientation Test. However, error analysis demonstrated that subjects with DLB and psychosis have more severe visual-perception (VH errors) impairments than subjects with DLB and predominant parkinsonian features, and AD subjects.
These results suggest that defective visual input caused by visual-system damage can result in hallucinations from defective visual processing or abnormal cortical release phenomena (176). Indeed, Imamura et al. (177) using positron emission tomography, found that visual hallucinations in DLB patients are associated with relatively preserved metabolism in the right temporoparietal association cortex and severe hypometabolism in the primary and secondary visual cortex.
A practical implication of the disproportionate impairment of DLB patients in visuospatial tasks has been demonstrated by Ala et al. (178). These authors analyzed accuracy in copying the interlocking pentagon item of the MMSE in patients with neuropathologically confirmed DLB and AD. They concluded that in patients with MMSE scores >13 an inability to accurately copy the pentagons suggests that the diagnosis is more likely DLB than AD with a sensitivity of 88% and a specificity of 59%.
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