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Aims: The purpose of the present study was to investigate the relationship between delusional thoughts (delusional ideation or misidentification) and frontal lobe function using the Japanese version of the Frontal Assessment Battery (FAB) bedside screening neuropsychological test in early stage Alzheimer's disease (AD) patients.
Methods: Forty-eight probable AD patients with Mini-Mental State Examination score ≧18 points and a clinical dementia rating score of either 0.5 or 1.0 were divided into two groups based on data obtained from interviews with their caregivers: a delusional thought group (n = 19) and a non-delusional thought group (n = 29). The FAB total and subtest scores were then compared for the two groups.
Results: Significant differences were found between the FAB total (P < 0.01) and subtest scores (similarities, motor series, conflicting instructions; P < 0.05) for the two groups. Multiple regression analysis showed that delusional thought was significantly associated with the FAB total score.
Conclusions: In addition to episodic memory disorders, a reduction in the FAB score may reflect frontal lobe dysfunctions, including executive function, in patients with AD, leading to delusional ideation.
ALZHEIMER'S DISEASE (AD) is mainly characterized by episodic memory disorders, visuospatial impairments, and executive dysfunction.1,2 With the progression of neuropathology or cognitive decline, delusional ideation or misidentification are frequently seen as behavioral and psychological symptoms of dementia (BPSD).3 AD with psychosis (delusional ideation or misidentification and hallucination) occurs in approximately 30–60% of patients with AD.4–6 Regarding the type of delusion experienced by AD patients, several studies have reported delusions of theft, persecution, infidelity, and abandonment.4–7 Delusional misidentifications such as believing that one's house is not one's home, that one's spouse is an imposter, and that televised images are actually present in the house have also been reported.4–7 Delusions in AD patients can create a distressful burden on their caregivers and can trigger or predict nursing home placement or death.8,9 Therefore, the assessment of delusions in AD patients is considered important, and some studies have investigated the incidence and risk factors for delusions in AD patients.10,11
In demented patients, the manifestation of psychosis, such as delusional thought or hallucination, is considered to result from the development of executive dysfunction.12–14 Frontal lobe function involves executive function and, reportedly, is significantly correlated with the total Frontal Assessment Battery (FAB) score.15–17 A previous study reported that the FAB score reflects frontal lobe dysfunction, including executive dysfunction, and is an easily administered test that can be completed at the bedside within 10 min.15,17 The FAB screening neuropsychological test consists of six subtests involving comparatively simple tasks that do not require any tools.17 FAB scores significantly differed between patients with frontotemporal dementia and those with AD, but MMSE scores did not, therefore the validity of FAB reflected frontal lobe function, as indicated in a study of two different types of brain region degeneration.18
With regards to neuroimaging, studies using single-photon emission computed tomography have found hypoperfusion in the bilateral dorsolateral frontal cortex, right anterior cingulate gyrus, inferior–middle temporal cortices, and right posterior parietal region in AD patients with delusional thoughts or psychosis.19–21 Blackwood et al. reviewed and stated that the presence of ‘reality distortion’, which leads to persecutory delusions, was correlated with the cerebral blood flow in the left lateral prefrontal cortex, ventral striatum, superior temporal gyrus, and parahippocampal region,22 and a positron emission tomography study found hypometabolism in the right prefrontal cortex and left medial occipital region in AD patients with delusional thoughts.23,24 Although various disputable results have been demonstrated in such previous studies, some neuroimaging studies support a relationship between neuropsychological frontal lobe dysfunction and delusional thoughts in patients with AD.23,20
The FAB has not been previously reported as a tool for performing neuropsychological evaluations in AD patients with delusional thoughts. In the present study we used the Japanese version of the FAB25 to investigate the relation between the manifestation of delusional thoughts and frontal lobe dysfunction in patients with AD.
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The present results showed that the FAB total and subtest scores (Similarities, Motor series, and Conflicting instructions) were significantly lower in AD patients with delusional thoughts than in those without delusional thoughts. Dubois et al. reported that the FAB scores were significantly correlated with the number of criteria and the number of perseverative errors on the Wisconsin Card Sorting Test (WCST), which was established to measure executive function.17 The WCST is considered to be sensitive to executive dysfunction30 and evaluates both conceptual ability and behavioral regulation.31
In previous studies, dementia patients with psychosis were characterized as having greater executive and visuoperceptual impairments in their neuropsychological functioning patterns.13,14 The present study showed that a decrease in the FAB score, including executive dysfunction, but not the MMSE score, was related to delusional thoughts. The MMSE has a few items of executive function but is heavily weighted toward orientation or memory items.18 Therefore, impairments in frontal lobe function, including executive function, rather than episodic memory or orientation disorder might be strongly related to the manifestation of delusional thoughts.
Of the six FAB subtests, the Similarities subtest includes the function of abstract reasoning, which can also be investigated using card-sorting tasks and proverb interpretation.17 Subjects have to conceptualize or integrate the links between some objects from the same category. In previous studies, patients with psychosis showed specific impairments on task requiring complex integration, that is, the capacity to form an overall impression by holding various fragments of information.31,32 Therefore, AD patients with delusional thoughts might manifest as the result of inaccurate memory with poor insight related to executive dysfunction, leading to errors in logic and comparing internal experiences with reality.13,23 The Motor series subtest measures the capacity to execute a sequence of actions successively in separate tasks; it resembles the ‘first-palm-edge’ task in Luria's motor series.17 The Conflicting instruments subtest resembles the Stroop test task and requires the capacity to perform a contrary reaction to each of two pattern directions.17 In order to carry out both tasks effectively, self-correcting or monitoring capacity that evaluates whether subject himself could perform the tasks accurately are required. Previous studies reported that alternations in self-monitoring were associated with psychosis with evidence of specificity for delusional ideation.31,33,34 Therefore, such impairments of self-correcting system might lead to delusion in AD patients in the present study.
Regarding the subtypes of delusional thoughts, the present study indicated that the ‘people are stealing things’ delusion and ‘suspiciousness’ ideation occurred frequently, similar to that found in previous studies.20,23 In contrast, typical delusional misidentification, such as ‘one's house is not one's home’ and ‘one's spouse is an imposter’, were not reported in the present study. Previous studies have reported that approximately 40–50% of delusional thoughts in patients with AD are delusional misidentifications.4,20,35 Lower levels of cognitive functions and lower average scores on cognitive tests, such as the MMSE, have been reported for AD patients with delusional misidentifications, compared with those with persecutory delusions.4,35
The limitations of the present study included the relatively small sample size and the use of only two neuropsychological tests: the FAB and the MMSE. If simple scales or tests measuring attentional, visuospatial function, or semantic memory had also been used, significant differences might have been obtained. The present study was limited to subjects with early stage AD whose MMSE scores were ≧18 points and who had a CDR of either 0.5 or 1.0 because even though the FAB tasks can be performed without tools or instruments, the tasks do contain relatively complex question forms that include several steps.
In spite of these limitations, the present study supports the hypothesis that frontal lobe dysfunction might be related to delusional ideation or misidentifications in patients with AD, supporting the results of neuropsychological and neuroimaging studies. Moreover, these results suggest that a simple neuropsychological screening test reflecting frontal lobe function and including mainly executive function – such as the FAB – might be useful for predicting the manifestation of delusional thoughts in patients with AD, providing important information regarding the selection of treatment stages that will reduce the early burden of caregivers.