Does executive impairment define a frontal variant of Alzheimer's disease?

BACKGROUND

People with Alzheimer's disease (AD) who present with prominent frontal features such as a dysexecutive syndrome may be difficult to differentiate clinically from subjects with frontotemporal lobar degeneration (FTLD). This study was performed to improve the differential diagnosis between AD and FTLD and to better characterize the AD subgroup with greater executive dysfunction.

METHODS

Using a well-defined prospectively studied cohort of cognitively impaired subjects, which included those with AD and with FTLD, we nominated a frontal variant of AD (FvAD) group as those AD subjects with the lowest quartile of scores on the Frontal Assessment Battery (FAB), indicating greatest executive dysfunction, and compared them with the rest of the AD cases (whom we called the AD group) and those with FTLD across several baseline variables including cognitive, functional and behavioral scales. We also compared the changes from baseline for these three groups at 6 and 12 months. Additionally, we controlled for dementia severity by matching AD and FTLD cases on a functional scale, the SMAF, and repeated the same comparisons with these severity-matched groups.

RESULTS

The 114 FvAD subjects had a mean age of 78.1 years and Mini-mental State Examination (MMSE) scores of 16.6, and the (remaining) AD group had a mean age of 78.4 years and MMSE of 22.4. There were 30 FTLD subjects with a mean age at baseline of 70.9 years and a mean baseline MMSE of 23.4. The FvAD group was significantly more severely impaired than the other two groups on all baseline assessments except the behavioral scale, the Neuropsychiatric Inventory (NPI), where there was insignificantly less impairment than in the FTLD group. In the analysis of subjects matched at baseline for functional impairment, the FvAD and FTLD groups were not significantly different on most assessment scales although on the FAB, clock-drawing and MMSE the FvAD subjects were still significantly more impaired. These two severity-matched groups were also similar in other baseline characteristics except for older age and less psychotropic use in the FvAD group. The severity-matched FvAD group was significantly different from the AD group in almost all assessment scales. All three unmatched and matched groups declined similarly over 12 months.

CONCLUSIONS

When groups were not matched for baseline severity, the use of the FAB defined a group of AD subjects with greater executive dysfunction that were distinguished from both the remainder of the AD and FTLD subjects in almost all domains except behavioral disturbance and probably were just more severely affected AD subjects. The FAB is thus more useful as a marker of dementia severity than as a scale to detect a frontal variant of AD or to distinguish AD from FTLD. Controlling for severity, however, did allow the definition of a subgroup of AD subjects that more closely resembled FTLD subjects than the remainder of the AD subjects. It is proposed that subjects with dementia presenting with greater executive impairment but without prominent behavioral symptoms are likely to have AD rather than FTLD, especially if they are quite functionally impaired. With time FTLD subjects develop increasing executive dysfunction and increasingly resemble the more severely affected AD subjects.