What did you expect?

Two recent papers have attempted, and failed, to distinguish Alzheimer’s disease (AD) from vascular dementia (VaD) on the basis of bedside executive measures (Matioli and Caramelli, 2010; McGuinness et al., 2010). Similarly, other studies have demonstrated that frontotemporal dementia (FTD) cannot be similarly distinguished from AD (Royall et al., 1994; Hooten and Lyketsos, 1998) or even that young adults with schizophrenia cannot be distinguished from geriatric cases when the two are equated on the basis of their functional autonomy (Royall et al., 1993). These attempts may betray a fundamental misconception of the relationship between executive function and dementia. I have argued that executive dyscontrol is essential to dementia. This is meant to suggest that dementia is unlikely to be diagnosed in the absence of executive impairment, while conversely, dementias may exist in the absence of any other cognitive impairment. I have labeled executive dominant dementias ‘‘type 2’’ to distinguish them from the (‘‘type 1’’) cortical dementia of AD. Thus, there may be more than one dementia syndrome and it becomes an empirical question which is most characteristic of conditions such as AD, VaD, and FTD. The special relationship between dementia and executive impairment arises from the latter’s unique ability to explain impairments of functional autonomy. The Executive Interview (EXIT25) for example, explains on average 25% of the variance in functional status measures, while some studies show associations as strong as r1⁄4 0.89 between EXIT25 scores and performance based functional capacity measures (Pereira et al., 2008). Mere cognitive impairment does not define ‘‘dementia.’’ Instead, there must also be disability. Moreover, the two must be related. Thus, the essential psychometric correlates of dementia can be resolved to the cognitive correlates of functional status. Nonexecutive domains are surprisingly weak correlates of functional capacity, especially if they are modeled after adjusting for executive control. Since functional impairment, and therefore executive impairment, is essential to dementia, it should not be possible to distinguish two dementing illnesses on the basis of executive measures, provided that they have been matched to functional capacity. In contrast,