Distinction from mild cognitive impairment

A syndrome obviously related to MCD in behavioral neurology is MCI (Petersen, 2011), which is being intensively studied as a putative precursor to AD. Introduced in the late 1990s (Petersen et al., 1999), the concept of MCI has proven to be highly influential in terms of both research on AD and clinical practice. As a concept signifying neuropathological changes in the brain, MCI is most commonly associated with cortical dysfunction, in particular hippocampal volume loss. Although not all cases of MCI in fact feature hippocampal atrophy, perhaps the major value of MCI has been in targeting what has been assumed to be the early stage of AD so that diagnosis and, at some point when more effective medications are available, treatment can be improved. Whereas many controversies continue as MCI is studied in greater detail, a focus on understanding the earliest manifestations of dementia, in this case AD, is surely a major step forward.

MCD is a parallel construct in the white matter disorders, and is conceptualized as the putative precursor to WMD. As might be expected, MCD does not readily correspond to one of the currently recognized subtypes of MCI (Petersen, 2011). Because MCD is founded on pathology in white matter, its profile naturally differs from that MCI, the most familiar form of which – amnestic MCI – is postulated to result from gray matter atrophy, most notably in the medial temporal regions, including the hippocampus. Thus the specific profile of deficits in MCD merits a separate category. Many differences in fact exist between MCD and MCI (Table 8.2), and, while the understanding of MCD is still preliminary, a syndrome that is relevant to the cognitive effects of white matter dysfunction of any etiology may prove useful in contrast to MCI as a syndrome of gray matter degeneration. Additional advantages of the use of MCD include its reliance on readily available in vivo neuroimaging technology, its applicability to any age, and its potential reversibility.

As discussed earlier, however, it can be questioned whether yet another attempt to capture the premonitory features of a dementia syndrome is warranted. Perhaps MCI should simply be applied to the white matter disorders causing dementia without the need for MCD. This view has merit, and medicine does risk subdividing its many disorders into so many categories that every patient may at times seem to deserve his or her own unique diagnostic label. But the distinct neurobiology of white matter tracts does pertain to a large portion of brain tissue that has undeniable importance for cognition, and impels a more specific descriptor of the pre-dementia stage of potentially all white matter disorders. Evidence supporting the concept has been adduced, and in the spirit of investigating the entire range of dementing disorders, MCD appears to have a legitimate role. At the very least, MCD can stimulate thinking about the relatively neglected area of white matter and cognition, just as the notion of MCI has done for cortical disease since its introduction.