Cognitive complaints are common in the population. In many such cases, history and examination reveal significant impairment in more than one cognitive domain and concomitant functional decline—and in these cases the findings provide for a diagnosis of dementia (see
Chapter 11). However, in many other cases, there may be involvement of just one cognitive domain and/or lack of any clear functional decline, and diagnosis of dementia may not be warranted. An “intermediate state,” neither representing dementia proper nor normality, must exist in the gradually progressive transition from normal brain health to dementia. There needs to be some means to categorize such a state as a diagnostic entity, even though it may simply be early stage of a degenerative brain disease. In the 20th century, a variety of nonspecific terms were invoked to describe this intermediate state, including “pure amnestic disorder,” “age-related memory impairment (ARMI),” “age-associated memory impairment (AAMI),” “questionable dementia (QD),” “cognitive impairment no dementia (CIND),” and “prodromal dementia.” Originally, such terms were predicated on the idea that perhaps this change was to be expected during normal aging. With the increased understanding of the biologic underpinnings of cognitive disorders, this view became less tenable. Thus for individuals with impairment not meeting dementia criteria, the term
mild cognitive impairment or MCI was coined. The original clinical definition of MCI was memory-based and required a subjective memory complaint, a documented objective deficit in memory, overall normal general cognitive function, and a lack of “significant” functional impairment precluding a diagnosis of dementia. This was codified by the American Academy of Neurology in 2001. A large proportion of these cases ultimately develop dementia, mostly due to Alzheimer disease, at a rate of between 7% and 20% per year, although over a few years, some persons remain relatively stable, nonprogressive, or even improve. The definition of MCI has been broadened to domains other than memory to reflect non-Alzheimer dementias, (
Table 49.1). These other types of MCI include single domain amnestic, amnestic multidomain, nonamnestic single domain, and nonamnestic multidomain impairments (
Table 49.2).
MCI has also been codified by the committees of the American Psychiatric Association in 2013. The formulation of the diagnostic criteria of
Diagnostic and Statistical Manual of Mental Disorders, 5th edition (
DSM-5), recognizes that for each dementing disorder, there must be a state in which the impairment does not reach the level of dementia. Thus, each dementing disorder is categorized as a “major neurocognitive disorder” or “minor neurocognitive disorder.” For example, Alzheimer dementia is “major,” whereas amnestic MCI is “minor.” For Alzheimer disease, which is the likely outcome for most amnestic MCI patients, biomarkers have allowed segmentation of the relatively slow neurodegenerative process into different diagnostic phases. The National Institute on Aging and Alzheimer’s Association workgroup in 2011 divided the Alzheimer neurodegenerative spectrum into three phases: “preclinical” disease in which persons are
asymptomatic but have biomarker evidence of an early Alzheimer process, a “symptomatic predementia phase” synonymous with MCI, and then the actual symptomatic dementia phase of Alzheimer disease (
Table 49.3 and
Fig. 49.1). In summary,
mild cognitive impairment is a diagnostic term referring to a cognitively impaired state of less extent or severity than a dementia. MCI is generally, but not always, an early symptomatic state of a neurodegenerative process.
