Mild Cognitive Impairment
Although MCI has proven problematic to define, Petersen [6] has described MCI as a transition between what is considered normal age-associated cognitive decline [7] and clinical dementia. However, he notes that this transitional area is not necessarily a continuum; overlaps at each end of normal cognition, MCI, and dementia are often evident, blurring the distinctions between the three [6]. While it is difficult to accurately pinpoint when someone transitions from one stage into the next, there is consensus that individuals with MCI experience one or more impairments in cognition that are not severe enough to warrant a diagnosis of dementia, and the most common impairment is in memory [6]. When compared to healthy peers, those with MCI are more likely to develop dementia, though the condition may also remain stable or even regress [8].
While a universally accepted set of diagnostic criteria for MCI does not exist, there have been attempts to provide guidance to healthcare professionals and researchers, the most well-known of which is the Petersen’s criteria [6, 9]. Because it precludes recommendation of specific diagnostic tools or cutoff scores, Petersen’s decisional framework relies heavily on clinical judgment, leaving considerable room for interpretation. For example, while the Montreal Cognitive Assessment (MoCA) is increasingly used by clinicians to operationalize general cognition in MCI, the mini-mental state examination (MMSE) is also used for the same purpose. Cutoff scores for MCI range from 23 to 26 on the MMSE [10]. For various other memory tests, cutoff scores of 1.0, 1.5, and 2.0 standard deviations from the mean and age norms [10] have also been reported. And while a cutoff score of 0.5 [11] on the Clinical Dementia Rating (CDR) Scale is regularly used within a research context to delineate MCI (both as a stand-alone tool or within the Petersen framework
[11]), most contend that this threshold denotes a broader group of individuals who may have MCI or possible/probable/very mild dementia [12] and does not capture all cases of MCI [13].
These inconsistencies in the clinical definition of MCI have led to variation in the reported prevalence of the condition [10]. In their systematic review on the prevalence and incidence of MCI, Ward et al. [14] identified ten studies that provided prevalence rates for MCI among older adults ranging from 3 to 42 % with a mean of
26.4 %. Similar variations were seen within the three incidence studies, where results ranged from 21.5 to 71.3/1000 person years for those 65 years and older. The statistics for individuals converting from MCI to dementia also demonstrate variation. A meta-analysis by Mitchell and Shiri-Feshki included 41 studies that reported conversion of MCI to dementia and identified progression rates ranging from 1 to 20 % annually [15].
In response to confusion over clinical diagnosis and the resulting inconsistencies within the literature, an updated diagnostic scheme was developed to include amnestic and non-amnestic subtypes of MCI and differentiate between single and multiple domain impairments [6, 16, 17]. There have been interesting findings surrounding prognoses in different groups. For example, those with amnestic types of MCI seem most likely to convert to dementia [18]. Reversion to a non-MCI state may be most likely for individuals with single domain types [18]. It has also been suggested that while most types of MCI progress to Alzheimer’s disease (AD), conversions to non- AD forms of dementia, such as Lewy body and vascular or frontotemporal dementia, are more likely for those with non-amnestic types of MCI [6, 11].
It is apparent that individuals with MCI remain a complex and heterogeneous group with a high level of variability across symptoms, severity, and outcomes. With such diversity, it should be expected that many individuals with MCI would be able to successfully handle a complex task such as driving a vehicle, while others would not. In the next section, we will explore some of the common skills required for safe driving, and take note of the potential for MCI to impact on these areas.
