Mild Cognitive Impairment

Excerpt

Memory loss is a common complaint among older adults, and cognitive decline can present in various ways. It is a part of the normal aging process, subjective cognitive impairment (symptomatic complaints with normal neurocognitive test results), mild cognitive impairment, or dementia. Although mild cognitive impairment (MCI) has been used in the literature since the 1960s, it was first fully characterized in 1997 by Petersen and colleagues at the Mayo Clinic. As part of their community aging study, they observed individuals with memory concerns beyond what was expected for their age. They demonstrated memory impairment on objective testing yet did not fulfill the criteria for dementia. MCI was deemed to represent a borderline between normal aging and very early dementia. The earlier criteria for MCI diagnosis were found inadequate when it was realized that not all patients with MCI progressed to dementia and that memory impairment was not the only cognitive domain affected. Thus, a broader definition of MCI was needed. In 2003, the first Key Symposium on MCI was held, leading to the publication of revised core clinical criteria. These criteria included cognitive complaints from the patient or family members, deficits in any cognitive domain and not only memory, preserved overall general function but increasing difficulties in activities of daily life, and no dementia. The concept of MCI traveled outside the realm of research to provide clinicians with a useful diagnosis, often for close monitoring. Further clinical research resulted in the development of novel clinical criteria for providers.

The American Psychiatry Association's Diagnostic and Statistical Manual of Mental Disorders 5th Edition (DSM-V) in 2013 classified MCI as one of the neurocognitive disorders (NCD). It is characterized by a decline in one or more cognitive domains, which is both subjectively and objectively observable. However, this decline does not interfere with the individual's ability to perform daily activities independently. Moreover, the deficits cannot be attributed to delirium or other psychiatric conditions. DSM-V does not propose specific neuropsychology test scores for diagnosing mild NCD. However, it is implied that such scores would help make this diagnosis. After diagnosing the syndrome as mild NCD, the next step is determining the etiology. The 11th Revision of the International Classification of Diseases (ICD 11) by the World Health Organization (WHO) in 2018 adopted the definition of mild neurocognitive disorder in alignment with the DSM-V diagnostic criteria.

The National Institute on Aging and the Alzheimer's Association (NIA-AA) convened a work group to discuss the MCI criteria. The group proposed metrics for the distinct definition of MCI due to Alzheimer Disease. The initial criteria were published in 2011 and revised in 2018. The initial criteria closely resemble the core clinical criteria proposed by the 2003 Symposium. The 2018 revision noted that neurobehavioral disturbance may be an important feature of the clinical presentation and that cognitive deficits may result in a mild but noticeable impact on the complex activities of daily living. The NIA-AA also suggested research criteria for MCI due to AD in 2011 to help determine the etiology of MCI established by the core clinical criteria. These included the use of biomarkers to differentiate MCI due to AD from MCI due to other reasons. The biomarkers are not used to define MCI but can be helpful in research settings. The two main helpful biomarkers are amyloid-beta (Aβ) deposition and neuronal injury.

MCI can also be classified based on the type of cognitive domain that is affected. MCI can potentially affect one or more of the following 6 cognitive domains: learning and memory, language, complex attention, executive function, social cognition, and visuospatial function. MCI can be classified as "amnestic" or "non-amnestic" with deficits in single or multiple cognitive domains. Amnestic MCI presents with predominant memory loss and the risk for progression to Alzheimer's Disease. In non-amnestic MCI, memory is relatively intact, and it is less common than the amnestic type and may progress to non-Alzheimer disease dementia. MCI can be further categorized into 4 subtypes:

In 2009, a comprehensive Neuropsychology classification system was introduced as the dichotomous classification was considered inadequate to represent the heterogeneous nature of MCI. These MCI classes are:

1. Amnestic - impaired recall and recognition

2. Mixed - impairments in various domains such as language, executive function, recall and recognition, and visuospatial functioning.

3. Dysexecutive - impairments in attention, executive, and visuospatial functions, but the memory remains intact.

4. Visuospatial - impairment in only a single measure of visual construction.

The phenotypic classification of MCI with clinical information and laboratory tests can guide the clinician in determining the probable cause of MCI and predicting its course. Patients with dysexecutive MCI (dMCI) are less likely to have Alzheimer - type dementia and more likely to have a stroke.