Mild cognitive impairment (MCI) is a neurocognitive disorder which involves cognitive impairments beyond those expected based on an individual’s age and education but which are not significant enough to interfere with instrumental activities of daily living.MCI may occur as a transitional stage between normal aging and dementia, especially Alzheimer’s disease. It includes both memory and non-memory impairments.Mild cognitive impairment has been relisted as mild neurocognitive disorder in DSM-5, and in ICD-11.
Classification
MCI can present with a variety of symptoms, but is divided generally into two types.
Amnestic MCI (aMCI) is mild cognitive impairment with memory loss as the predominant symptom; aMCI is frequently seen as a prodromal stage of Alzheimer’s disease. Studies suggest that these individuals tend to progress to probable Alzheimer’s disease at a rate of approximately 10% to 15% per year.[
Nonamnestic MCI (naMCI) is mild cognitive impairment in which impairments in domains other than memory (for example, language, visuospatial, executive) are more prominent. It may be further divided as nonamnestic single- or multiple-domain MCI, and these individuals are believed to be more likely to convert to other dementias (for example, dementia with Lewy bodies).
Causes
According to some experts, mild cognitive impairment (MCI) may be caused due to alteration in the brain triggered during early stages of Alzheimer’s disease or other forms of dementia. However, exact causes of MCI are still unknown.
Risk factors of both dementia and MCI are considered to be the same. They are ageing, genetic (heredity) cause of Alzheimer’s or other dementia, and risk of cardiovascular disease.
Individuals with MCI have increased oxidative damage in their nuclear and mitochondrial brain DNA.[12] A widely studied biomarker of DNA damage, 8-hydroxyguanine was found to be elevated in nuclear DNA of the frontal and temporal lobe of individuals with MCI and in mitochondrial DNA of the temporal lobe compared with age matched control subjects. Other oxidized DNA bases were also increased in the brains of individuals with MCI. These findings suggested that oxidative damage to DNA occurs in the earliest detectable phase of Alzheimer’s disease and thus may play a significant role in the pathogenesis of this disease.
Diagnosis
The diagnosis of MCI requires considerable clinical judgement, and as such a comprehensive clinical assessment including clinical observation, neuroimaging, blood tests and neuropsychological testing are best in order to rule out an alternate diagnosis. MCI is diagnosed when there is:
- Evidence of memory impairment
- Preservation of general cognitive and functional abilities
- Absence of diagnosed dementia
Neuropathology
There is evidence suggesting that although amnestic MCI patients may not meet neuropathologic criteria for Alzheimer’s disease, patients may be in a transitional stage of evolving Alzheimer’s disease; patients in this hypothesized transitional stage demonstrated diffuse amyloid in the neocortex and frequent neurofibrillary tangles in the medial temporal lobe. Alternatively, many individuals develop neurofibrillary tangles without amyloid, a pattern termed primary age-related tauopathy.
There is emerging evidence that magnetic resonance imaging can observe deterioration, including progressive loss of gray matter in the brain, from mild cognitive impairment to full-blown Alzheimer disease. A technique known as PiB PET imaging is used to clearly show the sites and shapes of beta amyloid deposits in living subjects using a C11 tracer that binds selectively to such deposits. Another tool aimed at predicting the progression of MCI to Alzheimer’s disease dementia, known as 18F PET with florbetaben shows promise but further study is needed to demonstrate diagnostic test accuracy with larger trials. Such tools may help greatly in assisting clinical research for therapies.
Outlook
MCI does not usually interfere with daily life, but around 50 percent of people diagnosed with it go on to develop the far more severe Alzheimer’s disease within five years (mainly for people diagnosed with the amnestic type). This diagnosis can also serve as an earlier indicator for other types of dementia. However, some instances of MCI may simply remain stable over time or even remit. Individuals diagnosed with MCI should seek reevaluation of symptoms every six months to ensure that the condition has not progressed In any case, it is recommended that people start seeking help as soon as any cognitive changes appear since this can lead to a diagnosis or possible treatments.
Prevalence
The prevalence of MCI varies by age. The prevalence of MCI among different age groups is as follows: 6.7% for ages 60–64; 8.4% for ages 65–69, 10.1% for ages 70–74, 14.8% for ages 75–79, and 25.2% for ages 80–84. After a two-year follow-up, the cumulative incidence of dementia among individuals who are over 65 years old and were diagnosed with MCI was found to be 14.9%.
Globally, approximately 16% of the population over the age of 70 experiences some type of mild cognitive impairment.
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