The paper reviewed here is ‘Incidence of Mild Cognitive Impairment: A Systematic Review’ by Riedell-Heller and colleagues and freely available here. Mild Cognitive Impairment (MCI) is an important concept in tackling the impending dementia epidemic. MCI involves an impairment in one or more cognitive domains but does not meet the criterion for dementia. Petersen developed the criteria for MCI and these include subjective experience of and objective evidence of cognitive impairment. There is the suggestion that by detecting MCI subsequent dementia can be prevented or at least delayed. However the evidence is equivocal. Not all people with MCI go on to develop dementia and in a number of cases there is a reversion back to functioning within normal limits.
The meta-analysis reviewed here was undertaken by a German group in the Department of Psychiatry and Psychotherapy in Leipzig. The aim of the meta-analysis is clearly outlined in the introduction and the authors note that it is the first meta-analysis examining the incidence rates of MCI that they are aware of. Firstly they outline the search strategy citing the relevant databases, search period and search terms which are straightforward. The authors segregate MCI into Amnestic MCI single and multidomain and Non-Amnestic single and multidomain. They included studies using Petersen’s criteria for MCI, ‘population or community-based samples’, use of person-at-risk measures and English or German language.
The authors include 9 studies in their meta-analysis. I wasn’t able to find a clear characterisation of the populations in all 9 studies. The authors note that one community sample was multiethnic and also that in only 2 of the studies were subjects younger than 65 included. There is some pooling of the results. Thus for the incident rates of MCI, the authors give either a range or values for individual studies although it would have been interesting to see a mean and standard deviation. The incidence rates are given below
– MCI – range 8.5-76.8 cases per 1000 person-years
– Amnestic MCI Single Domain – 8.5, 12.2 and 14 cases per 1000 person years
– Amnestic MCI Multiple Domain – 9 per 1000 person years
– Non-Amnestic MCI Single Domain – 23 per 1000 person years
– Non-Amnestic MCI Multiple Domain – 5 per 1000 person years
I found these rates difficult to interpret without the sample characteristics. How generalisable are these results for instance? The authors note that some of the variation obtained resulted from the methodology. Thus there were different approaches to objective outcome measures with some studies choosing 1.5 standard deviations below the norm as a cut-off point while others used a single standard deviation. The authors also note that in most of the studies, subjects were at least 65 years of age.
The figures above at least provide an initial answer to the question of what the incidence of MCI is. However the results for MCI suggest that much work remains to be done in characterising the epidemiology of MCI. There are many studies now examining biomarkers and looking specifically at conversion to dementia. Thus MCI has become a focus for research efforts into tackling dementia in the longer term and further studies such as this one will guide our understanding of this condition along with insights into therapeutic approaches.
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