Mild cognitive impairment: a nosological entity?

Mild cognitive impairment is not an established diagnosis but a concept for which different criteria have been proposed and modified over time. Mild cognitive impairment refers to the transitional zone between normal ageing and dementia. The mild cognitive impairment stage may be one of the optimum stages at which to intervene with preventive therapies. The heterogeneity of the term has been recognized. Multiple sources of heterogeneity have been described for mild cognitive impairment, including biological factors, clinical symptoms and clinical course. It is still not possible to consider mild cognitive impairment as an explicit predictor of Alzheimer's disease with the current criteria. The heterogeneity within mild cognitive impairment has been noted, and a classification has been proposed: amnestic or single memory mild cognitive impairment, multiple domains mild cognitive impairment, and single non-memory mild cognitive impairment. Future research should be focussed on redefining the criteria of the mild cognitive impairment entity. This could enable the better development of appropriate therapeutic interventions.