Mild cognitive impairment: An overview

In an ageing population with a projected large increase in the burden of Alzheimer's disease (AD) on patients, carers and health care provision, there has recently been great interest in the concept of identifying those high-risk individuals in the area between normal cognition and dementia for whom early treatment may be beneficial in slowing or even halting progression of cognitive decline. Mild cognitive impairment (MCI) is a syndrome defined as cognitive decline greater than expected for an individual's age and education level, but with preserved activities of daily living. Studies have confirmed an increased rate of progression to dementia in patients diagnosed with MCI (approximately 12% per year), with particular interest in the subtype of amnestic MCI (a-MCI) as a precursor to AD. Indeed some authors have described the clinical phenotype of amnestic MCI as mirroring that of fully expressed AD, only reflecting a milder stage. Diagnostic testing and elucidation of early disease markers are important evolving concepts in the field of MCI research. Neuroimaging studies with MRI have shown characteristic patterns of atrophy, especially in medial temporal lobes, and white matter changes in MCI patients. Metabolic imaging with positron emission tomography (PET) is a promising modality for detection of functional abnormalities. Cerebrospinal fluid biomarkers such as beta-amyloid and tau may help to identify those patients at risk of decline from MCI to AD. Other studies have associated severity of episodic memory deficits and neuropsychiatric symptoms with progression to dementia. MCI, however, is a heterogeneous condition which presents problems with respect to diagnosis and study variability. We discuss current thinking regarding the aetiology, diagnostic criteria and sub-typing of this syndrome. We also review research into therapeutic options for MCI, including risk factor management, identification of reversible causes, cholinesterase inhibitors and cognitive training. The clinical entity of MCI, if clearly defined and classified, is potentially of great use in predicting those patients at greatest risk of dementia. Improved understanding of early pathophysiological and clinical features, along with prompt systematic multidisciplinary assessment, will offer the best hope of effective early intervention.