A systematic review of near-infrared spectroscopy in dementia

Objectives This review aimed to evaluate previous studies using Near-infrared spectroscopy (NIRS) in dementia by summarising the results, determining the consensus in the literature, and delineating if, and how, NIRS experimental and analysis methods may be improved for future studies in dementia. Methods Three databases (PsychINFO, Medline, Embase) were searched for original research studies using NIRS in dementia and prodromal disease stages. We included both observational and randomised control trials, and studies published in English. Animal studies, conference abstracts, and reviews were excluded. Results From 759 identified records, 80 studies using NIRS in dementia and prodromal populations across a range of activation tasks testing memory (28), word retrieval (22), and motor (7) and visuo-spatial function (4), as well as in the resting state (29) were evaluated. Across these cognitive domains, dementia patients generally showed a blunted haemodynamic response, often localised to frontal regions of interest, and a lack of task-appropriate frontal lateralisation. Prodromal stages, such as Mild Cognitive Impairment, revealed mixed results and were associated with either diminished responses or hyperactivity, accompanied by reduced cognitive function, the latter suggesting a possible compensatory neural response which is not present at the dementia stage. Conclusion There is clear evidence of alterations in brain oxygenation in both dementia and prodromal stages across a range of cognitive domains and in the resting state, indicating an ability of NIRS to distinguish dementia from healthy ageing, or at-risk populations. A consensus as to the nature of these changes, however, is difficult to reach due to a lack of standardisation in optical techniques and processing methods. Further studies are required exploring more naturalistic settings and in a wider range of dementia subtypes.