Past, present and future EEG in the clinical workup of dementias.

Electroencephalography (EEG), as non-invasive, global measure of neuronal activity, is a prime candidate functional marker of synapse dysfunction and loss in dementias. Nevertheless, EEG currently has no established role in the clinical workup of individual patients. This opinion paper presents our critical view on why EEG has so far failed to keep its promise, and where we believe EEG will be clinically useful for patients threatened with cognitive decline in the future. Individual EEGs are an integral outcome of many causally intermixing upstream factors contributing to dementia. Therefore, EEG cannot become a clinically useful "simple" stand-alone biomarker of some pathognomic accumulations of specific brain proteins, but rather offer unique opportunities for more comprehensive and richly faceted insights into the functional status of brain systems. EEG may thus remain an essential window into the brain when it comes to the at-risk and presymptomatic phases of dementias, where it can be uniquely informative about concepts such as burdens of plasticity and repair, cognitive reserve, and sleep. Jointly with rapid gains in usability, portability, machine learning, closed loop systems, and understanding of the role of EEG-based sleep stages for memory and brain repair, EEG may come to keep its initial promise after all.