Rhythmic EEG patterns: The oldest idea in the EEG world, but without an obvious definition

The word "rhythmic" was quickly introduced in the vocabulary of the electroencephalographers with the discovery of the alpha rhythm and typical discharges of spike-and-waves at 3 Hz in childhood absence epilepsy, but without any definition until recently. In its last revision (2017), the International Federation of Clinical Neurophysiology proposed a specific definition. The word "rhythmic" is "applied to regular waves occurring at a constant period and of relatively uniform morphology." Unfortunately, there is a serious problem of ambiguity as this definition exactly corresponds in science to the definition of a periodic function (periodic waveform). This definition of regular patterns at constant intervals (constant period) can be applied to physiological rhythms such as alpha and mu rhythms but also to unusual rhythms such as rhythmic theta bursts of drowsiness. It can also be used to describe EEG patterns, such as frontal/temporal/occipital intermittent rhythmic activities or generalized rhythmic delta activities. Except with typical absence seizures, this definition cannot be used to describe most epileptic seizures and absence status epilepticus. Identifying 'periodic' versus 'rhythmic' features is crucial when analyzing an EEG in critically ill patients. The importance lies in a balanced approach that frequently promotes an interpretation of an acute encephalopathy when referring to diffuse periodic EEG patterns, and status epilepticus for unreactive bilateral rhythmic patterns. To include seizures and status epilepticus in the definition of rhythmic patterns, we suggest a more "dynamic" approach in addition to the regular waves at constant intervals without interdischarge intervals between waveforms. For the observed rhythmic patterns, we propose: repetition of consecutive waves with, in case of a variation in the pattern, a dynamic approach to these waveforms. Dynamic means spatiotemporal evolution of the patterns.