Hypsarrhythmia is very chaotic and disorganized brain electrical activity with no recognizable pattern, whereas a normal brain electrical activity shows clear separation between each signal and visible pattern. It is an abnormal interictal pattern, consisting of high amplitude and irregular waves and spikes in a background of chaotic and disorganized activity seen on electroencephalogram (EEG), and frequently encountered in infants diagnosed with infantile spasms, although it can be found in other conditions such as tuberous sclerosis.

Gibbs and Gibbs described hypsarrhythmia (originally spelled with one 'r') in 1952 as "...random high voltage waves and spikes. These spikes vary from moment to moment, both in time and in location. At time they appear to be focal, and a few seconds later they seem to originate from multiple foci. Occasionally the spike discharge becomes generalized, but it never appears as a rhythmically repetitive and highly organized pattern that could be confused with a discharge of the petit mal or petit mal variant type".

In most cases of infantile spasms, hypsarrhythmia either disappears or improves during a cluster of spasms and/or REM sleep. Hypsarrhythmia rarely persists beyond the age of 24 months.