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.

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".

[citation needed] Through the use of video EEG and continuous monitoring, five variants of the "classical" hypsarrhythmic pattern have been identified:[3] The "H" in PEHO syndrome stands for hypsarrhythmia.

[4] Together with developmental regression and infantile spasms, hypsarrhythmia is one of the diagnostic criteria for West syndrome.