Hypsarrhythmia is abnormal interictal high amplitude waves and a background of irregular spikes seen in electroencephalogram, mostly in infant diagnosed with infantile spasms. They both vary in duration and size, have no rhythm or pattern and can alternate between focal or multifocal. In most cases of infantile spasms hypsarrhythmia disappear during a cluster of spasms and/or REM sleep.
In simple terms, it is a very chaotic and disorganized brain wave behavior with no recognisable pattern, where a normal EEG shows clear separation between each signals and visible pattern.
Hypsarrhythmia rarely persists beyond the age of 24 months.