Human Phenotype Ontology entry HP_0002521 (Hypsarrhythmia) is abnormal interictal high amplitude waves and a background of irregular spikes. There is continuous (during wakefulness), high-amplitude (>200 Hz), generalized polymorphic slowing with no organized background and multifocal spikes demonstrated by electroencephalography (EEG). Also known as: Hypsarrhythmia by EEG.