I believe it is short for hypsarrhythmia.  From Dorlands Medical Dictionary:
"hypsarrhythmia (hyp?sar?rhyth?mia) (hi?s[schwa]-rith?me-[schwa]) [hyps- +
arrhythmia]  an electroencephalographic abnormality sometimes observed in
infants, with random, high-voltage slow waves and spikes that arise from
multiple foci and spread to all cortical areas. It is seen most commonly in
cases of jackknife seizures."

This site might help:
http://www.pediatricneurology.com/seizure_intro.htm