Hypsarrhythmia 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), frequently encountered in an infant diagnosed with infantile spasms, although it can be found in other conditions. In simpler terms, it is a very chaotic and disorganized brain electrical activity with no recognizable pattern, whereas a normal EEG shows clear separation between each signal and visible pattern.

Gibbs and Gibbs described hypsarrhythmia 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 spikes 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.

Classification

Through the use of video EEG and continuous monitoring, five variants of the "classical" hypsarrhythmic pattern have been identified:

1. Hypsarrhythmia with increased interhemispheric synchronization. Characterized by symmetic and synchronized activity, seen in patients with longstanding evolution, specially in those with West syndrome that changes to Lennox-Gastaut syndrome
   Lennox-Gastaut syndrome
   Lennox–Gastaut syndrome , also known as Lennox syndrome, is a difficult-to-treat form of childhood-onset epilepsy that most often appears between the second and sixth year of life, and is characterized by frequent seizures and different seizure types; it is often accompanied by developmental delay...
   
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2. Asymmetric hypsarrhythmia. Associated with a brain structural abnormality, and not necessarily predicts the affected hemisphere.
3. Hypsarrhythmia with a consistent focus of abnormal discharge.
4. Hypsarrhythmia with episodes of voltage attenuation. Commonly seen during nonrapid eye movement (NREM) sleep. When the episodes of voltage attenuation appear at the time as a epileptic spams does, they're called electrodecrements.
5. Hypsarrhythmia with little spike or sharp activity.

It can be associated with mental retardation

Mental retardation

Mental retardation is a generalized disorder appearing before adulthood, characterized by significantly impaired cognitive functioning and deficits in two or more adaptive behaviors...

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The "H" in PEHO syndrome

PEHO syndrome

PEHO syndrome is a progressive encephalopathy with edema, hypsarrhythmia and optic atrophy. It is a very rare disease, one of the Finnish heritage diseases, and has been reported also in Dutch and Swiss infants....

 stands for hypsarrhythmia.

Hypsarrhythmia is frequently found in patients with West syndrome

West syndrome

West syndrome or West's Syndrome is an uncommon to rare epileptic disorder in infants. It is named after the English physician, William James West , who first described it in an article published in The Lancet in 1841. The original case actually described his own son, James Edwin West...

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