What are the typical EEG findings in infantile spasms and the clinical syndrome they define?

• A. Normal EEG; no clinical signs
• B. Focal sharp waves; tremor
• C. Hypsarrhythmia on EEG; triad of infantile spasms, developmental regression, and chaotic EEG pattern
• D. Theta rhythm; irritability

Answer: (C)

Infantile spasms are classically accompanied by a distinctive EEG pattern called hypsarrhythmia. Hypsarrhythmia is a chaotic, disorganized rhythm with very high-amplitude, multifocal spikes and slow waves and no consistent background rhythm. This abnormal EEG signature helps define the syndrome alongside the clinical signs.

Clinically, the syndrome is described by a triad: clusters of infantile spasms, developmental regression or arrest, and the chaotic hypsarrhythmic EEG. The spasms themselves are sudden, brief contractions that can involve the trunk and limbs and often occur in clusters, commonly upon waking. The presence of this hypsarrhythmic pattern together with the spasms and regression makes this constellation characteristic of the condition.

Other options don’t fit as well because a normal EEG would not accompany infantile spasms, focal sharp waves point to localized epilepsy rather than the diffuse, chaotic pattern typical of this syndrome, and a theta rhythm with irritability does not capture the defining chaotic EEG and the triad of symptoms.