What is Lennox-Gastaut syndrome? The Lennox-Gastaut syndrome (LGS) is a type of epilepsy with multiple different types of seizures, particularly tonic (stiffening) and atonic (drop) seizures. Intellectual development is usually, but not always, impaired. The EEG shows a classic pattern of background slowing and spike-wave bursts at frequencies less than 2.5 per second.