Tonic seizures are very uncommon, especially when they occur without clonic jerking. They usually are manifest with Lennox-Gastaut syndrome or, less commonly, with multiple sclerosis. Tonic seizures most often develop in childhood, although they can occur at any age.

Tonic seizures are characterized by facial and truncal muscle spasms, flexion or extension of the upper and lower extremities, and impaired consciousness. Several types of tonic seizures exist. Those grouped with absence, myoclonic, and atonic seizures are non-convulsive and tend to be brief. The more prolonged seizures usually are convulsive and may manifest pupillary dilation, tachycardia, apnea, cyanosis, salivation, and the loss of bladder or bowel control. Tonic seizures are often followed by postictal confusion.