Tonic seizures are brief seizures (usually < 60 seconds) consisting of the sudden onset of increased tone in the extensor muscles. If standing, the patient typically falls to the ground. These seizures are invariably longer than myoclonic seizures. Occasionally tonic seizures terminate with a clonic phase.

The degree to which consciousness is impaired is often difficult to assess. In seizures lasting longer than a few seconds, impairment of consciousness is usually apparent.

Postictal impairment with confusion, tiredness, and headache is common. The degree of postictal impairment is usually related to the duration of the seizure.

Electromyographic activity is dramatically increased in tonic seizures.

Tonic seizures are frequently seen in patients with the Lennox-Gastaut syndrome, a disorder consisting of mixed seizure types, mental retardation, and the EEG findings of a slow spike-and-wave pattern. Seizures are usually more frequent at night.

Types of tonic seizures

Tonic seizures have been divided into four types:

  • Axial tonic seizures begin with a tonic contraction of the neck muscles, leading to fixation of the head in an erect position, widely opened eyes, and jaw clenching or mouth opening. Contraction of the respiratory and abdominal muscles often follows and may lead to a high-pitched cry and brief periods of apnea.
  • Tonic axorhizomelic seizures begin with a sequence similar to the axial type, but then the tonic contractions extend to the proximal musculature of the upper limbs, elevating the shoulders and abducting the arms.
  • In global tonic seizures, the tonic contractions extend to the periphery of the limbs. The arms are pulled upward to a semiflexed position in front of the head, and the fists are clenched, producing a body position similar to that of a child defending himself against a facial blow. The lower extremities also can be involved, leading to a fall if the child is in a standing position.
  • Asymmetric tonic seizures vary from a slight rotation of the head to a tonic contraction of all the musculature of one side of the body.

Other categorizations

In a study of epileptic falls in children, Ikeno et al described two types of tonic seizures:

  1. The tonic type is characterized by excessive flexion or extension of fingers, forced flexion of hand joints, jaw protrusion, shoulder elevation, upper arm abduction, and tonic flexion of the trunk. This hypertonic state continues unchanged even after the patient falls down.
  2. Flexor spasms have a different distribution of hypertonicity. The head is flexed forward, the shoulders are elevated, the arms are flung outward and forward, and the thighs are flexed at the hip. Unlike the tonic seizure type, fingers, hand joints, and elbows remain neither tonic nor atonic. Flexor spasms may resemble infantile spasms.

Egli et al described tonic seizures that lead to falls as "axial spasms." Reflecting a uniform pattern of movement, these seizures consist of moderate flexion of the hips, upper trunk, and head lasting from 0.5 to 0.8 second. The arms are almost always abducted, elevated, and in a semiflexed position. The fall is provoked by the rapidity and violence of the flexion in the hips.


Ictal manifestations of tonic seizures usually consist of bilateral synchronous spikes of 10 to 25 Hz, of medium to high voltage, with a frontal accentuation. Simple flattening or desynchronization also may occur. Occasional multiple spike-and-wave or diffuse slow activity may occur during a tonic seizure (See EEG).

Tonic Seizure EEG

Multiple spike-and-wave activity in a patient with tonic seizures.

Authored By: 
Gregory L. Holmes MD
Authored Date: