Tonic-clonic seizures, formerly known as "grand mal" seizures, are one of the most well-known and dramatic types of seizures. These seizures can be classified into two main categories: generalized tonic-clonic (GTC) seizures and focal evolving to bilateral tonic-clonic seizures.

Phases of Tonic-Clonic Seizures

A tonic-clonic seizure typically consists of two phases: the tonic phase and the clonic phase, followed by a postictal state.

1. Tonic Phase

The tonic phase is the first part of the seizure and is characterized by intense muscle rigidity. During this phase, the patient’s body becomes stiff and rigid, and they may fall if standing. The duration of the tonic phase is generally brief, lasting around 10-20 seconds.

  • Clinical signs: The patient’s body becomes stiff and may arch backward (called "opisthotonos"), and breathing may become irregular. There may be a brief cessation of breathing, leading to a blue or pale appearance (cyanosis).
  • EEG changes: On an EEG, the tonic phase shows significant myogenic artifact (muscle activity), with high-amplitude, fast-frequency spikes that reflect the ongoing muscle contractions.

2. Clonic Phase

Following the tonic phase, the seizure transitions into the clonic phase, which is marked by rhythmic jerking movements of the body.

  • Clinical signs: The jerking movements are typically bilateral, involving both sides of the body. These movements occur in bursts and tend to be less rigid than in the tonic phase.
  • EEG changes: On the EEG, the tonic phase evolves into intermittent bursts of spike/polyspike and slow waves, corresponding to the clonic jerks seen clinically. This transition marks the shift from muscle rigidity to rhythmic jerking.

3. Postictal State

After the seizure ends, the patient enters the postictal state. During this period, the brain activity gradually returns to normal.

  • Clinical signs: The patient may experience confusion, disorientation, fatigue, and soreness in the muscles. The postictal state can last anywhere from several minutes to hours, depending on the individual.
  • EEG changes: The EEG during this phase shows postictal attenuation, where the brain waves become slow or attenuated before eventually returning to the normal background pattern.

Classification of Tonic-Clonic Seizures

Tonic-clonic seizures can be classified into two broad categories:

1. Generalized Tonic-Clonic Seizures (GTC)

Generalized tonic-clonic seizures (GTC) are those that start as a generalized event, affecting both sides of the brain simultaneously. These seizures typically begin with the tonic phase, followed by the clonic phase, and then proceed into the postictal state.

  • Clinical features: The seizure begins abruptly and is characterized by the loss of consciousness, muscle rigidity, and rhythmic jerking movements.
  • EEG features: The EEG will show generalized spike-and-wave discharges that spread throughout both hemispheres, with a transition from tonic to clonic activity.

2. Focal Evolving to Bilateral Tonic-Clonic Seizures

When a focal seizure begins in one part of the brain but spreads to both hemispheres, it can evolve into a focal to bilateral tonic-clonic seizure. This type of seizure starts with focal onset, where the initial symptoms may be localized, but then it spreads across both sides of the brain.

  • Clinical features: The seizure initially presents with focal symptoms, such as jerking in one part of the body, but progresses to full-body tonic-clonic movements.
  • EEG features: The EEG will initially show focal discharges, which evolve into generalized spike-and-wave discharges as the seizure spreads to both hemispheres.

EEG and Temporal Evolution of Tonic-Clonic Seizures

In the case of a generalized tonic-clonic seizure, the EEG will show distinct temporal evolution, which can be used for diagnosis and monitoring of seizure activity.

  • Initial tonic phase: The EEG shows high-amplitude activity (myogenic artifact) as the muscle activity begins. This phase reflects the early, stiffening of the body.
  • Clonic phase: As the seizure progresses, the EEG will show bursts of spike/polyspike and slow waves, corresponding to the rhythmic jerking movements clinically seen.
  • Postictal phase: After the seizure ends, the EEG will demonstrate postictal attenuation, showing reduced brain activity before returning to baseline.

Conclusion

Tonic-clonic seizures are dramatic events that involve both tonic (stiffening) and clonic (jerking) phases. They can occur as generalized tonic-clonic seizures or evolve from focal seizures. The EEG plays a critical role in diagnosing these seizures by providing insight into their temporal evolution and helping clinicians differentiate between focal and generalized seizure activity. After the seizure ends, patients typically experience a postictal state characterized by confusion and fatigue.