Tonic-clonic seizure · Grey Matter

A tonic-clonic seizure is the convulsive seizure most people picture, in which both hemispheres are recruited, the body first stiffens and then jerks rhythmically, and awareness is lost throughout; it can begin generalised or start in one area and spread.

Tonic-clonic seizure. A tonic-clonic seizure is the convulsive seizure most people picture, in which both hemispheres are recruited, the body first stiffens and then jerks rhythmically, and awareness is lost throughout; it can begin generalised or start in one area and spread.

It runs in phases. In the tonic phase, lasting about 10 to 20 seconds, the muscles contract together and stay contracted: the person falls, air forced out past the closed vocal cords can make a cry, and breathing stops, so the skin may turn blue. The clonic phase follows, with jerks that start fast and small and become slower and larger until they stop, usually within a minute or two from the start. Then comes the postictal state, minutes to hours of deep sleep, confusion, headache and sore muscles while the brain recovers.

The EEG tracks the phases. The tonic phase shows a build-up of fast, low-voltage activity turning into high-amplitude polyspikes; in the clonic phase the discharge is interrupted by slow waves, one per jerk, as inhibition breaks in with growing length.

Spread runs through the thalamus. In a focal to bilateral seizure the discharge leaves its focus and engages loops between cortex and thalamus, which relay it to both hemispheres; how far it travels depends on inhibition along the way.

The motor signs come from motor cortex and its descending pathways being driven by the discharge, which is why a seizure that stays in one motor area jerks only the matching part of the body.

A seizure that keeps going past about five minutes, or repeats without recovery in between, is status epilepticus and is treated as an emergency.

A tug of war that the rope keeps winning is a fair picture of the clonic phase.

Each jerk is the discharge pulling, each pause is inhibition pulling back, and the pauses grow until inhibition holds.

Questions: What happens in the tonic and in the clonic phase of a convulsive seizure? In the tonic phase, about 10 to 20 seconds long, the discharge drives the motor system continuously, so muscles contract together and stay contracted; the EEG shows fast activity building into high-amplitude polyspikes. In the clonic phase the discharge is broken by pauses, each a slow wave on the EEG during which inhibition gains ground, and every burst between pauses is a jerk. The pauses lengthen until the seizure ends, usually within a minute or two, and the exhausted brain enters the postictal state of sleep and confusion. Why is a convulsive seizure that lasts more than five minutes treated as an emergency? Most tonic-clonic seizures stop within about two minutes, so one still running at five shows that the mechanisms that normally end a seizure have failed and it is unlikely to stop by itself. The international definition of 2015 sets that point, t1, at five minutes for convulsive status epilepticus, and a second point, t2, at 30 minutes, after which lasting consequences such as neuronal death become likely. Treatment starts at t1 because seizures become harder to stop the longer they run.