Absence seizure · Grey Matter

An absence seizure is a generalised seizure of a few seconds in which awareness switches off and back on, with a blank stare and an interrupted action, and it is the clearest case of a seizure produced by the loop between the cortex and the thalamus.

Absence seizure. An absence seizure is a generalised seizure of a few seconds in which awareness switches off and back on, with a blank stare and an interrupted action, and it is the clearest case of a seizure produced by the loop between the cortex and the thalamus.

The seizure begins and ends abruptly, usually lasting under 20 seconds, with no fall and no confusion afterwards; a child may have many in a day and they are easily mistaken for daydreaming. Onset is typically between 4 and 10 years. On the EEG each absence is a burst of spike-and-wave complexes at about 3 per second over both hemispheres at once, on an otherwise normal background, and breathing fast for a few minutes provokes one in most children who have them, which makes hyperventilation a routine test.

The rhythm comes from a circuit. Cortex excites the thalamus, the thalamus's inhibitory reticular nucleus hyperpolarises relay neurons, and their T-type calcium channels then fire a rebound burst back to the cortex; each turn of that loop is one spike-and-wave.

Drugs follow the circuit. Ethosuximide, which reduces T-type calcium currents, controls childhood absences about as well as valproate and better than lamotrigine in the largest trial, while some sodium channel blockers can make absences worse.

Most children outgrow them. Roughly 70 % of childhood absence epilepsy remits by adulthood, especially when it begins before about 9 years.

An absence is a rhythm the brain already knows, played too strongly.

The same thalamocortical machinery that produces sleep spindles is driven into a slow, synchronous oscillation that takes awareness with it.

Questions: Why do absence seizures appear as 3 Hz spike-and-wave over both hemispheres at once? The rhythm is generated by a loop between the cortex and the thalamus, which projects to both hemispheres, so the discharge starts everywhere the loop reaches instead of spreading from one spot. Cortex excites the thalamus, the inhibitory neurons of the thalamic reticular nucleus then hyperpolarise the relay cells, and the relay cells answer with a rebound burst carried by T-type calcium channels that drives the cortex again. Each turn of that loop takes about a third of a second, which sets the frequency near 3 per second: the spike is the cortex firing together, the wave is the inhibition that follows.