Seizure threshold · Grey Matter

The seizure threshold is how much of a push a brain needs before it produces a seizure, and it is the idea that explains why any brain can seize under enough stress while some brains seize with much less.

Seizure threshold. The seizure threshold is how much of a push a brain needs before it produces a seizure, and it is the idea that explains why any brain can seize under enough stress while some brains seize with much less.

Every brain can produce a seizure: give enough of a convulsant drug, drop the blood sugar far enough or raise the body temperature in a young child, and seizures follow. The threshold is set by the balance between excitation and inhibition, and by how well the tissue clears what firing leaves behind (potassium, glutamate) and pays for it in ATP. Hyperexcitability is the name for a threshold that has come down: neurons that respond to small inputs, fire in bursts, and recruit their neighbours easily.

Causes lower it for good. Mutations in sodium, potassium or calcium channels, altered GABA receptors, failing glutamate transporters, loss of interneurons, rewired circuits after injury (in the hippocampus, mossy fibres sprouting back onto their own cells) and scars from stroke, trauma or infection all leave a lasting tendency.

Triggers lower it for a while. Sleep deprivation, fever, alcohol withdrawal, low blood sugar, low blood sodium, lack of oxygen, intoxication and some medicines (bupropion and tramadol among them) can provoke a seizure in a person without epilepsy and make seizures more likely in a person with it.

The distinction matters for diagnosis. A seizure with a clear acute cause is called provoked and does not by itself establish epilepsy; epilepsy means an enduring predisposition, shown by unprovoked seizures.

Threshold is a useful word, and a simplification. There is no single number to measure, and it moves with time of day, sleep stage and hormonal state.

A dam with a waterline is the everyday picture.

Causes lower the wall, triggers raise the water for a time, and a seizure is the moment water goes over the top, whichever of the two moved.

Questions: What does it mean for a brain to have a seizure threshold, and what lowers it? The threshold is how hard a brain has to be pushed before excitation outruns inhibition and a seizure starts, so it is the excitation-inhibition balance seen from the side of its failure. Lasting causes lower it, such as mutations in ion channels, lost interneurons or a scar after injury, and that lasting lowering is what epilepsy means. Temporary triggers lower it for hours or days: sleep deprivation, fever, alcohol withdrawal, low blood sugar or sodium, and some medicines, and in diary studies of people with epilepsy, less sleep and more stress were each followed by more seizures. Why do sleep loss and the first hour after waking matter so much in juvenile myoclonic epilepsy? In juvenile myoclonic epilepsy the thalamocortical networks that switch the brain between sleep and waking are prone to generalised discharges, and the transition after waking, especially after too little sleep, is when they fire most easily. That is why the typical jerks come shortly after getting up and why sleep deprivation, together with alcohol, is the strongest trigger. The link between sleep, waking and these networks is described well and explained only in part, and it is why clinicians treat regular sleep as part of managing this epilepsy.