Epileptiform discharge · Grey Matter

An epileptiform discharge is a sudden, brief wave on the EEG that stands out sharply from the background and is typical of tissue prone to seizures, and reading these waves is how a neurologist supports a diagnosis of epilepsy in the long stretches between seizures.

Epileptiform discharge. An epileptiform discharge is a sudden, brief wave on the EEG that stands out sharply from the background and is typical of tissue prone to seizures, and reading these waves is how a neurologist supports a diagnosis of epilepsy in the long stretches between seizures.

The waves are called paroxysmal because they start and stop abruptly. A spike lasts about 20 to 70 ms and a sharp wave about 70 to 200 ms; both are often followed by a slower wave, and the pair is the spike-and-wave complex. Under the electrodes, the spike is a large population of neurons depolarising almost together, and the slow wave after it is the inhibition and hyperpolarisation that cut the burst off.

Interictal means between seizures. An isolated spike causes no symptom; it marks cortex that can synchronise, and where it appears on the scalp points to where focal seizures may begin. A single routine recording often catches none, which is why EEGs are repeated, recorded after a night without sleep, or extended over days.

Ictal means during a seizure, and an ictal pattern evolves. It starts as a rhythm at one frequency, grows in amplitude, changes frequency and spreads to neighbouring electrodes over seconds, then stops and leaves slowing behind; that evolution is what separates a seizure from a run of spikes.

Generalised patterns appear over both hemispheres at once. The 3 Hz spike-and-wave of absence seizures and the faster polyspike-and-wave of juvenile myoclonic epilepsy are the classic examples.

Spike counts rise and fall over days and weeks. Implanted devices that record from the cortex for years show interictal activity following multi-day cycles, most often of 20 to 30 days, with seizures more likely near their peaks.

An interictal spike is synchrony that inhibition still stops.

It shows the tissue can fire together, and the wave after it shows the brake working, which is why spikes mark risk without being seizures.

Questions: What does a spike on the EEG between seizures say about the brain? An interictal spike, a sharp wave of 20 to 70 ms standing out from the background, shows that a patch of cortex can make many neurons fire almost at once, the same synchrony a seizure is built from, while inhibition still cuts it short. It supports a diagnosis of epilepsy and its position on the scalp points to where focal seizures may begin, though it causes no symptom by itself. Its absence on a single recording proves little, and long recordings from implanted devices show spike rates rising and falling over cycles of weeks, with seizures more likely near the peaks.