Myth: every individual seizure causes permanent brain damage.
Reality: the effect of seizures on the brain depends on seizure type, duration, frequency, cause, age, underlying brain condition and other factors. A brief seizure is not automatically equivalent to permanent structural injury.
Why the idea persists
Seizures can temporarily disrupt memory, language, attention, mood and physical function, and prolonged seizures can be dangerous. Those facts are sometimes simplified into the claim that every seizure kills brain cells or causes cumulative permanent damage.
Where concern is justified
Status epilepticus and some prolonged or repeated seizures can cause neurological injury, particularly when accompanied by severe metabolic stress, hypoxia or an acute brain insult. Recurrent seizures may also be associated with network and cognitive changes in some epilepsies. But that is not the same as saying every brief focal or generalised seizure causes measurable permanent damage.
The underlying condition matters
Sometimes the brain disorder causing the seizures also causes cognitive or structural change. Separating the effect of seizures from the effect of the underlying epilepsy, medication, sleep disruption and other comorbidities can be difficult.
Sources
Review of seizure-related neuronal injury and epileptogenesis
Status epilepticus: mechanisms and consequences