Interictal epileptiform discharges
The IFCN operational definition lists six features. 3
- Di- or triphasic wave with a sharp or spiky shape
- Duration different from the ongoing background activity
- Asymmetry: a steep rise and a slower fall, or the reverse
- An after-going slow wave
- Disruption of the background around it
- A voltage field that suggests a source in the brain
In the clinical validation study, requiring five of the six gave a specificity of about 96% with a sensitivity of about 81%. 4 The practical rule: the fewer the criteria met, the stronger your reason to leave it uncalled.
Over-reading does more harm than under-reading. A wrong diagnosis of epilepsy brings years of medication, driving restrictions and a missed real diagnosis. If you are unsure, describe the waveform and say that it is not definitely epileptiform.
How often, and in whom
A first routine EEG shows epileptiform discharges in roughly half of people with epilepsy; repeated recordings and sleep raise the yield considerably. 11 They are also found in about 0.5% of healthy adults. 21 The EEG supports a clinical diagnosis. It does not replace one.
Syndrome signatures
| Syndrome | EEG |
|---|---|
| Childhood absence epilepsy | Generalized 3 Hz (2.5–4 Hz) spike-and-wave, provoked by hyperventilation |
| Juvenile myoclonic epilepsy | Generalized 3–5.5 Hz polyspike-and-wave; photoparoxysmal response common |
| Self-limited epilepsy with centrotemporal spikes | High-amplitude centrotemporal spikes, much more frequent in sleep |
| Lennox-Gastaut syndrome | Slow spike-and-wave at 2.5 Hz or less; generalized paroxysmal fast activity in sleep |
| Infantile epileptic spasms syndrome | Hypsarrhythmia: chaotic, very high amplitude, multifocal spikes; electrodecrement with spasms |
| Mesial temporal lobe epilepsy | Anterior temporal sharp waves; temporal intermittent rhythmic delta |
Names follow the 2022 ILAE classification; "self-limited epilepsy with centrotemporal spikes" replaces benign rolandic epilepsy. 23
Activation procedures
- Hyperventilation for three minutes: provokes absences in most untreated children with childhood absence epilepsy. Normal build-up of slowing is not an abnormality. 23
- Intermittent photic stimulation: a photoparoxysmal response is generalized spike-wave that may outlast the flashes. Photic driving is normal.
- Sleep and sleep deprivation increase the yield of focal and generalized discharges.
What makes a seizure
Not sharpness, and not amplitude: evolution. ACNS 2021 defines an electrographic seizure as either epileptiform discharges averaging more than 2.5 Hz for at least 10 seconds, or any pattern with definite evolution in frequency, morphology or location lasting at least 10 seconds. With a clinical correlate, or with clinical and EEG improvement after an intravenous anti-seizure medication, it is an electroclinical seizure whatever its duration. 1
Non-convulsive status epilepticus
The Salzburg criteria, for patients without a known epileptic encephalopathy: 2
- Epileptiform discharges faster than 2.5 Hz, or
- Epileptiform discharges at 2.5 Hz or slower, or rhythmic delta or theta above 0.5 Hz, plus one of: clinical and EEG improvement after intravenous anti-seizure medication; subtle clinical ictal signs; typical spatiotemporal evolution.
EEG improvement without clinical improvement, or fluctuation without definite evolution, is "possible" non-convulsive status. For non-convulsive status with impaired consciousness, the ILAE operational time to start treatment is 10 minutes. 24
First-line treatment of convulsive status epilepticus is a benzodiazepine at a full dose (for example intravenous lorazepam 0.1 mg/kg, up to 4 mg per dose, repeated once if needed), followed by levetiracetam, fosphenytoin or valproate, which performed equally in ESETT. Follow your local protocol.16,27