Epileptiform discharges are brief, abnormal electrical bursts recorded on an electroencephalogram (EEG) that resemble the patterns seen during epileptic seizures, even though they occur between seizures or sometimes in people who have never had one. They include sharp waves, spikes, and spike-and-wave complexes, and their presence on an EEG is one of the strongest markers clinicians use to assess seizure risk. But their meaning is far from straightforward: the same spike on a screen can signal active epilepsy in one person and something completely harmless in another.
What Epileptiform Discharges Actually Look Like on EEG
On an EEG tracing, epileptiform discharges stand out from the normal background rhythm. A “spike” lasts about 20 to 70 milliseconds, while a “sharp wave” lasts 70 to 200 milliseconds. Both are pointy, high-amplitude deflections that interrupt the smoother background oscillations. Many are followed by a slow wave, creating the classic “spike-and-wave” complex. Clinicians reading EEGs evaluate features like the sharpness of the waveform, how much taller it is than the surrounding activity, and whether a slow wave follows. A scoring system developed for sharp discharges found that amplitude, slope, difference from the background, and the area of the slow after-wave all help predict whether someone has epilepsy.1PubMed Central. A New Score for Sharp Discharges in the EEG Predicts Epilepsy
The location of these discharges matters too. A spike firing repeatedly in the left temporal lobe suggests a different clinical picture than generalized spike-and-wave bursts across both hemispheres. Focal discharges often point toward a structural or localized abnormality, while generalized discharges are more typical of genetic or widespread epilepsy syndromes.
What Happens Inside the Brain During a Discharge
Each spike you see on the EEG reflects an event happening across thousands of neurons simultaneously. At the cellular level, a group of neurons fires in an abnormally synchronized burst called a paroxysmal depolarization shift. This is a large, sustained depolarization followed by a period of inhibition, and it is considered the cellular equivalent of the interictal spike seen on the surface EEG.2PubMed Central. The paroxysmal depolarization shift in epilepsy research The burst depends heavily on calcium channels in the neuron’s dendrites, particularly a type called L-type voltage-gated calcium channels. Research on rat hippocampal neurons has shown that these channels are essential for generating the burst, and that it appears to start in the dendritic branches of the neuron before spreading to the cell body.3PubMed. On the Origin of Paroxysmal Depolarization Shifts: The Contribution of Ca(v)1.x Channels as the Common Denominator of a Polymorphous Neuronal Discharge Pattern
These bursts can also spread to nearby neurons through electrical fields, not just through chemical synapses. Recent work has shown that the electrical signal from a paroxysmal depolarization shift can be recorded up to 16 micrometers from the network, with its strength dropping off exponentially with distance. Blocking L-type calcium channels completely abolished this spread, while blocking another calcium channel subtype reduced it.4PubMed. Ephaptic Coupling Contributes to the Propagation of Paroxysmal Depolarization Shifts In Vitro This matters because it shows that epileptiform activity does not need active synaptic connections to propagate; the electric field itself can recruit neighbors.
Interictal Discharges Versus Actual Seizures
The word “interictal” means “between seizures.” Most epileptiform discharges detected on routine EEGs are interictal: they are isolated pops of abnormal electricity that last a fraction of a second and do not produce any visible symptom like a convulsion or a staring spell. An actual seizure, or ictal event, looks different on EEG. It shows a sustained, evolving pattern of rhythmic activity that builds, spreads, and eventually resolves, usually lasting at least several seconds and often accompanied by clinical symptoms.
But the line between the two is not always clean. In absence epilepsy, for instance, the hallmark pattern is a generalized three-per-second spike-and-wave discharge. Studies using magnetoencephalography have shown that these spike-and-wave bursts begin with high synchronization in the frontal regions and then alternate between generalized activity during the wave component and more localized activity during the spike component.5PubMed. Onset and propagation of spike and slow wave discharges in human absence epilepsy: A MEG study There is even a longstanding hypothesis that these spike-and-wave discharges develop from the same brain circuitry that normally generates sleep spindles, where cortical neurons become hyper-responsive to signals from the thalamus and transform what should be normal sleep rhythms into epileptic ones.6PubMed. Spike-and-wave discharges of absence seizures as a transformation of sleep spindles: the continuing development of a hypothesis
Epileptiform Discharges in People Without Epilepsy
Finding epileptiform discharges on an EEG does not automatically mean a person has epilepsy. Interictal epileptiform discharges are rarely seen in healthy adults without a history of seizures, but the rates are higher in children. A review of the evidence found that while these discharges appear infrequently in healthy volunteers, the risk of subsequently developing unprovoked seizures remains low in both healthy volunteers and non-epileptic patients.7PubMed. Interictal epileptiform discharges in persons without a history of seizures: what do they mean?
In children specifically, the numbers are striking. A prospective study using digital EEG in healthy children found epileptiform discharges in about 6.5% of them, with focal discharges being the most common pattern.8PubMed. Prevalence of epileptiform discharges in healthy children–new data from a prospective study using digital EEG In younger children between 12 and 60 months old, a separate study found a lower prevalence of less than 1%.9PubMed. Prevalence of Epileptiform Discharges in Healthy Infants The discrepancy likely reflects differences in the age range studied and the sensitivity of the recording equipment. The broader point is that a spike on a child’s EEG, taken without other context, does not mean they have or will develop epilepsy.
Benign Patterns That Look Epileptiform But Are Not
One of the biggest pitfalls in EEG interpretation is mistaking a harmless brain pattern for an epileptiform discharge. Benign epileptiform variants are EEG patterns that look sharp, spiky, or rhythmic enough to fool the reader into thinking they are pathological, but they carry no increased risk of epilepsy. A study of 3,000 EEGs found these benign variants in about 10% of recordings, with small sharp spikes being the most common, appearing in nearly 4% of patients.10PubMed. Benign epileptiform variants in EEG: A comprehensive study of 3000 patients Wicket waves, 6 Hz spike-and-slow-wave complexes, and 14-and-6 Hz positive bursts were also relatively common.
A larger analysis spanning 35 years and over 35,000 EEG recordings found benign variants in about 3.4% of all subjects and confirmed that, unlike true focal epileptic spikes and generalized spike-and-wave discharges, these patterns do not predict epilepsy.11Clinical Neurophysiology. Prevalence of benign epileptiform variants Accurate identification of these variants avoids misdiagnosis and unnecessary treatment.
The problem is that misidentification happens often. An Indian EEG laboratory study found that benign epileptiform variants were over-interpreted as true epileptiform abnormalities in about 30% of records that were available for review, and that these variants had not even been mentioned in the original EEG reports.12PubMed. Prevalence of benign epileptiform variants from an EEG laboratory in India and frequency of their misinterpretation That is not a trivial error. A misread EEG can lead to an incorrect epilepsy diagnosis, years of unnecessary medication, driving restrictions, and considerable psychological burden.
Cognitive Effects Even Without Visible Seizures
Even when epileptiform discharges do not trigger a clinical seizure, they are not always silent. Research has shown that interictal spikes can transiently interfere with brain function, a phenomenon sometimes called transitory cognitive impairment. In animal studies, hippocampal spikes occurring during memory retrieval strongly impaired performance, though spikes during memory encoding or maintenance did not have the same effect. The spikes also slowed response times by roughly half a second per event.13PubMed Central. Hippocampal Interictal Spikes Disrupt Cognition in Rats
In humans, the picture is nuanced. Short nonconvulsive seizures have been shown to produce large cognitive effects, ranging from half to a full standard deviation across measures of processing speed, memory, and overall cognitive function. Frequent epileptiform discharges in children without seizures during testing showed more subtle but still measurable effects, independent of whether those children also had nonconvulsive seizures.14PubMed Central. The cognitive effects of interictal epileptiform EEG discharges and short nonconvulsive epileptic seizures The type of cognitive disruption depends on where in the brain the spike occurs and what task the person is performing at that moment. Frequent and widespread spikes are a particular concern in developing brains, where animal data suggest they can produce lasting cognitive harm even after the discharges themselves stop.15PubMed. Interictal Spikes as an EEG Biomarker of Cognitive Impairment
What Epileptiform Discharges Mean for Seizure Recurrence
For someone who has had a first unprovoked seizure, the presence of epileptiform discharges on their EEG is one of the most clinically meaningful findings. A meta-analysis from the International League Against Epilepsy found that the presence of interictal epileptiform discharges roughly doubled the odds of seizure recurrence, with the association being even stronger in children than in adults.16PubMed Central. Presence of interictal epileptiform EEG discharges implies increased risk of recurrence after the first unprovoked seizure
A ten-year prospective cohort study put numbers on this more concretely. Among patients with epileptiform abnormalities on video-EEG, about 72% had seizure recurrence. Epileptiform discharges on EEG and a history of prior brain injury were the two independent predictors of recurrence, with epileptiform discharges carrying a relative risk of 2.5.17PubMed. Risk of recurrence after a first unprovoked seizure with different risk factors: A 10-year prospective cohort study This is why many clinical guidelines incorporate the first EEG result into the decision about whether to start antiseizure medication after a first seizure. The EEG alone does not settle the question, but it shifts the probability estimate meaningfully.
Techniques That Provoke Hidden Discharges
A routine EEG might look perfectly normal even in someone with epilepsy, because interictal discharges are not firing every minute. To increase the chances of capturing them, technicians use activation procedures: hyperventilation, where the patient breathes rapidly for several minutes, and intermittent photic stimulation, where a strobe light flashes at various frequencies. Both can provoke epileptiform activity that would otherwise stay hidden.
A study examining the added value of these procedures found that they provided additional clinically useful information in about 10% of abnormal EEG recordings, helping to either establish a diagnosis of epilepsy or further characterize the epilepsy syndrome. This benefit was seen exclusively in patients younger than 36 and was more common in women.18PubMed. Reexamining the added value of intermittent photic stimulation and hyperventilation in routine EEG practice Sleep deprivation is another common activator; having a patient stay awake the night before an EEG often brings out discharges that wakefulness suppresses.
The Ictal-Interictal Continuum in Critical Care
In intensive care units, the distinction between interictal spikes and seizures becomes especially blurry. Critically ill patients with brain injuries from stroke, trauma, or infection sometimes show rhythmic or periodic EEG patterns that do not meet the full criteria for seizures but are clearly more than benign interictal spikes. These patterns live in what is called the ictal-interictal continuum, and they may be associated with ongoing metabolic stress on the brain and neuronal injury.19Acta Epileptologica. Ictal-interictal continuum: a review of recent advancements
The clinical challenge is that there are no clear guidelines on how aggressively to treat these patterns.20PubMed. Understanding and Managing the Ictal-Interictal Continuum in Neurocritical Care They are associated with an increased risk of developing definite seizures and with worse outcomes, but treating them with aggressive anti-seizure drugs carries its own risks, including sedation, low blood pressure, and respiratory depression.21PubMed. The Ictal-Interictal Continuum: To Treat or Not to Treat (and How)? Many neurointensivists approach it on a case-by-case basis, weighing the pattern’s features against the patient’s overall condition.
Epileptiform Patterns Unique to Children
Some epileptiform patterns have special significance in pediatric neurology. Self-limited epilepsy with centrotemporal spikes, formerly called benign rolandic epilepsy, is one of the most common childhood epilepsy syndromes. Children show distinctive sharp waves over the central and temporal regions of the brain, especially during sleep. Traditionally, these children have a very good prognosis, with many outgrowing the condition by adolescence even without medication.22PubMed Central. The Clinical Spectrum of Benign Epilepsy with Centro-Temporal Spikes: a Challenge in Categorization and Predictability However, the range of outcomes is broader than the “benign” label implies, with some children experiencing subtle cognitive and language difficulties linked to their nighttime spike burden.
At the more severe end of the spectrum, infantile epileptic spasms syndrome produces a chaotic EEG pattern called hypsarrhythmia, characterized by high-amplitude disorganized slow waves mixed with multifocal sharp and spike discharges. Developmental stagnation and decline can occur in these children, partly because of the abundant interictal epileptiform and slow-wave abnormalities disrupting normal brain development. The specific variations of hypsarrhythmia are not tied to a single cause but can reflect the severity of the underlying condition.23PubMed Central. Electroclinical Features of Infantile Epileptic Spasms Syndrome
Medications That Can Cause or Worsen Epileptiform Activity
Not all epileptiform discharges are generated by an underlying brain disorder. Some medications can provoke them, including, paradoxically, certain anti-seizure drugs themselves. Multiple classes of central nervous system medications have been linked to increased epileptiform activity in people both with and without known seizure disorders.24PubMed Central. Iatrogenic Epileptogenicity Caused by CNS Drugs: A Short Case Series and Narrative Review
Among antipsychotics, clozapine stands out. A systematic review of EEG changes in patients on antipsychotic therapy found that clozapine was the drug most consistently accompanied by both EEG slowing and epileptiform discharges, with substantially elevated odds compared to patients not taking antipsychotics.25PubMed. EEG changes in patients on antipsychotic therapy: A systematic review In one clinical case, discontinuing the psychotropic medications mirtazapine and donepezil resolved the epileptiform discharges and restored a faster, more normal background rhythm on EEG.26American Epilepsy Society. EPILEPTIFORM ENCEPHALOPATHY AS A RESULT OF PSYCHOTROPIC MEDICATIONS The practical implication is that whenever epileptiform discharges appear on an EEG, clinicians should review the patient’s medication list before jumping to an epilepsy diagnosis.
Conversely, anti-seizure medications can suppress epileptiform discharges. A study in patients with idiopathic generalized epilepsy found that anti-seizure medication reduced the rate of interictal epileptiform discharges by a median of about 64% between the highest and lowest medication-load windows, an effect that held up even when benzodiazepines were excluded from the analysis.27PubMed. Investigating short-term antiseizure medication effects on interictal epileptiform discharges in idiopathic generalized epilepsy This means an EEG recorded while a patient is already on medication may underestimate their underlying level of epileptiform activity.
Genetic Roots of Epileptiform Excitability
For many people with epilepsy, the tendency for neurons to fire epileptiform discharges traces back to their genes, specifically to mutations in ion channel genes. These channels control the flow of sodium, potassium, and calcium across nerve cell membranes, and when they malfunction, neurons can become hyperexcitable. Epilepsy is a frequent consequence of acute brain injuries like stroke and trauma, but in many cases, especially in children, the cause is genetic rather than structural.28PubMed Central. Prognostic models for seizures and epilepsy after stroke, tumors and traumatic brain injury
A large genetic study of over 2,000 patients with epilepsy found disease-causing variants in about 23% of them. Among those patients, roughly a third carried mutations in voltage-gated channel genes. Rarer channel mutations were identified in sodium and potassium channel subtypes not typically included in standard genetic panels, and about 60% of these patients were diagnosed with developmental and epileptic encephalopathy. Seizure freedom was eventually achieved in just over half, while seizures persisted in the rest.29PubMed Central. Unusual Voltage-Gated Sodium and Potassium Channelopathies Related to Epilepsy Identifying the specific genetic defect can occasionally guide treatment, because some channel mutations respond better to certain medications than others.
Automated Detection With Artificial Intelligence
Reading EEGs for epileptiform discharges is time-consuming and subjective. Two experienced neurophysiologists reviewing the same recording may disagree on whether a particular waveform qualifies as epileptiform. This has motivated considerable investment in automated detection using artificial intelligence. A deep learning system developed for magnetoencephalography, a cousin of EEG that records magnetic rather than electrical signals, achieved sensitivity near 80% and specificity above 99% for detecting epileptiform spikes, with the location of the detected activity coming within about 6 millimeters of where expert neurophysiologists placed it.30PubMed. Fully-Automated Spike Detection and Dipole Analysis of Epileptic MEG Using Deep Learning
A follow-up multi-center study tested a similar deep-learning model across data from different hospitals and found high accuracy on internal data and good accuracy on external data, with median localization distances of about 4 to 7 millimeters from expert-determined positions.31Scientific Reports. Deep learning based automatic detection and dipole estimation of epileptic discharges in MEG: a multi-center study Another pipeline managed to accomplish the full analysis, from spike detection to source localization, in about 12 minutes compared to roughly four hours for the traditional manual process, with lobar-level localization accuracy matching invasive electrode findings about 87% of the time.32Journal of Neural Engineering. An artificial intelligence–based pipeline for automated detection and localisation of epileptic sources from magnetoencephalography These tools are not replacing human readers yet, but they promise to reduce both the time and the inter-reader variability that currently limit EEG-based diagnosis.
Brain Stimulation as a Way to Quiet Discharges
Beyond medication, non-invasive brain stimulation is being explored as a way to reduce epileptiform discharges. Low-frequency repetitive transcranial magnetic stimulation, which uses a magnetic coil placed on the scalp to modulate cortical excitability, has shown promise. A systematic review pooling data from hundreds of patients found that this stimulation approach significantly reduced epileptiform discharges and also improved cognitive function.33PubMed Central. Systematic reviews of low-frequency repetitive transcranial magnetic stimulation on cognition and epileptiform discharge in patients with epilepsy In a randomized clinical trial of patients with refractory epilepsy, active stimulation produced a significant decrease in epileptiform discharges both immediately after treatment and at four weeks of follow-up, while sham stimulation did not.34PubMed. A randomized clinical trial of repetitive transcranial magnetic stimulation in patients with refractory epilepsy The dual effect on both discharges and cognition is particularly interesting, given the evidence that frequent discharges contribute to cognitive problems even in the absence of seizures.