An abnormal EEG can result from dozens of different conditions, ranging from epilepsy and head injuries to liver failure, medication effects, and even normal variants that look alarming but are harmless. About 10% of healthy people show nonspecific EEG abnormalities, and roughly 1% have spike patterns that resemble epilepsy despite never having a seizure. So an abnormal result doesn’t automatically point to a serious diagnosis. What matters is the type of abnormality, where it appears on the scalp, and how it fits with your symptoms.
Epilepsy and Seizure Disorders
Epilepsy is the most well-known reason for an abnormal EEG, and different seizure types produce distinct patterns. Generalized spike-and-wave discharges, where a sharp spike is immediately followed by a slow wave across both sides of the brain, are the hallmark of idiopathic generalized epilepsies. In childhood absence epilepsy (the kind where a child briefly “zones out”), these spike-and-wave bursts are the primary diagnostic finding. In juvenile myoclonic epilepsy, which causes sudden muscle jerks, the pattern shifts to multiple rapid spikes clustered together before the slow wave.
Focal epilepsies show spikes or sharp waves limited to one brain region. Children with benign Rolandic epilepsy, for instance, have characteristic spikes over the central-temporal area of the scalp. More severe conditions like Lennox-Gastaut syndrome, a childhood epileptic encephalopathy, produce slow spike-and-wave discharges that appear simultaneously across both hemispheres.
It’s worth knowing that a single routine EEG catches epileptic activity only 20 to 50% of the time, dropping to just 17% in adults after a first unprovoked seizure. The brain doesn’t always cooperate during the 20 to 30 minutes of recording. A normal EEG does not rule out epilepsy, and repeat recordings or longer monitoring often follow when suspicion remains high.
Metabolic and Toxic Encephalopathy
When the brain’s chemical environment goes wrong, the EEG often shows a pattern called triphasic waves: three-phase, blunt waveforms that appear across both hemispheres. These were first described in patients with liver failure, and hepatic encephalopathy remains one of the top causes, with triphasic waves appearing in about 25% of those patients. Kidney failure is the second major trigger, typically producing a more abrupt decline in mental function. Oxygen deprivation (anoxic injury) rounds out the three most common causes.
The list of metabolic triggers extends further: abnormal sodium levels (too high or too low), elevated calcium, low blood sugar, and thyroid dysfunction can all produce these EEG changes. Sepsis, where a body-wide infection disrupts brain function, generates triphasic waves in more than 10% of affected patients.
Certain medications and toxins cause their own recognizable patterns. Lithium toxicity, serotonin syndrome, and some antibiotics like cefepime have all been linked to triphasic wave encephalopathy. Overdoses of common pain relievers can do it too. These findings typically resolve once the underlying metabolic problem or toxic exposure is corrected.
Medication Effects
Even medications taken at normal doses can change your EEG in ways that look abnormal. Benzodiazepines (drugs like diazepam, commonly prescribed for anxiety or sleep) and similar sedatives produce a paradoxical increase in fast beta activity across the brain. This means the EEG shows more high-frequency waves than expected, not less, despite the sedating effect of the drug. This beta enhancement is one of the most common reasons for an unexpected EEG finding, and it’s essentially a pharmacological fingerprint rather than a sign of brain disease.
Anticonvulsant medications, anesthetics, and even high caffeine intake can alter EEG patterns. If you’re having an EEG, letting your doctor know every medication and supplement you take helps them separate drug effects from genuine abnormalities.
Structural Brain Lesions
Any physical damage to brain tissue can produce focal slowing on an EEG, meaning one area of the brain generates slower-than-normal waves while the rest looks fine. This pattern points directly to the damaged region and appears with a wide range of causes: strokes (both from blocked blood flow and bleeding), brain tumors, traumatic injuries, abscesses from bacterial infection, abnormal blood vessel formations, and malformations that were present from birth.
The slowing typically shows up as delta waves (the slowest type, under 4 cycles per second) concentrated over the affected area. A 35-year-old with a temporal lobe tumor, for example, would show delta slowing over that specific part of the scalp while the opposite side reads normally. This asymmetry is what makes the finding clinically meaningful and often prompts brain imaging to identify the underlying cause.
Traumatic Brain Injury
Head injuries have their own characteristic EEG timeline. Immediately after a mild concussion, the EEG may show sharp waves or rapid discharges, followed by a brief period of suppressed brain activity lasting one to two minutes, then diffuse slowing that typically clears within 10 minutes to an hour.
In the weeks and months that follow, a subtler change persists: the normal resting alpha rhythm at the back of the head slows slightly, then gradually speeds back up by one to two cycles per second as recovery progresses. The majority of EEG abnormalities after mild traumatic brain injury resolve within three months, and 90% are gone within a year. More severe injuries can produce longer-lasting changes, including persistent focal slowing over the area of greatest damage.
Neurodegenerative Diseases
Alzheimer’s disease and Parkinson’s disease both produce a gradual shift in the EEG toward slower frequencies. In healthy people, the brain’s resting rhythm hums along at around 9 to 10 cycles per second (the alpha range). In Alzheimer’s, this slows and the brain produces more theta waves (4 to 8 cycles per second), particularly over the temporal regions. Parkinson’s patients with early cognitive decline show the same pattern, with even greater theta increases than Alzheimer’s patients at a comparable stage.
One study found the median frequency dropped from 9.2 Hz in healthy older adults to 8.8 Hz in Alzheimer’s patients and 8.1 Hz in Parkinson’s patients. These differences are subtle but measurable, and EEG slowing is now considered a potential biomarker for the early stages of cognitive decline in both diseases. Creutzfeldt-Jakob disease, a much rarer condition, produces a distinctive pattern of periodic sharp wave complexes that can help distinguish it from other dementias.
Sleep Disorders
Sleep-related EEGs, recorded during overnight studies, have their own set of abnormal findings. The most diagnostically useful is the sleep-onset REM period, where a person enters dream sleep within 15 minutes of falling asleep instead of the typical 90 minutes. Rapid transitions from wakefulness directly into REM sleep are a diagnostic hallmark of narcolepsy. A nighttime sleep-onset REM period is highly specific for narcolepsy (99%), meaning it almost never happens in people without the condition, though it’s only moderately sensitive (around 45%), so many narcolepsy patients won’t show it on any given night.
Narcolepsy also disrupts normal sleep architecture, producing fragmented nighttime sleep and unusual transitions between sleep stages that show up clearly on EEG monitoring.
Benign Variants That Mimic Abnormalities
Not every unusual EEG pattern means something is wrong. At least 10 recognized benign variants can look like pathological findings to an untrained eye. Wicket spikes, which appear over the temporal lobes, can resemble epileptic sharp waves. The 14-and-6 Hz positive spikes pattern occurs in 20 to 60% of the normal population, predominantly in adolescents during drowsiness and light sleep. Six-Hz spike-and-wave discharges show up in young adults during relaxation or drowsiness and can be mistaken for seizure-related activity.
Other benign patterns include benign sporadic sleep spikes (more common in adults), rhythmic temporal theta bursts during drowsiness, and subclinical rhythmic discharges that primarily affect older adults. Recognizing these variants is critical because misidentifying them as epileptic activity can lead to unnecessary medication or driving restrictions. An EEG interpreted by a neurologist with training in electroencephalography is far less likely to result in these errors than one read by a generalist.
Infections and Inflammation
Brain infections produce EEG abnormalities that vary with the type and location of the infection. Viral encephalitis, particularly herpes simplex encephalitis, causes focal slowing and sometimes periodic discharges over the affected temporal lobe. Bacterial infections that form an abscess produce focal slowing similar to what a tumor would cause. Septic encephalopathy, where a systemic infection impairs brain function without directly infecting the brain, produces diffuse slowing or triphasic waves.
Autoimmune encephalitis, where the immune system attacks the brain, can produce a range of EEG changes including diffuse slowing, seizure activity, and a unique pattern called extreme delta brush that is particularly associated with anti-NMDA receptor encephalitis. These findings often appear before brain imaging shows anything abnormal, making the EEG an important early diagnostic tool in these conditions.