EEG Patterns in Clinical Neurology
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Direct answer
EEG patterns in clinical neurology are characteristic waveforms that point towards a diagnosis, for example 3 Hz spike-and-wave in absence epilepsy, hypsarrhythmia in infantile spasms and triphasic waves in metabolic encephalopathy. Periodic complexes signal acute cortical injury, as in herpes simplex encephalitis or prion disease. The pattern is always read alongside the clinical picture rather than on its own.
What you must remember
- 3 Hz spike-and-wave: a generalised, symmetrical discharge at about 3 per second in typical absence epilepsy, with abrupt onset and offset and hyperventilation as the usual trigger.
- Hypsarrhythmia: chaotic, high-amplitude, multifocal spikes on a disorganised background, seen in infantile spasms, where early treatment protects development.
- Triphasic waves: large, frontally predominant waves with three phases, classically seen in hepatic and uraemic encephalopathy.
- Lateralised periodic discharges: periodic lateralised epileptiform discharges over the temporal lobe in herpes simplex encephalitis, where intravenous acyclovir should not wait for the EEG.
- Periodic sharp wave complexes: recurring about once a second in Creutzfeldt-Jakob disease, alongside rapidly progressive dementia and myoclonus.
- Generalised periodic complexes: seen in subacute sclerosing panencephalitis, recurring every few seconds across both hemispheres.
- Burst suppression: alternating bursts and flat periods seen in deep coma, anaesthesia or severe hypoxic-ischaemic injury, usually with a poor prognosis.
- Photoparoxysmal response: generalised spike-wave discharges provoked by intermittent photic stimulation, associated with juvenile myoclonic epilepsy.
Reading the tracing in context
An EEG is read as a pattern in context, not as a lone waveform. The steps below show how a trained reader moves from the tracing to a diagnosis.
- Check the background: Establish whether the background is normal for age, slowed, disorganised or suppressed, since it sets the context for any discharge.
- Identify the discharge: Note the morphology, frequency, distribution (focal, lateralised or generalised) and whether it is rhythmic or periodic.
- Note the provocation: Hyperventilation, photic stimulation and sleep bring out specific patterns, which helps confirm childhood generalised epilepsy.
- Match the clinical setting: A 3 Hz pattern in a child with staring spells points to absence epilepsy, while periodic lateralised discharges in a febrile, confused adult call for urgent imaging and empirical acyclovir.
- Avoid overreading: Benign variants, such as wicket spikes and sleep spindles, must be distinguished from genuinely abnormal discharges.
Worked example: a 7-year-old girl has brief staring spells several times a day with normal development. The EEG shows a generalised 3 Hz spike-and-wave burst that starts and stops abruptly and is provoked by hyperventilation, so the diagnosis is typical absence epilepsy.
Where the examiner tests the tracing
Questions usually give a short EEG description with a clinical history and ask for the diagnosis or the drug.
- Trap: Diagnosing epilepsy from a single spike. Correct: Benign variants and sleep phenomena produce spikes in healthy people, so the clinical event matters more than one discharge.
- Trap: Confusing 3 Hz spike-and-wave with frontal intermittent rhythmic delta. Correct: Absence discharges are generalised and about 3 per second, whereas frontal intermittent rhythmic delta is slower activity linked with encephalopathy.
- Trap: Waiting for the EEG before starting acyclovir in suspected herpes encephalitis. Correct: Treatment starts on clinical suspicion, and periodic lateralised discharges support the diagnosis afterwards.
- Trap: Treating hypsarrhythmia with routine antiepileptic drugs alone. Correct: Infantile spasms need ACTH or prednisolone, with vigabatrin used in tuberous sclerosis.
Frequently asked questions
What does a 3 Hz spike-and-wave pattern indicate?
A 3 Hz spike-and-wave pattern indicates a generalised discharge typical of childhood absence epilepsy. It appears as a symmetrical burst at about 3 per second with abrupt onset and offset. Hyperventilation usually provokes it, and ethosuximide is a first-line drug for this condition.
What is hypsarrhythmia and in which condition is it seen?
Hypsarrhythmia is a chaotic, high-amplitude, multifocal pattern of spikes on a disorganised background. It is characteristic of infantile spasms, also called West syndrome. Treatment with ACTH or prednisolone is started promptly, because delay worsens the developmental outcome.
Which EEG pattern suggests Creutzfeldt-Jakob disease?
Periodic sharp wave complexes recurring about once a second are the EEG pattern associated with Creutzfeldt-Jakob disease. They appear alongside a rapidly progressive dementia and myoclonus. Creutzfeldt-Jakob disease is a prion disease, and the EEG is always read with the clinical picture.
What is the EEG pattern of herpes simplex encephalitis?
Herpes simplex encephalitis typically shows periodic lateralised epileptiform discharges over the temporal lobe on EEG. These discharges support the diagnosis but are not needed before treatment. Intravenous acyclovir should be started at once when the clinical picture is suspicious.