What Is an EEG (Electroencephalography)? How Is It Performed and When Is It Used?

Created: 16.01.2023 · Last Updated: 19.08.2026 · Category: Neurology · Prepared by the Academic Hospital Web and Editorial Board.

What Is an EEG?

Electroencephalography (EEG) is a non-invasive neurophysiological test that records electrical activity generated by the brain using electrodes placed on the scalp.

The electrical activity generated by brain cells is detected at the scalp and displayed as waveforms on a computer. The resulting recording is called an electroencephalogram.

EEG is particularly important in the assessment of suspected epileptic seizures, classification of seizure types or epilepsy syndromes and selected disorders of brain function.

An EEG does not produce an image of the brain. MRI and CT primarily assess brain structure, whereas EEG provides information about electrical activity during the recording period.

What Does an EEG Show?

An EEG allows specialists to evaluate the rhythm, frequency, distribution and characteristic patterns of electrical brain activity.

The recording may demonstrate:

  • Normal activity appropriate for age and state of alertness
  • Epileptiform discharges
  • Focal or generalised slowing
  • Changes associated with sleep
  • Electrical changes occurring during a clinical event

EEG findings should be interpreted together with the patient's symptoms, neurological examination and other investigations where appropriate.

How Is an EEG Performed?

Small electrodes are positioned at defined locations on the scalp. They are usually attached using electrode paste, gel or an electrode cap.

The electrodes do not send electricity into the brain. They only detect electrical activity that can be measured at the scalp.

The patient is asked to remain as relaxed and still as possible. Depending on the purpose of the test, periods with the eyes open and closed, wakefulness and sleep may be recorded.

The EEG is recorded by appropriately trained neurophysiology staff and interpreted by a specialist in conjunction with the relevant clinical information.

How Long Does an EEG Take?

The duration depends on the type and clinical purpose of the EEG.

Current IFCN and ILAE minimum recording standards suggest, excluding preparation time:

  • At least 20 minutes of technically satisfactory recording for a routine EEG
  • At least 30 minutes for a sleep EEG

The overall appointment is longer because electrode preparation, application and removal also take time. NHS patient guidance notes that a standard EEG appointment commonly takes around one hour in total.

Ambulatory and prolonged video EEG recordings may continue for many hours or several days depending on clinical need.

When Is an EEG Used?

EEG is most commonly used in the assessment of seizures and suspected epilepsy.

It may be requested to:

  • Assess events suspected to be epileptic seizures
  • Help classify seizure type or epilepsy syndrome
  • Investigate recurrent seizure-like events
  • Assess selected unexplained episodes of altered awareness or consciousness
  • Evaluate certain encephalopathies and disorders of brain function
  • Assess selected sleep-related events that may resemble seizures
  • Correlate clinical events with electrical activity during prolonged video EEG monitoring
EEG is not routinely required for every neurological symptom. For example, routine EEG is not recommended for the standard evaluation of typical primary headache disorders unless the clinical history raises concern about possible seizures.

What Is the Role of EEG in Epilepsy?

Epilepsy is fundamentally a clinical diagnosis. The description of the event, witness information and neurological assessment are central to diagnosis.

When an epileptic seizure is suspected, EEG can support the diagnosis and provide information about seizure type or epilepsy syndrome.

NICE recommends considering a routine awake EEG when the clinical history and examination suggest an epileptic seizure.

If EEG is requested after a first suspected seizure, performing it promptly may improve diagnostic yield. NICE recommends that it is ideally performed within 72 hours of the seizure.

Does a Normal EEG Exclude Epilepsy?

No.

A person with epilepsy may have a completely normal routine EEG because the test records electrical activity only during the recording period.

A normal EEG therefore must not be used by itself to exclude a diagnosis of epilepsy.

Similarly, not every EEG abnormality means that a person has epilepsy. Findings need to be interpreted in the context of the clinical history.

An EEG result should not be interpreted on its own as “epilepsy present” or “epilepsy absent.” Clinical correlation is essential.

What Are the Different Types of EEG?

  • Routine EEG: A standard-duration EEG recording.
  • Sleep EEG: Includes recording during sleep.
  • Sleep-deprived EEG: Performed after a prescribed reduction in sleep before the examination.
  • Ambulatory EEG: Uses a portable recorder to monitor electrical activity over a longer period during usual daily activities.
  • Video EEG: Records video and EEG simultaneously.

What Is a Sleep or Sleep-Deprived EEG?

Some epileptiform abnormalities are more likely to appear during sleep. A sleep EEG may therefore be considered when routine EEG has not provided sufficient information or when the suspected events are related to sleep.

For a sleep-deprived EEG, the patient is instructed to reduce sleep before the examination. This can make sleep during the recording more likely and can increase the likelihood of detecting certain epileptiform abnormalities.

Because sleep deprivation can affect seizure susceptibility, patients should follow the specific instructions provided by the EEG service rather than deciding independently how much sleep to miss.

What Is Video EEG?

Video EEG records the patient's behaviour and movements at the same time as the EEG.

This allows clinicians to compare what happens clinically during an event with any simultaneous electrical changes in the brain.

Prolonged video EEG monitoring can be useful in distinguishing epileptic seizures from non-epileptic events and for detailed assessment of selected people with epilepsy.

How Should You Prepare for an EEG?

Instructions vary according to the type of EEG.

For a routine EEG, patients are generally advised to:

  • Wash their hair and keep it clean
  • Avoid hair gel, oil, mousse, spray and similar products
  • Eat normally and avoid skipping meals unless specifically instructed otherwise
  • Tell the clinical team about all medicines being taken
  • Not stop regular medication unless specifically instructed by a clinician
  • Follow the EEG service's sleep instructions when a sleep-deprived EEG is planned
Anti-seizure medicines and other regular medication should not be stopped without medical instructions.

What Activation Procedures May Be Used During EEG?

Activation procedures may be used to assess electrical brain activity under different conditions.

These can include:

  • Opening and closing the eyes
  • Hyperventilation: breathing more deeply and rapidly for a defined period
  • Intermittent photic stimulation: exposure to flashing lights at different frequencies

Not every procedure is appropriate for every patient. The potential benefits and risks are considered according to the clinical situation.

Hyperventilation or flashing lights can rarely provoke a seizure in susceptible individuals, which is why these procedures are carried out in a controlled clinical setting.

Is an EEG Painful or Harmful?

A routine EEG is non-invasive and generally painless.

The electrodes only record electrical activity and do not deliver electricity into the brain or body.

Occasionally, mild temporary redness or skin irritation may occur at an electrode site.

Sleep deprivation, hyperventilation and photic stimulation carry a small risk of provoking a seizure in susceptible individuals. Whether these methods are used is determined according to the clinical situation.

EEG at Academic Hospital

According to the Academic Hospital Neurology service, routine EEG and sleep/wake EEG recordings are available according to clinical need.

The appropriate type of EEG is selected according to the patient's symptoms, event history and neurological assessment.

EEG Results Must Be Interpreted in Clinical Context

EEG is an important investigation in the assessment of seizures and epilepsy, but a normal EEG does not exclude epilepsy and an EEG result alone is not sufficient to establish a diagnosis.

Frequently Asked Questions

What is an EEG?
EEG is a non-invasive neurophysiological test that records electrical activity generated by the brain using electrodes placed on the scalp.
Is an EEG painful?
No. A routine EEG is generally painless. The electrodes do not deliver electricity to the brain; they only record electrical activity detected at the scalp.
How long does an EEG take?
IFCN and ILAE minimum standards suggest at least 20 minutes of recording for routine EEG and at least 30 minutes for sleep EEG, excluding preparation. The overall appointment is longer because electrode preparation and removal also take time.
Does a normal EEG mean that a person does not have epilepsy?
No. A person with epilepsy may have a normal routine EEG. EEG should not be used by itself to exclude epilepsy and the result must be interpreted together with the clinical history.
Does an abnormal EEG always mean epilepsy?
No. Not every EEG abnormality indicates epilepsy. Findings must be interpreted together with the patient's events, neurological examination and other clinical information.
How should hair be prepared before an EEG?
Hair should be clean and products such as gel, oil, spray or mousse should be avoided so that the electrodes can make good contact with the scalp.
Should anti-seizure medicines be stopped before an EEG?
No. Anti-seizure medicines and other regular medication should not be stopped without medical instructions. Any medication changes should be directed by the referring clinician or EEG team.
Is EEG routinely used for headache?
Routine EEG is not recommended as part of the standard evaluation of typical primary headache disorders. It may be considered when the clinical history includes features suggesting a possible seizure disorder.
Academic Hospital note: The meaning of an EEG result depends on the patient's symptoms and clinical history. EEG findings should not be interpreted in isolation without specialist assessment. You can book an appointment.

References

  1. Academic Hospital. What Is EEG (Electroencephalography)? Which Patient Groups Are EEGs Taken?
  2. Peltola ME, et al. Routine and Sleep EEG: Minimum Recording Standards of the IFCN and ILAE
  3. NICE. Epilepsies in Children, Young People and Adults: Diagnosis and Assessment
  4. NHS. EEG (Electroencephalogram)
  5. Cambridge University Hospitals. Routine EEG