Electroencephalography

Original Editor - Swati Singh

Top Contributors - Swati Singh, Stacy Schiurring and Ewa Jaraczewska  

Introduction

Electroencephalography (EEG) is a safe and non-invasive method to monitor the brain's electrical activity by placing sensors on the scalp. It is crucial in detecting irregular brain wave patterns that may signal neurological conditions such as epilepsy, sleep disorders, or brain injuries. EEG captures tiny electrical impulses generated by neurons, which are then amplified and recorded as waveforms for medical analysis. This makes EEG particularly effective in identifying seizures and other disruptions in brain function. Its wide clinical application and ease of use have made EEG a valuable tool in diagnosis and ongoing neurological care. The visual output of the EEG procedure is known as an 'electroencephalogram'. This recording displays brain waves as wavy lines, reflecting the electrical activity of the brain.

Physiological basis of electroencephalography( EEG) functioning

Electroencephalography (EEG) records electrical activity from the brain by placing electrodes on the scalp. These electrodes detect changes in electrical potential produced by neurons in the cerebral cortex. To register a visible signal on an EEG, roughly 10 cm² of cortex must be synchronously active. The primary contributors to EEG signals are pyramidal neurons, predominantly located in layers III and V of the cortex.

When neurotransmitters bind at synaptic junctions, they trigger either excitatory postsynaptic potentials (EPSPs) or inhibitory postsynaptic potentials (EPSPs or IPSPs). EPSPs involve sodium ions entering the neuron, resulting in an extracellular negative charge due to depolarisation. IPSPs, on the other hand, involve hyperpolarisation, making the inside of the neuron more negative. These localised potentials combine to form a dipole field, aligned with the vertical orientation of pyramidal neurons. The EEG captures this collective electrical field rather than the activity of individual neurons.

Cortical neurons work in concert with subcortical structures through well-organised feedback loops. In a resting or relaxed state, the EEG typically displays a 'posterior dominant rhythm' (PDR), especially over the occipital lobe. This rhythm reflects rhythmic communication between the visual cortex and the thalamus. When a person becomes alert or stimulated, this synchronised activity decreases, and the EEG shows faster, lower-amplitude waves. In contrast, seizures produce abnormal, large-scale synchronous discharges, which can be localised and analysed using EEG.

EEG waveform examples

EEG waveforms are categorised by frequency:

• Alpha waves (8–12 Hz) dominate during relaxed wakefulness.
• Beta waves (13–30 Hz) occur during active mental engagement.
• Theta waves (4–7 Hz) are seen during drowsiness or early sleep.
• Delta waves (<4 Hz) are most common in deep sleep or certain pathological states.

Waveform patterns change with age and level of consciousness. Early in development, EEGs have discontinuous backgrounds that gradually evolve into more stable, continuous activity. The posterior dominant rhythm, averaging about 8.5 Hz, typically matures by age eight. There’s also a typical 'anterior-to-posterior gradient', with faster rhythms found in the front of the brain and slower ones in the back.

Special features like sleep spindles and K-complexes emerge in the first year of life and help differentiate sleep stages. For accurate interpretation, it is important to recognise normal variants and artifacts, such as those caused by eye movement or muscle activity, to avoid misdiagnosis.

EEG interpretation

Purpose and technique

The following are several indications for the use of EEG:

Primary Indications

• Seizure Evaluation and Epilepsy Diagnosis: EEG is instrumental in classifying seizure types, localising seizure onset zones, and confirming epilepsy diagnoses. It is particularly useful when clinical assessments and standard EEGs yield inconclusive results.
• Monitoring Treatment Efficacy: EEG helps assess the effectiveness of anti-seizure and anti-epileptic therapies, especially in patients with refractory seizures.
• Assessment of Altered Mental Status: In cases of unexplained changes in consciousness, confusion, or coma, EEG can detect non-convulsive seizures or status epilepticus, guiding appropriate management.
• Brain Death Confirmation: EEG is used as an adjunctive test to confirm brain death in comatose patients, particularly when clinical criteria are insufficient.

Additional Clinical Applications

• Evaluation of Neurological Conditions: EEG aids in diagnosing conditions such as encephalopathy, encephalitis, brain tumours, and stroke by revealing characteristic patterns of brain activity.
• Investigation of Sleep Disorders: EEG is a component of polysomnography, assisting in the diagnosis of sleep disorders like sleep apnea and narcolepsy.
• Pediatric Assessments: In children, EEG is utilised to evaluate developmental delays, unexplained behavioural changes, or suspected seizure activity.
• Pre-Surgical Evaluation: For patients with intractable epilepsy, EEG, including long-term video monitoring, helps localise epileptogenic zones to guide surgical interventions.
• Monitoring in Intensive Care Units (ICU): Continuous EEG monitoring in ICUs assists in detecting non-convulsive seizures, guiding sedation levels, and assessing prognosis in comatose patients.

The following 2-minute video provides an introduction to interpreting EEGs, discussing basic patterns and what they signify.

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EEG Preparation and Procedure: A Clinical Overview

Patient Preparation

Before undergoing an EEG, patients are provided with specific instructions to ensure optimal recording quality. Hair should be clean and free of conditioners, gels, or sprays, as these can interfere with electrode adhesion and increase impedance. Ideally, electrode impedance should be kept below 5 kΩ to ensure accurate signal capture.

Patients are also advised to:

• Avoid caffeine for 8–12 hours before the test, as stimulants can alter brain wave activity.
• 'Eat a proper meal' before the procedure to prevent low blood sugar, which may influence results.
• Report any medications they are currently taking. Depending on the clinical context, certain drugs may need to be discontinued under medical supervision.
• 'Limit sleep' the night before if a sleep-deprived EEG is scheduled—typically 4–5 hours of sleep for adults and 5–7 hours for children.
• Remove metal objects, such as jewellery, which may interfere with EEG signals.

In intensive care settings, additional care is taken to minimise electrical and mechanical artifacts from nearby devices. While chemical restraints are generally avoided', mechanical restraints may occasionally be necessary to reduce movement and obtain a quality recording.

EEG Technique and Setup

A routine EEG is conducted in a quiet, controlled environment with adjustable lighting. The procedure is carried out by a trained EEG technician or neurophysiologist using the internationally recognized 10–20 system for electrode placement. Typically, 21 or more electrodes are placed on the adult scalp, including reference and ground electrodes.

Before recording begins:

• Electrode impedance is verified and maintained below 5 kΩ.
• A calibration check is performed using a square wave signal and biological calibrations (such as eye movements and blinks).

To provoke potential abnormalities, various' activation procedures' may be used:

• Eye opening and closure
• Hyperventilation (used cautiously or avoided in patients with certain conditions)
• Photic stimulation
• Sleep induction or sleep deprivation, especially in epilepsy evaluations

EEG Recording and Interpretation

EEG recordings display the electrical potential difference between pairs of electrodes, organised into different ' montage formats' for accurate interpretation. Common montage types include:

• Referential montages: Compare active electrodes to a standard reference (e.g., ear or average reference).
• Bipolar montages: Show the difference between two adjacent electrodes, useful for detecting asymmetries between hemispheres.
• Laplacian montages: Use a weighted average of surrounding electrodes as a reference, enhancing detection of focal discharges.

Digital EEG systems now allow for easy reformatting and montage reconfiguration, improving the accuracy and flexibility of interpretation.

Procedure Execution

The patient lies on a bed or reclining chair while electrodes are attached to the scalp using conductive paste, adhesive, or a preconfigured cap. As the brain generates electrical activity, the electrodes detect these signals and transmit them to an EEG machine, which records them as waveforms.

In certain complex cases, such as pre-surgical epilepsy evaluation, intracranial EEG may be performed, where electrodes are placed directly on or within the brain through a surgical procedure by a neurosurgeon.

Once recording is complete, the data is analysed by a neurophysiologist, who reviews brain wave patterns to identify abnormalities that may indicate neurological disorders, such as seizures, sleep disturbances, or brain injuries. EEG findings assist clinicians in diagnosis and treatment planning for a wide range of neurological conditions.

Complications and considerations for EEG testing

While EEG is generally considered a safe and non-invasive procedure with no absolute contraindications, certain patient conditions require caution or modification of standard protocols. For instance, hyperventilation, often used as an activation method during EEG, may pose risks and should be avoided or modified in individuals with a history of stroke, cerebrovascular disease, myocardial infarction, severe respiratory or cardiovascular conditions, Moyamoya disease, or sickle cell anaemia. In such cases, inducing rapid or deep breathing could exacerbate underlying conditions or trigger complications.

Additionally, structural factors may complicate electrode placement. Patients who have undergone craniotomy, or those with open scalp wounds or skull defects, may present technical challenges for secure and accurate electrode application. Therefore, a thorough clinical history and assessment should precede EEG testing, particularly if there is a concern for epilepsy or seizures. When necessary, activation procedures (e.g., hyperventilation or photic stimulation) should be adjusted or omitted based on the individual’s medical background to ensure patient safety.