Electroencephalogram (EEG): Measuring Brain Activity

An electroencephalogram (EEG) is a noninvasive neurophysiological test that records the brain's spontaneous electrical activity through electrodes placed on the scalp. The procedure is central to the diagnosis and monitoring of epilepsy and seizure disorders, sleep disorders, encephalopathies, and other conditions affecting cerebral function. Understanding how EEG works, when it is ordered, and how its findings are interpreted helps clarify its role within the broader landscape of neurological diagnostic testing.

Definition and Scope

An EEG captures voltage fluctuations produced by the synchronized firing of neurons — primarily cortical pyramidal cells — measured in microvolts (µV) across multiple scalp locations. The American Clinical Neurophysiology Society (ACNS) publishes standardized guidelines governing EEG recording and interpretation, most recently updated in its Guideline 1: Minimum Technical Requirements for Performing Clinical EEGs (ACNS, 2016).

The International Federation of Clinical Neurophysiology (IFCN) further establishes standards for electrode placement through the internationally adopted 10–20 electrode system, which uses 19 to 21 active scalp electrodes in a standard clinical recording. Electrode positions are defined as percentages of measured skull distances — hence the name "10–20" — ensuring reproducible spatial coverage across the frontal, temporal, parietal, and occipital lobes.

The regulatory context for neurological diagnostics in the United States places EEG equipment under U.S. Food and Drug Administration (FDA) Class II medical device classification (21 CFR Part 882), with substantial equivalence pathways applicable to most commercial EEG amplifiers and electrode systems (FDA Device Classification, 21 CFR §882.1400).

EEG variants span a clinically significant range:

• Routine EEG: 20–40 minutes; captures resting brain activity with standard activation procedures (hyperventilation, photic stimulation)
• Ambulatory EEG: 24–72 hour recording worn by the patient; detects infrequent events in outpatient settings
• Video-EEG (vEEG): Continuous synchronized video and EEG; the gold standard for pre-surgical epilepsy evaluation and seizure semiology classification
• Intraoperative EEG / Electrocorticography (ECoG): Electrodes placed directly on cortical surface during neurosurgery; captures sub-centimeter spatial resolution
• Continuous EEG (cEEG): Extended monitoring used in intensive care units for detecting non-convulsive seizures in critically ill patients

How It Works

EEG signal generation begins at the cellular level. Postsynaptic potentials across thousands of cortical neurons sum to produce measurable dipoles. When neuronal activity synchronizes across a sufficient cortical area — estimated at roughly 6 to 10 cm² of cortex — the summed electrical field is detectable at the scalp surface.

Acquisition follows a structured sequence:

1. Scalp preparation: Skin impedance is reduced to below 5 kilohms (kΩ) at each electrode site using abrasive gel or paste, a requirement specified in ACNS technical guidelines to ensure signal fidelity.
2. Electrode application: Electrodes are applied according to the 10–20 system and secured with conductive paste or cap-based systems.
3. Montage configuration: Signals are recorded as differential amplification between electrode pairs (bipolar montage) or between each electrode and a reference (referential montage). Neurophysiologists typically review both.
4. Activation procedures: Hyperventilation for 3 minutes and intermittent photic stimulation at frequencies from 1 to 30 Hz are performed to provoke abnormal activity.
5. Signal digitization: Modern systems sample at 256–2000 Hz per channel. The ACNS recommends a minimum sampling rate of 256 Hz for clinical routine EEGs.
6. Review and interpretation: A board-certified neurologist or clinical neurophysiologist analyzes waveforms for frequency bands (delta: 0.5–4 Hz; theta: 4–8 Hz; alpha: 8–13 Hz; beta: >13 Hz), morphology, symmetry, and epileptiform features.

Safety classification by the FDA places standard scalp EEG among minimal-risk procedures; no ionizing radiation is involved, and no electrical current is delivered to the patient during standard recording.

Common Scenarios

EEG is ordered across a defined set of clinical indications recognized by professional bodies including the American Academy of Neurology (AAN) and the ACNS:

• First unprovoked seizure: AAN practice parameters support EEG as part of the initial evaluation to estimate recurrence risk and characterize seizure type (AAN Practice Parameter, Neurology, 2000).
• Established epilepsy: Ongoing monitoring for classification, medication titration, and surgical candidacy assessment.
• Encephalopathy evaluation: Distinguishing metabolic, toxic, infectious, or autoimmune encephalopathies based on characteristic EEG patterns (e.g., triphasic waves in hepatic encephalopathy).
• Altered consciousness in the ICU: Detection of non-convulsive status epilepticus (NCSE), which can account for altered mental status in 8–10% of comatose ICU patients, according to data reviewed in the Journal of Clinical Neurophysiology.
• Brain death determination: EEG demonstrating electrocerebral silence (ECS) is one of the ancillary tests used when clinical determination is equivocal, per American Academy of Neurology guidelines.
• Sleep disorders: Polysomnography incorporates EEG channels to stage sleep and identify abnormal events.

Decision Boundaries

EEG interpretation requires precise understanding of both sensitivity limits and artifact recognition. A single routine EEG yields a normal result in approximately 50% of patients with confirmed epilepsy, according to the ACNS; the yield increases to roughly 80–90% after three serial recordings with sleep deprivation.

Key contrast: interictal vs. ictal recording. Interictal EEG captures brain activity between seizure events and may show epileptiform discharges (spikes, sharp waves) without a seizure occurring. Ictal EEG records the seizure itself and provides definitive electrographic evidence of seizure onset zone — critical for surgical treatment for epilepsy planning.

EEG does not replace neuroimaging. Conditions producing focal structural lesions — such as tumors or cortical dysplasias — require MRI brain and spine for anatomical characterization. EEG and MRI findings are frequently complementary rather than interchangeable.

Interpretation is bounded by technical and physiological confounders:

• Electrode artifact (muscle, eye movement, electrode pop) can mimic epileptiform activity
• Normal variants (e.g., benign epileptiform transients of sleep, 14-and-6 Hz positive spikes) must be distinguished from pathological discharges
• Sedating medications, particularly benzodiazepines and barbiturates, suppress EEG amplitude and alter frequency content, requiring documentation in the clinical record

Clinical neurophysiologists credentialed through the American Board of Clinical Neurophysiology (ABCN) or the American Board of Psychiatry and Neurology (ABPN) with subspecialty training are the recognized interpreting physicians for clinical EEG in the United States. The ABCN administers the subspecialty certification examination in clinical neurophysiology, with candidates required to demonstrate competency in EEG, evoked potentials, and EMG and nerve conduction studies.

References

• American Clinical Neurophysiology Society (ACNS) — Guidelines
• International Federation of Clinical Neurophysiology (IFCN)
• FDA Electronic Code of Federal Regulations — 21 CFR Part 882 (Neurological Devices)
• American Academy of Neurology (AAN) Practice Parameters
• American Board of Clinical Neurophysiology (ABCN)
• American Board of Psychiatry and Neurology (ABPN)

The law belongs to the people. Georgia v. Public.Resource.Org, 590 U.S. (2020)