An electroencephalogram (EEG) is an essential tool that studies the brain's electrical activity. Despite the development of more advanced imaging techniques, EEG remains the essential paraclinical tool for seizure evaluation. It is primarily used to assess seizures and conditions that may mimic seizures. It is also useful to classify seizure types, assess comatose patients in the intensive care unit, and evaluates encephalopathies, among other indications. The electrical properties of the brain were first discovered by an English scientist, Richard Caton, in 1875, and about 50 years later, the first human EEG was recorded by the German psychiatrist, Hans Berger.
The EEG recording electrodes are placed over the scalp. They measure the absolute electrical potentials generated by the neurons of the underlying cerebral cortex. An estimated cortical area of 10 cm2 discharging synchronously is required to generate a deflection on scalp EEG. The pyramidal cell bodies are mostly present in layers III and V of the cerebral cortex. Following the release of neurotransmitters at the endplate, excitatory or inhibitory postsynaptic potentials (EPSP/IPSP) are generated secondary to neuronal depolarization (in the case of EPSP with intracellular sodium influx resulting in extracellular negativity) or hyperpolarization (in the case of IPSP intracellular negativity). The summation of EPSPs and IPSPs over a selected cortical region with synchronous discharge creates an electrical field with positive and negative ends (dipole). The dipole is typically parallel to the pyramidal cell orientation. This summation is what's measured by the EEG.
The cortical neurons and the subcortical structures are systematically connected through well-developed feedback linkages. During the resting or relaxed state, the EEG records a sinusoidal rhythmic activity called the posterior dominant rhythm (PDR) that is believed to be due to oscillatory interaction between the cortex (visual cortex in this instance) and subcortical structures (thalamus). During activation, the cortical activity desynchronizes, and the oscillatory activity is replaced with a lower amplitude faster frequency activity. In the event of a seizure, a large super-synchronous neuronal discharge is created from an abnormal brain network. EEG evaluation provides important information about the localization and the spread of such discharges.
The commonly encountered waveform frequencies in EEGs are alpha (8 to 12 Hz), beta (13 to 30 Hz), theta (4 to 7 Hz), and delta (<4 Hz). The predominance of waveforms in an EEG varies based on the age and state of wakefulness of the individual. The EEG waveforms start with discontinuous backgrounds during the prenatal phase and mature to be continuous at a later age. The normal adult resting PDR of 8.5 Hz in the posterior head regions is noted after eight years of age. The slower waveforms are less during the wakeful state and dominate during later stages of sleep. There is also an anterior-to-posterior distribution of waveforms with faster frequencies present in the anterior and slower frequencies in the posterior head regions.
Other notable components/waveforms that appear during the first year of life and are useful to differentiate sleep stages are sleep spindles and K-complexes. There are also several benign EEG variants and artifacts that one should learn in order to avoid reporting a false-positive test.
There are no clear contraindications to performing an EEG. However, electrode placement could be challenging following a craniotomy, and in case of breaches in the skull or open wounds. The EEG should be performed after a detailed history and if there are concerns for seizures or epilepsy. Activation procedures should be omitted in individuals with certain underlying conditions. For example, hyperventilation is a relative contraindication in patients with a history of strokes, myocardial infarction, surgeries (transplants), acute respiratory distress syndrome, asthma, Moyamoya disease, and sickle cell anemia.
The basic equipment includes electrodes (Silver/Silver-chloride electrodes are the most widely used), an amplifier, and an EEG system (monitor and processor). Previously, mechanical pen and paper recording devices were used for plotting the EEG recordings (analog recording). In current clinical practice, a standard EEG system has the capacity to obtain information from at least 128 channels, with greater than 10 kHz sampling rate from all the channels along with a 24-bit resolution at each of the amplifiers.
Other important components of the electrode placement are the gel and salts that are applied to improve the scalp conductivity and thereby to record waveforms. These days ‘dry electrodes’ are also used that have improved and hastened the scalp preparation.
The EEG is performed by the EEG technician/technologist, who is a trained professional with appropriate under-grad education and training. They undergo rigorous training and certification process. Once a study is completed, the recordings are reviewed, and a report is generated by the clinical neurophysiologist, who is typically a board-certified/eligible neurologist who undergoes additional subspecialty training in EEG/epilepsy.
The EEG performance in the ICU and the epilepsy monitoring units require the participation of additional trained staff (nurses, support staff, monitor techs. and others) in order to provide proper and safe evaluation and care of all patients.
When the EEG is performed in an outpatient setting, clear instructions are provided to the patients prior to their EEG appointment. They are recommended not to use conditioner or other substances that might affect the quality of the recording (electrode impedance). The scalp is usually cleaned well to obtain proper recording with low impedance. Typically, the impedance should be lower than 5KOhm. For ICU patients, several measures are typically taken to reduce the disturbances from various medical instruments, devices, and lines used. Mechanical restraints and not chemical restraints might be necessary at times and ensure a proper EEG recording.
A routine EEG is performed in a quiet room with controllable lighting levels. The test should be performed by an EEG technician with appropriate and relevant training. The 10 to 20 international system is most widely used for scalp electrodes placement. Typically at least 21 electrodes are placed on an adult scalp, including a reference and ground electrodes. Once the electrodes are placed, the impedance of all electrodes should be measured and ensured that it is less than 5 KOhms. Calibration should be performed prior to beginning the study. These include recording a square wave signal and biological calibration. During the recording, various activation procedures are performed in order to trigger epileptiform abnormalities and other EEG changes. These including eye-opening and closure, hyperventilation, and photic stimulation. Drowsy and sleep recordings are important components of any EEG procedure. Sleep deprivation is also used as a provocative technique.
The EEG channels are displayed following different montages, and each channel records the electrical potential difference between the two components (electrodes) of each channel. EEGs should be reviewed using different types of montages (mainly bipolar and referential montages) in order to accurately isolate and localize abnormal discharges. The digitalization of EEG has significantly improved the ease of reformatting and re-montaging per the electroencephalographer's requirements for interpretation purposes.
Referential montages: The channels display the potential difference between the recording/active electrode and a preselected reference (another electrode over a body area or the average of a certain number of selected electrodes).
Bipolar montages: The channels, arranged in a longitudinal or transverse manner, display the potential difference between two contiguous electrodes. These montages easily detect asymmetry between the two brain hemispheres.
Laplacian montages: In this montage, the reference is a combined weighted average of the potentials surrounding a particular electrode or region of interest. It is typically useful to study or assess focal discharges that have a minimal field.
Unexpected complications can occur if due diligence is not performed while screening patients before performing the activation or provocative procedures like hyperventilation in certain individuals as mentioned previously. Long-term EEG monitoring in epilepsy monitoring units and the intensive care units are associated with skin injury and appropriate care needs to be provided.
EEG is an important tool to investigate central nervous system pathologies associated with seizures and altered mental status in routine practice. It is a complementary test to the more advanced imaging studies. EEG is widely used in the evaluation of epilepsy patients, altered mental status or altered consciousness, parasomnias, encephalopathies secondary to various metabolic and toxic derangements, dementias, and strokes presenting as seizures. EEG is also useful to assess for prognostication in patients with anoxic brain injury, traumatic brain injuries, determining brain death, and drug toxicities. EEG is fundamentally a universal tool to assess any interictal brain wave activity and to better understand the underlying progress in an unresponsive or comatose individual. It is also useful to assess patients with behavioral or psychogenic spells that appear to be similar to seizures.
The activation procedures help or facilitate capturing abnormal discharges that are useful to classify the areas of the brain involved in focal epilepsies or determine if the individual has a genetic or primary type of epilepsy. Long-term EEGs with video are useful to capture seizures and characterize their semiology. From a diagnostic and treatment standpoint, this information would be useful for presurgical work with curative intent if the patient's seizures tend to be medically intractable. The more invasive form of EEGs using the grid and depth electrodes is applied to assess the brain's electrical activity from the surface of the cortex and subcortical white matter, respectively.
A multidisciplinary team is essential for the management of patients who require a diagnostic EEG. A team of well-trained EEG technicians, nurses, clinical neurophysiologists, and neurologists are required to appropriately screen the patients, ensure a proper test is performed in a safe manner, and interpreted correctly based on guidelines, thus facilitating the best treatment decision to be made by the treating physicians. The EEG interpreting physicians should be board certified and follow the guidelines provided by the appropriate clinical neurophysiology society for EEG reporting.
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