Tools
Electroencephalogram
Introduction
An electroencephalogram (EEG) is an essential tool that studies the brain's electrical activity. Despite developing 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 evaluate 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 German psychiatrist Hans Berger recorded the first human EEG.[1][2]
Anatomy and Physiology
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Anatomy and Physiology
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.[3] The pyramidal cell bodies are mostly present in layers 3 and 5 of the cerebral cortex.[4] 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. The EEG measures this summation.[5][6]
The cortical neurons and the subcortical structures are systematically connected through well-developed feedback linkages.[7] During the resting or relaxed state, the EEG records a sinusoidal rhythmic activity called the posterior dominant rhythm that is believed to be due to oscillatory interaction between the cortex (visual cortex in this instance) and subcortical structures (thalamus).[8][9] During activation, the cortical activity desynchronizes, and the oscillatory activity is replaced with lower amplitude and 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 (less than 4 Hz). The predominance of waveforms in an EEG varies based on the individual's age and state of wakefulness. The EEG waveforms start with discontinuous backgrounds during the prenatal phase and mature to be continuous at a later age. The normal adult resting posterior dominant rhythm of 8.5 Hz in the posterior head regions is noted after 8 years. 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 in the anterior and slower frequencies in the posterior head regions. Sleep spindles and K-complexes are other notable components/waveforms that appear during the first year of life and are useful to differentiate sleep stages. One should learn several benign EEG variants and artifacts to avoid reporting a false-positive test.[10]
Indications
There are several indications for an EEG.[11][12][13] A brief list of various indications includes:
- To classify the type of seizure and localize the onset of seizures[14][10]
- Sodium amobarbital or Wada test to determine the hemisphere dominance for language and memory[15]
- Management of status epilepticus and inducing therapeutic coma[16][17][16]
- Patients with altered mental status from various etiologies like toxic metabolic encephalopathies[18][19]
- Encephalopathic patients with unexplained etiologies to assess the degree of encephalopathy[20]
- Syncope or symptoms of loss of consciousness with a negative cardiac workup[21]
- Comatose patients in the intensive care unit with impaired or persistent confusion or decreased responsiveness[22][23]
- Prognostication after cardiac arrest[24]
- Identify delayed ischemic changes after subarachnoid and intracranial hemorrhage[25][26]
- Anesthetic procedures to monitor the depth of anesthesia[27]
- Brain death determination[28][29]
Contraindications
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.[30][31]
Equipment
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 to plot the EEG recordings (analog recording). In current clinical practice, a standard EEG system can obtain information from at least 128 channels, with a greater than 10 kHz sampling rate from all the channels, along with a 24-bit resolution at each amplifier.[5] Other important components of the electrode placement are the gel and salts that are applied to improve the scalp conductivity and thereby record waveforms. Nowadays, dry electrodes are also used to improve and hasten scalp preparation.[32]
Personnel
The EEG is performed by the EEG technician/technologist, who is a trained professional with appropriate undergraduate 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.[33] The EEG performance in the ICU and the epilepsy monitoring units requires the participation of additional trained staff (nurses, support staff, monitor technicians, and others) to provide proper and safe evaluation and care of all patients.[34]
Preparation
When the EEG is performed in an outpatient setting, clear instructions are provided to the patients before their EEG appointment. They are recommended not to use conditioners or other substances that might affect the recording quality (electrode impedance). The scalp is usually cleaned well to obtain proper recording with low impedance.[35] Typically, the impedance should be lower than 5 kohm. Several measures are typically taken for ICU patients to reduce the disturbances from various medical instruments, devices, and lines used. Mechanical restraints and not chemical restraints might sometimes be necessary to ensure a proper EEG recording.
Technique or Treatment
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 widely used for scalp electrode placement.[36] 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 before beginning the study. These include recording a square wave signal and biological calibration. Various activation procedures are performed during the recording to trigger epileptiform abnormalities and other EEG changes. These include eye-opening and closure, hyperventilation, and photic stimulation. The recordings of drowsiness and sleep are important components of any EEG procedure. Sleep deprivation is also used as a provocative technique.[37]
The EEG channels are displayed following different montages, and each channel records the electrical potential difference between the 2 components (electrodes) of each channel. EEGs should be reviewed using different types of montages (mainly bipolar and referential montages) to isolate and localize abnormal discharges accurately.[38] The digitalization of EEG has significantly improved the ease of reformatting and re-montaging per the electroencephalographer's requirements for interpretation purposes.[39] Commonly used montages are:
- Referential montages (eg ear reference, average reference)
- Bipolar montages (longitudinal and transverse)
- Laplacian montages[40][41]
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 longitudinal or transverse channels display the potential difference between 2 contiguous electrodes. These montages easily detect asymmetry between the 2 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 with minimal field.[42]
Complications
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 intensive care units is associated with skin injury, and appropriate care needs to be provided.[43]
Clinical Significance
EEG is an important tool for investigating 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 for assessing interictal brain wave activity and understanding underlying progress in an unresponsive or comatose individual. It better is also useful to assess patients with behavioral or psychogenic spells that appear to be similar to seizures.[11][12][13]
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.[44][45]
Enhancing Healthcare Team Outcomes
An interprofessional team is essential for managing patients who require a diagnostic EEG. A team of well-trained EEG technicians, nurses, clinical neurophysiologists, and neurologists is required to appropriately screen the patients and ensure a proper test is performed safely and interpreted correctly based on guidelines, thus facilitating the best treatment decision to be made by the treating providers.[46] The EEG interpreting physicians should be board-certified and follow the guidelines provided by the appropriate clinical neurophysiology society for EEG reporting.[47][48]
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