Clinical aspects

Description

EEG studies are conducted either as routine (short duration) or continuous inpatient and outpatient studies. Although routine EEG can help establish the diagnosis of epilepsy, continuous EEG with video (video-EEG) has an important role in the assessment of patients who present diagnostic or management difficulties following clinical evaluation and a routine EEG (32). The initial step in performing EEG involves measuring and marking the patient’s head; this is a vital first step that needs to be taken by the technologist. After adequate scalp preparation, electrodes should be placed according to the international 10-20 system. The ACNS recommends using a minimum of 21 electrodes for recording EEG for clinical purposes (22). Each electrode site is designated by a letter and a numerical value. Letters represent which lobe of the brain the electrode position overlies (eg, F = frontal, T = temporal, P = parietal, and O = occipital). Odd-numbered electrode sites appear on the left side of head; even-numbered electrodes are on the right side; “z”-labeled sites are in the sagittal midline from anterior to posterior; and “A” or auricular sites are on the mastoid processes or ears. New electrode technologies, including dry electrode cap models that don’t require gel, are being tested to reduce the delay for obtaining an EEG in acute neurologic settings and eliminate the need for trained EEG technologists to place the electrodes (16). In addition to the scalp electrodes, the EEG is capable of monitoring other parameters such as heart rate, respiration, and muscle activity (polygraphic recording) to enhance the recording when needed (20).

The electrode pins are inserted into a jack box. A shielded, multiple connector input cable connects the electrode wiring in the jack box with the assigned input channels of the EEG machine. This pairing of electrode pins from the jack box to the assigned input channels allows the selection of a bipolar or referential montage.

The recording begins with a mechanical calibration, which verifies the integrity of the amplifiers. Calibration is an integral part of every EEG recording. It gives a scaling factor for the interpreter and tests the EEG machine for sensitivity, high-frequency and low-frequency response, noise level, and pen alignment and damping (for analog systems). At the outset, all channels should be adjusted, if necessary, so that they respond equally and correctly to the calibration signal. Standard calibration includes at least 10 seconds (or the duration needed to reach a stable recording) of a square wave calibration. The calibration is performed with the standard settings and a known input of 50 µV. When doubt as to correct functioning of any amplifier exists, a repeat calibration run should be made. Calibration voltages must be appropriate for the sensitivities used during the recording. A biological calibration reflects the recording of the most anterior electrode, which is usually Fp1, to the most posterior electrode, which is O2 is no longer required in digital recordings but can be useful in troubleshooting mechanical calibrations. After calibration, visual review of a 30-second run on the system reference montage without the notch filter is also recommended (22).

The subject’s behavior and occurrence of artifacts should be the object of constant and vigilant attention during the entire recording, and the team should react appropriately when a problem arises (eg, artifact correction, clinical examination in case of seizures). Thus, the presence of an experienced technologist with experience in neurophysiology is highly recommended during the entire EEG recording. The minimal duration of a standard EEG is 20 minutes. It should include at least one 3-minute hyperventilation stimulation and 1 intermittent photic stimulation, when clinically appropriate. The technologist must update (annotate) these stimulations during the EEG test.

Use of standard equipment provided by most vendors allow for 64-256 input amplifiers though these may not suffice for higher density recording. Electrodes placed in accordance with the 10-20 International system of electrode placement may serve to limit identification of regions that may be enhanced by the use of a 10-10 system. The benefits of many of the proprietary systems are that most equipment will interface within the EEG laboratory, most include clinical applications involving software (eg, spike and seizure detection) and hardware (eg, push-button activation, video etc.), and most are capable of remote access. Furthermore, most EEG technologists will be familiar with the digital EEG instrumentation and equipment beyond routine information technology or network support services and do not require extensive training on performance and upkeep of standard equipment.

Indications

An EEG is a crucial test in the differential diagnosis between epilepsy and other disorders associated with intermittent paroxysmal disturbances that resemble epileptic seizures (23; 03; 32). EEG can help support a clinical diagnosis of epilepsy when it demonstrates interictal epileptiform discharges. The presence of interictal epileptiform discharges in the EEG can provide an objective means of supporting the clinical diagnosis of epilepsy when the history is unclear in delineating a seizure (08). In addition, an EEG in patients with known seizures can serve to classify whether the seizures are focal or generalized and part of a specific epilepsy syndrome (09). Although the presence of interictal epileptiform discharges is highly specific for people with epilepsy, the sensitivity of EEG is only moderate, with 10% to 15% of individuals having repeatedly normal EEGs despite having been diagnosed with epilepsy (28). Nevertheless, the repeated absence of interictal epileptiform discharges may lead to questions of alternative diagnoses that may be unmasked when ictal recordings obtained through video-EEG monitoring are undertaken. In this manner, video-EEG monitoring may help differentiate those patients with nonepileptic behavioral events from epileptic seizures to obtain a definitive diagnosis. Similarly, video-EEG monitoring may help quantify seizures and spikes and identify patients for epilepsy surgery by characterizing the site of seizure onset as part of a comprehensive presurgical evaluation.

EEG may also help sort out different neurologic conditions that may present as altered mental status (26). For example, an EEG may suggest diffuse brain dysfunction or encephalopathy (eg, metabolic or toxic etiologies) or may suggest focal or multifocal dysfunction (eg, multiple strokes). There are also some specific conditions that have characteristic EEG patterns, including herpes encephalitis, subacute sclerosing panencephalitis, or Creutzfeldt-Jakob disease. In addition, nonconvulsive status epilepticus, a prolonged seizure without overt clinical manifestations, may present only as altered mental status and may only be diagnosed with EEG. Continuous EEG may help demonstrate subclinical seizures in nearly 20% of critically ill patients admitted to the intensive care unit and is crucial in the management of patients with status epilepticus. EEG is also useful in predicting the prognosis of comatose patients (26). The presence of a response in the EEG to activation procedures, such as somatosensory stimulation, is favorable compared with those that are unreactive in the appropriate clinical context. In addition, confirmation of neocortical death can be documented by at least 30 minutes of electrocerebral inactivity. Any brain-wave activity above 2 microvolts per millimeter precludes a diagnosis of brain death, and recommendations for a repeat study merits consideration. In the operating theater, EEG monitoring has been useful to portend impending ischemic complications during endarterectomy. Ipsilateral loss of fast frequencies or hemispheric slowing on intraoperative EEG during carotid clamping may be averted by temporary shunt placement to maintain adequate cerebral blood circulation (33).

Contraindications

There are essentially no contraindications or risks associated with the proper use of EEG because it is a noninvasive procedure. However, an EEG should not be performed if the patient requires high concentration oxygen as the electrical currents from the EEG machine can ignite the oxygen, resulting in an explosion. Patients with indwelling catheters are also at risk of electrical shock because they can provide a direct path to the heart for the passage of current.

Results

Reviewing EEG for interpretation is typically conducted at the hospital or the clinic where the EEG laboratory is located. However, advances in computer science and internet technology with faster broadband connections and networking have enabled remote access to EEG. In addition, advances in digital EEG have allowed the electroencephalographer to reformat the record post hoc for greater flexibility during interpretation.

Following assessment of the technical parameters of recording, the initial goal in the visual analysis of the EEG is determination of the background. In the normal awake adult with eyes closed, a 8.5 to 12 Hz alpha rhythm appears maximal in the posterior part of the head, referred to as the posterior-dominant rhythm. The interpretation should take into account the conditions of examination (eg, number of electrodes and patient’s alertness), clinical data, patient treatment at the time of the recording, and reason for the treatment (suspected pathology and types of electrical abnormalities). Abnormal EEG findings include those patterns that are nonepileptiform, interictal epileptiform discharges, and electrographic seizures. Nonepileptiform EEG abnormalities may be characterized by several distinct patterns, including focal slow activity, regional or generalized bisynchronous slow activity, generalized asynchronous slow activity, and focal or generalized suppression of the background activity. These patterns provide valuable information in the investigation of patients with epilepsy and patients with signs of acute brain insults, toxic-metabolic encephalopathy, neurodegenerative conditions, coma, and brain death (08). Interictal epileptiform discharges are characterized by the presence of spikes and sharp waves, with or without after-going slow waves and are strongly associated with epilepsy. However, in rare cases these discharges may also be seen in healthy subjects without seizures (24). Identifying focal or generalized interictal epileptiform discharges on EEG can support the clinical diagnosis of an epilepsy syndrome and can help guide the management using antiseizure medication. Specific types of focal interictal epileptiform discharges are associated with specific types of epilepsy syndromes that are localization-related (eg, temporal lobe epilepsy) and may provide information about treatment options (eg, surgery) when co-localized to other areas of abnormal neuroimaging. Continuous video-EEG recordings including video-EEG monitoring in an epilepsy monitoring unit have a substantially greater yield than standard EEG recordings for detection of seizures and epileptiform activity (07). The absence of interictal epileptiform discharges may also provide useful information when epilepsy is in doubt though a normal EEG does not exclude the possibility of epilepsy. Non-epileptiform EEG abnormalities may be characterized by several distinct patterns, including focal slow activity, regional or generalized bisynchronous slow activity, generalized asynchronous slow activity, and focal or generalized suppression of the background activity. These patterns provide valuable information in the investigation of patients with signs of acute brain insults, toxic-metabolic encephalopathy, neurodegenerative conditions, coma, and brain death, as well as in patients with epilepsy while recording electrographic seizures provides a definitive diagnosis.

Adverse effects

EEG is a safe and relatively easy procedure without a major risk; however, hospital patients are at higher risk of electrocution due to all of the many devices that are being used simultaneously. Low-resistance routes to the body are created with electrodes and intravenous lines that are connected to the patient as well as potential spillage of the intravenous fluids in proximity to the electrical outlets (06). Added precautions such as proper grounding of the electrical main should be adhered to in the intensive care setting. For safety reasons, EEG amplifiers are required to be isolated from earth ground; however, computer data acquisition system cases are typically connected directly to earth ground. Therefore, the EEG amplifier system, including instrumentation amplifiers and power supplies, is grounded to an "isolated common" voltage plane and chassis, which is at a floating potential with respect to earth. The maximum allowable current leakage on an electrode with ground connected is 10 µA. This requires a current limiter on every lead or electrical isolation. A ground-fault interrupter should be in use in the power line to the operating room as well as other locations of the hospital (06). Even with the substantial improvements that have occurred in electrical safety, multiple patient grounds should be avoided to reduce the risk of electrical shock.

Neuroimaging safety is another issue where heating or burning may occur as radiofrequency or gradient fields during imaging produce current flow in electrode wires through induction. The use of Teflon or conductive plastic electrodes (using silver epoxy) is optimal though pure metals including silver (99.9%), gold, and copper also limit the potential for magnetization. Short lead wires may overcome coiling, which can induce electrical currents or become tangled on the MRI or CT equipment. Most connectors, however, are not MRI compatible.

Special considerations

There are similarities between recording techniques, strategies of recordings, and interpretations of EEG features performed for both pediatric patients and adults. However, there are some differences in the different types of recording techniques and monitoring patients in different age groups (30; 31). For example, neonatal EEGs are typically performed utilizing a reduced number of electrodes per montage with lower gain compared to adults. In addition, noncephalic DC channels (polygraphy) for respiratory monitoring and pulse oximetry are often used in pediatric patients. Neonates also present with thinner skin, and special care needs to be taken when applying the electrodes (11). Neonates are subject to a greater amount of artifacts in the EEG because they are usually in an incubator that controls oxygen and temperature regulation, which can negatively impact the technical quality. Similarly, EEGs obtained in special care units are challenging to record without artifacts due to interference by multiple electrical motorized devices (29).

In situations where EEG acquisition is difficult (eg, patients in the intensive care unit, agitated and delirious patients), electrodes with rapid application including nickel titanium, stainless steel, or silver wire electrodes may be considered (14). EEG caps or templates are becoming popular for rapid use in special care units to reduce the time and amount of technical support required (13).

Clinical vignette

An 18-year-old female with migraine was a cheerleader for her high school football team. She had just come from a practice when she experienced an initial event that was felt to represent a seizure and she was treated for epilepsy with antiseizure medication. While going to the water fountain prior to returning home from school, she was witnessed by her peers to “pass out” and was reported to have “seizure activity” with a loss of bladder control. She recovered and was sent to the local emergency department for evaluation. There, she was seen and diagnosed with syncope and released and told to follow-up with her primary care physician. She was evaluated with both a general and cardiovascular examination that were normal. She was referred to a neurologist. A brain MRI was normal. An EEG performed in the office was reported to be abnormal due to generalized spike-and-slow waves. She was diagnosed with epilepsy and placed on valproate.

She presented with concern regarding chronic administration of antiseizure medication with complaints of weight gain and hair loss. Review of her history disclosed recurrent dizzy spells since she was 16 years of age. She reported that the event occurred after skipping lunch the day of her “black-out.” She recalled becoming dizzy and nauseated, experienced visual blurring, and then collapsed with loss of consciousness. Her best friend reported a few jerks when she was done on the ground. Recovery was immediate without chest pain, palpitations, or racing of the heart rhythm. She had no risk factors for epilepsy. The general and neurologic examinations were normal. The EEG was repeated and compared with the “abnormal” one, which had similar features.

Case conclusions. Review of the “abnormal” EEG in addition to interpretation of the follow-up interictal EEG was performed.

The findings demonstrate a 1-second burst of 4.5 Hz pseudo-generalized spike-and-waves (red arrows) due to superimposition of artifacts. Using an anterior-posterior longitudinal bipolar montage, in the frontal head regions eye flutter (validated by the eye movement monitors with an “out-of-phase” polarity) coinciding with myogenic spikes derived from the frontalis musculature was detected to create the false artifactual appearance of spike-and-waves. Photic stimulation was the trigger that produced the myogenic spikes that were time-locked to the frequency of the photic flash.

Case discussion. This case illustrates several important points to underscore the importance of appreciating the technical aspects of EEG recording. First, it illustrates the importance of the EEG in the misdiagnosis of epilepsy. Next, it reveals the pitfalls that may lead to misinterpretation from artifact (and in this case a combination of artifacts) as a common trap that may beguile even the most experienced electroencephalographer. Lastly it underscores the importance of the technical application of eye movement monitors to validate the “slow-waves” in revealing the eye as the generator. By placing an electrode above and below an extracerebral generator, such as the eye, this allows for dipolar localization (the cornea is relatively electropositive relative to the retina). An “out-of-phase” deflection identifies the eye as the source of generating the waveforms. This is opposed to “in-phase” electrodes that commonly signal the brain as the generator. In addition, the frequency of a “spike” that is shorter than 15 msec is unlikely to be generated by the brain and instead more likely to be derived from a muscular source. When sustained, this may be recognized as a normal feature of the EEG known as the photomyogenic response.

This patient was “undiagnosed” with epilepsy and instead diagnosed with neurocardiogenic syncope after cardiology evaluation. A series of jerks in healthy individuals is a normal physiological response and when notable is referred to as convulsive syncope. She was tapered off valproate and given instructions for syncope prevention. Furthermore, the use of valproate as an antiseizure medication has been associated with a high risk of teratogenesis when exposure has taken place during the first trimester of pregnancy and can lead to birth defects in an unborn baby. This is because a significant number of pregnancies are unplanned, obtaining the correct diagnosis involving seizure treatment (or migraine) needs to encompass gender-specific consequences that could occur from treatment. Fortunately, syncope did not recur with conservative treatment, and she has been event-free off antiseizure medication.

Scientific basis

Source generators. The biological signals represented by EEG are represented by current movement of electrical charge (measured in coulombs) that flow within the nervous system. Charges are either positive or negative depending on the direction electrons are flowing. Separation of charge (polarization) is associated with the inherent ability of biological tissue. Current flows through electrical conductors (measured in amperes) such as the brain and also produces magnetic fields. Current (I) flow is equal to the change in charge (Q) over change in time (t) within a circuit where: I = dQ/dt.

The amount of potential energy (measured in joules) between 2 points determines the voltage. Electronic circuits in devices such as an EEG machine are designed to accommodate rapidly switching current flow in a bidirectional fashion (alternating current) as opposed to flow in only one direction (direct current). As current flows through the brain tissues it experiences what is known as impedance (Z) that inhibits the flow of electrons. Flow of electrical current is proportional to voltage (V) and inversely related to resistance (R): I = V/R.

Resistance does not account for capacitor and inductor impedances, whereas impedance is the sum of the circuit's resistance and reactance. Reactance represents the impedance from the electrical and electromagnetic fields generated from capacitors and inductors.

Capacitors store energy in electrical fields, and inductors store energy in electromagnetic fields. The basic analogue component of the EEG recording system is a circuit predicated on resistors and capacitors (RC circuit). The flow of electrical current in the circuit varies over time when a change in voltage occurs.

Traditional EEG displays electrophysiological fields on the surface of the scalp that vary in amplitude, frequency, and morphology and are related to both local cortical and distant subcortical generators (18). Scalp-recorded EEG oscillations are thought to be primarily generated by the summation of excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) by cortical pyramidal neurons in the cerebral cortex (layer 3, 5, and 6) that are oriented perpendicularly to the brain’s surface. The diameter of an EEG electrode (10 mm) is several orders of magnitude larger than the size of single neurons (20 µm). Therefore, it requires pooled synchronous electrical activity derived from a source area of approximately 10⁸ neurons in a cortical area of a minimum of 10 cm² for most EEG signals to be detected at the scalp (19; 27). The brain waves are captured in microvolts, and the amplifier is designed to magnify the signal in order to see the signal by multiplying an input voltage by a constant range.

Electrodes and conductive media. Electrodes provide a critical first step in recording from biological tissues such as the brain to interface with metallic recording electrodes as part of an EEG system. Most electrodes are composed of silver-silver chloride sensors (nonpolarizing electrodes) and are good for recording high frequencies as well as DC potentials. These are produced by a thin layer of silver coating surrounding plastic substrates with an outer layer of silver converted to silver chloride. The principle of silver-silver chloride use lies in the conversion of oxidized silver cationic currents at the surface of the scalp to electron current that traverses the lead wire to reach the instrument for processing. Gel or paste, therefore, contain anions (free chloride) and serve as a conductive media. With chloride (anion) ions move to the electrode to bond with the silver; free electrons are then released in the process and are delivered to the lead wire to transmit the electrophysiological charge for amplification. An undesirable half-cell potential is created when there is an ionic imbalance of charge between the electrolyte gel and the metal electrode. Silver-silver chloride electrodes have a low half-cell potential and lower resistance to charge flow. The Nernst equation is an important concept in evaluating the relative degree of reduction potential from various electrochemical properties of metals. New neuroimaging-compatible electrodes are available and composed of disposable plastic material (14). High impedance and unbalanced inter-electrode impedance have been proven to compromise the integrity of the EEG and allow noncephalic slower frequencies to be recorded (12). This is particularly challenging in the intensive care unit setting due to the abundance of other equipment that emits electrical noise (02). Current flowing through a conductor between 2 points is directly proportional to the voltage. Impedance to current flow within brain tissue causes a reduction. Paired electrodes through modern equipment can usually tolerate higher impedances identified at the scalp-electrode interface. Most electrodes should have an output impedance of 5 kOhms or less. In order to have an measurable and acceptable voltage, input impedances should have a minimum of 100 mOhms given Ohms Law equation (voltage = current x resistance).

A/D converter and grounding. Analog to digital conversion (jack box) converts a continuous analog signal in the EEG into a digital numerical series.

Time and amplitude in “steps” (expressed as bits) allows EEG to be quantified. Step size depends on the bit-value (expressed in a binary system of 1 or 0) of the computer and maximal and minimal value of the steps that is possible by the amplifier to be transformed. The smaller the step size used (usually 0.5 to 1 microvolts in most systems), the more accurate the digitization that is seen. New recommendations require a minimum of 16-bit conversion with a minimum amplitude resolution of 0.5 microvolts and dynamic range of +/- 1.638 millivolts with a minimum sampling rate of 256 samples per second and an anti-aliasing filter cutoff at two thirds the Nyquist frequency (10). Discrete time intervals sample a continuous waveform to express the samples as an integer. This 2-dimensional representation of time versus voltage has a sampling rate that determines how accurately the signal will be reproduced. The sampling rate is the rate at which the analog to digital conversion samples the signal to represent it digitally as a numerical value. Under sampling will underrepresent the waveform and make it appear as an “alias” waveform. This is unable to be corrected after the digitization though some machines may have an anti-aliasing filter with a steep roll-off.

Grounding is a method used to reduce noise from the electrical main. The use of a ground is essential to eliminate the risk of unsafe potential passage of current through the patient leading to electrical injury. Patients are not connected to a “true” ground but instead use a ground electrode that electrically (or optically) isolates ground from the power supply. For patient safety, an electrode board having an isolated ground or isoground can be employed instead of the commonly used unit. The isoground limits current flow to ground via the EEG ground electrode to safe levels regardless of any faults that might develop in other connections to the patient. During EEG recording, both the active and reference leads undergo analysis during referential or bipolar recording relative to the common ground electrode.

Amplifiers and filters. One essential function of the EEG machine is to amplify biological signals in voltage that are generated by the brain (reflected in the gain of an amplifier). The EEG amplifiers are not designed to distort the EEG but, rather, to increase the voltage of the signal and control the low frequency filter, high frequency filter, the notch filter (60 Hz), and the baseline adjustment for each channel (06). Amplifiers receive input voltage in a dynamic range. They are flexible due to the sensitivity of the units measured in either microvolts per millimeter or millivolts per centimeter. For example, when an EEG is performed using a sensitivity of 7 µV/mm, the defection of the waveforms ranges from 3 to 20 mm for input voltages of 20 to 140 µV (06). In addition, the EEG amplifiers are differential amplifiers and measure the potential difference between paired electrodes. The amplifiers also have the capability to suppress common voltages recorded by 2 electrodes (input 1 and input 2). Electrodes that record a 60 Hz artifact do so because of the capacitance generated between the metallic sensors on the body and electrical sources located in close proximity to each other. Because the power main contains a common voltage, the amplifier function reduces “noise” by common-mode rejection of the common in-phase signals. The common-mode rejection ratio (usually 10,000:1) reflects the relative suppression of the common signals relative to that which is generated by the brain. The Common Mode Rejection Ratio (CMRR) is the ratio of the powers for the differential gain divided by the common-mode gain. This is measured in decibels. Modern EEG machines CMRR ranges from 80 dB to 100 dB though unable to suppress all common signals. The CMRMR is measured by connecting a known voltage source to 2 connectors.

The power supply transforms alternating current acquired from the main power to a lower voltage direct current, usually in the range of 1 to 24 volts (34). The power cord must maintain an intact ground connection to the chassis and a low capacitance in case the connection fails. Today, EEG machines have multiple channels, isolated grounds, high-capacity storage, and software applications designed to enhance source localization. Modern amplifiers include up to 512 referential channels; some have DC low frequency filters and up to 30,000 Hz sampling rates (bandpass 3000 to 10,000 Hz). Newer digital filters and finite impulse response filters may not produce the phase distortion seen with prior filters. Matrices and software to integrate internal switching is possible now to combine functions. Visualization of the signal can be manipulated by gain as the amplification factor of the EEG that reflects output voltage in decibels (dB). Wireless communication now exists from the jackbox to the amplifier to “externalize” the system more completely. Additional outputs (with USB ports available) are now able to record oximetry, carbon dioxide monitors, and DC channels among others to integrate more physiological functions with EEG for comprehensive assessment.

Filters are devices that remove some unwanted components of an electrical signal. These are usually specific frequencies. Filters act as a circuit combining a capacitor and resistor (like a voltage divider). Highpass filters facilitate high frequency recording by combining the effects of a capacitor and resistor and block lower frequencies. A lowpass filter is designed with the resistor and capacitor in reverse positions in the circuit. Low pass filters facilitate both low and high frequency components of the signal to pass through. Both highpass and lowpass filters have a targeted cutoff frequency. This is defined by the point at which the output in volts of the filter is −3 dB (or 0.707) relative to the voltage of the input signal.

Recording conventions. Electrode impedances should be less than 5 microvolts and balanced with no more than 10 kOhm difference between each electrode pair (22). Polarity convention with differential amplifiers is represented as follows: (a) if input 1 is negative with respect to input 2, there will be an upward deflection; (b) if input 1 is positive with respect to input 2, there will be a downward deflection; (c) if input 2 is negative with respect to input 1, there will be a downward deflection; and (d) if input 2 is positive, there is an upward deflection. Scalp EEG typically captures brain wave signals in frequencies ranging from 1 Hz to 70 Hz. If the frequency range of the recording is too narrow, certain wave forms will not be captured, and if the range is too high, too much noise will be recorded. Thus, EEG machines have the capability to filter high frequencies as well as low frequencies. Using the recommended settings of 1 Hz for low frequency filters and 70 Hz for high frequency filters will result in low probability of a distorted recording. The notch filter or 60 Hz filter should be used only when 60 Hz is observable and all nonessential equipment has been unplugged in attempts to alleviate line noise. Incorrect filter settings will distort both amplitude as well as interchannel phase of signals so should be changed sparingly.

Digital EEG display. Monitor display of the EEG signal quality depends on adequate resolution. The number of pixels that reflect the points used to represent the display and pitch between them (diagonal distance) have software as a foundation for controlling any video that is combined. Screen sizes of at least 20 inches should be used though often dual monitoring is implemented when video-EEG monitoring is instituted. Screen resolution (eg, 1280 x 1024) may prevent aliasing created by too few pixels and under sampling (minimum required is 100 pixels/second).

Conclusions. Technical aspects of recording optimal EEG signals may be challenging. Knowledge of the fundamental aspects of instrumentation involved in performing EEG is essential to optimal recording and to facilitate accurate interpretation for clinical use. EEG technology is ever-changing and advancing; however, the basic technical aspects of recording EEG lie at the foundation of clinical neurophysiology.