LEARNING OBJECTIVES

Introduction

Electroencephalography allows for the continuous, real-time monitoring of cerebral function in critically ill children. However, the recording of electrical activity from the brain is subject to interference from noncerebral electrical potentials, which can create artifacts in the EEG trace. Artifacts can obscure cerebral potentials or, alternatively, be mistaken for abnormal electrocerebral activity. However, artifacts can also provide useful information; for example, artifacts associated with eye movements can assist in the staging of sleep-wake cycles. Distinguishing artifacts from electrocerebral activity represents one of the greatest challenges in critical care EEG monitoring; however, this is essential to accurate EEG interpretation in the intensive care unit.

Artifacts are typically categorized as physiologic and nonphysiologic. Physiologic artifacts arise from electrical potentials generated by the patient’s noncerebral biologic activity, for example, the QRS complex of the cardiac cycle. Artifacts generated by patient movement are also considered physiologic artifacts. Nonphysiologic artifacts are generated in the EEG recording system or in the environment and include interference from equipment and personnel at the patient’s bedside. In the intensive care unit, common sources of nonphysiologic artifact include, but are not limited to, mechanical ventilation, extracorporeal membrane oxygenation, respiratory therapy, and daily cares (Figure 12.1). Artifacts can be mistaken for electrographic seizures, rhythmic and periodic patterns, or focal slowing, among others. The failure to identify artifacts as noncerebral potentials can lead to unnecessary diagnostic testing and unwarranted treatment.

Differentiating Artifact From Electrocerebral Activity

Distinguishing artifact from electrocerebral activity is essential to the correct interpretation of a critical care EEG recording. To do this, electroencephalographers rely on two basic principles of EEG interpretation. The first is the electric field and the second is wave form morphology. In the case of suspected seizure activity, evolution of the pattern is also important in distinguishing artifact from electrocerebral activity.

Electric Field

Applying basic principles of spatial analysis can assist in distinguishing artifact from electrocerebral activity. Electrocerebral activity exhibits a distribution over the cortical surface such that one can identify a site of maximum negative voltage at a specific electrode or electrode pair. The voltage, or amplitude, then gradually decreases with increasing distance across the scalp away from the electrode with the maximum negative potential. This is the basic principle of the electric field (Figure 12.2). As an example, if the maximum negative voltage is at C3, the neighboring electrodes, for example, Cz and P3, should be within the electric field of the discharge and should demonstrate a negative deflection similar in shape but lower in amplitude than the deflection seen at C3. This principle holds true for seizures, epileptiform discharges (i.e., spikes and sharp waves), and nonepileptiform abnormalities of cerebral origin (e.g., focal slowing). Of note, there are exceptions to this rule, primarily in the setting of positive electric potentials with little to no electric field, as can be seen in neonatal seizures (see Chapter 7). Artifacts, in contrast, are often present at only a single electrode without an adjacent electric field. In other words, there is no gradual decrease in voltage from the site of maximum negativity. As such, a prominent waveform that is recorded from only a single electrode should raise suspicion for an artifact. Similarly, waveforms that appear simultaneously in unrelated head regions are typically not of cerebral origin. Evaluation of the electric field should be performed on both a bipolar and referential montage.

Morphology

It is also important to consider the shape of the wave form when an artifact is suspected. Spike wave discharges should have a well-formed spike followed by an after-going slow wave, while sharp waves should range from 70 to 200 ms in duration. Discharges that are highly spiculated or excessively 436narrow or that recur extremely rapidly should raise suspicion for muscle activity or other artifact.

Evolution

In trying to determine whether a pattern seen on EEG represents an artifact or a seizure, the definition of a seizure must be considered, in addition to the principles of the electric field and morphology discussed above. To qualify as a seizure, a pattern must last for a minimum of ten seconds and demonstrate evolution defined as at least two sequential changes in frequency, morphology, or location. Patterns with an abrupt, rather than incrementing, onset and offset and patterns that are extremely regular with little variation should raise suspicion for an artifact.

One of the benefits of cEEG is that it is typically accompanied by a video recording. When an artifact is suspected, this can often be confirmed by close review of the video, including the sound in the room. In some cases, direct evaluation of the patient at bedside by the neurophysiologist or EEG technologist can also be helpful.

Physiologic Artifacts

Eye Movement

Eye movement artifacts are common in EEG recordings from conscious patients. The eyeball is an electrical dipole with a positive charge over the cornea and a negative charge over the retina. Movement of the eye generates an electrical potential over the anterior leads of the EEG, with the morphology of the potential varying based on the direction and speed of eye movement. Eye movement artifacts can usually be identified by their distribution over the anterior electrodes, their symmetry, and their characteristic shape. In some instances, eye movement artifact may resemble electrocerebral activity. For example, slowly repetitive eye movements may resemble generalized rhythmic delta activity with a bifrontal predominance.

EYE CLOSURE

Eye closure results in an upward movement of the eye, known as Bell’s phenomenon. This, in turn, results in an upward movement of the positively charged cornea (Figure 12.3A schematic). This then produces a downward deflection over the frontal leads with a maximum deflection over Fp1 and Fp2 (the positively charged cornea is moving towards Fp1 and Fp2, making them increasingly more positive) (Figure 12.3).

RAPID EYE MOVEMENTS

Rapid vertical eye movements, for example, rapid blinking and eyelid flutter, can produce waveforms as frequent as 6–13 Hz that can also be mistaken for abnormal electrocerebral activity (Figure 12.4). In a patient with vertical nystagmus, the upward phase of the eye movement appears as a downward deflection at both Fp1 and Fp2, as for eyelid closure. This is then followed by the reverse as the negative retina is brought in proximity to the frontopolar electrodes during the downward phase of the eye movement. This manifests as an upward (negative) deflection at Fp1 and Fp2. There can be an immediate transition between the two phases in the case of downbeat nystagmus (Figure 12.5).

LATERAL EYE MOVEMENTS

In the case of lateral eye movements, the greatest potential change is recorded over F7 and F8. With eye movement to the left, an increase in positivity is seen at F7. This manifests as an upward deflection in Fp1-F7 and a downward deflection in F7-T3, which is visualized on a bipolar montage as a positive phase reversal at F7. At the same time, there is an increase in negativity maximal over F8. This manifests as a downward deflection at Fp2-F8 and an upward deflection at F8-T4, which is visualized as a negative phase reversal at F8. The opposite pattern is seen when the eyes move to the right. (Figure 12.6A-D). Of note, lateral eye movements are often preceded by a single sharp muscle potential or lateral rectus spike (Figure 12.6E). The lateral rectus spike, in combination with lateral eye movement artifact, can be mistaken for abnormal epileptiform spike and wave activity.

EYE MOVEMENT ABNORMALITIES

Decreased movement of one eye, as may be seen in the setting of brainstem injury or cranial nerve palsy, can result in unilateral or asymmetric eye movement artifacts, which can complicate the interpretation of potentials generated by eye movement. In cases where it is challenging to distinguish eye movement artifact from electrocerebral activity, additional electrodes can be placed at the lateral canthi (left upper, right lower) to record eye movements.

Electroretinogram

Electroretinogram (ERG) artifact occurs during photic stimulation. It is time-locked to the stimulus and manifests as a low-voltage discharge in the frontopolar leads. This can be distinguished from the photoelectric effect (see below) by shielding the frontopolar leads from the flash stimulus; this will 437abolish an artifact caused by the photoelectric effect without impacting the electroretinogram. In contrast, covering the eyes blocks the input to the retina and abolishes the electroretinogram. High rates of photic stimulation can also fatigue the retinal response, further helping to distinguish the ERG from a cerebral potential. This artifact can be especially prominent in recordings of electrocerebral inactivity due to the increased sensitivity of the recording and the absence of cerebral potentials that might otherwise obscure the ERG.

Electromyographic Artifact

In addition to eye movement artifact, muscle artifact is most commonly seen in EEG recordings from conscious patients. Muscle artifact is high-frequency and spiky and is seen most commonly over the frontal and temporal electrodes (Figure 12.7A). This can appear rhythmic when caused by chewing (Figure 12.7B) or biting on an endotracheal tube.

Electrocardiogram

The EKG is recorded as part of the EEG to enable detection of heart rate abnormalities that may occur in association with abnormal cerebral potentials. Channel 1 of a standard EKG is approximated with electrodes linking the left and right chest and displayed at the bottom of the EEG tracing. Cardiac potentials can also appear as artifact in the scalp lead (Figure 12.8). Cardiac interference may appear in one or in all leads but often appears maximally over the left posterior head region as a positive sharply contoured waveform accompanied by a lower-amplitude negative waveform over the right anterior head region. This reflects the distribution of the main cardiac vector producing the R wave of the QRS complex. EKG artifact can readily be identified based on the precise synchrony of the artifact with the QRS complex in the EKG lead. In patients with pacemakers, a spike of variable amplitude may be seen immediately prior to the QRS complex.

Pulse and Ballistocardiographic Artifact

Placement of an electrode on or near an artery can result in the appearance of a pulse artifact. This appears as periodic waves that are smooth or triangular in shape and results from pulse waves producing changes in the electrical contact between the electrode and the scalp. Pulse artifact is typically seen over a single electrode in the frontal or temporal region and is time-locked to the cardiac rhythm, typically appearing 200–300 ms after the QRS complex. This may be accentuated in the setting of skull defects (Figure 12.9). Widespread periodic waves may be caused by vibrations of the body in association with systolic pulse waves. This is referred to as ballistocardiographic artifact and appears as low voltage, rhythmic delta activity that is typically maximal over the posterior head regions and synchronized to the EKG activity.

Sweat Artifact

Perspiration can also create EEG artifacts. These appear as low-frequency (<0.5 Hz), low-amplitude, undulating potentials. Sweat artifact is typically seen in multiple or all electrodes and is due to changes in electrode impedance causing slow shifts of the electrical baseline.

Glossokinetic Artifact and Artifacts Associated With the Movement of Other Oropharyngeal Structures

Similar to the eyeball, the tongue is a dipole with the tip of the tongue negative relative to the base of the tongue. As such, oropharyngeal movements may be accompanied by an electrical potential that is seen most commonly over the frontal and temporal electrodes. Vertical movements of the tongue that occur with speaking or swallowing may be seen as delta frequency waves over the frontal electrodes and may mimic generalized rhythmic delta activity with a bifrontal predominance (Figure 12.10A, B).

Artifact due to tongue movement can often be distinguished from RDA by the presence of superimposed muscle artifact. Horizontal tongue movements can produce phase reversals similar to those seen with lateral eye movements.

Palatal myoclonus, which is rare in children, can also cause artifacts manifest as 1–2 Hz paired deflections best seen in a referential montage. Palatal myoclonus can also be associated with the appearance of rhythmic sharp waves at Cz or Pz.

In addition to movement artifacts associated with oropharyngeal movements, in patients who have dental fillings with dissimilar metals, spike-like artifacts may appear whenever the metals are moved against each other during chewing, speaking, or swallowing.

Patient Movement

Patient movement causes the electrodes or leads to move, which can, in turn, create prominent artifact in the EEG recording. Artifacts can arise from a variety of patient movements and, at times, may appear rhythmic. Movements that are especially common in the critical care setting include tremor, shivering, sucking, patting, cardiopulmonary resuscitation, and chest physical therapy, among others (Figure 12.11A–L). Patient movement can be readily visualized on the accompanying video recording. The EKG channel can also 438assist in the identification of patient movement (Figure 12.11D, G, H, K) and in some cases, the movement artifact is more prominent in the EKG lead than in the scalp leads (Figure 12.11D, H, K).

Breach Rhythm

A breach rhythm can be seen in patients with a skull defect. The skull attenuates electrocerebral activity, especially fast frequency activity. In patients with a skull defect, there is less attenuation of electrocerebral activity in the area of the defect as compared to elsewhere in the recording. This typically manifests as an increase in the voltage and sharpness of the electrocerebral activity in the area of the skull defect, with fast activity being more affected than slow activity (Figure 12.12A, B). While this is unlike other physiologic artifacts in that the breach rhythm is generated by underlying electrocerebral activity, we include it here for simplicity.

Nonphysiologic Artifacts

Nonphysiologic artifacts are generated in the EEG recording system or in the environment.

Electrode Artifacts

Electrode artifacts are one of the most common types of nonphysiologic artifact and are typically restricted to a single electrode. The integrity of the scalp-electrode interface is critical to the accurate recording of electrocerebral activity. Disruption of this interface can result in the generation of artifact and can also result in the increased recording of artifacts from noncerebral sources.

ELECTRODE IMPEDANCE

Poor electrode contact with the scalp results in the creation of a scalp-electrode interface with abnormal impedance. A poor or fluctuating contact between the electrode and the scalp can result in electrode “pops.” These appear as sudden positive discharges with an initial high voltage and very steep deflection followed by an exponential decay (Figure 12.13). They may be single or repetitive. In some cases, a poor or fluctuating contact between the electrode and the scalp can also produce low amplitude rhythmic or semirhythmic slow activity that resembles a focal seizure or focal slowing. This is caused by the changing impedance associated with movement of the electrode and, again, is restricted to a single electrode, distinguishing it from electrocerebral activity. High electrode impedance can result in a photoelectric effect during photic stimulation wherein each flash stimulus produced causes a photochemical reaction in the electrode that appears as spike-like discharges time-locked to the flash. A noted above, this can be abolished by shielding the affected electrode from the flash stimulus. A low impedance contact may form in the case of a salt bridge. Salt bridges are created when electrode paste or gel is smeared between two electrode locations, and these can lead to spuriously low amplitude recordings. Proper electrode application and maintenance with impedances less than 5000Ω can significantly reduce salt bridge formation and electrode artifact. Maintaining appropriate impedances can be especially challenging in the setting of the prolonged recordings typically carried out in the intensive care unit setting and can require intermittent reapplication of problematic electrodes.

ELECTRODE PLACEMENT

As detailed in Chapter 1, precise interelectrode distances are also essential to the accurate recording of electrocerebral activity. If two electrodes are placed too close together, the EEG activity in the channel comprised of those two electrodes will appear spuriously low in amplitude (Figure 12.14). In contrast, if two electrodes are placed too far apart, the EEG activity in the channel comprised of those two electrodes will appear spuriously high in amplitude. Continued recording after removal of electrodes can also create a pattern that can lead to inaccurate assessment of artifact as electrocerebral activity or a lack thereof (Figure 12.15).

Environmental Artifacts

A variety of artifacts arise from the electronic and other devices in the patient’s room. These can have an array of morphologies, and it is often difficult to identify the exact source of the artifact. However, the waveforms produced by external devices have a very different morphology than those produced by electrocerebral activity, thereby allowing for the identification of a waveform as an artifact, even if the precise source of the artifact cannot be determined. The most common cause of electrical interference is the 60 Hz alternating current that comprises the main power supply (Figure 12.16). This typically causes interference via electrostatic effects; power sources couple to other nearby conductors, including the patient, and induce a small alternating potential, which may appear large relative to the recorded electrocerebral activity. On EEG, this appears as a 60 Hz sinusoidal artifact that is typically most prominent in electrodes with high impedance. This artifact can often be reduced by replacing high impedance electrodes, by removing electrical equipment 439from the vicinity of the patient’s head, and/or applying a 60 Hz notch filter. Electrostatic artifacts can also be produced by movement of electrode wires, resulting in the generation of electrostatic currents within the wires. This can occur during patting or chest physical therapy or during the use of a percussive bed or oscillatory ventilator, for example. Electrostatic artifacts can also be produced by the movement of people and objects at the bedside, even if the electrode wires remain stationary. For example, in intubated patients in the intensive care unit, artifacts can result from the movement of water in respiratory tubing. These typically appear as high-voltage slow waves with superimposed fast components with a broad field over the frontal or temporal regions closest to the respiratory tubing (Figure 12.17). Similarly, electrostatic charges on drops of fluid in intravenous drips can cause spike-like potentials coinciding with the drips of the infusion. Electric equipment can also cause electromagnetic interference from a current flowing through cables and other components of the device. Sources of such artifact can include pagers, cellular telephones, computer screens, motors in infusion pumps, and other motorized equipment.

Instrument Artifacts

In addition to the above, malfunction of the EEG machine can result in the generation of artifacts. For example, thermal agitation of electrons in the amplifier circuit can result in amplifier “noise.” This typically manifests as mixed-frequency activity and should be less than 2 μV, making it problematic primarily during recordings of electrocerebral inactivity. However, higher levels of noise may be produced if a component of the amplifier is malfunctioning, thereby distorting the recording of cerebral activity. A faulty or absent ground connection to the patient or defects in the power supply or other parts of the EEG machine can result in excessive 60 Hz artifact in all leads.

The widespread use of digital technology for the recording and analysis of EEG can result in artifacts specific to the limitations of digital technology. For example, sampling at a rate less than twice the highest frequency present in the signal can result in aliasing, causing the EEG signal to contain artefactual slow frequencies. Similarly, using a monitor with inadequate resolution can also result in aliasing and the introduction of artefactual slow frequencies.

Conclusion

The recording of electrical activity from the brain is subject to interference from noncerebral electrical potentials, and this can be especially problematic in the complex environment of the intensive care unit. Distinguishing artifact from electrocerebral activity is essential to the correct interpretation of a critical care EEG recording. Applying basic principles of spatial analysis can assist in distinguishing artifact from electrocerebral activity. Moreover, artifacts often have a characteristic waveform that can assist in identification of the potential as an artifact.

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Templates and Treatment Algorithms Cases Questions and Answers by Chapter Brief Rhythmic Discharges, Seizures, and Mimics in Critically Ill Neonates Seizures in Critically Ill Children Convulsive and Nonconvulsive Status Epilepticus in Critically Ill Children

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