Artifact in Pediatric Electroencephalography
Erika Takle Axeen and Phillip L. Pearl
Surface recordings of electrophysiological brain activity are subject to artifact from a variety of sources (1). In an EEG recording, artifact is present in any tracing or changes in the tracing when it is attributed to a noncerebral source. Artifact can obscure a recording, rendering the electrocerebral activity unreadable. Familiarity with commonly seen patterns of artifact can help avoid misinterpretation, which when falsely attributed to cerebral sources, may have significant clinical implications. The pediatric EEG is subject to the same technical artifacts as seen in adults. Common examples include alternating current artifact (60 Hz) and vertical eye blink artifact during wakefulness. However, there are also artifacts that are unique to the pediatric population. Common examples of pediatric EEG artifacts are rhythmic patting and rocking artifact in the neonate and infant, which may be misinterpreted as an electrographic seizure without clinical signs. Therefore, video correlation of the EEG tracing may be indispensable to identify the potential source of artifact. This chapter includes common artifacts that are encountered in the EEG for a pediatric population. Specific pediatric patterns of artifact in the EEG that simulate seizures may be similar to those seen in an adult (see Chapter 7). Ictal mimics, ranging from benign epileptiform variants to induced cerebral activities, such as the texting rhythm (2–4), must be discriminated from common patterns associated with artifact.
PHYSIOLOGIC SOURCES OF ARTIFACT
Artifacts of physiologic or biologic origin are those arising from the patient’s body, including eye movements, muscle activity (including glossokinetic), cardiac function, and sweat (Table 6.1).
Eye movements represent a common physiologic source of artifact seen in nearly every pediatric EEG tracing containing periods of wakefulness. This common artifact emanates from the frontal head regions and is best seen in the frontopolar and frontal electrodes (Figure 6.1). The inherent dipole in the eye is the generator of this artifact with the retina being relatively electronegative and the cornea electropositive. A useful mnemonic is “pupil positive, nerve negative.” Attention to eye movements may be informative regarding the patient’s state of alertness, with eye blink artifact signifying wakefulness and slow-roving eye movements suggesting drowsiness. Lateral rectus spikes can commonly be seen with lateral visual scanning. With rapid eye movement, the artifact producing the “spikes” from contraction of the lateral rectus muscles signifies a motor unit potential.
In the active child, muscle artifact is a pervasive feature that can obscure the EEG tracing. This is identified as high-frequency activity most prominently seen over the bilateral temporal and frontal regions. Predictable rhythmic patterns such as chewing or tongue movements can be identified based on their rhythmicity (Figures 6.2 and 6.3). Close attention to the timing of muscle artifact with event analysis can help discern whether movements are related to a seizure discharge.
• Eye movements
• Tongue movements
A frequently encountered artifact in all ages is the electrocardiogram. Pulse artifact can also be seen depending on positioning and can be mistaken for rhythmic slowing (Figure 6.4). Hiccups can also produce potentially confusing rhythmic artifacts requiring clinical correlation (Figure 6.5). The phenomenon known as sweat sway, with low-voltage undulating delta usually seen diffusely, is another example of an endogenous artifact from a physiologic but extracerebral source.
NONPHYSIOLOGIC SOURCES OF ARTIFACT
As in adults, commonly encountered nonphysiologic artifacts, such as electrode artifacts, are related to the technique and recording process of obtaining an EEG (Table 6.2).
In the inpatient setting, particularly where surrounding machines and equipment are abundant, such as in intensive care units or the operating room, 60-Hz artifact is frequently seen. A 60-Hz notch filter may be necessary to eliminate this artifact despite the potential loss of detecting underlying epileptiform activity. Figures 6.6A and B show 60-Hz artifact in an infant, which is attenuated utilizing the notch filter, after which electrode artifact remains. Figure 6.7 shows a periodic discharge attributable to the movement of the ventilator in a child who otherwise has electrocerebral inactivity on EEG. Nebulized respiratory therapy can serve as an exogenous source of artifact as well (Figure 6.8).
Electromagnetic disturbances can similarly occur in the outpatient setting, whether in the EEG laboratory or during ambulatory studies. A prominent EEG artifact simulating generalized spike-and-wave may be associated with ringing from a cellular telephone. In this case, proper grounding is needed to reduce these machine-related artifacts (5).
• Electrodes and lead wires
• Machines (e.g., ventilator)
• Environment (e.g., 60 Hz)
• Behavior (patient or caregiver)
• Other devices (e.g., pacemakers)
Rhythmic movement artifacts are commonly seen in the newborn, infant, and young child associated with patting and other soothing behaviors provided by caregivers (Figure 6.9). The rhythmicity, polarity, and location of the artifact can look suspiciously like an electrographic seizure. This is especially true for neonates who are prone to focal seizures that may be seen in only a single electrode. Use of video to identify movement patterns is critical in these situations as is close observation of the EEG for patterns of evolution and propagation, which are less likely to be seen in a purely artifactual pattern. Chest physical therapy introduces another artifact in the EEG produced by the exogenous repetitive behavior (Figure 6.10) (6,7).
Video 6.1 demonstrates very prominent EEG waveforms during which the video confirms the source as the child playing with a balloon! The active child will create frequent movement artifacts. This is seen as a myogenic artifact (surface electromyogram [EMG]) on the EEG recording. Occasionally, these behaviors may appear rhythmic, creating an unusual artifact (Figure 6.11).
ROLE OF ARTIFACT IN EVENT ANALYSIS AND SPECIALIZED RECORDING TECHNIQUES
EEG artifact is often frustrating in that it can obscure the cerebral recording, with loss of valuable electroclinical and localizing information. Alternatively, close correlation with video may be instrumental in 97identifying the source of the artifact as a psychogenic, nonepileptic seizure (Video 6.2). In addition, there are circumstances where an epileptic seizure can be initially detected by a stereotypical artifact. Examples include a rhythmic artifact in the frontopolar region caused by nystagmoid eye movements. Similarly, an extracerebral artifact may demonstrate characteristic patterns associated with EMG, electrocardiographic, or electrooculographic activity in an evolving fashion despite the absence of an electrocerebral change (Figure 6.12).
Specialized pediatric EEG recordings require meticulous attention to artifacts and may pose particular challenges. Ambulatory EEG recordings may produce a bewildering array of artifacts that may be especially difficult to explain or verify, and corresponding videography may be too obscure or unavailable for analysis. Unrestricted activity during ambulatory recordings and relative absence of observer information compounds the problem. Although many artifacts on ambulatory recordings are similar to those seen in conventional EEG (e.g., smiling, chewing, swallowing, eye and head movements, cable movement, coughing, startling, and talking), there are other artifacts that are relatively unique to ambulatory recordings (8). These include movements throughout sleep such as turning the head on a pillow, movements of others near the patient, walking, and poor connections at the preamplifier input. These have been mitigated by performing the initial portion of ambulatory EEGs in a controlled laboratory, documenting the appearance of common movements and other sources of artifact, in addition to verifying the integrity of the connections between the input electrode cables and recording unit.
The feasibility of novel, highly compact, wireless multichannel EEG in NICU has been challenging to interpret due to the prominent artifact. Also known as micro-EEG, this is a miniature, wireless recording device that digitizes the EEG signals for wireless transmission and represents an attempt to record EEG from an electrode cap. This method is used to achieve full-channel recordings with user-friendly application, enhanced portability, and ease of use. In a study of preterm newborns between 24 and 33 weeks gestational age with a postnatal age less than 30 days and at least two episodes of apnea, bradycardia, or oxygen desaturation within a 12-hour period, over half of the recordings were uninterpretable due to artifact in patients with a corrected gestational age of 35 weeks (9). Yet only 10% were uninterpretable due to artifacts in infants prior to 35 weeks corrected age. Negative predictors adversely affecting accurate interpretation included infants that were older and heavier and those with a large head circumference, but the primary reason for low interpretability was disconnection of the cap from the scalp. Another form of EEG recording that has gained increasing acceptance in the NICU is that of amplitude-integrated EEG (aEEG), which is highly practical although limited to 2 to 4 electrodes and is thus poorly representative of the scalp EEG and less informative for detecting focal epileptiform discharges (10–12). A higher rate of artifacts has been reported in aEEG recordings compared with scalp EEG recordings due to the presence of muscle activity, suboptimal electrode positioning, and unstandardized electrode application (13,14).
Artifacts of EEG recording are pervasive in pediatric recordings similar to adults; however, some are unique to this age period. Accurate discrimination of an artifact from cerebral sources is a critical skill for the encephalographer. Knowledge of common patterns of artifact is a useful tool that ensures appropriate interpretation of the pediatric EEG. Use of concurrent, time-locked video is indispensable for determining the source of some artifacts and differentiating them from interictal epileptiform discharges and electrographic seizures, particularly in the active child. Additionally, use of known artifact patterns can prove useful for determining the state of the patient (e.g., vertical eye blink artifact reflecting wakefulness) as well as the analysis of patterns during event monitoring, separating epileptic from nonepileptic seizures.
Video 6.1: Artifact from a balloon near the EEG equipment. Note that the artifact changes frequency and amplitude depending on how the balloon is moved.
Video 6.2: A psychogenic nonepileptic spell can be diagnosed by careful analysis of artifact. In this video, note the muscle artifact is time-locked with the movements. Other features such as abrupt onset/offset (clinically and electrographically) and spell semiology are suggestive of a nonepileptic spell.