Pediatric Clinical Neurophysiology
Gloria Galloway and Thoru Yamada
Artifacts in the pediatric population are often similar in many ways to those encountered in adults. However, some artifacts are more commonly seen in the pediatric population. One reason for this is the short distance used between stimulation and recording electrodes in the very young patient with a small body size. Therefore, this chapter attempts to describe a number of age-specific artifacts that can cause significant difficulties in interpretation of clinical neurophysiologic studies involving both electromyogram (EMG) recordings and intraoperative monitoring.
Obtaining an EMG in a child or young adult patient can be challenging due to the discomfort involved and the need to avoid inadvertent movement. The presence of artifacts during an EMG procedure increases the challenge involved and likely increases the amount of time needed for the procedure to eliminate these artifacts (1). Artifacts can often be difficult to eliminate completely and may cause misinterpretation of the data. This can be particularly problematic in pediatric cases and especially in the very young patient where cooperation cannot always be depended upon during the procedure or over the length of time it takes to accomplish adequate real-time artifact reduction. In young children, certain artifacts (e.g., movement) may be seen more commonly than in adults. These may be unintentional despite attempting to exert volitional control. In this chapter, several case examples are provided in which the presence of an artifact led to confusion in the interpretation of the EMG evaluation. It is important first to recognize when an EMG finding is due to the presence of an artifact, and then to determine what measures need to be taken to eliminate or reduce the frequency or intensity of that artifact, optimizing interpretation of the EMG evaluation.
Several clues help guide the examiner to suspect that certain features in an EMG are physiologic or due to the presence of an artifact. The first clue a waveform is artifactual involves having a finding which is unexpected from the clinical scenario. It is important to remember that the EMG is an extension of the physical, neurological, and neuromuscular examinations (2). Therefore, the EMG should be interpreted with the clinical context in mind. It is unusual that after a thoughtful history and neuromuscular examination, the EMG finding is contrary to or inappropriate for the clinical situation. The second clue that the EMG features are due to the presence of an artifact stems from observation of the patient. This involves observing whether the patient is participating in movement that may cause the corresponding artifact either voluntarily or involuntarily. The third clue is recognized by observing the patient surroundings. In observing the patient surroundings, it is important to observe what equipment is attached to the patient, as well as what electrophysiologically active sources may be in the surrounding environment that could introduce artifacts in the EMG signal. These sources may include other machinery, devices, or people. The strongest clue involves a successful attempt to minimize a suspected artifact that correlates to reduction or elimination of the artifact, such as unplugging an electrical device or moving equipment away from the patient in order to eliminate interference. Artifacts therefore can be divided into intrinsic sources involving the patient and external sources that are not patient induced.
42ARTIFACTS IN THE ELECTROMYOGRAM
Intrinsic sources of artifacts involve electrophysiological activity generated by the patient that interferes with the recording. Common sources of artifacts in the EMG are summarized in Table 3.1 and include the following:
Voluntary Muscular Contraction
Muscular contraction can be attributed to voluntary or involuntary muscle activity. One example of a patient-induced myogenic artifact shown in Figure 3.1 is due to voluntary muscle contraction. This may initially go unnoticed as it can involve small muscles and be unintentional. In some cases, particularly in a young patient, muscle contraction can be a response to pain or in anticipation of pain despite the absence of pain itself. During voluntary muscle contraction, the EMG response is sustained for a variable period of time but can be eliminated by relaxation of the muscle. This can often be accomplished by speaking calmly to the patient if an older child or by distracting a younger patient. Elimination of the muscle contraction through relaxation is critical to interpretation (3). This is critical not only because muscle contraction makes it difficult to recognize abnormal spontaneous electrical activity, but excessive involuntary muscle contractions can be seen in several neuromuscular disorders as well and therefore may confuse rendering an appropriate interpretation. In disorders involving hypocalcemia, hypomagnesemia, and hyperkalemia, cramps induce electrical discharges in the EMG during the needle examination. These are briefly sustained involuntary muscular contractions lasting seconds to minutes in duration. Because they are involuntary, eliminating the contraction through distraction or relaxation can help differentiate these from voluntary muscle contractions. Myoclonus may introduce another type of movement artifact that may be confused with involuntary muscle contraction. Myoclonus is a sudden brief and often intense involuntary muscle contraction which can be peripheral, cortical, or spinal in origin. In addition, in the congenital form of Stiff person syndrome, sustained muscle contractions are seen. Therefore, recognizing this finding as a voluntary muscle contraction as opposed to an artifact by making unsuccessful attempts to eliminate it will allow appropriate interpretation of the EMG (4).
Another example of an intrinsic source of artifact is produced by a startle response, as shown in Figure 3.2. Startle occurs most commonly in infants without an inciting factor. When it is associated with anxiety disorders, an increased auditory startle response, along with increased sweat response and enhancement of other autonomic responses, may be seen in young adults (5). In general, a startle response can be differentiated from an abnormal neuromuscular or myogenic response in that it is transient, becomes habituated, and usually is associated with a sudden movement involving the upper body of the patient. It generally does not recur. If not recognized it may be misinterpreted as a polyphasic motor unit in the EMG suggesting the presence of chronic change or reinnervation in that muscle. Since some polyphasic motor unit activity is expected at percentages that vary depending on the sampled muscle, when they occur infrequently, this could lead to a misdiagnosis (6).
Chewing and sucking movements can be subtle and missed clinically as young patients may make chewing movement with their tongues without much mouth movement being noticed (7,8). If obtaining an EMG recording near the mouth or along upper shoulder girdle muscles, one may notice artifact appearing in bursts of motor unit activity that varies in amplitude and duration (see Figure 3.3). Complex repetitive discharges in the EMG in contrast have a polyphasic pattern of discharge that is uniform from one waveform to another with a constant firing rate starting and stopping abruptly. These complex repetitive discharges can be seen in the chronic neuropathies including various polyneuropathies, motor neuron disease (including spinal muscular atrophy and muscular dystrophy), as well as chronic radiculopathies (9,10). Chewing or sucking movements may be misinterpreted as complex repetitive discharges if the configuration of the EMG pattern is not closely analyzed, or if the patient is not observed to have apparent mouth or tongue movements (9).
The next group of artifacts is one caused by objects or electrical activity in the environment or around the patient that interferes with the recording. Common causes of artifact in the EMG include the following extrinsic sources:
Cardiac monitors attached to the patient may produce a regular cardiac rhythm superimposed upon the EMG signal, as shown in Figure 3.4. These can be recognized by the regular periodic pattern that is synchronous with the patient’s heart rate. Furthermore, it is able to be eliminated by temporarily removing the cardiac leads or by moving the cardiac monitoring equipment further away from the surface electrode or EMG needle recording electrode. If not recognized, these could be misinterpreted as fibrillation potentials, suggesting the presence of denervation and leading to inaccurate diagnosis. Fibrillation potentials may be seen in a wide variety of neuromuscular disorders and usually indicate active, ongoing denervation. These may occur in acute radiculopathies, inflammatory myopathies, axonal degeneration in peripheral neuropathies, and motor neuron disease (11).
60 Hz Artifact
A common source of EMG artifact is due to electrical equipment near the patient. Interference from nearby equipment may include mechanical ventilators, blood warming machines, and infusion pumps. A common type of artifact in EMG is 60-Hz artifact. This artifact occurs at a frequency of 60 cycles per second or a multiple of 60 Hz as seen in Figure 3.5. Similar to artifacts produced by cardiac monitors, these artifacts are recognized by their regular rate and successful elimination if the offending equipment is removed or moved further away from the patient (12). If not recognized during the EMG procedure, these superimposed discharges may be misinterpreted as positive sharp discharges or fibrillations suggesting the presence of denervation, leading to inaccurate diagnosis. Positive, sharp wave discharges and fibrillations can be seen in a number of neuromuscular disorders including axonal degeneration and motor neuron diseases, acute and subacute neuropathies, and inflammatory myopathic disorders.