As in adults, the use of electrodiagnostic studies in pediatric patients provides an extension of the neurologic examination facilitating evaluation of the physiology of the neuromuscular system. However, pediatric electrodiagnostic studies require further consideration by the practitioner due to variations in pathology, normative values, technical factors, and diagnostic approaches relative to adult assessments. For example, diagnoses such as focal entrapment neuropathies that are commonly seen in adults are not encountered as frequently in children. Additionally, many neuromuscular disorders including congenital myopathies present in childhood require consideration by the electrodiagnostician. Due do physiologic and anatomic differences of nerves in infants and children compared with adults, skilled electromyographic (EMG) techniques are essential for accurate interpretation and diagnosis. Incorporating approaches to improve cooperation of pediatric patients helps to build the foundation for consistently obtaining complete and well-tolerated studies. This chapter explores various aspects of pediatric electromyography in order to provide readers with useful information that can be implemented to optimize their practices.

While the pathophysiology of nerves in children is similar to that of adults, there are some important differences that must be considered when performing pediatric studies. Starting from conception, nerves undergo a maturation process during which the nodes of Ranvier achieve their peak internodal distances around 5 years of age. Nerve myelination begins between the tenth and fifteenth weeks of gestation and is typically completed by the child’s fifth birthday. Pediatric patients have a smaller nerve diameter relative to adult nerves.¹ Given the short stature of pediatric patients, the nerve lengths of children are frequently shorter than comparable segments in postpubescent individuals. These structural differences, along with shorter nerve lengths, may affect the nerve conduction study (NCS) values obtained in pediatric studies. The nerve conduction velocity becomes a very sensitive parameter in children due to such factors. These aspects make pediatric studies more susceptible to measurement error. Further discussion of technical considerations may be found in the section “Nerve Conduction Study.”

In relation to nerve injury, both pediatric and adult nerves can sustain demyelinating, axonal, or combination injuries. In demyelinating injuries, myelin destruction occurs first and is then followed by a process of repair. In axonal injuries, the nerve axon distal to the affected region undergoes Wallerian degeneration, and depending on the extent of axonal damage, complete regeneration may not be possible. In pediatric patients, in utero neuronal insults also should be considered. In one study of in utero alcohol exposure, the authors found decreased motor amplitudes and conduction velocities in newborns that had not improved by 1 year of life. These findings reflect sequelae of both prenatal myelin and axonal injury.² When interpreting pediatric data, especially in newborns and infants, the risk of prenatal and/or postnatal nerve insults therefore must be considered.

The estimated time course for nerve recovery may be less in children than in adults due to shorter nerve lengths. Animal studies demonstrated faster axonal regeneration in younger rats compared with older rats as well as shorter lag time before healing initiation in younger rats. This finding supports age-related variation in nerve recovery.³ When performing pediatric studies, these physiologic differences are important to be cognizant of when interpreting electrodiagnostic data.

In order to facilitate the acquisition of necessary information in pediatric EMG studies, there are differences in the technical approach compared with adult studies. A larger examination room or area may be needed to accommodate parents/family, adaptive equipment, and additional support staff for some pediatric patients. The diagnostic examination room should be electrically shielded to minimize outside interference that may potentially lead to erroneous interpretation of the child’s study. Accessible cabinets and drawers should be stocked with bandages, gauze, alcohol swabs, abrasive paste, measuring tape, vapocoolant spray, and scissors. Scissors are required for trimming disposable electrode pads to optimize fit for small limbs. In an adult study, the use of ethyl chloride spray for upper limb needle examination resulted in higher patient satisfaction and decreased pain ratings compared with the control group with water spray.⁴ A comparison of topical lidocaine cream with ethyl chloride spray used for adult EMG of the gastrocnemius muscle had similar findings.⁵ Vapocoolant sprays are commonly used in pediatric studies to minimize pain and improve patient tolerance. Although there are no studies of vapocoolant spray use in pediatric EMG, there are various reports demonstrating its use in pediatric immunization and venipuncture procedures. The manufacturer’s safety precautions and instructions should be followed when selecting and using a vapocoolant spray to maximize safety of the pediatric patient.⁶

In children, minimizing distress is an essential factor in obtaining a complete study. The goal is to accurately address the electrodiagnostic question while minimizing the duration and discomfort of the test. Child life specialists, music, games, toys, or other distractions are often used to assist pediatric patients and their families through challenging evaluations. In a survey, it was found that 2- to 6-year-old patients exhibit the most extreme behavioral stress. This pattern includes screaming, flailing, and attempting to get off the examining table. This age group may need extra care to minimize the stress of the study. Most practitioners surveyed perform pediatric electrodiagnostic studies with parents present, although the benefits of parents being present during the study are controversial. It is often a useful and common practice to demonstrate nerve stimulation on the examiner or a parent to alleviate anxiety in the patient. Depending on the child, observing the screen may either provoke more anxiety or assist with distraction. While anesthesia may be considered, most physicians prefer to attempt the study with the child awake to obtain optimal EMG data.⁷

The instrumentation for pediatric studies is similar to that used in adult settings. Equipment includes recording electrodes, amplifier, converter, filters, and an EMG device capable of delivering 20-, 30-, and 50-Hz stimulations. In infants and young children, high-rate repetitive nerve stimulation may be required because they are unable to volitionally activate muscles. If this is necessary, repetitive nerve stimulations are best completed under anesthesia for patient comfort. There are pediatric stimulators that have smaller distances between the stimulator heads for more accurate nerve depolarization and measurement (Fig. 66–1). Disk or disposable electrodes may be used. The electrode wires should be short and shielded if the room cannot be shielded. Monopolar or concentric needles may be used for needle EMG, which is further discussed in the section “Electromyography.”

Nerve conduction studies are performed, as in adults, on both sensory and motor nerves. Several modifications in testing procedures are used due to the smaller size of a child’s limb. In addition, it is essential to be familiar with the process of nerve maturation, which affects the normative data based on the age of the child. It is also important to be aware of various technical pitfalls and challenges that can occur when performing studies on children.

Nerve stimulation is typically performed at similar anatomic locations in children as in adults. Given that the limbs are smaller in a young child, standard adult measurements for distal stimulation sites are not used because these would result in stimulation sites that are too proximal and deep to achieve adequate stimulation with a surface probe. The published normative data tables list a range for both distal latencies and distal segment measurements that were used. Subsequently, unless a distal latency is clearly in the abnormal range, it may be difficult to differentiate a truly prolonged distal latency. In practice, this rarely presents a problem. Because the majority of pediatric nerve conduction studies evaluate generalized rather than focal neuropathies, more emphasis is placed on amplitudes and nerve conduction velocities than on latencies.⁸ The contralateral limb serves as a good reference in young children with focal nerve lesions and unilateral weakness. The distal motor latency (DML) can be corrected to a standard distance using the formula of Slomic et al:

where L is the distance between the stimulating cathode and the active recording electrode, X is the standard distance (4 cm for upper limb nerves, 5 cm for lower limb nerves), and MCV is the motor conduction velocity.¹³

Normal values of corrected DML have been published for children of ages 0 to 72 months for the median, ulnar, peroneal, and tibial nerves.⁹ In infants and children younger than 2 years of age, a pediatric nerve stimulator probe is used (see Fig. 66–1). The interelectrode distance between cathode and anode on a pediatric stimulator is less than 15 mm, which allows the current to be better directed over the desired nerve, thus decreasing the incidence of costimulation of adjacent nerves.

Disposable electrodes are helpful for infants and small children. They may be easily trimmed to a smaller size to facilitate placement on a small hand or foot. The disposable electrodes may stay positioned better due to their adherent contact surface. Standard metal disk electrodes also may be used, although it can be difficult to keep these taped in place, especially on the sweaty skin of irritable infants in a warm room.

Normative data for latencies, conduction velocity, and amplitudes depend on the age of the child due to incomplete myelination of the peripheral nerves at birth. Miller and Kuntz published normative reference data for children aged 0 to 15 years in 1986,¹⁰ and Parano published data for children aged 0 to 14 years in 1993 (see Table 66-1).¹¹ The process of peripheral myelination begins at approximately 15 weeks of conceptual age.¹² Sensory and motor nerve conduction velocities in a full-term newborn are approximately 50% of the adult values. As myelination proceeds, conduction velocities typically reach adult values by 3 to 5 years of age. The greatest increase in motor conduction velocity is noted in the first year of life, with mean conduction values reported in the lower-adult range in the lower limbs at 12 months and in the upper limbs at 24 months. Motor amplitudes in a term infant are reduced to one-half of adult values for the abductor hallucis (AH) and to one-third of adult values for the abductor pollicis brevis (APB), abductor digiti minimi (ADM), and extensor digitorum brevis (EDB). Adult values were reached by ages 2 to 4 years for AH and the hand muscles, respectively. In contrast, the EDB continued to remain smaller than the adult values at age 14 years.⁹ There is reported variability in the rate of maturation of individual nerves in early childhood. The median nerve may be delayed in maturation of conduction velocity relative to the ulnar and peroneal nerves in the first 3 years of life.¹ In infants younger than 2 years of age, the posterior tibial motor nerve conduction velocity (MNCV) has been noted to be slower than the ulnar, median, and peroneal MNCV.¹⁰ There are differences in the appearance of waveforms as well in pediatric nerve conduction studies. The configuration of the sensory nerve action potential has two distinct peaks in infants and children. This morphology has been attributed to differences in the rate of maturation of the two groups of fibers.¹³ Ulnar motor compound motor action potentials (CMAPs) have been noted to have a double-negative peak in one-third of children aged 0 to 6 years.⁹ When interpreting the data, the electrodiagnostician should not be alarmed by these possible pediatric waveform variations.

F-wave latencies are helpful in pediatric studies to evaluate for proximal demyelination in conditions such as polyradiculoneuritis associated with Guillain-Barré syndrome. Given that F-wave studies do not require measurements, they are also beneficial in young children because measurement error may be introduced when short distal distances are necessary.¹⁰ F-wave latencies increase in a linear fashion from 6 months to 6 years of age.¹¹ They are shorter in infants compared with adults despite the slower nerve conduction velocities because the limb length is smaller. The H-reflex is easily recorded in hand muscles of newborns and infants up to 1 year of age.¹

It is important to be aware of various technical pitfalls when performing nerve conduction studies on children. Potential sources of error include accidental nerve costimulation, volume conduction of muscle responses, measurement error associated with short segments, and temperature changes in small digits and limbs. Nerve costimulation may occur at even low current settings, and one should therefore closely monitor nerve configuration changes as the current is increased. It is also helpful to observe the muscle contraction pattern associated with stimulation to ensure that only the intended nerve is stimulated. Given the small limb size in pediatric patients, inadvertent stimulation of other nearby nerves may occur. A small positive deflection on the motor curve is another possible indicator of costimulation. This finding can also indicate that the active electrode is not ideally placed over the muscle motor point.

There are other technical factors to consider in pediatric nerve conduction studies. As mentioned previously, measurements should be made carefully to minimize measurement error that will be exaggerated in studies of short nerve segments. As in adults, temperature may affect nerve conduction study values. In all nerve conduction studies it is very important to monitor temperature with goal targets of greater than 32°C for the hand and greater than 30°C for the foot.¹⁴ While temperature correction factors may be used, it is best to warm a cool limb with a hot pack or warmed blanket because the accuracy of adult temperature correction factors for pediatric patients is unknown. Additionally, the accuracy of temperature correction factors for diseased nerves has not been determined. Newborns and very young infants are not able to maintain their core body temperature, as well as adults. To prevent unnecessary cooling of an infant, the trunk and limbs not directly being examined for the study should be kept covered if possible.

Many pediatric electrodiagnostic referrals involve a diagnosis of weakness in which a neuromuscular junction disorder cannot be excluded. Repetitive stimulation is performed in children based on the same protocols recommended in adults. Because it may be difficult for children to cooperate, the greatest difficulty is ensuring that the child’s limb is adequately immobilized to avoid movement artifact. An intravenous (IV) board may be helpful for immobilizing the hand. Achieving a quiet baseline for proximal studies in a young child is often challenging, so sedation may be required. Sedation is beneficial in younger children who are unable to cooperate with the recommended exercise protocol for neuromuscular junction testing. In these cases, sedation facilitates rapid-rate stimulation, which is quite painful. Temperature must be carefully monitored with a target goal of 35°C, as recommended by the American Association of Neuromuscular and Electrodiagnostic Medicine (AANEM).¹⁵ Neuromuscular junction assessment is further discussed in the section “Special Studies.”

Needle EMG is an important component in the electrodiagnostic evaluation of a pediatric patient. As in adults, pediatric needle EMG is used to assess myopathic diseases, to localize nerve injuries, and to identify signs of denervation related to neuropathies and motor neuron diseases. By their very nature, some diagnoses such as congenital myopathy, birth brachial plexus injury, and spinal muscle atrophy will be frequently encountered by the pediatric electromyographer.¹⁶ While many practitioners feel comfortable performing studies on adolescents and older children, the approach for neonates and infants may seem more challenging. By increasing one’s understanding of techniques and variances seen in children, pediatric EMG can be an important component in the diagnostic armamentarium of the neuromuscular specialist.

Building rapport with the child or adolescent is an important first step in obtaining a reliable EMG study. Some patients may be too young or have cognitive deficits that may impair their understanding of the testing. However, it is important to provide an explanation of the study to those who can comprehend in clear, simple language to demystify the study and help alleviate anxiety.¹⁷ Avoiding words such as “needle” that may have negative connotations and using alternative descriptors such as “recording electrode,” “fine wire,” or “antenna” can further minimize fear. Some individuals find the idea that the sounds of the EMG are their “muscles talking” quite interesting and change their mood to one of interest.⁷ Effective communication to explain and educate the patient and family may ease some of the child’s concerns. However, many young patients will benefit from distraction, which may be in the form of games, videos, or music. A child life specialist, music therapist, or assisting nurse can be an incredible resource to occupy the child’s attention during an EMG study.

Little data regarding the incidence of complications related to EMG studies are available in pediatric populations. As with adult studies, the insertion of the recording electrode through the skin and into the muscle of a pediatric examinee carries the risk of pain, infection, and bleeding. Topical lidocaine/prilocaine creams may be used to decrease discomfort. These creams are often impractical due to their delayed onset, the number of sites to be pretreated, and the limited maximal total dose for infants and children. Topical vapocoolant sprays may be a better alternative to decrease pain related to insertion of the recording electrode. Cleaning the skin with alcohol swabs and avoiding needle placement near areas of skin breakdown or infection can minimize infection risk. Given the small body habitus and thin musculature of infants and young children, great care must be taken when evaluating areas near the thorax to avoid a pneumothorax. For patients on anticoagulation or with a coagulopathy, evaluating superficial muscles and applying sustained pressure after studying the muscle may minimize the risk of bleeding or hematoma. However, the true risk of such complications is minimal as in adults.¹⁸

The preference for concentric or monopolar recording electrodes is generally based on the training and experience of the electromyographer. Both types of recording electrodes may be used in pediatric EMG studies. A 26- to 30-gauge disposable recording electrode that is 25 mm long is often used for evaluations. As with adults, longer electrodes may be needed in adolescents depending on muscle depth and body habitus. It is not uncommon for the pediatric electromyographer to perform diagnostic testing in a neonatal or pediatric intensive care unit (ICU) given the clinical severity of some neonatal and infantile diseases.¹⁶ Concentric recording electrodes may offer the benefit of less susceptibility to outside noise than monopolar electrodes. Concentric recording helps minimize electrical interference from nearby life support devices and other equipment in the hospital or ICU setting. However, concentric electrodes will yield shorter measured motor unit action potential (MUAP) durations and decreased MUAP amplitudes.¹⁴

Optimal pediatric EMG evaluation is obtained from a patient who is able to participate with the practitioner, allowing complete muscle relaxation and graded activation. This type of cooperation can be difficult to obtain in infants and young children. While nerve conduction studies may be obtained from sedated patients, MUAP assessment is generally not possible. Use of an anesthesia or ICU team to provide conscious sedation may allow for some MUAP evaluation depending on the child’s arousal and ability to follow instructions. However, these studies typically provide only limited information about motor unit recruitment. In neonates and infants, use of primitive reflexes such as the Moro reflex may activate muscles for evaluation. Close examination of the young child’s movements will guide the examiner to which muscles will likely provide adequate MUAPs for assessment. Equivalently, muscles that are noted to be less active may provide ideal sites for assessing insertional and spontaneous activity. In young children, providing a task or activity that activates the desired muscle may improve MUAP recruitment. When planning a pediatric EMG study, one should strive to limit the number of muscles tested to only the essential muscles needed for an accurate electrodiagnosis and to minimize patient distress and discomfort. Additionally, the study should avoid bilateral assessments when possible to preserve potential sites for future muscle biopsies.

EMG assessment of the MUAP is the same for both pediatric and adult patients. Typically, the electrodiagnostician evaluates the insertional activity of the muscle, spontaneous activity of the muscle, motor unit morphology, motor unit recruitment, and interference pattern.¹⁹ The MUAPs of children are quantitated with regard to amplitude, duration, and number of phases. The jiggle (or stability) of the pediatric MUAP can theoretically be measured but requires a cooperative patient able to provide a steady, slow firing rate for accurate assessment. Studies of normal neonates and 3-month-old infants suggest that biphasic more than triphasic morphology predominates with MUAP durations nearly 25% shorter than those found in adults.^20,21 However, while Sacco et al found smaller amplitudes in younger patients, do Carmo’s data indicated larger MUAP amplitudes in his pediatric sample. Spontaneous activity including fibrillation potentials and positive sharp waves may be observed in anterior horn cell diseases, axonal neuropathies, and certain myopathies.²² Characteristic findings such as the myotonic discharges of congenital myotonic dystrophy may not initially be present in infants or appear atypical.²³ While the short-duration, low-amplitude, early-recruitment pattern of myopathies may be seen in children with muscle diseases, it is not uncommon to observe a normal infant EMG study in confirmed myopathic disorders.²⁴ Congenital muscular dystrophies, in contrast, have been reported to consistently demonstrate typical myopathic changes.²⁵ Regardless of the technical aspects of performing the needle EMG study in pediatric patients, the information obtained from the needle examination can greatly assist in the diagnosis.