Pediatric Considerations in Electromyography

Rosalind J. Batley

Introduction

Electrodiagnosis in the pediatric population differs from that in adults with respect to the diagnoses, clinical presentations, set-ups, and normal values. In fact, it is probably easier to describe what is similar between pediatric and adult studies than what is different. The instruments and electrodes are similar, but the latter are smaller than those used with the adult. The underlying physiology is similar, but development of the myelin and motor unit is incomplete in the pediatric patient. Due to the developing nervous system, normative data vary, as do the presenting abnormalities. The pediatric electromyographer must have an extensive knowledge of growth and development and a familiarity with pediatric differential diagnosis. The knowledge of growth and development helps to narrow down the differential. The pediatric examiner’s understanding of “primitive” reflexes can be used to “trick” an infant into reflex movement, which will allow more rapid analysis of the motor unit potentials (MUPs). In keeping with this book’s title, “Practical Electromyography,” this chapter will approach the practical aspects of the pediatric electrodiagnostic medicine evaluation.

Development of the Motor Unit

The changes in the motor unit during growth are the basis for the differences between pediatric and adult normative data. Human fetal muscle development was studied in the 1960s, and with the current regulations regarding study of fetal tissue it is not likely to be studied again in the near future. Dubowitz found that muscle fiber diameters prior to 6 months of gestation are 10 to 25 μm, and after 6 months the diameter increases to 20 to 50 μm. He described three phases of muscle fiber development in utero. During phase I, from 12 to 20 weeks of gestation, the enzyme activity is equal in all fibers. In phase II, from 20 to 26 weeks, differentiation between fiber types commences. However, only a small number of type I fibers are present during this period. In phase III, later than 30 weeks of gestation, the fetus shows the same differentiation in muscle fiber types as are found in adult muscle (1).

Evaluation of fetal tissue also revealed that myelination of peripheral nerve axons begins toward the end of the first trimester. Myelination begins between weeks 10 and 15 and is completed by 2 years of age (2). Nerve conduction velocities (NCVs) increase with the diameter of the nerve fibers and with the increases in the distances between the nodes of Ranvier. The internodal distances reach their maximum lengths by age 5 years (2).

It is also important to be familiar with the normal maturation of the MUP as seen with needle electromyography (EMG). Normal infant MUPs are small in amplitude and duration, making it a challenge to differentiate them from potentials found in myopathies. Jablecki (3) and
Sacco et al (4) reported that MUPs are similar in appearance to adults but increase 20% in both amplitude and duration between age 3 months and adulthood. The amplitudes of MUPs in nerve studies in children have been well researched. Nerve conduction velocities (NCV) increase 100 μV to 700 μV (5), with occasional units to 1,600 μV (5,6). The MUP durations range from 2 to 10 ms (6). The electromyographer must have experience to differentiate between the small motor units and fibrillations. One must focus on the initial deflection of the MUP and whether the potential is under voluntary control.

Many infants and children rapidly learn that contracting a muscle causes pain when a needle electrode is present in it. They will cease muscle movement as soon as an electrode is inserted. This further challenges the examiner, who must use normal infantile reflexes (and sorcery) to induce movement in the infant or child. Conversely, when evaluating resting membrane potentials, the child will frequently decide to move. Holding the joint in such a position that the muscle is at its shortest length is helpful in diminishing voluntary contraction (7).

Nerve conductions in children have been well studied. Motor and sensory NCVs increase with postconceptual age. Bougle reported that NCVs in preterm infants (≤31 weeks of gestation at birth) are about half that of adult normal values (usually about 25 m/s) and that the lower levels of adult normal values are reached by age 4 years (8). Full-term newborns have also been shown repeatedly to have NCVs half the adult normal values, and the NCVs also reach low adult normal values by age four years (2,8,9,10,11,12,13,14). Premature infants have been shown to have slower NCVs than full-term babies. The motor velocities are correlated with postconceptual age and are not affected by intrauterine growth retardation (13). A small study suggests that sensory NCVs are sensitive to growth retardation and cannot be used to determine the postconceptual age, which is possible with motor nerve conduction studies (8,14). Thus, it is possible to determine the postconceptual age of a small baby by motor nerve conduction studies. Gestational age is estimated by combining the velocities of the ulnar and posterior tibial nerves, as this is thought to be more accurate than that of a single nerve (8). Care in the positioning and measurement of the distances is critical. Skin temperature must be controlled (8). The technique is rarely if ever used as it is expensive and time-consuming. Alternative methods, such as the Dubowitz maturity scale, have been adequate for clinical purposes.

Setting up the EMG Laboratory

Once the physiology, growth, and development are mastered, the pediatric electromyographer must have appropriate space, instrumentation, and personnel to make an accurate diagnosis. The pediatric electromyographic laboratory should be large enough to accommodate a standard wooden plinth, electromyographic instrument, chairs, cabinets, warming lamps, sphygmomanometer, oxygen saturation instrument, and a table. The space must come equipped with a sink, suction, oxygen, and a telephone. Electrical shielding is often not necessary with modern instruments, but with the small distances encountered in pediatric patients, it is very helpful. Likewise, a telephone may not be needed in an adult laboratory, but if the physician is paged and is alone in the laboratory, he or she cannot leave the child to answer.

The pediatric laboratory must have space and seating for two or more parents or caregivers. There must be room for a nurse to help sedate, monitor, and distract the child. A resident or fellow will often be present as well. Additional room for a crib or wheelchair will smooth patient flow by preventing the need to move furniture. Blankets and gowns for different age groups should be stored close by. An otoscope, tongue blades, and stethoscope must be available for pre-sedation examinations. Of course, some means of cleansing toys and documenting this work must be developed. Scissors are needed to cut disposable electrodes into infant or small child sizes. The pediatric laboratory must have medicine cups and oral syringes available to administer sedatives and analgesics. Standard measuring devices and a calculator (to adjust medication doses) must be present as well. One must not forget a step-stool to help children reach the plinth.

Cupboards for toy storage are helpful because tubs of toys on the floor become cumbersome. Toys are necessary for distraction during the history but are just as valuable during the physical
and the electrodiagnostic examination. A series of “cubbies” with toys for different ages would be ideal. Bubbles make a mess but are good distraction for most young children. Contact your hospital’s Child Life Department; their members will be very helpful, as one of their goals is to help children through difficult procedures. A TV and a radio are beneficial, especially if they use batteries, as extra power cords create electrical noise. Older children and adolescents sometimes respond well to music CDs with headphones.

To become more specific regarding equipment, the pediatric EMG instrument must have the ability to deliver 20, 30, and 50 Hertz repetitive stimulation, as the diagnosis of botulism is encountered routinely in most pediatric laboratories. Be wary of the sales representative or purchasing department who assume that because you are studying children you will not need the capability for sophisticated testing. The less expensive instruments may not offer rapid repetitive stimulation. The sales representative and the purchasing department will try to make the most economical choice and during price negotiations may unknowingly acquire an instrument that is inappropriate for your needs.

Small, slender needles, 25 mm in length, are most frequently used, although the adult standard lengths of 37 mm are routinely used with adolescents. Both concentric and monopolar needles of small gauges must be available. The laboratory must carry electroencephalograph abrasive paste and alcohol swabs for cleansing. Cleaning the skin with the abrasive paste will decrease the impedance, which is helpful when dealing with the small stimulator-to-electrode distances encountered with pediatric patients. Disposable electrodes with embedded gel are almost a necessity for a pediatric laboratory. They are less likely to shift position than do simple metal electrodes, which require electrode gel and tape. Self-adhering electrodes are easier to use, so their use shortens the length of the examination, which secondarily allows the child to tolerate a more thorough study.

In summary, a large space in an electrically quiet room, but with a TV, telephone, and cupboard storage, is secondary only to the choice of appropriate instrument and electrodes. A skilled nurse is invaluable in teaching, sedating, and reassuring patients and parents.

Techniques to Approach the Pediatric Patient

The patient’s history, as with all medical evaluations, is critical. Physicians must ask about the birth history and development. Questions regarding whether a child is slowly progressing in development or actually regressing will often narrow the differential diagnosis. Age of presentation is helpful, as well as the reason for the first physician visit. Slowly progressive, painless disorders are often picked up by teachers or coaches when the child cannot keep up with peers. Extended family members who do not see the child except at holidays may also bring slowly progressive diseases to the parents’ attention.

Painful diseases are noticed immediately by parents. Questions regarding a child’s choice of activity after school can be helpful. Chronically weak children will not choose fatiguing activities. Parents may notice changes in their children’s choices, but this history is rarely offered spontaneously. Specific questions must be used to elicit information regarding changes in a child’s endurance and strength that parents may not be aware of initially. Inability of the child to enjoy field trips or visits to the mall is worrisome. Subacute presentations will be revealed in normally athletic children having difficulty participating in their chosen sport. Congenital problems usually prevent children from choosing vigorous sports. Once again, this history can direct an examiner toward a specific item in the differential diagnosis.

Physical Examination

The physical exam is a challenging portion of pediatric electrodiagnosis. Observing children in a parent’s lap and watching them play with toys is very helpful. I try to watch the child while getting a history from the caregiver and then proceed with watching him or her crawl, walk, and run. Playing with toys will allow examination of coordination and strength. Sometimes we are lucky enough to be able to watch the child walk to the waiting room before he is even aware of us. Obtain as much of the physical examination as possible before touching a pediatric patient, because from that point on, the
examination may reveal only what can be obtained from a crying and kicking child.

Observation of children’s movement patterns is critical since it substitutes for the child cooperating with the examiner during the physical examination. Children move in specific patterns when sitting down and arising from the floor normally, and these patterns differ in the presence of weakness. The most familiar example is the Gower’s sign, when a child will walk his hands up his legs when arising to stand. Though associated with Duchenne muscular dystrophy, this phenomenon occurs with any chronic proximal lower limb weakness. A quarter-turn from a sit to a prone four-point position is a more subtle hint and frequently occurs prior to the development of a full Gower’s sign. If you ask a chronically weak child to stand up from the floor, he or she will crawl over to a chair or a parent’s leg to pull up to stand rather than fall. In contrast, a child with a new-onset weakness will fall. This is often helpful to determine whether to proceed down a differential diagnosis for acute-onset abnormalities versus longstanding weakness that has just recently been noticed.

Children with weak necks will prop their occiput upon their shoulders; they appear to have lost their necks. This is seen in young children, as their heads are bigger in proportion to the rest of their body than are adult heads. Children will easily teach themselves to walk their fingers up a wall to turn on a light switch when they have weakened proximal upper limb strength. Children will also accommodate for weakness by using “primitive reflexes” in a functional manner. Children learn to use an asymmetric tonic neck reflex to facilitate elbow flexion. When in a supine position a child will reach out for a toy while looking toward it, but will look toward the opposite shoulder when raising the toy to his or her chest. Watching a child sit up from a supine position is also helpful. A weak child will rotate his body around so his shoulders will come behind his pelvis for stability.

The Electrodiagnostic Exam

As with an adult examination, planning is critical to efficient performance of the procedure. Sedation is a controversial and important subject in this planning. An examiner will develop a sense for when distraction and verbal reassurance will be sufficient or whether sedation will be necessary. If sedation is going to be used, it is more effective to use it from the outset than to start it after a child has become agitated. It helps some children and their parents tolerate a more detailed examination than is possible without it. Parents know their child’s ability to tolerate procedures and are often the best source to consult if there is any question regarding the use of sedation. A pediatric laboratory must have medicine cups and oral syringes available to administer sedatives and analgesics. A policy and procedure for sedative use must be developed and followed to ensure safety. New pediatric electromyographers must become familiar with the conscious sedation protocol for their hospital and make sure that they have any certifications necessary. Some hospitals require pediatric life support (PALS) training, and specific medical staff privileges are necessary. Our pharmacy has put together a “tackle box” with our medications and documentation forms that our nurse signs out prior to each EMG clinic.

Sedation has the unfortunate potential to cause disinhibition and can occasionally make a child more easily agitated. I have found this to be more frequent with chloral hydrate (CH) than some other medications. CH is a pure sedative and in my experience does not offer relaxation: the child either sleeps or not. I have found the combination of lorazepam (0.07–0.1 mg/kg PO) and acetaminophen with codeine (10 mg/kg of acetaminophen and 1 mg/kg of codeine PO) in liquid form to be the most helpful. It reduces stranger anxiety, and children will often relax during the set-ups between studies. Nasal midazolam provides excellent anxiolysis for short studies, but its duration of action is not long enough to do a thorough study. The nasal administration of midazolam causes nasal burning, and rectal administration can be used as an alternative route with similar doses. The oral route is also available.

Occasionally, general anesthesia is useful when there is need for extensive nerve conduction studies in cases of complicated peripheral injury or when performing repetitive nerve stimulation. Usually these children will have significant anxiety when the physical examination is done. Obviously,
one cannot evaluate MUPs under general anesthesia. Anesthesiologists will usually be willing to use heavy conscious sedation for examination of MUPs and then put the child under general anesthesia to complete the nerve conduction studies. This technique is used infrequently, but occasional children have an aversion to needles or have been so traumatized by frequent and painful procedures that they cannot be studied while awake.

Technical Considerations

An electrically silent room, preferably with shielding, is helpful. All accessory electrical equipment should be unplugged and on a battery to prevent 60 Hz interference. If this is not possible, electrode wires need to be as short as possible and shielded. Needles can be monopolar or concentric. It is generally thought that monopolar needles are less painful, but short concentric needles used for small children are also very narrow in gauge. I do not believe they are more uncomfortable than monopolars, but this is a personal opinion based upon trial and clinical experience. Concentric needles cause more bleeding, and some children are very upset by the sight of blood, but the electrical baseline is so much quieter with a concentric needle that the examination can frequently be accomplished in a much shorter time.

In an intensive care unit (ICU) setting, concentric needles are usually necessary. ICUs have transport monitors available that run on batteries, and these can be used to diminish electrical interference. Respiratory therapists can also be asked to bag-ventilate a patient during an EMG. The ventilator should be shut off and disconnected from the AC source if interference continues to be a problem despite the use of the 60 Hz notch filter. Despite all these efforts, however, interference from equipment in neighboring cubicles is often a problem, and it is always easier to transport the child to the EMG laboratory if the child is stable enough.

Instrumentation and Set-Ups

In young children, distances are shorter between the reference and active electrodes than they are in adults. This is also true for the distances between the distal and proximal stimulation sites. Not only does stimulator interference increase with shorter distances, but shorter distances also introduce greater measurement error. An error of 1 cm in measuring an 8-cm latency introduces a 15% error in the NCV (3).

Sensory nerve conduction set-ups in adults are designed to have 4 cm between the reference (E2) and active (E1) electrodes. The latency measured to the first take-off varies less than the peak latency measured to the peak as the distance between the E1 and E2 electrodes narrows (15,16). On infants and young children, it is often impossible to obtain a distance of 4 cm between the electrodes; therefore, it is critical to evaluate the latency to the take-off as well as the peak.

Sensory nerve conduction responses are more easily obtained in children if the skin is prepared prior to starting the procedure. Rubbing the skin with a clean emery board or rubbing with EEG paste and then cleaning the skin with alcohol to remove oil and lotions increases the ability to record reproducible sensory nerve action potentials (SNAPs) by decreasing the skin impedance. Another trick is to hold the foot or hand while stimulating with the other hand. This enables the examiner to determine when the child’s foot is relaxed so that the stimulation can be given when there is less background electrical interference from the patient’s voluntary activity. This takes some practice to feel. An alternate approach is to use the auditory feedback from the surface EMG signal to determine the amount of voluntary motor unit activity.

Temperature

Attention to temperature is critical when examining children. Babies have a greater surface area per kilogram than adults, which facilitates heat loss. The room must be warm and the child must be covered whenever possible. Heating lamps are helpful to maintain body temperature in infants as well as warming limbs of older children prior to nerve conduction studies. Heating lamps must be turned off and preferably unplugged during the actual examination, however, as they cause a
great deal of 60-Hz interference. Pressure-activated heating pads can be helpful for local warming.

Control for skin temperature improves the accuracy of motor and sensory latencies as well as that of NCVs. Studies in adults have shown that NCVs change from 1.4 to 2.4 m/s/°C in the upper limbs and from 1.1 to 2.0 m/s/°C for the lower limbs (16). One may assume that temperature will affect the NCVs in young children, but the actual amount is not known. It is therefore critical to maintain a skin temperature close to the skin temperature of the normal children studied when the pediatric values were developed. Correction factors to adjust for skin temperature changes have not been developed for young children.

Volume conduction must also be considered, as it causes decreased latencies (16), which will affect the NCV calculation. Care must be taken not to overstimulate when trying to obtain a supramaximal compound motor action potential. The smaller size of our patients makes this error an even greater concern than it is in adults (see Fig. 3-22B).

Normative data are essential in interpreting electrodiagnostic studies. It is difficult to collect such data due to the reluctance of both parents and physicians to cause discomfort to children. We must thank investigators who have published the data that they have collected. Careful attention to their protocols regarding electrode set-up and distances used is necessary to use their data clinically. It is best to refer to the original papers published to ensure the closest reproduction of their techniques possible (2,8,9,11,16,17,18). An invaluable reference is the chapter entitled “Pediatric Electromyography” in Neuromuscular Disorders of Infancy, Childhood, and Adolescence: A Clinician’s Approach (15), in which the authors collected normative data for children. From this list, the original articles can be found to determine the specific protocols for the normative data collected.

Clinical Problems

As in adult laboratories, there are groups of clinical problems that arise frequently. Children are frequently referred in the first year of life for evaluation of brachial plexus injuries, floppy tone, and decreasing strength. Less frequently, a child will be referred with regression of milestones in both cognitive and motor spheres.

Brachial Plexus Injury

Babies with brachial plexus injuries are frequently referred for evaluation during their second or third week of life. Injuries occur perinatally and are usually of the Erb type, an upper trunk injury (19). In more severe injuries the entire plexus will be involved. The classic Klumpke presentation of a lower trunk injury is rarely seen. The majority of Erb palsy injuries recover within the first 4 months (19,20), although Pondaag’s review (21) has refuted this premise. All EMG laboratories are referred a selected population of the more severe injuries. Microsurgery continues to be controversial, but children with the global brachial plexus injuries and a group of the severe classic Erb (C5–6 ± C7) will have a poor prognosis without surgery (21,22,23). The electrodiagnostic studies will be helpful to determine which babies are more likely to have a spontaneous recovery. The parents’ history of the child’s movement and clinical observation are primary sources of information. Use of the Moro reflex is very useful to activate the muscles innervated by the upper trunk, including the shoulder abductors, external shoulder rotators, elbow flexors, and wrist extensors. If significant motion is elicited, an electrodiagnostic study may not be indicated, as spontaneous recovery early frequently occurs. Heisse et al (24) recently published a study that determined that the compound muscle action potential (CMAP) ratio (CMAP of the affected side as a percentage of the CMAP on the unaffected side) was predictive of motor prognosis.

The infant brachial plexus study starts with evaluation of the amplitude of the CMAPs of the axillary, radial, and suprascapular nerves. The stimulations are done from a supraclavicular position and responses are quite easy to elicit. If the amplitudes are questionable, then the contralateral side is stimulated for a comparison of amplitude and latency. Normal values for amplitude can vary up to 50% from side to side (24), but as Heisse reports, other normal values are not readily available (24). Sensory nerve conduction studies were previously thought to be necessary, as a normal response was considered diagnostic of a
root avulsion. However, it has been found that they are not always helpful, as their absence does not exclude a root avulsion (23). A needle EMG is also necessary to identify active MUAPs. The electromyogram for a brachial plexus study is started within the sensory distribution of the axillary nerve. Babies with a significant upper trunk lesion cannot feel within the sensory distribution of the axillary nerve, so examination of the deltoid can be accomplished without discomfort to the child. It is, in fact, a good clinical piece of information if the child feels the electrode insertion, as it implies a less severe injury. Once again, if the electromyographer has any question regarding the size and recruitment of the MUAPs seen, then the child’s contralateral arm can be used as a control. The clinician should examine muscles that are obviously weak. Those muscles that are questionably weak will probably recovery spontaneously. The prognosis and the discrimination among children who may benefit from surgery and those that will not are the goals of the study. Repeat studies can be done to see whether reinnervation is occurring electrically, if there is question left after the clinical examination. The preoperative study will not need to completely determine what surgical procedure is necessary, as an intraoperative study will be done to direct the surgical decisions.

I have refused to do repeat electrodiagnostic studies for medicolegal reasons, as I do not believe it is ethical to hurt a child for that purpose. Prediction of disability is based upon a clinical evaluation of function rather than upon an electrical analysis. Those examinations must wait until the child’s neurologic function has had a chance to stabilize.

The Hypotonic Infant

A frequent referral to a pediatric EMG laboratory is a request to evaluate a “floppy infant.” Since DNA testing is now available for many congenital syndromes, the frequency of these referrals has dropped.

Once again, a thorough history and physical examination should direct the examination. Primary central nervous system dysfunction causes 80% of cases of newborn hypotonia (13,14,25). A history of a difficult delivery frequently accompanies these children. Knowledge of the normal patterns of development is again necessary in the performance of the physical examination. Examination of muscle stretch reflexes and of distal motor control versus proximal motor control helps to guide the electrical examination. Children with central hypotonia almost invariably have muscle stretch reflexes. A spontaneous and persistent Babinski response is abnormal. All newborns have a positive Babinski response unless they are under influence of maternal sedation or they have severe weakness secondary to a peripheral neuropathy or another lower motor neuron lesion. Lower motor neuron paraplegia is usually secondary to myelomeningocele, but tumors are also possible.

Evaluate the alertness of the infant. Children with central hypotonia are usually not as alert and tend to have obligatory or persistent primitive reflexes. Distal strength is more difficult to appreciate in infants than in older children, as distal hand control does not develop until the latter half of the first year. Distal ankle strength in children is usually examined during observation of gait, so special care must be taken to look for it in an infant. Most infants examined for a peripheral neuropathy will have generalized weakness; the weakness is just more profound distally than it is proximally.

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