Electromyography in Pediatrics




Keywords

Electromyography, pediatrics, neural conduction, infant, neonate

 




Introduction


This chapter discusses the use and techniques of pediatric electromyography (EMG). In a comprehensive text on neuromuscular disease such as this, targeted towards clinicians in neuromuscular medicine, the intention of this chapter is not to make readers experts in the art of the EMG, but rather to present a brief summary of the use of EMG in children to foster awareness of what is possible. In that way, readers will be able to anticipate what they are going to expose their patients to.




Dealing with Misconceptions


It may seem strange to begin a chapter on a diagnostic investigation by discussing the misconceptions that exist regarding its use. This kind of discussion is unlikely to take place when discussing muscle biopsy, muscle MRI, or genetic testing, but it is a significant problem for EMG. Some bodies of opinion in the neuromuscular world hold that EMG has no role in the future, that it has perhaps gone the way of the myelogram, which has been superseded by MRI. This is disappointing, because EMG provides, very quickly and with, in reality, a minimum amount of discomfort, information that can have a major influence on the management of children with neuromuscular disorders. The misconceptions held fall into one of three categories: first that it is too painful, secondly that it is too difficult, and finally that it is redundant in the era of molecular genetics.


It Is too Painful


Most pediatric electromyographers have experienced the referring practitioner telling parents their child was about to have “the most painful test they will have ever experienced.” This is an extraordinary thing for any colleague to communicate to a parent, but paradoxically the fact that they consider sending the child for such an appalling investigation suggests that they, perhaps subconsciously, realize that the test has an important role in the investigation of the child. It is not hard to imagine the kind of anxiety this produces in the parents and child and the effort needed to persuade them of the benign nature of the investigation.


In considering where this misconception arose, one has to acknowledge that the first pioneers of pediatric EMG, in the early 1970s, were working in much less favorable conditions. The initial needles used were far larger in diameter than those used currently and were reused multiple times. Those needles became blunt after repeated use and, despite admonishments to sharpen them, it was often difficult to do so. To add further difficulty, inserting the needles repeatedly often meant that they would bend and become permanently bent with the possible risk of breakage. This was particularly a problem if they were used in children. Today, disposable needles are universally used in the Western world. However, larger-bore needles are still in widespread use in adult EMG laboratories, perpetuating misconceptions about how the testing is performed on children in the hands of a skilled and thoughtful examiner.


The way we regard children has changed a great deal over the years. In England and the United States, the traditional view was that children were to be “seen but not heard,” but enlightenment regarding the rights of children has changed that attitude considerably in recent decades. Children are very much allowed to be heard, and if an investigation is uncomfortable for them we should recognize that and make strenuous efforts to diminish the discomfort. There is also now a realization, rather reluctant, that some of the discomfort produced in children resulted from inadequate knowledge among the people performing the pediatric EMG. There was a lack of appreciation of how children react and of the diseases they suffer. As an example, the correct investigational strategy to be used when discovering anterior horn cell disease in children depends on a differential diagnosis between whether the condition is segmental or generalized, which is quite different from encountering these findings in adults where the diagnostic question often involves motor neuron disease versus cervical and lumbar radiculopathy. In children, this question can be answered straightforwardly by sampling as few as two muscles.


So what is the reality with regard to the pain of pediatric EMG? Some studies have tried to answer this question. At Great Ormond Street Hospital in London, an informal analysis asked participants to rate the pain on a sliding scale after the examination had been completed, comparing it with blood tests. We also included venous cannulation as a further comparison, but the numbers for that comparison were less. To our satisfaction, the results of this unpublished analysis showed that the discomfort rests somewhere just above a blood test and a long way short of the cannulation. As a further observation, it was interesting to talk to parents about the stress of other maneuvers, some of which would not immediately be associated with discomfort. Among that list is the insertion of a nasogastric tube, which causes huge discomfort to both parents and children alike and is one of the major factors that upset children when lying down for investigations such as bulbar EMG and single fiber EMG. Since the first of these tests is often requested when a child is having feeding difficulty, it is important to realize the association and to be able to reassure parents.


What can one do about the pain of EMG? A variety of strategies is available to the practitioner. The first perhaps is the use of local anesthetic. This is now commonplace for nearly all children having blood tests and some very effective local anesthetics exist, some based on amethocaine and others using xylocaine as the active agent. The problem with their application in EMG is that in many ways the needle EMG, however it might be regarded by the parents or child, is not really the most painful experience. Motor nerve stimulation can be more painful. Perhaps because needle EMG takes longer than the motor stimulation and the child often becomes more upset, it is hard not to feel that local anesthetic would help. Of course, it only helps the passage of the needle through the skin, which if done skillfully only takes a matter of milliseconds.


Next there is a tendency to use sedation, sometimes termed conscious sedation. The problem of sedation, which is well recognized, is that a child who is asleep will be awakened by any painful experience and this situation is no different for a child who is sedated and having EMG. To achieve a degree of analgesia with sedation is only possible when you approach an anesthetic application. Clearly this will have important implications with regard to respiratory depression and inhalation and other serious complications. It is quite possible for children to be nearly anesthetized and then left to recover from their sedation with no awareness of how severely depressed they are. So, if sedation is used it is mandatory that care is taken to ensure there is adequate training for staff and adequate monitoring. Resuscitation equipment also needs to be readily available.


The next stage in the escalation of methods of reducing the discomfort is the use of inhalation anesthetics such as nitrous oxide. This in many ways is a logical step, as it is an anesthetic with significant analgesic effects, which are perhaps more than its sedative effects. It is perhaps salutary to remember that there are requirements for trained personnel to deliver this and significant dangers if the equipment is not maintained. During the administration of nitrous oxide to mothers in labor, they are required to clasp the mask across their face; when the anesthetic reaches a certain level the sedative effects cause the mother to then be unable to hold the mask against her face. This is a built-in safety measure, which prevents overdose. Unfortunately, children will not cooperate in this technique as well as adults and therefore the mask has to be held on the face by an anesthetic assistant. There are dangers if attendants do not realize how deep the children are.


At Boston Children’s Hospital, propofol general anesthesia has been used for EMG for over a decade. This medication is generally safe in the proper circumstances, which typically include monitoring by trained anesthesiologists and/or nurse anesthetists with cardiorespiratory telemetry in a properly equipped day surgery procedure room. The only significant effect on the quality of nerve conduction studies is that F responses are typically obliterated by propofol.


The downside of sedation and general anesthesia, if the previous discussions are not sufficient to curb enthusiasm, is that while they help the quality of nerve conduction studies, they have a deleterious effect on the needle EMG examination, as children will not respond to commands and will not contract as much as desired under these circumstances.


None of the above methods is entirely safe, even the use of local anesthetic, as a child may develop an allergy to the anesthetic and, at worst, an anaphylactic reaction, although fortunately this is very rare. This is more common with amethocaine as opposed to xylocaine.


At this juncture, it is perhaps opportune to make a case for not using sedation at all. In order for this to work, one must engage very actively with the children themselves, if they are old enough, and, if not, with their parents. In truth, regardless of the child’s age, it is imperative that the parents be brought on board with the procedure. Clearly it is essential that they not be excluded from the examination room and, indeed, any person accompanying the child should be encouraged to join in. The first step towards engagement is the requirement of complete honesty. There is no point in telling children or their parents that the procedure to follow will produce no feelings of discomfort. Certainly it does produce discomfort and it is important to be candid about this. Reassurance that it is perhaps no worse than a blood test is a good starting point. It is also very important to put all parties at their ease, and the social interactions which would normally be employed when talking to children or their parents outside a medical setting are perfectly acceptable methods. Some might regard this as unprofessional, but it can be very effective. Children will respond after a certain age to questions about what they love to do or their favorite subjects at school, and an anxious child can quite suddenly change and sometimes smile while discussing enjoyable experiences. It is preferable to talk to a child about their dreams of becoming a disco dancer rather than try to calm them by repeated reassurances that what they are experiencing is not uncomfortable, when they clearly disagree. Getting children to talk about favorite subjects can be immensely effective. Many instances of needle-phobic children passing through the EMG without any anxiety have occurred using this approach.


Another invaluable distraction technique is the use of portable electronic devices. Almost every parent will have games or music or even videos on their mobile phone, and many children will come with electronic tablets or similar devices. Whatever amuses them must be employed. Our experience has been that battery-powered devices (i.e. those powered by direct current) generate little to no electrical artifact, whereas nearly any device that is plugged into an electrical outlet (i.e. those powered by alternating current) tends to generate 60/50 Hz interference during the study. Although toys seem to be rather old-fashioned nowadays, a good supply of these is also important.


Another point that is important to emphasize, particularly in the younger patients, is that not all reactions are pain related. A lot of children become angry when they are investigated and asked to do things they do not want to do, even such things as simple as lying down on the couch. Parents recognize this kind of upset and can easily distinguish it from that caused by pain. If they appreciate that the practitioner working with them also understands this, it will firmly establish yet another measure of trust. Many of us have examples of instances when this has occurred. One of the authors (MCP) will always remember a child about two years old, who could only be described as “creating” during the EMG. The noise and anger generated were truly remarkable. Her grandmother, who was of Italian descent, was circling around the doctor, who feared the worst, anticipating a tirade of fury against him for being so unkind to her granddaughter. Instead, all she said was, “The Latin temperament!”


It Is too Difficult


The next misconception is that EMG is difficult. In part this leads from the first misconception, and certainly if the EMG is regarded as a painful experience it is more likely to feel like a struggle. In particular, the EMG practitioner may become frightened of pursuing the examination with adequate determination if they feel they are hurting the child. Whatever one’s beliefs about the discomfort of the test, there is a measure of general acceptance that recording reliable results in children is a more skillful exercise than in adults. There are many ways in which skill can be enhanced, not least of which is being slick but also by determining which tests are essential and not performing unnecessary tests that perhaps only corroborate others that have already been done. Modifications to the equipment are important but one of the major problems in pediatric EMG is training. The number of pediatric EMGs performed in any center is small by comparison with the number of adult EMGs. Even in large referral centers with dedicated pediatric electromyographers, the volumes do not compare to those found in many adult EMG laboratories. Thus, the opportunities for a debutant EMG practitioner to learn from the examination of children, whether compliant or not, are limited for a variety of reasons, the most important of which is that children should not have to undergo repeated investigations because the first one was not performed well. Therefore pediatric electromyographers, unlike in adult practice, cannot leave trainees loosely supervised, no matter how valuable it might be for improving confidence and gaining experience. This means that EMG training in pediatrics is unique among all forms of physician training, as a period of observation of the techniques and strategies is absolutely crucial. In this way perhaps pediatric EMG is more like a surgical speciality than a medical one. At Great Ormond Street the period of observation varies according to the aptitude of the debutant. They are gradually allowed to handle cases on their own, but with a senior member of staff either observing and helping them, or near enough to be able to step in and complete the examination seamlessly if the situation becomes untenable.


Another important point with regard to acquiring skills in pediatric EMG is interpretation of the results. While the recording and analysis of the nerve conduction studies is a skill that can be attained over a reasonable period, that of analysis of the EMG is particularly difficult. A specific difficulty is that the muscle fibers in children are smaller than in adults and increase their diameters over the period of maturation. Thus, for example in infants, a normal EMG pattern can appear myopathic compared to those of older children and adults. The measurement of the motor unit duration is absolutely crucial in these circumstances.


To help trainees acquire the experience needed to distinguish pathological abnormalities from normal findings, one of the best methods is to learn from an archive of previously studied cases. Many EMG systems now allow storage of all the data collected including the EMG. At Great Ormond Street Hospital data from selected studies are archived prior to analysis. These data are graded according to difficulty into either easy , moderate , or difficult . In the meantime the study is reported in the usual way and stored in a patient results database. The trainees select what level they feel they want to study and choose a patient from that category. They are then able to go to the patient results database and bring up the data supplied at the time of the referral. Next they go to the EMG database, assess the abnormality, and then return to the database to see if their conclusions concur with those of the consultant or attending neurophysiologist. If they do concur, then the trainee can move on to another case and simply repeat the process, noting instances where there is disagreement, which can then form the basis of in-depth discussions of key topics. This is an invaluable resource, as even trainees who are assigned to a pediatric EMG laboratory for only short periods can still acquire a great deal of knowledge. Regardless of how busy the clinical service is, it is certainly possible for a keen trainee to review the data from upwards of 20 cases in a morning. The benefits to their skill and confidence are hard to overestimate.


EMG Has no Future in the Molecular Age


The impact of molecular genetics on all diagnostic tests has been considerable. One of the reasons that it has not been used perhaps to its fullest extent is the cost of the sequencing and the intensity of the data storage and analysis required. As with many technological advances, this cost will come down with time. Next-generation sequencing technologies are already being used in clinical practice, especially targeted sequence capture panels, which are less expensive and more efficient than traditional Sanger sequencing panels. There are some concerns about the consistency of the depth of coverage for exome sequencing to be used routinely in clinical practice, but exome sequencing will soon become a widespread clinical resource as well, followed by whole genome sequencing. However, these powerful new techniques do not eliminate the dreaded “variant of unknown significance” seen in genetic test reports. Such ambiguities will, if anything, proliferate as more genetic data are generated for each patient. Accurate phenotype information, including EMG, will thus continue to be crucial to help interpret genetic test data.


One common example seen in pediatric EMG laboratories is the child with pes cavus in whom the question of Charcot-Marie-Tooth disease has been raised. There are clinical findings that can help predict whether the child has a polyneuropathy but in many instances an EMG is crucial to determine definitively whether this is the case and, if so, whether the neuropathy is demyelinating or axonal. Unfortunately, because genetic tests are promoted as being less invasive and painful than studies such as EMG, children with pes cavus or toe-walking sometimes undergo an expensive genetic test that reveals a variant of unknown significance in an obscure Charcot-Marie-Tooth disease gene, only to be found later by a pediatric neuromuscular specialist and/or pediatric electromyographer not to have a polyneuropathy at all. Similar situations occur when congenital myasthenic syndrome and various channelopathies are considered in the differential diagnosis, as valuable data regarding the presence and likely subtypes of these disorders can still be obtained more readily from EMG testing than from other sources. It is salutary to note that in a highly specialized congenital myasthenia clinic run at one of our hospitals, some children whose condition has defied genetic classification, when eventually referred for assessment of the neuromuscular junction, are found to have normal jitter. It is therefore highly likely that, while the role of EMGs will continue to evolve, it will still have an important function in defining the phenotype along with clinical examination. Another way of looking at this might be that its role will change from “phenotype down” to “genotype up” analysis, as available genetic data become more widespread among patient populations.


EMG will also remain central to the diagnostic evaluation of children and adolescents with acquired neuromuscular disorders, including inflammatory/autoimmune, toxic, and traumatic insults. These include Guillain-Barré syndrome, chronic inflammatory demyelinating polyradiculoneuropathy, myasthenia gravis, toxic neuropathies and neonatal brachial plexopathies.


Lastly, more clinical research is being conducted on diseases such as Charcot-Marie-Tooth disease and spinal muscular atrophy, and EMG will likely be an important outcome measure. A specialized nerve conduction study technique, motor unit number estimation, is being used routinely now in clinical research on spinal muscular atrophy.




Normative Data


It is a fundamental problem, and it does not matter what parameter is measured, whether in clinical neurophysiology or in any other quantitative speciality, that the dataset of normative values available is never truly comprehensive. Factors that inhibit the collection of rigorous normative data for pediatric EMG include the cost, ethical issues (it is hard to imagine a modern institutional review board approving such a study in children and hard to imagine that many young children and their families would volunteer), and the many variables that would have to be accounted for. If we look at, for example, nerve conduction it is recognized that such varying factors as handedness, height, age, and sex might all influence the measurement. Temperature is well recognized as being something that affects velocity and yet the truth is that over the range of temperatures seen in most laboratories, its influence most probably is minimal, particularly as the relationship is nonlinear with little if any change over the normal temperatures encountered in the laboratory. Yet this is something we all correct for, perhaps just because we can. However, when considering some of the factors that we do not correct for, again returning to conduction velocity, it is almost certain from our experience in a central London hospital with a wide range of ethnic subgroups that the Caucasian population has the slowest conduction velocity of any group. The velocities experienced by one of the authors (MCP) when visiting Bangladesh were truly astounding when compared with the normal population in the United Kingdom. Trying to cover every single parameter with every single factor that might affect it could, if one is not careful, become a life’s work.


Some medical specialities have abandoned the slavish collection of normative data. For example, when a new sequence is devised in MRI it is uncommon for the findings in normal populations to be studied. Instead, what is more likely to happen is that the reporting clinicians make a judgment based on previous data and also the incidence of disease being studied as to what is likely to be abnormal and what is likely to be normal. Experience in these circumstances is the absolute key.


A further problem, perhaps peculiar to children, is that collecting any data on normal children by techniques perceived as being uncomfortable (even if not as painful as some would have it) is highly unlikely to be granted ethical permission. Fortunately, we are lucky to have some historical data, which is well validated, including the studies on nerve conduction velocity changes with age. There are many different studies showing the increase in conduction velocity over the first years of life and, although there are many different sources, they all roughly show the same change and are a useful basis for determining normal velocity. Another group on which exhaustive studies have taken place is the motor unit duration values that were first produced by Buchthal and his team in Copenhagen. This was truly a gargantuan piece of work, with individual muscles studied in innumerable normal subjects from very early in life to old age. Unfortunately, this data had every danger of being redundant as it was performed using the standard EMG needle, as the facial needle was either not available or not used in the study. Pediatric electromyography was the particular beneficiary when work by Sanjev Nandedkar and colleagues discovered that the facial needle when used to measure the duration of the motor unit produces a value which is very close to that produced with either a monopolar needle or standard concentric needle but there are significant differences with regard to the amplitudes recorded. This means this precious resource is available for pediatric electromyography to use, hugely improving the accuracy of assessment of conditions such as myopathy and neurogenic change.


However, not such a rosy view can be taken of other normative data. One of the particular problems is amplitude of both the compound muscle action potential and also the sensory nerve action potentials. It is one of the mysteries to all collected wisdom in EMG that, while the EEG world made as its top priority, at the inception of the speciality, standardization not only of the position of the electrodes but also the dimensions of the electrodes themselves, the EMG world did no such thing. EEG is a specialty where amplitude, which is affected by the size of the recording surface, is not as crucial as it is in the nerve conduction studies and yet standardization across the whole specialty in EMG and nerve conduction studies has not yet taken place, even if some laboratories have their own internal standards. Measurements therefore exist but if the recording system used is not similar to your own, the results are essentially unusable. To make the problem worse, in publications detailing the recordings of the amplitude measurements, the exact dimensions of the recording electrodes may not be given.


If there are not enough problems already in the measurement of amplitude, the range of amplitudes encountered between subjects may not be appreciated. To take one example, in certain ethnic groups seen in London, median sensory amplitudes reaching as high as 160 μV have been recorded regularly using an orthodromic palm-to-wrist technique. However, in other ethnic groups without any evidence of peripheral nerve damage values of 30 to 35 μV are more common. This means that a sensory nerve action potential that reaches a modest 40 μV might be passed as normal in one of the former group, with the tester completely unaware that this is perhaps one quarter of what it should be. In fact the child may have lost 75% of their sensory fibers.


From what has been said previously, it is obvious that the interpretation of results is very difficult indeed. It would be wonderful to be able to look at the single criterion and say this is abnormal, but this is an extremely dangerous technique. All of us in pediatric EMG, perhaps no more than our adult colleagues, recognize that only if there is a concordance between our results are we able to accurately assess individual measurements, but only in that context. Needle EMG has been described as being a technique combining auditory pattern recognition with semiquantitation, but there are elements of this approach that apply to the whole electrodiagnostic examination. It is important to remember this. Unfortunately—and this is one of the major difficulties with the specialty and not the first time that it has been said—experience is key.




Techniques


Technical advances have improved the quality of data obtained from pediatric EMG studies and have improved the comfort of patients who undergo this testing. First and foremost has been the huge improvement in the recording electrodes, which are placed on the surface. The days when every laboratory had its own homemade recording electrodes are rapidly disappearing. Now there are single-use self-adhesive pre-gelled electrodes, which are high quality at a relatively low price. The recording area is of standard size; it is possible to cut down the whole electrode for use in infants without affecting this, but it is often not necessary even in the smallest limbs ( Figures 3.1 and 3.2 ). Stimulating electrodes also are now better than those formerly used. Unfortunately, there is a tendency to produce what can be best described as “cattle prod” electrodes, large devices often with an intensity control within their handles, which are placed on the limb. While these are probably acceptable in an adult environment, where the patient can be relied upon to remain still, in a child environment they are not always practical. One of the authors (MCP) has a personal preference for using somewhat older electrodes, which allow the limb to be encircled at the same time as delivering the stimulus ( Figure 3.3 ). Even if the child should move, the relationship between stimulus and the nerve is maintained and the test can be performed successfully.




Figure 3.1


Ulnar nerve motor studies recording from abductor digiti minimi. Stimulation at the wrist.



Figure 3.2


Stimulation above the elbow.



Figure 3.3


Stimulation of the median nerve at the wrist recording from abductor pollicis brevis for motor studies demonstrating the use of the stimulating electrode.


The needles have been touched on already, but they are now very high quality, so sharp that just the weight of the electrode cable will allow their passage through normal muscle. For one of the authors (MCP), a key factor in success when performing stimulated single fiber EMG (StimSFEMG) has been that it is now possible to obtain the stimulating electrode at 15 mm lengths rather than the standard 30 mm, which means that the electrode will remain in position without having to be held ( Figure 3.4 ). This leaves the practitioner with both hands free to manipulate the recording electrode. For some time now, this procedure has been performed with facial concentric needle electrodes using various manipulations of the low frequency bandpass to produce something that approaches, but is not exactly, single fiber electromyography. The single fiber electrodes are now being produced as single-use devices, with the price coming down as the technology improves. This development would be a huge advantage for the continued use of StimSFEMG, but the lack of availability of these needles has not been the only reason facial needles are preferred. The problem particularly relevant in children is that, because the recording area is set back from the needle in a child or particularly a baby with a very thin muscle, it is very possible to be in the muscle itself or even through it, but not have the recording surface in the muscle. With the facial needle, where the recording surface is at the leading edge, this is not a problem. Earlier, when using reusable needles it was feasible to sand down the needle to shorten the distance between its tip and the recording area, but clearly this is not possible when using single-use electrodes, which are not meant to be resterilized.


Jun 25, 2019 | Posted by in NEUROLOGY | Comments Off on Electromyography in Pediatrics

Full access? Get Clinical Tree

Get Clinical Tree app for offline access