CONCENTRIC NEEDLE JITTER ON STIMULATED FRONTALIS AND EXTENSOR DIGITORUM IN 20 MYASTHENIA GRAVIS PATIENTS

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1 CONCENTRIC NEEDLE JITTER ON STIMULATED FRONTALIS AND EXTENSOR DIGITORUM IN 20 MYASTHENIA GRAVIS PATIENTS JOÃO ARIS KOUYOUMDJIAN, MD, PhD, 1 ADRIANA CRISTINA DA SILVA FANANI, MD, 1 and ERIK V. STÅLBERG, MD, PhD 2 1 Department of Neurological Sciences, Neuromuscular Investigation Laboratory, Faculdade de Medicina de São José do Rio Preto, Rua Luiz Antônio da Silveira 1661, São José do Rio Preto, São Paulo, Brazil 2 Department of Clinical Neurophysiology, Institute of Neurosciences, Uppsala University, Uppsala, Sweden Accepted 9 June 2011 ABSTRACT: Introduction: Our objective was to study jitter parameters using a concentric needle electrode (CNE) in the extensor digitorum (ED) and frontalis (FR) muscles. Methods: Twenty myasthenia gravis (MG) patients, mean age 44.5 years, were studied. Percutaneous (FR) and intramuscular needle (ED) stimulation approaches were used. Jitter was expressed as the mean consecutive difference (MCD). The filter settings were from 1000 HZ to 10 khz. Results: Abnormal MCD was found in 85% for both ED and FR and in 90% when combining the two muscles. An abnormal percentage of outliers was found in 90% for ED and 85% for FR. The mean MCD did not show a difference for ED and FR, but the percentage of outliers and blocking were higher in FR. Abnormality was found in 93.7% (generalized) and in 75% (ocular) of MG cases. For ED outliers abnormality was greater than the MCD. Conclusion: CNE jitter is reliable for investigation of MG, although borderline findings should be judged with caution. Muscle Nerve 44: , 2011 Myasthenia gravis (MG) is an autoimmune disorder of the neuromuscular junction (NMJ) associated with the presence in most patients of antibodies to the postsynaptic acetylcholine receptor (AChR). In some proportion of seronegative AChR patients, antibodies to muscle-specific tyrosine kinase (MuSK), another protein at the NMJ, can be found. Single-fiber electromyography (SFEMG), a technique developed in Sweden in the early 1960s by Erik Stålberg and Jan Ekstedt 1 3 for measuring neuromuscular jitter, is the most useful parameter, together with impulse blocking, for the electrodiagnosis of neuromuscular transmission, including MG. Jitter represents the variation in time intervals between pairs of single-fiber action potentials (SFAPs) in the voluntarily activated technique or the time measured between stimulation pulse and SFAPs in the stimulated technique. With high jitter values, the neuromuscular transmission is so disturbed that occasional impulse blocking occurs. Abbreviations: AChR, acetylcholine receptor; AChR-Ab, antibodies to acetylcholine receptor; ASFAP, apparent single-fiber action potential; CNE, concentric needle electrode; ED, extensor digitorum; FR, frontalis; MCD, mean consecutive difference; MG, myasthenia gravis; MGFA, Myasthenia Gravis Foundation of America; MuSK, antibodies to muscle-specific tyrosine kinase; NMJ, neuromuscular junction; OOc, orbicularis oculi; RNS, repetitive nerve stimulation; SFAP, single-fiber action potential; SFEMG, single-fiber electromyography; s-jitter, stimulated jitter; ULN, upper limit of normal Key words: concentric needle electrode, extensor digitorum, frontalis, jitter, myasthenia gravis Correspondence to: J. Aris Kouyoumdjian; jaris@terra.com.br VC 2011 Wiley Periodicals, Inc. Published online in Wiley Online Library (wileyonlinelibrary.com). DOI /mus The sensitivity of SFEMG has been found to be 88% in ocular MG and % in generalized MG 4 if any muscle, limb or facial, were tested. In a systematic review, Benatar 5 confirmed a high specificity of SFEMG in the diagnosis of generalized MG, whereas for ocular cases there was more variation, ranging from as low as 66% to as high as 98%. Mercelis and Merckaert 6 found a specificity of 97% and a sensitivity of 80% for ocular MG in stimulated SFEMG for the orbicularis oculi (OOc) muscle. Due to increasing concerns for the transmission of infections, 7,8 disposable concentric needle electrodes (CNEs) have been tested for measurements of jitter. Some studies have presented normative data and the diagnostic value of the test in MG Farrugia et al. 11 found no difference in mean jitter values for extensor digitorum (ED) and OOc muscles in 24 MG patients using both a single-fiber electrode and CNE. To successfully use a CNE for jitter measurement, the low-frequency filter should typically be raised from 500 HZ to 1 or 2 khz to suppress the activity from distant muscle fibers. 7,8 A filter setting with a 1-kHZ high-pass filter, rather than higher, has been suggested for optimal quality. 12 As the signals obtained with CNE recording do not always represent one SFAP, but rather a summation of many, jitter recording with CNE from apparent single-fiber action potentials (ASFAPs) is a more accurate description, rather than SFEMG with CNE. So far, relatively few studies have been done using CNEs in the diagnostic work-up of patients with MG. No studies have used electrical stimulation. The aim of this investigation was to confirm that a stimulated CNE in the ED and frontalis (FR) muscles in MG patients gives results similar to those described with a SFEMG electrode. METHODS Patients. Twenty MG patients recruited from the outpatient clinic of the Medical School (Faculdade de Medicina de São José do Rio Preto, Brazil) were studied between August 2009 and May 2010 for jitter measurement using a CNE. All patients had confirmed MG based on the clinical picture (fluctuating weakness), unequivocal clinical response to 912 Stimulation CNE Jitter in MG MUSCLE & NERVE December 2011

2 edrophonium and/or pyridostigmine, positive acetylcholine receptor antibody (AChR-Ab) titer, or decrement of at least 10% on slow (2 3-HZ) repetitive nerve stimulation (RNS) studies from the first to fourth response. None of the patients had previous jitter measurements. All patients stopped pyridostigmine at least 24 hours before testing. Cholinesterase inhibitors may mask abnormal jitter when the abnormality of neuromuscular transmission is mild. 13 The MG group consisted of 13 men and 7 women with a mean age of years (range years). The mean duration of MG symptoms was months (range months). The mean time since diagnosis was months (range months). The severity of disease was determined at each clinic visit according to the clinical classification of the Myasthenia Gravis Foundation of America (MGFA) from I (ocular weakness) to II V (generalized weakness). 14 In the worst period classifications were as follows: class I, 4 patients; class IIb, 4 patients; class IIIa, 3 patients; class IIIb, 3 patients; class IVa, 3 patients; class IVb, 1 patient; and class V, 2 patients. The classification profile at the time of CNE jitter measurement was as follows: class I, 4 patients; class IIa, 2 patients; class IIb, 1 patient; class IIIa, 2 patients; class IIIb, 2 patients; and class IVa, 2 patients. Seven patients were asymptomatic at this time. CNE jitter recordings and antibody measurements, either to AChR-Ab or MuSK, were studied on the same day. The most pronounced RNS decrements were taken from the files at random timepoints of the MG diagnosis and included several nerve muscle settings, either distal, proximal, or facial. Controls. Twenty healthy subjects, employees of the Medical School (FAMERP, Brazil), were studied prospectively for jitter measurement using stimulated CNE of the FR muscle. None were diagnosed with a neuromuscular disorder, an unrelated medical condition, or were taking medication that could have potentially interfered with the study, such as calcium channel blockers. 15 Reference jitter parameters for the CNE in stimulated ED muscle were taken from our previously published data 16 as follows: upper limit of normal (ULN), 95%, for the mean consecutive difference (MCD) ¼ 22.6 ls and for outliers MCD ¼ 28.7 ls. Jitter Recording. For all studies a electromyographic device (KeypointNet; Medtronic Skovlunde, Denmark) with built-in jitter software was used for recording and analysis, using a peak detection algorithm for time measurements. The recordings were performed using a CNE with a diameter of 0.30 mm and a recording area of mm 2 (this CNE is the smallest facial needle from Medtronic/Alpine BioMed, Denmark). CNE measurement of jitter was performed by the first author (J.A.K.) and, when necessary, the third author (E.V.S.) revised the digital recordings. Stimulation Technique for Frontalis Muscle. The temporal branch of the facial nerve was stimulated with a bar electrode over the nerve. Stimulation frequency was set at 10 HZ. The stimulus was delivered as rectangular pulses of 0.10-ms duration, and the intensity was adjusted to produce a slight visible twitch of the FR muscle. In general, this could be achieved at about 5 7 ma. The stimulus intensity was adjusted to give a minimum number of spikes in the response. For each spike accepted for analysis, supraliminal stimulation for that particular spike was assured. The CNE was inserted in the mid-forehead at variable distances from the bar stimulator and positioned to record clearly defined spike components. To be accepted for measurements, the ASFAP, isolated or summated, should have a fast rising phase without notches or shoulders, should have parallel rising phases seen on superimpositions of consecutive discharges for signals separated by >150 ls, and should have a welldefined peak. Jitter for each of the control subjects was expressed the mean consecutive differences (MCDs) of analyzed potentials. For each jitter analysis, a minimum of 50, and ideally 100, consecutive traces were recorded. The filter settings were 1 khz to 10 khz. Keypoint software also calculated the mean peak latency. Stimulation Technique for Extensor Digitorum Muscle. Intramuscular axonal stimulation was delivered by a 28G, mm disposable monopolar needle electrode (Medtronic, Denmark), inserted near the motor endplate zone, between the proximal and middle thirds of the ED. A 30G, mm disposable scalp needle electrode (Medtronic) was used as the anode and was inserted subcutaneously about 2 cm away from the cathode. Stimulation was delivered at 10 HZ; the stimulus was delivered as rectangular pulses of 0.04-ms duration, and the intensity was adjusted to produce a slight twitch of the muscle. In general, this could be achieved at about 2 6 ma. Other parameters were similar to those for FR muscle stimulation. Abnormal Jitter Parameters. Stimulation jitter was considered abnormal, after technical jitter due to subliminal stimulation was excluded, if: (1) the mean MCD was above the ULN obtained in normal subjects for the FR muscle or as previously described by us for the ED muscle 16 ; or (2) >10% Stimulation CNE Jitter in MG MUSCLE & NERVE December

3 of individual jitter values for the ASFAPs for each study were above the ULN (outlier) obtained in the normal subjects for the FR muscle or as previously described by us for the ED muscle. 16 Statistical analysis was performed with Minitab 15 software (Minitab, Inc., State College, Pennsylvania). MCD parameters (jitter) were analyzed using the mean and standard deviation (SD) for Gaussian distributions, or median and percentage for non-gaussian distributions. To check for Gaussian distribution, the Anderson Darling test was applied. The ULN was set at the 97.5th percentile for non-gaussian distributions and the mean þ 2 SD for Gaussian distributions. Ethics Approval. The study was approved by the ethics committee of the Faculdade de Medicina de São José do Rio Preto, and informed consent was obtained from each subject. RESULTS Controls for Frontalis Muscle. The mean jitter was analyzed according to a previously used method: calculation of the mean MCD of 30 isolated potentials for stimulated jitter (s-jitter). 2 There was no correlation with age (P > 0.5). There were no recordings with impulse blocking. The mean of the 20 MCD (s-jitter) values had a Gaussian distribution (Anderson Darling, P ¼ 0.786) and was ls (range ls); the median was 16.2 ls. The upper 95% confidence limit was 21.5 ls. The mean of all 600 ASFAP MCD (s-jitter) values had a non-gaussian distribution (Anderson Darling, P < 0.005) and was ls (range ls); the median was 14.6 ls. The upper 95% confidence limit was 28 ls. The mean latency between stimulus and each spike was ms ( ms). Patients. AChR-Ab was positive in 17 cases (85%) and negative in 3 (worst MGFA classification: I, IIIa, and IIb, respectively). The mean abnormal AChR-Ab value was nmol/l (range nmol/l). Antibodies to muscle-specific tyrosine kinase (MuSK) were negative in all 3 seronegative MG patients. Antibodies to striated muscle tissue were found in 2 of 20 MG patients (10%) with values of 1:80 and 1:20, respectively. The latter patient (1:20) had thymoma. In a retrospective analysis of the patients, RNS studies were as follows: ulnar ADM in 19 of 20; median APB in 6 of 20; accessory trapezius in 6 of 20; facial OOc in 4 of 20; facial nasalis in 3 of 20; radial anconeus in 3 of 20; and facial orbicularis oris in 2 of 20. Abnormal decrement was found in 16 of 20 patients (80%). Seven patients were in remission (asymptomatic) at the time of jitter and antibody studies. Thymectomy was performed in 9 of 20 patients (45%), and thymoma was found in 4 of them. Nineteen patients (95%) were treated with pyridostigmine from 90 to 360 mg/day. Nine patients (45%) were treated with prednisone: 6 from 5 to 20 mg every other day and 3 from 5 to 60 mg/day. Three patients (15%) were taking azathioprine from 100 to 150 mg/day. Jitter Parameters in Extensor Digitorum. The total number of ASFAPs analyzed was 546, varying from 13 to 34 for each patient (mean ¼ 28, median ¼ 30), except in 1 case in whom only 5 ASFAPs with very high jitter were obtained. The mean MCD was ls (range ls). Abnormality was found in 85%, meaning that 17 of 20 patients had an MCD >22.6 ls (95%). Using the criterion for outliers (see earlier) abnormality was found in 90% of the patients, meaning that 18 of 20 patients had at least 10% of ASFAPs with an MCD >28.7 ls (95%). In 2 of 20, both MCD and outliers remained normal, so the overall abnormality was 90%. The mean latency between stimulus and each spike was ms (range ms). Detailed results from jitter parameters are given in Table 1. No hematomas were found in any of the patients studied; the test duration was about minutes and was more painful than in FR, because more needle insertions and position corrections (stimulation and recording) were necessary. Jitter Parameters in Frontalis. The total number of ASFAPs analyzed was 629, varying from 25 to 34 for each patient (mean ¼ 31.6, median ¼ 32). The mean MCD was ls (range ls). Abnormality was found in 85% of the patients, meaning that 17 of 20 patients attained an MCD of >21.5 ls (95%). Abnormality based on outliers was found in 85% of the patients, meaning that 17 of 20 patients had at least 10% of ASFAPs with an MCD >28.0 ls (95%); in the other 3 patients both MCD and outliers were normal, so overall abnormality remained at 85%. The mean latency between stimulus and each spike was ms (range ms). Detailed results from jitter parameters are given in Table 1. Some recordings from ED and FR, with normal jitter, abnormal jitter, and impulse blocking, are shown in Figure 1. The relationships of jitter parameters from ED and FR and antibodies, worst RNS since diagnosis, and MGFA classification are presented in Table 1. Comparisons of jitter abnormalities between ED and FR from 95% confidence intervals are shown in Table 2. When ED or FR are considered 914 Stimulation CNE Jitter in MG MUSCLE & NERVE December 2011

4 Table 1. Summary of stimulation concentric needle jitter parameters from 20 myasthenia gravis patients in extensor digitorum (ED) and frontalis (FR). Case MCD Outliers ED FR ED FR AChR-Ab (nmol/l) RNS-w (ab 10%) MGFA-p class MGFA-w class <0.10 n I I ab I IIb ab Asymp V ab IIa IVb ab Asymp I ab IIa IIIa ab IIIb IIb ab Asymp IIIa ab IIIa IVa ab Asymp IIIb ab IIb IIIb ab Asymp I n Asymp I ab I IIb <0.10 n I IIb ab IIIb V ab IVa IVa n Asymp IIIb ab IIIa IIIa ab IVa IVa 95%* 22.6 ls 21.5 ls 28.7 ls 28 ls Ab 85% 85% 90% 85% 85% 80% Muscle-specific tyrosine kinase antibody (MuSK) was negative in all AChR-Ab negative cases. MCD, mean consecutive difference (in microseconds); AChR-Ab, acetylcholine receptor antibody; RNS-w, worst repetitive nerve stimulation since diagnosis; MGFA-w/MGFA-p, Myasthenia Gravis Foundation Association classification, worst/present; ab, abnormal; n, normal. *Limit. Percentage above limit (abnormal 10%). FIGURE 1. Stimulation concentric needle jitter in myasthenia gravis patients. (A, B) Highly abnormal jitter (161 and 130 ls) with impulse blocking. (C) Normal jitter (9.4 ls) with fast-rising phase spikes without notches or shoulders and well-defined peak without shape changes in consecutive discharges. (D) Highly abnormal jitter (66 and 48 ls) without impulse blocking. Stimulation CNE Jitter in MG MUSCLE & NERVE December

5 Table 2. Comparison of stimulated CNE jitter parameters in extensor digitorum (ED) and frontalis (FR) in 20 myasthenia gravis patients. MCD ASFAP n Mean Range Ab* n Mean Range Ab ED ls ls 85% ls ls 90% FR ls ls 85% ls ls 85% ED or FR 90% 90% MCD, mean consecutive difference; ASFAP, apparent single-fiber action potential; Ab, abnormal. *Greater than 22.6 ls (ED) and >21.5 ls (FR). Outliers: 10% >28.7 ls (ED); 10% >28.0 ls (FR). together, abnormality was found in 90%, either as MCD or outliers. Jitter Parameters in Ocular vs. Generalized Cases. In a comparison between initial ocular MG (MGFA class I, 4 patients) and generalized MG (MGFA class II V, 16 patients), we obtained values of 38.9 and 67.1 ls for mean ED MCD and 39.8 and 69.1 ls for mean FR MCD, respectively. For outlier frequency, the values were 75% and 93.7% for the two MG types for ED, and 75% and 87.5% for FR. Impulse blocking was found in 21.6% and 31% in ED, and 23.3% and 38.3% in FR. Abnormal AChR-Ab values were found in 75% and 87.5%, and abnormal RNS values in 50% and 87.5%, respectively. Thymectomy was performed in 9 of 16 of the generalized MG patients but in none of the ocular patients. Over a mean period of 74 months (5 288 months), 3 patients were asymptomatic and 1 was unchanged among the ocular cases, and 4 were asymptomatic, 8 improved, 3 unchanged, and 1 became worse among the generalized cases. DISCUSSION Our MG cohort was small and heterogeneous regarding age, MGFA class, time of disease, thymus pathology, use of acetylcholinesterase inhibitors, and ongoing immunosuppressive treatment. Seven patients were asymptomatic (clinical remission). but only 1 was completely free of medication. Jitter analysis and AChR-Ab titer determination were done at the same time after months or years from MG diagnosis. The study was not designed to study sensitivity and specificity, but rather to assess the general feasibility of stimulation CNE jitter analysis in MG. RNS was abnormal in 50% of cases of ocular MG and in 87.5% of generalized MG cases. The results give an impression of the general severity of our patients. An abnormal decrement is found in a hand or shoulder muscle in 75% of patients with generalized MG, and in 50% of those with ocular MG. 13 The AChR-Ab titer was abnormal in 75% for ocular MG (mean 2.85 nmol/l) and in 87.5% for generalized MG (mean nmol/l). In later published reports, the AChR-Ab titer was elevated in 55% of those with ocular MG and in 80% of those with generalized MG. 13 Despite the different techniques and needles and the small number of patients, the general pattern of abnormalities was similar to that in voluntary or stimulation SFEMG studies (for review, see Stålberg et al. 13 ) in a large cohort. In our entire group of patients, ED and FR had abnormal jitter in 90% in either or both, similar what has been described by others 13 who found abnormality with voluntary SFEMG in either or both ED or FR in 92%. The patients with normal jitter studies had a current classification of asymptomatic (patients 8 and 13) or ocular (patient 15). Patients with ocular MG had abnormal mean MCD and outliers in 75% for each of ED and FR. Sanders and Howard 4 found abnormality in 63% in ED and in 88% if any, ED, and facial (FR, OOc, or orbicularis oris) muscles were tested. Patients with generalized MG had abnormal jitter in 93.7% of instances when both ED and FR were considered. Others found abnormality ranging from 81.4% to 95% in ED. 13 Impulse blocking was found in 21.6% (ED) and 23.3% (FR) in ocular MG; in generalized MG, the values were 31% (ED) and 38.3% (FR). Jitter parameters after therapy (similar to this study) showed impulse blocking in 5.2% (ED) and 20.6% (FR) for ocular MG, and in % (ED) and % (FR) for generalized MG. 13 The data from that study 13 differ slightly from ours, but detailed statistics could not be performed because of the large size difference between the study populations. It should be emphasized that blocking percentage is calculated for fiber pairs in voluntary and for individual fibers in stimulated studies. Thus, 20 jitter recordings obtained during voluntary contraction represent 40 endplates. So the percentage of patients with abnormal jitter/blocking may be the same in the voluntary and stimulated techniques, but the percentage of blocking may be higher in voluntary recordings in which a block in one of two fibers will be counted as a blocking pair. In the stimulation technique, each spike is assessed on its own. 916 Stimulation CNE Jitter in MG MUSCLE & NERVE December 2011

6 In our study abnormal MCD and abnormal percentage of outliers were the same for FR; in contrast, the percent of abnormal outliers was greater in ED. For voluntary SFEMG, the MCD value is a somewhat more sensitive index of abnormality than the number of outliers, that is, individual potential pairs with increased jitter. 13 A theoretical possibility for the sensitivity in the outlier number in the stimulation technique is the fact that jitter in voluntary recording is obtained with two motor endplates involved. If one is very normal, and the other is barely above the normal value for individual neuromuscular junctions, the combined jitter of the two would be normal. With stimulation, the abnormality in one of them would have been detected. Technically it was easier to measure jitter and impulse blocking in FR than in ED. This is probably due to a difference in motor unit fiber organization, as spikes are more separated and easier to record cleanly in FR, and there is also a difference in stimulation method. There is no risk of direct stimulation in FR (percutaneous stimulation) compared with ED (intramuscular microaxonal stimulation). With CNE, as with SFEMG, one must be aware of jitter due to insufficient stimulation. Reliable jitter measurement is dependent on the intensity of the stimulus above threshold, and acceptable spikes are dependent on the physician s ability to avoid subliminal stimulation. To be accepted for analysis, the spike must fulfill the ASFAP criteria and also be checked for supraliminal stimulation. With stimulation there is also a high risk of summation, because many axons are frequently stimulated; in addition, individual spikes obtained with CNE are not always obtained from single muscle fibers, 12,17 but rather represent a summation of more than one SFAP. Thus, we can get SFAPs and ASFAPs (summated), and therefore separate normative data should be collected for CNE recordings, as has already been done for SFEMG recordings. A fast-rising phase without notches or shoulders, parallel rising phases on consecutive discharges with spikes separated by more than 150 ls, and a well-defined peak without shape changes on consecutive discharges are desirable, and the recordings should show clear spikes and reliably increased jitter and impulse blocking. In our previous reports we have already collected control data for ED and OOc muscles, both for voluntary and stimulated techniques. 10,16,18,19 The few studies that have compared jitter values from SFEMG and from CNE in healthy controls and patients with MG reported a good correlation of results. 7 9 In previous work on stimulation jitter analysis with CNE we concluded that the method is difficult to use because of summation of recorded signals from a given motor unit and problems with stimulation when there is activation of more than one axon. With more experience using CNE in healthy subjects and patients with MG, we realize the marked difference in responses obtained in ED compared with those in FR or orbicularis oculi. In the latter two muscles the individual single-fiber signals may be shorter and definitively more separated in time from each other (probably due to endplate scatter and variation in fiber conduction velocity). This temporal dispersion makes it nearly always possible to obtain perfectly acceptable ASFAP signals. Our previous recommendations still remain that voluntary activation is preferred when performing CNE jitter analysis, and that large limb muscles are not recommended for stimulation studies. We add further that facial muscles, with careful adjustment of stimulus strength, can also give quite acceptable responses for analysis with the stimulation technique. In general, our study has confirmed that CNE can be used for jitter analysis and impulse blocking in definite MG patients in a small cohort, but larger studies should be done for sensitivity and specificity in suspected cases. We found a very high rate of abnormality for MCD (85% for both ED and FR), outliers (90% for ED and 85% for FR), and blocking (22.3% for ED and 40.2% for FR) in our 20-patient MG cohort. This is similar to what has been reported in larger series with SFEMG. In spite of the promising results with regard to detection of abnormal jitter, some caution must be maintained before a study using CNE is declared abnormal or normal in situations of borderline jitter values. Technically, jitter measurement was much more easily obtained, less painful, and less time-consuming for FR than ED. Finally, this technique may also be utilized in pediatric patients, 20 after collection of reference values for younger age groups. This study was supported by Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), Brazil. REFERENCES 1. Ekstedt J. Human single muscle fiber action potentials. Acta Neurol Scand 1964;61: Stålberg E, Trontelj JV. Single fiber electromyography. Studies in healthy and diseased muscle, 2nd ed. New York: Raven Press; p Sanders DB, Stålberg E. AAEM minimonograph #25: Single-fiber electromyography. Muscle Nerve 1996;19: Sanders DB, Howard JF. AAEE minimonograph #25: Single-fiber electromyography in myasthenia gravis. Muscle Nerve 1986;9: Benatar M. A systematic review of diagnostic studies in myasthenia gravis. Neuromuscul Disord 2006;16: Mercelis R, Merckaert V. Diagnostic utility of stimulated single-fiber electromyography of the orbicularis oculi muscle in patients with suspected ocular myasthenia. Muscle Nerve 2011;43: Benatar M, Hammad M, Doss-Riney H. Concentric-needle singlefiber electromyography for the diagnosis of myasthenia gravis. Muscle Nerve 2006;34: Stimulation CNE Jitter in MG MUSCLE & NERVE December

7 8. Sarrigiannis PG, Kennett RP, Read S, Farrugia ME. Single-fiber EMG with a concentric needle electrode: validation in myasthenia gravis. Muscle Nerve 2006;33: Ertasla M, Baslo MB, Yildiz N, Yazici J, Öge AE. Concentric needle electrode for neuromuscular jitter analysis. Muscle Nerve 2000;23: Kouyoumdjian JA, Stålberg E. Concentric needle single fiber electromyography: normative jitter values on voluntary activated extensor digitorum. Arq Neuropsiquiatr 2007;65: Farrugia ME, Weir AI, Cleary M, Cooper S, Metcalfe R, Mallik A. Concentric and single fiber needle electrodes yield comparable jitter results in myasthenia gravis. Muscle Nerve 2009;39: Stålberg E, Sanders DB. Jitter recordings with concentric needle electrodes. Muscle Nerve 2009;40: Stålberg EV, Trontelj JV, Sanders DB. Myasthenia gravis and other disorders of neuromuscular transmission. In: Single fiber EMG, 3rd ed. Fiskebäckskil, Sweden: Edshagen; p Jaretzki A III, Barohn RJ, Ernstoff RM, Kaminski HJ, Keesey JC, Penn AS, et al. Myasthenia gravis. Recommendations for clinical research standards. Task force of the Medical Scientific Advisory Board of the Myasthenia Gravis Foundation of America. Neurology 2000;55: Ozkul Y. Influence of calcium blocker drugs in neuromuscular transmission. J Clin Neurophys 2007;118: Kouyoumdjian JA, Stålberg E. Concentric needle single fiber electromyography: comparative jitter on voluntary-activated and stimulated extensor digitorum. Concentric needle single fiber electromyography. Clin Neurophysiol 2008;119: Stålberg E, Daube JR. Electromyographic methods. In: Stålberg E, editor. Clinical neurophysiology of disorders of muscle and neuromuscular junction, including fatigue. Amsterdam: Elsevier; pp Kouyoumdjian JA, Stålberg E. Reference jitter values for concentric needle electrodes in voluntarily activated extensor digitorum and orbicularis oculi muscles. Muscle Nerve 2008;37: Kouyoumdjian JA, Stålberg E. Concentric needle jitter on stimulated orbicularis oculi in 50 healthy subjects. Clin Neurophysiol 2011;122: Tidswell T, Pitt MC. A new analytical method to diagnose congenital myasthenia with stimulated single-fiber electromyography. Muscle Nerve 2007;35: Stimulation CNE Jitter in MG MUSCLE & NERVE December 2011