Myopathic changes detected by quantitative electromyography in patients with MuSK and AChR positive myasthenia gravis

Journal of Clinical Neuroscience xxx (2016) xxx–xxx

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Clinical Study

Myopathic changes detected by quantitative electromyography in patients with MuSK and AChR positive myasthenia gravis Ana Nikolic a,b,⇑, Ivana Basta a,b, Vidosava Rakocevic Stojanovic a,b, Zorica Stevic a,b, Stojan Peric a, Dragana Lavrnic a,b a b

Neurology Clinic, Department for Neuromuscular Disorders, Clinical Center of Serbia, Dr Subotica 6, 11000 Belgrade, Serbia Medical Faculty, Belgrade University, Belgrade, Serbia

a r t i c l e

i n f o

Article history: Received 4 March 2015 Accepted 30 August 2015 Available online xxxx Keywords: Electrophysiology Multi-MUP analysis MuSK Myasthenia gravis Quantitative EMG Turns amplitude analysis

a b s t r a c t Myopathic changes are frequent a electrophysiological finding in patients with muscle specific tyrosine kinase (MuSK) positive myasthenia gravis (MG). The aim of this study was to explore the importance of quantitative electromyography (EMG) in the detection of myopathic changes in MuSK MG patients. Classical and quantitative EMG were performed in 31 MuSK and 28 acetylcholine receptor (AChR) positive MG patients, matched by sex, age, disease duration and severity. Classical EMG revealed the presence of myopathic changes more frequently in MuSK MG compared to AChR MG patients, especially in the facial muscles. Quantitative EMG registered myopathic lesions more frequently than classical EMG, but the frequency was similar between MuSK and AChR MG patients. Quantitative EMG revealed myopathic changes in the majority of both MuSK and AChR positive MG patients. This examination is sensitive, but it cannot be used to differentiate between MG patients belonging to the different disease groups. It should not be used in isolation. Rather, it should complement classical EMG in the detection of myopathic changes. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Myasthenia gravis (MG) is an autoimmune disease in which antibodies against targets on the postsynaptic muscle membrane cause neuromuscular transmission failure. About 85% of patients with MG have autoantibodies against acetylcholine receptor (AChR) [1]. In AChR negative MG patients antibodies to muscle specific tyrosine kinase (MuSK) have been identified in 40–50% of cases [2]. MG associated with anti-MuSK antibodies is considered to be a distinct clinical entity [3] and it differs in many aspects from the typical presentation of AChR positive MG (AChr MG). Unlike AChR MG patients, who experience weakness and fatigue in electrophysiologically normal muscles, myopathic changes have been described in patients with MuSK positive MG [4,5], as well as visible wasting of the tongue and facial muscles [3,6]. Moreover, MRI studies demonstrated fatty degeneration in the cranial muscles of MuSK positive MG (MuSK MG) patients, similar to patients with proven muscle disease such as myotonic dystrophy, while such changes were not present in patients with AChR MG [6]. Therefore, the muscle atrophy observed in the tongue and facial muscles of MuSK MG patients has been thought to be secondary ⇑ Corresponding author. Tel.: +381 66 830 1254; fax: +381 11 268 4577. E-mail address: [email protected] (A. Nikolic).

to the myopathic process [7]. There is also evidence of significant muscle ultrastructural changes in MuSK MG patients. Myopathic and mitochondrial abnormalities are more prominent in MuSK MG with giant, swollen, and degenerated mitochondria with fragmented cristae. The most common changes in AChR MG muscles are fiber atrophy, myofibrillar disarray, and Z-line streaming, consistent with mild neurogenic abnormalities [8]. Since the origin of myopathic electromyography (EMG) changes in MuSK MG patients is still unclear, we aimed to further address this issue. Quantitative EMG methods, including turns/amplitude (T/A) and multi-motor unit potential (MUP) analysis, are more sensitive than regular EMG, which only qualitatively analyzes interference pattern and depends on subjective interpretation of the findings. The aim of this study was to explore the importance of quantitative EMG techniques in the detection of myopathic EMG changes in MuSK MG patients. 2. Methods 2.1. Patients This is a prospective study of patients already diagnosed with MG. The study group consisted of 31 MuSK MG patients (25 women and six men) who were treated at the Neurology Clinic,

http://dx.doi.org/10.1016/j.jocn.2015.08.041 0967-5868/Ó 2015 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Nikolic A et al. Myopathic changes detected by quantitative electromyography in patients with MuSK and AChR positive myasthenia gravis. J Clin Neurosci (2016), http://dx.doi.org/10.1016/j.jocn.2015.08.041

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A. Nikolic et al. / Journal of Clinical Neuroscience xxx (2016) xxx–xxx

Clinical Center of Serbia in Belgrade, Serbia. The control group consisted of 28 AChR MG patients (25 women and three men) with the generalized form of the disease, who were matched with MuSK MG patients by sex, age, disease duration and severity. The diagnosis of MG was established on the basis of a typical clinical presentation in the form of fluctuating weakness and fatigability of various skeletal muscles, positive decrement response of more than 10% on the repetitive nerve stimulation test or increased jitter on single fiber electromyography. Positive neostigmine or edrophonium tests were carried out to confirm the diagnosis. The disease severity was graded using the Myasthenia Gravis Foundation of America (MGFA) classification [9].

tus. The EMG finding was defined as normal or myopathic with short-duration, small-amplitude MUP. Quantitative EMG analysis included T/A and multi-MUP analysis. T/A analysis was performed as the patient increased up to the maximal voluntary contraction of the EDC, deltoid and OO muscles. Multi-MUP analysis was performed during sustained mild voluntary muscle activation of deltoid and OO muscles. In every examined muscle, at least 20 MUP were collected and included in the analysis. Individual MUP durations were measured using an automatic cursor setting, which was manually adjusted when needed in order to mark the precise onset and termination of the MUP. Amplitudes and number of phases for each MUP were automatically measured.

2.2. Clinical data 2.5. Statistical analysis At the time of the study, MuSK MG patients were an average of 49.4 ± standard deviation (SD) 14.8 years old, ranging from 19–75 years, while the average age of AChR MG patients was 42.5 ± SD 15.8 years, with a range of 19–74 years. Mean disease duration of the MuSK MG group was 6.4 ± SD 6.8 years, with a range of 1–34 years, while it was 6.8 ± SD 5.6 years, varying from 0.1–20 years in AChR MG patients. Disease severity, at the time of the study, was also similar between the two groups. Five (16.1%) MuSK MG patients and three (10.7%) AChR MG patients were in remission. MGFA class IIA was registered in 10 (32.3%) MuSK MG and 11 (39.3%) AChR MG patients, class IIB was registered in 10 (32.3%) MuSK MG and 10 (35.7%) AChR MG patients, class IIIA in one (3.2%) MuSK MG and also in one (3.6%) AChR MG patient and class IIIB was registered in five (16.1%) MuSK MG and three (10.7%) AChR MG patients. This study was approved by the Ethics Committee of the Medical Faculty of Belgrade University. All patients gave informed consent for participation in the study. 2.3. Antibody analysis Anti-AChR antibody analysis was performed in sera of all patients by radioimmunoassay. The test was considered positive if the antibody titer was more than 0.2 nM. Anti-MuSK antibody titer was obtained in all AChR negative MG patients. The test was considered positive if the antibody titer was more than 0.02 nM. 2.4. EMG EMG was performed by concentric needle electrode during relaxation and voluntary muscle activation on extensor digitorum communis (EDC), deltoid and orbicularis oculi (OO) muscles using a CareFusion Synergy EMG machine (CareFusion, San Diego, CA, USA). The EMG examiner was blinded to the patient’s antibody sta-

Statistical analysis was performed using the Statistical Package for the Social Sciences version 15.0 (IBM, Armonk, NY, USA). The results of the study were statistically analyzed by means of the chi-square tests (or exact test), Student’s t-test and Spearman rank correlation test, as appropriate. A p value of <0.05 was considered significant. 3. Results In our previously published paper [10], EMG examination revealed the presence of myopathic EMG changes in around onethird of MuSK MG patients in the extremity muscles and in more than 80% in the facial muscles. Conversely, myopathic EMG changes were registered in around one-third of AChR MG patients in the facial muscles and were extremely rare in the extremity muscles. T/A analysis was performed in the EDC, deltoid and OO muscles. In one AChR MG patient this examination could not be performed in the EDC and deltoid muscles due to severe weakness of these muscles. For the same reason, T/A analysis could not be performed in the OO muscle in two MuSK MG patients. The results of T/A analysis are presented in Table 1. T/A analysis registered similar results between MuSK MG and AChR MG patients. Number of MUP turns was the only registered parameter which was lower in the MuSK MG patients compared to the AChR MG patients (p = 0.043). Multi-MUP analysis was performed in the deltoid and OO muscles and the results are presented in Table 2. Values of all analyzed MUP parameters were similar between MuSK and AChR MG patients (p > 0.05). When the results of our MG patients were compared to the published reference values for MUP parameters in the OO [11] and deltoid [12] muscles, a myopathic lesion was confirmed in the OO muscle in 26 (83.9%) MuSK MG and 24 (85.7%) AChR MG patients,

Table 1 Results of T/A analysis in MuSK positive and AChR positive MG patients T/A analysis

MuSK MG

EDC MUP amplitude (lV) MUP number of turns

Range 155–946 112–1193

Mean ± SD 378.2 ± 209.3 515.2 ± 222.6

Range 206–980 173–1050

AChR MG Mean ± SD 425.7 ± 178.4 629.1 ± 191.8

Statistical significance p = 0.360 p = 0.043

Deltoid MUP amplitude (lV) MUP number of turns

Range 190–992 236–1108

Mean ± SD 372.5 ± 213.8 547.6 ± 180.6

Range 191–1106 415–806

Mean ± SD 460.1 ± 292.9 592.1 ± 98.0

p = 0.195 p = 0.259

OO MUP amplitude (lV) MUP number of turns

Range 143–570 127–1552

Mean ± SD 254.1 ± 89.7 713.6 ± 359.8

Range 114–696 109–1945

Mean ± SD 293.1 ± 135.5 651.3 ± 384.6

p = 0.204 p = 0.530

AChR = acetylcholine receptor, EDC = extensor digitrum communis, MG = myasthenia gravis, MUP = motor unit potential, MuSK = muscle specific tyrosine kinase, OO = orbicularis oculi, SD = standard deviation, T/A = turns amplitude.

Please cite this article in press as: Nikolic A et al. Myopathic changes detected by quantitative electromyography in patients with MuSK and AChR positive myasthenia gravis. J Clin Neurosci (2016), http://dx.doi.org/10.1016/j.jocn.2015.08.041

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A. Nikolic et al. / Journal of Clinical Neuroscience xxx (2016) xxx–xxx Table 2 Results of multi-MUP analysis in MuSK positive and AChR positive MG patients Multi-MUP

MuSK MG

AChR MG

Statistical significance

Deltoid MUP amplitude (lV) MUP duration (ms) MUP number of phases

Range 329.7–1205.3 3.7–12.2 2.4–4.6

Mean ± SD 618.4 ± 223.9 6.5 ± 2.3 3.6 ± 0.5

Range 280.6–829 3.4–13.3 2.4–4.5

Mean ± SD 523.5 ± 143.2 6.4 ± 2.9 3.4 ± 0.6

p = 0.061 p = 0.868 p = 0.116

OO MUP amplitude (lV) MUP duration (ms) MUP number of phases

Range 147.8– 810.3 1.2–8.3 2.2–7.8

Mean ± SD 346.6 ± 153.8 3.3 ± 1.3 3.5 ± 1.1

Range 132.2– 2271.0 1.2–7.5 2.0–4.8

Mean ± SD 376.8 ± 386.7 3.4 ± 1.6 3.3 ± 0.7

p = 0.227 p = 0.765 p = 0.425

AChR = acetylcholine receptor, MG = myasthenia gravis, MUP = motor unit potential, MuSK = muscle specific tyrosine kinase, OO = orbicularis oculi, SD = standard deviation.

while in the deltoid muscle myopathic findings were recorded in 26 (83.9%) MuSK MG and in 21 (75%) AChR MG patients, mainly in the form of short duration MUP. Neither of these differences were statistically significant. In the MuSK MG patients, lower values of MUP amplitude in the OO muscle were recorded in patients with longer disease duration (p = 0.031), but such a correlation was not found in the deltoid muscle (p = 0.919). In AChR MG patients, there was no correlation between the disease duration and the MUP amplitude, neither in the OO (p = 0.936) or the deltoid muscle (p = 0.695). 4. Discussion Myopathic EMG changes are frequently detected in patients with MuSK MG, unlike in AChR MG patients [10]. However, these EMG changes are not always accompanied by myopathic histopathological findings. It has been shown that mitochondrial abnormalities seem to be more prominent in MuSK MG, whereas neurogenic atrophy is observed in AChR MG patients [8]. Since the origin of myopathy in MuSK MG patients is still unclear and there are discrepancies between EMG and histopathological findings, we wanted to further analyze this issue. In order to be more sensitive and to quantitate myopathic changes in different forms of MG, we studied multi-MUP analysis, the most accepted method of analysis of individual MUP, and T/A analysis, the best studied method of automatic interference pattern analysis. Since multi-MUP analysis is performed under weak muscle contraction and examines only motor units activated at weak effort, it is sampling a higher percentage of type I fiber activity, while abnormalities of motor units activated at greater effort cannot be revealed. T/A analysis can be performed under different degrees of muscle activation. As the force of contraction increases, more and larger motor units are recruited and the electrical potentials they generate are summated. Because of this, the mean change in amplitude per turn of the interference pattern increases with force of contraction to the maximum possible, whereas the number of turns per second initially rises and then plateaus at higher force levels. As the maximum force of voluntary contraction varies between individuals and disease states, it is difficult to make comparisons between different patients. Therefore, Stalberg et al. proposed T/A analysis modification [13], which allows quantitative EMG interference pattern analysis disregarding estimation of muscle force. Since this method analyzes the T/A at various forces, this analysis samples a higher percentage of type II muscle fiber activity. A number of MUP studies on limb muscles of MG patients have shown a reduction in MUP duration [14–16], as occurred in our study. Nevertheless, pathohistological examination of muscle biopsies of MG patients who had myopathic features on MUP analysis revealed no abnormalities in most cases [17,18]. Therefore, it was suggested that this discrepancy was most likely due to the functional shortening of MUP due to neuromuscular transmission dysfunction rather than anatomical changes. Contrary to this

explanation, Somnier and Trojaborg [15] showed no consistent evidence to support the view that blocking of muscular fibers by itself could account for the reduction of MUP duration. Previously published studies have shown superior sensitivity of T/A analysis to demonstrate myopathic changes as compared to individual MUP analysis [19–21]. Cruz Martinez et al. [22] studied seven patients with MG and reported that all showed myopathic change with T/A analysis using force monitoring. Lo Monaco et al. [14] found a myopathic pattern in four and neurogenic pattern in two out of 17 MG patients by T/A analysis. Again, muscle biopsy did not support the EMG findings of these patients with neurogenic and myopathic change by T/A analysis. T/A analysis at low force failed to differentiate MG patients from controls whereas ratio of turns to mean amplitude was increased in seven out of eight patients compared to controls at maximum force [14]. These changes were attributed to fatigability as abnormal findings appeared during maintained contraction or after activation. The results of quantitative EMG techniques in subgroups of MG patients, according to their antibody status, have rarely been published. One such study showed similar quantitative EMG results in the facial muscles of MuSK MG patients and patients with primary muscle diseases, such as myotonic dystrophy [7]. In this study, myopathic EMG changes were registered in 50% of MuSK MG patients and in 40% of AChR MG patients. Facial muscles, both in the MuSK MG and AChR MG group, had short-duration MUP, without increased polyphasia, similar to those reported in patients with known myopathic changes secondary to myotonic dystrophy [11]. Since the dominant finding was that of short duration MUP, it led to the hypothesis that the facial muscle atrophy in MuSK MG patients is secondary to a myopathic process, probably as the result of uniform muscle fibre shrinkage or loss of muscle fibers from motor units. In our study, classical EMG showed myopathic changes more frequently in patients with MuSK MG, compared to AChR MG patients. However, results of the quantitative EMG techniques (multi-MUP and T/A analysis) were similar between MuSK MG and AChR MG patients. This finding can be explained by the fact that quantitative EMG is a much more sensitive technique, and can detect very mild changes which cannot be detected during classical EMG examination. Also, as there was pronounced fatigue of patients belonging to both groups during the examination, patients were more likely to activate only small motor units, which gives a myopathic appearance on quantitative EMG examination. This myopathic picture was especially pronounced in the OO muscle. In this muscle, larger motor units are recruited early, but with unsustained firing. The preponderance of type II muscle fibers in the OO muscle may also produce fatigue in prolonged contraction. 5. Conclusion Quantitative EMG techniques are very sensitive, but they cannot be used to differentiate MG patients belonging to the different disease subtypes. These techniques should not be used in isolation

Please cite this article in press as: Nikolic A et al. Myopathic changes detected by quantitative electromyography in patients with MuSK and AChR positive myasthenia gravis. J Clin Neurosci (2016), http://dx.doi.org/10.1016/j.jocn.2015.08.041

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during evaluation of MG patients. Rather, they should complement classical EMG and supplement the detection of myopathic or neurogenic changes in the examined muscles. Conflicts of Interest/Disclosures The authors declare that they have no financial or other conflicts of interest in relation to this research and its publication. Acknowledgement This study was funded by the Ministry of Science of the Republic of Serbia (grant #175083). References [1] Vincent A, Palace J, Hilton-Jones D. Myasthenia gravis. Lancet 2001;357:2122–8. [2] Evoli A, Padua L, Monaco ML, et al. Clinical correlates with anti-MuSK antibodies in generalized seronegative myasthenia gravis. Brain 2003;126:2304–11. [3] Lavrnic D, Losen M, Vujic A, et al. The features of myasthenia gravis with autoantibodies to MuSK. J Neurol Neurosurg Psychiatry 2005;76:1099–102. [4] Sanders DB, El-Salem K, Massey JM, et al. Clinical aspects of MuSK antibody positive seronegative MG. Neurology 2003;60:1978–80. [5] Padua L, Tonali P, Aprile I, et al. Seronegative myasthenia gravis: comparison of neurophysiological picture in MuSK+ and MuSK patients. Eur J Neurol 2006;13:273–6. [6] Farrugia ME, Robson MD, Clover L, et al. MRI and clinical studies of facial and bulbar muscle involvement in MuSK antibody associated myasthenia gravis. Brain 2006;129:1481–92. [7] Farrugia ME, Kennett RP, Hilton-Jones D, et al. Quantitative EMG of facial muscles in myasthenia patients with MuSK antibody. Clin Neurophysiol 2007;118:269–77. [8] Cenacchi G, Papa V, Fanin M, et al. Comparison of muscle ultrastructure in myasthenia gravis with anti-MuSK and anti-AChR antibodies. J Neurol 2011;258:746–52.

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Please cite this article in press as: Nikolic A et al. Myopathic changes detected by quantitative electromyography in patients with MuSK and AChR positive myasthenia gravis. J Clin Neurosci (2016), http://dx.doi.org/10.1016/j.jocn.2015.08.041