Myasthenia gravis with anti-acetylcholine receptor antibody or anti-muscle specific kinase antibody

Patients with myasthenia gravis (MG) are divided into 3 groups: anti-acetylcholine receptor antibody-positive MG (AChR MG), anti-muscle specific kinase antibody-positive MG (MuSK MG), and AChR-and MuSK-negative MG (double seronegative MG; double SNMG). A recent study on the detection of low-affinity antibodies binding to AChR showed the presence of AChR antibodies in about 70% of double SNMG patients. There is accumulating evidence that double SNMG patients are similar to AChR-MG patients with respect to clinical features and thymic pathology. Since most MG patients are thought to belong to the AChR-MG or MuSK-MG group of patients, this article reviews the pathophysiology, clinical features, and treatments in these 2 groups of MG patients. The pathophysiology of AChR-MG is closely related to the abnormal thymic pathology, such as thymic hyperplasia or presence of thymoma, and thy(mo)mectomy is recommended in patients with generalized AChR-MG. On the contrary, little thymic abnormality in patients with MuSK-MG discourages thymectomy; however, MuSK-MG patients do respond to thymectomy and therefore studies to define the indications of thymectomy in MuSK-MG patients are required. The responses to cholinesterase inhibitors are poor in patients with MuSK-MG, and these patients tend to show hyperactivity to the Tensilon test, such as fasciculation of facial muscles and stuffy sensation in the throat. The adverse responses to a small dose of intravenous edrophonium chloride injection may support the clinical diagnosis of MuSK-MG. Further, only small doses of acetylcholinesterase inhibitors are administered to patients with MuSK-MG in order to avoid cholinergic hyperactivity. In general, both types of MG patients respond well to treatments with immunosuppressants, including steroids, but some patients with MuSK-MG show persistent bulbar symptoms.     

我国华南地区MuSK抗体阳性重症肌无力患者的临床特点

目的分析我国华南地区骨骼肌特异性酪氨酸激酶抗体(MuSK-Ab)阳性重症肌无力(myasthenia gravis,MG)的临床特点。方法回顾性收集2017年8月至2019年8月在中山大学附属第一医院确诊的住院MuSK抗体阳性MG(MuSK-MG)患者26例,并选取同期收治的乙酰胆碱受体抗体(AChR-Ab)阳性MG(AChR-MG)患者157例作为对照组,比较两组患者临床特点以及对治疗反应的差异。结果与AChR-MG患者比较,MuSK-MG患者女性构成(80.8%比58.0%,P<0.05)、平均发病年龄〔(43.12±13.02)岁比(36.04±17.97)岁,P<0.05〕高,球部肌受累(96.2%比70.1%,P<0.01)和肌无力危象(myasthenic crisis,MC;61.4%比28.7%,P<0.05)更常见。两组患者新斯的明试验阳性率(61.5%比70.7%)、低频重复神经电刺激(RNS)阳性率(78.3%比83.3%)比较差异无统计学意义(均P>0.05)。在治疗反应方面,胆碱酯酶抑制剂(AChEIs)对MuSK-MG患者的有效率低于AChR-MG患者(16.0%比58.6%,P<0.01)。两组患者发生MC期间,对血浆置换(PLEX)的反应优于静脉注射丙种球蛋白(IVIG)。结论我国华南地区MuSK-MG以40岁左右女性多见,与AChR-MG比较易累及球部肌和呼吸肌,容易发生MC。MuSK-MG对AChEIs的反应性低于AChR-MG。RNS对我国华南地区MuSK-MG诊断的敏感性与AChR-MG比较无统计学差异,但高于欧美人群。PLEX在MuSK-MG患者的危象前或危象状态中的应用可显著缓解病情。     

Anti-MuSK-positive myasthenia gravis: neuromuscular transmission failure in facial and limb muscles

The presence of antibodies against muscle-specific receptor tyrosine kinase (MuSK) appears to define a subgroup of patients with myasthenia gravis (MG) characterized by weakness predominant in bulbar, facial and neck muscles compared with anti-acetylcholine receptor (AChR) antibody-positive MG. To investigate the patterns and severity of neuromuscular transmission failure in different muscles in MuSK-positive MG, we performed single fiber electromyography (SFEMG) in the facial (frontalis) and limb (extensor digitorum communis, EDC) muscles in three anti-Musk-positive patients, and compared results with those of 11 anti-AChR-positive patients. Only one of the three MuSK-positive patients had abnormal jitter in EDC, but all the three showed clearly increased jitter in the frontalis. By contrast, the AChR-positive patients showed similarly abnormal jitter for the two muscles. These results suggest that when the diagnosis of anti-MuSK-positive MG is suspected, SFEMG should be performed in most prominently affected muscles.     

Comparison of muscle ultrastructure in myasthenia gravis with anti-MuSK and anti-AChR antibodies

Patients with myasthenia gravis (MG) with antibodies to muscle-specific receptor tyrosine kinase (MuSK) differ from acetylcholine receptor (AChR)-positive MG patients, as they frequently present with severe oculobulbar muscle weakness or with neck, shoulder, and respiratory muscle involvement. The neuromuscular junction (NMJ) has been confirmed to be the main target of both AChR- and MuSK-MG. However, histopathological investigation disclosed that muscle fiber atrophy was prevalent in AChR-MG, whereas mild myopathic changes and mitochondrial abnormalities were more frequently observed in MuSK-MG. As the pathogenetic mechanism in MuSK-MG remains unclear, this study investigated the submicroscopic pattern of muscle histopathology to establish a possible correlation between clinical involvement and subcellular morphological findings. Muscle biopsies from seven MuSK-MG patients and from seven patients with AChR-MG were analyzed by transmission electron microscopy. Myopathic and mitochondrial abnormalities were more prominent in MuSK-MG and show giant, swollen, and degenerated mitochondria with fragmented cristae. The most common changes in AChR-MG muscles were fiber atrophy, myofibrillar disarray, and Z-line streaming, consistent with mild neurogenic abnormalities. A different pathogenetic mechanism is emerging in MuSK-MG compared to AChR-MG. Mitochondrial abnormalities seem to be more prominent in MuSK-MG, whereas neurogenic atrophy is observed in AChR-MG.     

Muscle-Specific Receptor Tyrosine Kinase Antibody Positive Myasthenia Gravis Current Status

Muscle-specific tyrosine-kinase-antibody-positive myasthenia gravis (MuSK-MG) has emerged as a distinct entity since 2001. This disease has been reported worldwide, but with varying rates among patients with generalized acetylcholine-receptor-antibody-negative MG. MuSK-MG was detected in approximately 37% of generalized acetylcholine receptor antibody-negative MG. MuSK-MG patients were predominantly female with more prominent facial and bulbar involvement and more frequent crises. Disease onset tended to be earlier. Patients tended to have a relatively poor edrophonium response but showed prominent decrement in the repetitive nerve stimulation test in the facial muscles. Patients were more likely to display poor tolerance of, or a lack of improvement with, anticholinesterase agents. Somewhat better response was observed with steroids and plasma exchange. Most were managed successfully with aggressive immunomodulatory therapies, although a higher proportion of MuSK-MG patients had a refractory course when compared with other forms of generalized MG. I present here an up-to-date overview on MuSK-MG based on our experience at the University of Alabama at Birmingham and the existing literature.     

Epidemiological and immunological profile of muscle-specific kinase myasthenia gravis in Greece

Objectives:  The purposes of this study were to determine the epidemiological characteristics of muscle-specific kinase-myasthenia gravis (MuSK-MG) in Greece and the IgG subclass of the anti-MuSK antibodies.
Methods:  This population-based study was performed on MuSK-MG patients in Greece between 1 January 1986 and 30 June 2006. Epidemiological and clinical data for 33 patients were collected. In addition, the distribution of anti-MuSK IgG autoantibody subclasses in the sera of 14 patients was determined by immunoprecipitation.
Results:  The average annual incidence was 0.32 patients/million population/year. On 1st July 2006, there were 33 prevalent cases, giving a point prevalence rate of 2.92/million (women 4.56 and men 1.25). In females, onset of MuSK-MG occurred after the age of 30, whilst, in males, the disease appears in any decade. The female:male incidence ratio was 3.33:1, whilst the prevalence ratio was 3.65:1. Most patients presented with involvement of the facial and bulbar muscles. Amongst about 800 MG patients seropositive for antibodies against either the AChR or MuSK, one patient was found to be seropositive for anti-MuSK antibodies and ambiguous for anti-acetylcholine receptor (anti-AChR) antibodies. The vast majority of anti-MuSK antibodies were IgG4, whilst total IgG4 levels in these patients were similar to those in two healthy controls.
Conclusions:  The incidence and prevalence of MuSK-MG in Greece are amongst the highest reported previously for other countries. MuSK-MG in Greece affects both sexes, but mainly females. The main epidemiological indices were calculated. The vast majority of anti-MuSK antibodies were IgG4.     

Single fiber EMG in the frontalis muscle in ocular myasthenia: specificity and sensitivity.

Patients (n = 41) with isolated weakness of the eyelids or extraocular muscles, who had been referred for single fiber electromyography (SFEMG), were followed up after 4 to 24 months, At follow-up the patients were classified as "definite ocular myasthenia gravis" (MG), "definite other diagnosis," or "no definite diagnosis" on the basis of the completed investigations and subsequent course. The original SFEMG findings in the frontalis muscle were then reviewed. The specificity and sensitivity of SFEMG for "definite ocular MG" could be maximized by using as criteria for abnormality greater than 8/20 pairs with jitter greater than 45 microseconds, or a mean jitter of 20 pairs of greater than 50 microseconds. Patients with abnormal SFEMG according to these criteria have MG, and are likely to require treatment in the immediate future. Patients who have normal SFEMG according to these criteria (and no other demonstrated disorder) may have MG, but it is so mild that they are unlikely to require treatment. Two patients whose final diagnosis was progressive external ophthalmoplegia had normal SFEMG according to these criteria.     

Quantitative EMG of facial muscles in myasthenia patients with MuSK antibodies.

OBJECTIVE: Our aim was to study the pathophysiological process leading to facial muscle atrophy in 13 patients with MuSK antibody positive myasthenia gravis (MuSK-MG), and to compare with findings from 12 acetylcholine receptor antibody positive myasthenia patients (AChR-MG), selected because they suffered from the same degree of disease severity and required similar treatment. METHODS: Motor unit action potential (MUAP) and interference pattern analysis from orbicularis oculi (O oculi) and orbicularis oris (O oris) muscles were studied using a concentric needle electrode, and compared with findings in 20 normal subjects, 6 patients receiving botulinum toxin injections (representing a neurogenic model) and 6 patients with a muscle dystrophy (representing a myopathic model). The techniques and control data have been reported previously. RESULTS: The mean MUAP durations for O oculi and O oris were significantly reduced (p<0.001) in both MG cohorts when compared with healthy subjects, and were similar to those in the myopathic control group. They were significantly different from those obtained from the neurogenic control group (p<0.001 for both O oculi and O oris). The MUAP findings in O oculi occurred independently from neuromuscular blocking on single fibre EMG (SFEMG) in the same muscle. On turns amplitude analysis (TAA), 50% of MuSK-MG patients and 42% of AChR-MG patients had a pattern in O oculi which was similar to that in the myopathic control group, and 62% of MuSK-MG patients and 50% of AChR-MG patients had a pattern in O oris that was also similar to that in the myopathic control group. The TAA findings for O oculi and O oris in both MG cohorts were different from those obtained from the neurogenic control group. CONCLUSIONS: Facial muscle atrophy in MuSK-MG patients is not neurogenic and the pathophysiological changes are akin to a myopathic process. The selected AChR-MG patients also show evidence of a similar pathophysiological process in the facial muscles albeit to a lesser degree. SIGNIFICANCE: We propose that muscle atrophy in MuSK-MG is a myopathic process consisting of either muscle fibre shrinkage or loss of muscle fibres from motor units. The duration of disease and long-term steroid treatment may be further contributory factors.     

Effect of thymectomy and immunosuppressive therapy on anti-neuroblastoma antibody levels in patients with myasthenia gravis

Antibodies reacting with human neuroblastoma cells (NBL) are distinct from the "classical" anti-acetylcholine receptor (AChR) antibodies in myasthenia gravis (MG). The influence of therapeutic interventions on serum anti-NBL antibody levels was followed in 42 MG patients. Thymectomy alone was performed in 28 patients while immunosuppressive medication was given to 14 patients out of whom 10 also had a thymectomy. In most patients serum anti-NBL antibody titers declined after thymectomy and/or during immunosuppressive treatment, though individual variations in the antibody response could be observed. Sequential examinations of individual patients revealed an association between the clinical severity of MG and anti-NBL antibody levels. No correlation between the treatment-induced changes of anti-NBL and anti-acetylcholine receptor (AChR) antibody titers could be observed during the follow-up period in MG patients positive for both types of antibodies. These findings further emphasize the immunological complexity of MG. Anti-NBL antibodies represent a pathogenic marker of the disease and display a regulation different from that of the anti-AChR antibodies.     

重症肌无力患者病情变化对单纤维肌电图jitter值的影响

目的:探讨重症肌无力(MG)病情变化和单纤维肌电图(SFEMG)颤抖(jitter)值变化之间的关系。方法:对MG患者治疗前后的病情进行绝对评分并计算相对评分,同时行SFEMG检查,统计分析相对评分和jitter值变化率之间的关系。结果:MG患者随病情好转或痊愈,jitter值相应变小,病情变化和jitter值变化率总体呈弱相关(非Ⅰ型MG患者相关参数r=0.617、R2=0.380、P=0.008)。结论:病情的变化影响jitter值,但jitter值的改变不能及时反应病情的变化,对于预测病情的转归帮助有限。     

Seronegative myasthenia gravis: disease severity and prognosis

Around 10-20% of myasthenia gravis (MG) patients do not have acetylcholine receptor (AChR) antibodies (seronegative), of whom some have antibodies to a membrane-linked muscle specific kinase (MuSK). To examine MG severity and long-term prognosis in seronegative MG compared with seropositive MG, and to look specifically at anti-AChR antibody negative and anti-MuSK antibody negative patients. Seventeen consecutive seronegative non-thymomatous MG patients and 34 age and sex matched contemporary seropositive non-thymomatous MG controls were included in a retrospective follow-up study for a total period of 40 years. Clinical criteria were assessed each year, and muscle antibodies were assayed. There was no difference in MG severity between seronegative and seropositive MG. However, when thymectomized patients were excluded from the study at the year of thymectomy, seropositive MG patients had more severe course than seronegative (P < 0.001). One seropositive patient died from MG related respiratory insufficiency. The need for thymectomy in seronegative MG was lower than in seropositive MG. None of the seronegative patients had MuSK antibodies. This study shows that the presence of AChR antibodies in MG patients correlates with a more severe MG. With proper treatment, especially early thymectomy for seropositive MG, the outcome and long-term prognosis is good in patients with and without AChR antibodies.     

重症肌无力患者抗骨骼肌抗心肌抗体与心脏损害

目的探讨重症肌无力（ＭＧ）合并心脏损害的发病机制。方法采用无创性心脏检查对５６例ＭＧ患者进行心电图、彩色多普勒二维超声心动图和心肌酶学检查;并应用免疫荧光技术,对其中４９例ＭＧ患者测定抗骨骼肌抗心肌抗体（ＳＨ－Ａｂ）。以正常人作对照检查。结果心电图异常率３０．３５％,超声心动图异常率１０．７１％,心肌酶学异常率４１．０７％。ＳＨ－Ａｂ阳性率４８．９％,对照组无１例有ＳＨ－Ａｂ。还发现伴心肌酶异常的ＭＧ患者ＳＨ－Ａｂ阳性率６９．６％（１６／２３）,心肌酶正常的患者ＳＨ－Ａｂ阳性率仅为３０．８％（８／３０）,伴胸腺瘤者（ＭＧＴ）ＳＨ－Ａｂ阳性率为４１．６％（５／１２）,不伴胸腺瘤者（ＭＧＮＴ）ＳＨ－Ａｂ阳性率仅为２４．３２％（９／３７）。另外,ＭＧＴ心电图、心肌酶学异常率均为５０％,均明显高于ＭＧＮＴ。结论ＭＧ的心脏损害可能与ＳＨ－Ａｂ滴度增高抗原抗体结合等自身免疫机制有关。     

Concentric needle electrode for neuromuscular jitter analysis

We used a concentric needle electrode (CNE) with 2 kHZ low-cut filter and a single fiber electrode (SFE) in the same subjects for neuromuscular jitter measurement in the extensor digitorum communis (EDC) and orbicularis oculi (OOc) muscles. At the same session, 20 jitter values were obtained from each subject with each electrode. For EDC (during voluntary contraction), mean jitter values with SFE and CNE were 23.4 +/- 8 micros and 23.3 +/- 8 micros in 10 normals; and 56.8 +/- 28 micros and 57.4 +/- 33 micros in 10 myasthenics. For OOc (during electrical stimulation), mean jitter values with SFE and CNE were 17.9 +/- 5 micros and 16.3 +/- 4 micros in 11 normal subjects, and 41.2 +/- 29 micros and 36.7 +/- 27 micros in 10 myasthenics. For both muscles, the numbers of individual abnormal jitter values with SFE and CNE were highly comparable. Both needles labeled the same patients as having "normal" or "abnormal" neuromuscular transmission. CNE may be an alternative to SFE in neuromuscular jitter analysis.     

Cholinergic hyperactivity in patients with myasthenia gravis with MuSK antibodies: A neurophysiological study

ObjectivePatients with myasthenia gravis associated with muscle-specific tyrosine kinase antibodies (MuSK-MG) often manifest signs of cholinergic hyperactivity with standard doses of acetylcholinesterase inhibitors (AChE-Is). Aim of the study was to investigate whether repetitive compound muscle action potential (R-CMAP), the neurophysiological correlate of cholinergic hyperactivity, was present in MuSK-MG irrespective of AChE-I treatment.MethodsPatients with confirmed diagnosis of MuSK-MG were consecutively enrolled during follow-up visits, from January 2019 to April 2020. All these subjects underwent the same neurophysiological protocol, including motor nerve conduction studies and repetitive nerve stimulation. In patients taking pyridostigmine, neurophysiological testing was performed at least 12 hours after the last dose. For comparison, the presence of R-CMAP was investigated in 20 consecutive acetylcholine receptor antibody positive myasthenia gravis (AChR-MG) patients.ResultsWe enrolled 25 MuSK-MG patients (20 females), aged 16–79 years at the study time, with disease duration ranging 0.6–48.8 years (median: 17.7 years). R-CMAP was detected in 12/25 (48%) MuSK-MG cases and in none of the AChR-MG controls (p = 0.0003). In the MuSK-MG population, a history of muscle cramps and fasciculations, during low-dose pyridostigmine therapy, was significantly more frequent in R-CMAP positive than in R-CMAP negative patients (100% vs 31%, p = 0.001). At the time of the study, the proportion of patients still symptomatic for MG was higher among R-CMAP positive cases (92% vs 23%, p = 0.0005).ConclusionsCholinergic hyperactivity is a relatively common finding in MuSK-MG patients, independent of AChE-I treatment, and may constitute an intrinsic feature of the disease.SignificanceR-CMAP detection can represent a useful diagnostic clue for MuSK-MG and predicts poor tolerance to AChE-Is.     

Cholinergic neuromuscular hyperactivity in patients with myasthenia gravis seropositive for MuSK antibody

A 75-year-old man with severe oculobulbar myasthenia gravis (MG) treated with acetylcholine esterase inhibitors (AChEIs) was found to have muscle-specific tyrosine kinase (MuSK) antibodies. Neurophysiological examination displayed extra repetitive discharges after the compound motor action potential (CMAP) at low-frequency stimulation, possibly triggered by AChEI. This indicates an abnormal sensitivity to acetylcholine in patients with MuSK antibodies and may be a useful indicator of the adverse effect of AChEI treatment in these patients.     

Myasthenia gravis: disease severity and prognosis

Objectives –  To examine myesthenia gravis (MG) severity and long-term prognosis in seronegative, seropositive, and thymoma MG.
Materials and Methods –  Four series of patients were studied retrospectively. Severity and treatment were assessed each year, and muscle antibodies were assayed.
Results –  Seropositive MG patients had a more severe course than seronegative MG patients. MG severity was higher in non-thymectomized compared to thymectomized early-onset MG patients. MG severity did not differ between thymectomized and non-thymectomized late-onset patients. There was no significant difference in MG severity between thymoma and non-thymoma MG patients.
Conclusions –  MG is more severe in seropositive MG patients. With proper treatment, especially early thymectomy, the long-term prognosis is good in seropositive MG patients. The present studies indicate a benefit of thymectomy in early-onset MG, but no dramatic benefit in late-onset MG. Similar MG severity and outcome was seen in thymoma and non-thymoma MG.     

Electrophysiological profile of the patients with MuSK positive myasthenia gravis

Abstract

Objectives:

To determine the electrophysiological profile of our cohort of patients with muscle-specific tyrosine kinase (MuSK) positive myasthenia gravis (MG).

Methods:

Repetitive nerve stimulation test (RNS) and jitter analysis using concentric needle electrode were performed in 31 MuSK and in 28 acetylcholine receptor (AChR) positive MG patients.

Results:

Pathological RNS was verified in 16 (51·6%) MuSK and 26 (92·9%) AChR MG patients (P < 0·01). Pathological jitter analysis was registered in 28 (90·3%) MuSK and 26 (92·9%) AChR MG patients (P > 0·05). Increased jitter was present in extensor digitorum communis (EDC) in 23 (74·2%) MuSK and in 25 (89·3%) AChR MG patients (P > 0·05) as well as in orbicularis oculi (OO) muscle in 24 (85·7%) MuSK and 22 (81·5%) AChR MG patients (P > 0·05). Lower mean value of mean consecutive difference (MCD) and fewer potential pairs with increased jitter were registered in MuSK MG compared to AChR MG patients only in EDC muscle (P < 0·05). In MuSK MG patients, increased jitter was observed to be more frequent in patients with longer disease duration (P < 0·05) and also in those patients exhibiting more severe disease forms (P < 0·01) only in EDC muscle.

Discussion:

Repetitive nerve stimulation test has low sensitivity in MuSK MG patients, while jitter analysis shows high sensitivity, especially in facial muscles. The EDC muscle in MuSK MG patients usually shows increased jitter in more severe disease forms and later in the course of the disease.     

Clinical and experimental features of MuSK antibody positive MG in Japan

We investigated the presence of antibodies (Abs) against muscle-specific tyrosine kinase (MuSK) in Japanese myasthenia gravis (MG) patients. MuSK Abs were found in 23 (27%) of 85 generalized seronegative MG (SNMG) patients but not in any of the ocular MG patients. MuSK Ab-positive patients were characterized as having female dominance (M:F, 5:18), age range at onset 18 to 72 (median 45) years old, and prominent oculobulbar symptoms (100%) with neck (57%) or respiratory (35%) muscle weakness. Limb muscle weakness was comparatively less severe (52%), thymoma absent. Most patients had good responses to simple plasma exchange and steroid therapy. MuSK IgG from all 18 patients was exclusively the IgG 4 subclass and bound mainly with the MuSK Ig 1–2 domain. Serial studies of 12 individuals showed a close correlation between the variation in MuSK Ab titers and MG clinical severity ( P  = 0.01 by Kruskal–Wallis). MuSK Ab titers were sharply decreased in patients who had a good response to early steroid therapy or simple plasma exchange, but there was no change, or a rapid increase on exacerbation after thymectomy. Measurement of MuSK Ab titers aids in the diagnosis of MG and the monitoring of clinical courses after treatment.     

Jitter analysis with concentric needle electrode in the masseter muscle for the diagnosis of generalised myasthenia gravis

Objectives

The purpose of our study was to show neuromuscular transmission abnormality in the masseter muscle of generalised myasthenia gravis (MG) patients and to compare motor end-plate failure of the masseter with the extensor digitorum communis (EDC) and periocular muscles.

Methods

Motor end-plate function was evaluated during voluntary contraction of the masseter muscle of 20 generalised MG patients aged between 16 and 63 years, as well as 20 age-matched healthy volunteers. The mean jitter value was calculated for each group and compared. The upper limit of normal jitter was also calculated and the number of jitters exceeding this cut-off value was counted for each group for comparison. In MG patients, jitter analysis was also performed in periocular and EDC muscles along with the masseter and the number of single fibre-like potentials with abnormal jitter was counted for each muscle. All tests were performed during the same session with a concentric needle electrode (CNE).

Results

For the masseter muscle, the mean jitter of all potential pairs was significantly higher in the patient group (24.7 ± 9.6 μs in healthy volunteers, 71.9 ± 41 μs in patients). The calculated mean jitter for the 18th highest value in healthy volunteers was 33.8 ± 5.9 μs (upper 95% confidence limit was 45.6 μs). The number of abnormal jitters (?46 μs) was significantly higher in the patient group (276 out of 402 jitters) compared to healthy volunteers (10 out of 400 jitters). In the patient group, the number of single fibre-like potentials with abnormal jitter was found to be similar for the masseter, periocular and EDC muscles.

Conclusion

The masseter muscle has diagnostic importance in generalised MG. The ratio of high jitters to all of the calculated jitters in a particular muscle was similar for masseter, periocular and EDC muscles.

Significance

Jitter analysis of the masseter muscle during voluntary contraction is easy to perform and it was found as informative as other muscles in patients with generalised MG.     

Single fiber electromyography in myasthenia gravis during pregnancy

We report the use of single fiber electromyography (SFEMG) to demonstrate changes in the physiologic abnormality of myasthenia gravis (MG) during pregnancy. A 23-year-old became pregnant 15 months after the onset of mild ocular weakness. On initial evaluation, SFEMG jitter measurements demonstrated a slight abnormality of neuromuscular transmission. There was no change in severity of clinical disease or jitter measurements until the third trimester, when she improved. Jitter measurements at that time were normal. Labor was normal and she delivered a normal male. Three days postpartum, myasthenic weakness recurred temporarily and jitter measurements showed worsening. At 16 days and 6 weeks postpartum, she had only minimal medial rectus weakness and jitter studies were normal. Three months postpartum, ocular symptoms recurred and jitter measurements were slightly abnormal. She continued to worsen, developing limb muscle and severe ocular muscle weakness at 4 months postpartum. She was treated with plasma exchange and thymectomy. Prednisone was added 2 months after thymectomy due to continued worsening and development of oropharyngeal weakness. Three years postpartum she was taking prednisone 10 mg every other day and had only slight weakness of neck flexors, and jitter studies were again normal. © 1993 John Wiley & Soncs, Inc.