AAEE Minimonograph #33: electrodiagnostic approach to defects of neuromuscular transmission

Clinical testing for neuromuscular dysfunction is supported by an extensive amount of excellent basic information about normal and abnormal subcellular physiology and ultrastructure. This information provides an essential frame of reference for describing the rationale of single-fiber electromyography (SFEMG). SFEMG in turn helps to explain the more conventional clinical testing of neuromuscular function by repetitive nerve stimulation (RNS). Electrical findings in myasthenia gravis, Lambert-Eaton myasthenic syndrome, and botulinum intoxication are discussed from the subcellular level via the cellular level (SFEMG) to the integrated responses of whole muscle (RNS) as a rational means of understanding the technique of clinical repetitive nerve stimulation.     

Extensor digitorum brevis test and resistance to botulinum toxin type A

We studied 22 patients with dystonia to determine the normal range of values for the extensor digitorum brevis (EDB) test, and to determine its sensitivity and specificity in detecting resistance to botulinum toxin type A (BTX-A). Three compound muscle action potentials (CMAPs) elicited by peroneal nerve stimulation were averaged before and 2 weeks after injection of 20 units of BTX-A into the EDB. Amplitude and area ratios were calculated by dividing the averaged postinjection CMAP by the averaged preinjection CMAP values. The difference in means of this ratio between clinically sensitive and resistant subjects was statistically significant (P < 0.002). A normal range of <0.45 for each ratio was determined by adding two standard deviations to the ratio mean of 14 clinically sensitive subjects. Four of five resistant patients had values outside the normal range. The EDB test is a simple quantitative method of detecting resistance to BTX-A, with a sensitivity of 80% and specificity of 94%.     

Nerve stimulation boosts botulinum toxin action in spasticity.

Spasticity leads to functional and structural changes in nerves and muscles, which alter skeletal muscle function. To evaluate whether short-term electrical nerve stimulation (NS) improves the effect of botulinum toxin in spastic skeletal muscle, we studied changes in the amplitude of the compound muscle action potential (CMAP) recorded from the extensor digitorum brevis (EDB) muscle in response to peroneal nerve stimulation at the ankle after injection of botulinum toxin type A (BTXA) alone or combined with short-term NS. In paraparetic patients, both EDB muscles were injected with BTXA; and NS was applied to one EDB muscle alone. All patients received a 30-minute session of electrical NS once a day for 5 consecutive days after BTXA injection. We used two different stimulation frequencies (low-frequency, 4 Hz; and high-frequency, 25 Hz). EDB-CMAP amplitudes were evaluated before BTXA injection (day 0) and changes in CMAP amplitude, expressed as a percentage (CMAP%), were measured at various time points over a 30-day period after BTXA injection. We compared changes in the CMAP% amplitude on the stimulated and contralateral nonstimulated sides. We also studied the electromyographic activity recorded from EDB muscles over a 30-day period. CMAP% amplitudes measured at all time points after BTXA injections were significantly reduced in both EDB muscles. On days 4, 10, and 15, the CMAP% amplitude reduction was significantly greater for the low-frequency stimulated EDB than for the contralateral nonstimulated EDB. No significant differences in CMAP% were observed for the high-frequency stimulated and nonstimulated EDB. After BTXA injection, spontaneous activity appeared in both EDB muscles; but it appeared earlier and involved larger areas in the stimulated than in the nonstimulated EDB. In conclusion, short-term NS accelerates the effectiveness of intramuscular BTXA injections on the neuromuscular blockade in patients with spastic paraparesis and could induce a rapid and persistent improvement in spasticity. Its action probably arises mainly from low-frequency NS.     

Initial Repetitive Nerve Stimulation Test Predicts Conversion of Ocular Myasthenia Gravis to Generalized Myasthenia Gravis

Background and PurposeA major concern with ocular myasthenia gravis (MG) is the potential conversion to generalized MG. This study was conducted to determine if the repetitive nerve stimulation (RNS) test could predict the conversion from ocular to generalized MG.MethodsThe RNS test was conducted in a consistent manner on five muscles in the face and limbs in every patient. Subjects were divided into those who remained as ocular MG (ROMG group) and those who experienced conversion to generalized MG during follow-up (GOMG group).ResultsConversion to generalized MG occurred in 24 (21.4%) of 112 MG patients with ocular onset. The proportion of patients displaying abnormal decreases in responses in the trapezius, abductor digiti minimi, or flexor carpi ulnaris muscles on the RNS test was higher in the GOMG group (p<0.001, p=0.002, and p<0.001, respectively). The Cox proportional-hazards model revealed that an abnormal result on the RNS test was significantly associated with conversion to generalized MG [hazard ratio (HR)=3.13, 95% confidence interval (CI)=1.18–8.32]. Notably, the HR was higher for abnormal results on the RNS test for the limb muscles, at 5.19 (95% CI=2.09–12.90).ConclusionsAn abnormal result on the RNS test, especially in the limb muscles, is an independent predictor of the conversion from ocular to generalized MG. Applying the RNS test to limb muscles could be useful for predicting the conversion to generalized MG in patients with ocular onset.     

Botulinum toxin type B in antibody-induced botulinum toxin type A therapy failure

Recently, it was reported that botulinum toxin type B complex (BoNT/B) (NeuroBloc®, Elan Pharmaceuticals) can produce an adequate therapeutic response in patients with antibody induced failure of botulinum toxin type A complex (BoNT/A) therapy. We wanted to study whether this effect is transient or sustained. For this, 10 consecutive patients (6 males, 4 females, age 54.6 ± 14.3 years, duration of illness 15.8 ± 7.0 years) with complete BoNT/A therapy failure and BoNT/A antibody titres in excess of 10mU/ml in the mouse diaphragm assay (MDA) received BoNT/B in an initial dose of 12370 ± 1804MU. After the first BoNT/B application the Toronto Western Spasmodic Torticollis Rating Scale (TWSTRS) improved from 20.1 ± 3.0 to 11.9 ± 3.4. In all patients systemic anticholinergic side effects occurred. Three patients had stable continuous responses to two, three and five subsequent BoNT/B applications. Six patients showed complete secondary therapy failure to the second or third subsequent BoNT/B applications. Side effects did no longer occur. In four of them the BoNT/B doses were doubled without producing any therapeutic benefit or any side effects. In five of them MDA testing was performed and revealed BoNT/B antibody titres in excess of 1mU/ml. One patient lost half of her initial BoNT/B responsiveness indicating partial secondary BoNT/B therapy failure. This partial therapy failure was seen on two consecutive application series and has not proceeded to complete therapy failure so far. BoNT/B seems to be only temporarily effective in the majority of patients with BoNT/A antibody induced therapy failure. Whether the formation of BoNT/B antibody points to a high antigenic potency of BoNT/B, to an increased immunoreactivity in BoNT/A antibody carriers or whether it is due to the large amount of protein applied in BoNT/B therapy needs to be studied.     

重症肌无力患者低频重复神经电刺激的特点

目的研究重症肌无力(myasthenia gravis,MG)患者低频重复神经电刺激(repetitive nerve stimulation,RNS)的特点。方法回顾性分析1995-2010年作者医院住院的275例MG患者的低频RNS检查资料。结果 275例患者中RNS检查结果异常216例,异常率为78.55%。其中Ⅰ型RNS异常率最低(48.91%),且与其他临床分型异常率之间存在统计学差异(P<0.05)。刺激腋神经RNS异常率最高(58.68%),面神经次之(56.18%),尺神经最低(18.88%)。在眼轮匝肌和三角肌进行记录时所测得的RNS异常率高于在小指展肌所测(P<0.01)。结论 MG患者RNS异常率与临床分型及检测部位有关。Ⅰ型患者RNS异常率低于其余各分型,于近端肌肉记录时所测的RNS异常率明显高于在远端肌肉所测。     

重症肌无力的电生理研究

目的探讨重症肌无力（ＭＧ）患者的电生理特征及其诊断价值。方法应用针极肌电图（ＥＭＧ），重复神经电刺激（ＲＮＳ）和激发重复神经电刺激（ＡＲＮＳ）方法，对４７例ＭＧ患者的肌肉和神经肌肉传递功能进行研究。结果部分患者的肌肉见到病理自发电位及短小动作电位，具近端肌分布特征。ＡＲＮＳ阳性率（９０．６％）较ＲＮＳ阳性率（８３．０％）高。Ⅰ型患者肢体肌群存在多项电生理异常。结论适当、综合应用电生理检测技术有助于ＭＧ的早期诊断     

Ten-second exercise is superior to 30-second exercise for post-exercise facilitation in diagnosing Lambert-Eaton myasthenic syndrome

An incremental response after brief exercise or high-rate stimulation on the repetitive nerve stimulation (RNS) test is a critical diagnostic criterion for Lambert-Eaton myasthenic syndrome (LEMS). This prospective study was performed to determine what duration of exercise shows the highest diagnostic sensitivity for LEMS. The compound muscle action potential amplitude in the abductor digiti quinti muscle was obtained at rest and after 5 s, 10 s, 15 s, 20 s, 25 s, and 30 s of exercise. Incremental responses were compared for the different exercise durations in 24 studies performed in nine LEMS patients. The increment was highest with 5-s and 10-s exercises (244%-243%) and lowest with 30-s exercise (84%). A gradual decrease in the increment was noted from 5- to 30-s exercise. A significant difference in the increment was noted between 5- to 10-s and 20-s to 30-s exercise. There was significantly higher diagnostic sensitivity with the 10-s exercise compared with 30-s exercise at 100% increment and 60% increment levels. Higher increment and diagnostic sensitivity were achieved with 10-s exercise than with 30-s exercise. Thus, 10-s exercise should be the standard protocol for the RNS test for LEMS.     

Decremental motor responses to repetitive nerve stimulation in ALS

Repetitive nerve stimulation (RNS) of the trapezius muscle at slow rates was performed on 192 patients with amyotrophic lateral sclerosis (ALS). Fifty-six patients (29%) showed classical neuromuscular decrement of 10–43% (mean 16.8%) while 44 patients (23%) had a borderline decrement of 5–9%. The trapezius was significantly more sensitive in revealing the defect than the distal hypothenar muscles. In 30 patients followed serially, the decremental response remained constant or increased with time. However, 25% of patients continued to show no decrement in spite of progression of disease. No statistical correlation was found between decrement and clinical severity, disease staging, or disease progression. The finding that at least 50% of ALS patients show some degree of decrement on RNS of the trapezius muscle suggests that functional alterations of the neuromuscular junction accompany this disease. © 1994 John Wiley & Sons, Inc.     

Importance of pathway formation for nodal sprout production in partly denervated muscles

Experiments were carried out to investigate possible factors controlling nodal sprout growth in partly denervated mouse gluteal muscles. Pretreatment of the muscle with botulinum toxin for up to 20 days, which produces denervation-like changes and elicits terminal⁵ and nodal¹⁰ sprouting, increased reinnervation by terminal sprouting after partial denervation but did not alter the rate of reinnervation by nodal sprouts. This implies that nodal sprout growth is not limited by the development of an adequate growth stimulus from denervated muscle. A distro-proximal gradient of degeneration was observed in denervated intramuscular nerves in the electron microscope, suggesting that nodal sprout growth may be modulated by the availability of endoneurial pathways sufficiently degenerated to permit reinnervation by nodal sprouts, although the initial outgrowths from nodes of Ranvier may appear in response to a growth stimulus from regenerated.     

50例重症肌无力患者近端神经低频重复电刺激的比较

对50例确诊为重症肥无力（MG）患者腹神经和副神经低频（3Hz、5Hz）重复电刺激（RNS）结果表明，在不同频率刺激下，所有患者腋神经的平均衰减幅度均大于副神经的平均衰减幅度，50例患者腋神经RNS均为阳性，而副神经RNS阳性者仅33例，说明应用RNS检查诊断MG时，腋神经的敏感性高于副神经。此外，对两种检查方法及其结果的可靠性进行了讨论。     

癌性Lambert-Eaton肌无力综合征四例临床及电生理研究

目的研究癌性Lambert－Eaton肌无力综合征（LEMS）临床及电生理特征。方法回顾性地评价了4例病理证实的癌性LEMS（例1、2为小细胞性肺癌，例3为肾上腺癌，例4为乳腺癌）的临床和电生理资料。结果面神经、腋神经及尺神经重复电刺激示，诱发电位起始波幅减小（0．4～0．7mV）；5Hz以下低频刺激后波幅递减16％～54％（对照：小于15％），但在10Hz以上高频刺激后波幅递增120％～657％（对照：小于65％）；例2和例3肌电图和神经传导速度测定示，多发周围神经源性损害，以感觉神经纤维轴索损害为主。结论观察结果表明，部分癌性LEMS患者的神经肌肉接头和周围神经可同时受累。     

强直性肌营养不良的激发重复神经电刺激研究

对９例强直性肌营养不良患者行激发重复神经电刺激检查。１例ＡＲＮＳ前见序列复合肌肉动作电位波幅递减，８例ＡＲＮＳ后出现首位ＣＭＡＰ波幅降低，其中６例伴序列ＣＭＡＰｓ波幅递减。首位ＣＭＡＰ波幅降低出现率和平均降低程度随病程延长而加重。     

眼轮匝肌睑部重复电刺激在眼肌型重症肌无力诊断中的价值研究

目的探讨不同面神经支配肌尤其是眼轮匝肌睑部重复电刺激(RNS)在眼肌型重症肌无力(MG)的诊断价值。方法分析61例眼肌型MG患者,比较不同面神经支配肌RNS阳性率。结果额肌的RNS阳性率为8.47%(5/59例),眼轮匝肌睑部疲劳试验前RNS阳性率为73.77%(45/61例),疲劳试验后RNS阳性率提高至88.52%(54/61例);眼轮匝肌眶部和鼻肌的RNS阳性率分别为50.82%(31/61例)和42.62%(26/61例)。眼轮匝肌睑部RNS阳性率高于其他面神经支配肌(P=0.000~0.009),且疲劳试验后RNS阳性率高于疲劳试验前(P=0.037)。结论眼肌型MG患者眼轮匝肌睑部RNS阳性率高于其他面神经支配肌,疲劳试验可进一步提高其阳性结果 。     

Single‐fiber EMG and clinical correlation in Lambert‐Eaton myasthenic syndrome

Objectives: We analyzed 82 single‐fiber EMG (SFEMG) tests in the extensor digitorum communis muscle in 30 Lambert‐Eaton myasthenic syndrome (LEMS) patients to study the relationship between electrodiagnostic findings and clinical severity. Methods: The repetitive nerve stimulation test was performed in the abductor digiti quinti and flexor carpi ulnaris muscles. SFEMG was performed in the extensor digitorum communis muscle using the conventional method. Results: Fiber density was normal in all patients. Jitter was abnormal in all patients at the first evaluation regardless of clinical severity. The jitter was increasingly abnormal with worsening disease severity. Mean MCD correlated well with clinical and electrophysiological severity. In 5 potential pairs in 3 patients, MCD analysis in relation to firing rate showed improvement with increasing firing rates, which is consistent with presynaptic neuromuscular transmission disorders. Conclusions: In all LEMS studies, SFEMG was abnormal on the first evaluation. The mean MCD correlated well with clinical and electrophysiological disease severity on the repetitive nerve stimulation test. Muscle Nerve 47: 664–667, 2013