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Abstract The nerve terminal and muscle membrane compose the neuromuscular junction. After opening the voltage-gated calcium channels, action potentials from the motor axons provoke a cascade for the acetylcholine release from synaptic vesicles to the synaptic cleft, where it binds to its receptor at the muscle membrane for depolarization. Low amplitude compound muscle action potential typically presents in presynaptic disorders, increasing by more than 100% after a 10-second effort in the Lambert-Eaton myasthenic syndrome and less in botulism. Needle electromyography may show myopathic motor unit action potentials and morphological instability ("jiggle") due to impulse blocking. Low-frequency repetitive nerve stimulation (RNS) is helpful in postsynaptic disorders, such as myasthenia gravis and most congenital myasthenic syndromes, where the number of functioning acetylcholine receptors is reduced. Low-frequency RNS with a decrement >10% is abnormal when comparing the 4th to the first compound muscle action potential amplitude. High-frequency RNS is helpful in presynaptic disorders like Lambert-Eaton myasthenic syndrome, botulism, and some rare congenital myasthenic syndromes. The high-frequency RNS releases more calcium, increasing the acetylcholine with a compound muscle action potential increment. Concentric needle records apparent single-fiber action potentials (spikes). A voluntary activation measures the jitter between spikes from two endplates. An electrical activation measures the jitter of one spike (one endplate). The jitter is the most sensitive test for detecting a neuromuscular junction dysfunction. Most neuromuscular junction disorders are responsive to treatment.

Resumo O nervo terminal e a membrana muscular compõem a junção neuromuscular. Após a abertura dos canais de cálcio dependentes de voltagem, os potenciais de ação do axônio motor provocam uma cascata de eventos que libera acetilcolina das vesículas para a fenda sináptica, ligando-se ao receptor na membrana muscular para despolarização. O potencial de ação muscular composto de baixa amplitude ocorre nas desordens pré-sinápticas, aumentando em mais de 100% após esforço de 10 segundos na síndrome miastênica de Lambert-Eaton e menos no botulismo. A eletromiografia pode mostrar potenciais de ação da unidade motora miopáticos e instabilidade morfológica ("jiggle") devido ao bloqueio do impulso. Estimulação nervosa repetitiva (ENR) de baixa frequência é útil nos distúrbios pós-sinápticos, como miastenia gravis e a maioria das síndromes miastênicas congênitas, quando há número reduzido de receptores de acetilcolina funcionantes. ENR de baixa frequência com decremento >10% é anormal comparando-se à amplitude do quarto com o primeiro potencial de ação muscular composto. ENR de alta frequência é útil nas doenças pré-sinápticas, como síndrome miastênica de Lambert-Eaton, botulismo e algumas síndromes miastênicas congênitas raras. ENR de alta frequência libera mais cálcio, aumenta acetilcolina, resultando em incremento do potencial de ação muscular composto. O eletrodo de agulha concêntrico registra potenciais de ação aparente de fibra única (PAAFU). Ativação voluntária mede jitter entre dois PAAFUs (duas junções neuromusculares). Ativação elétrica mede jitter de um PAAFU (uma junção neuromuscular). Jitter é o teste mais sensível para detectar disfunção de junção neuromuscular. A maioria dos distúrbios juncionais é responsiva ao tratamento.

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The aim of this study was to measure the muscle fiber conduction velocity (MFCV) in situ in the tibialis anterior muscle in healthy subjects. A total of 36 subjects matched for age and sex were studied. The MFCV was measured with a concentric needle by intramuscular monopolar needle electrical activation at a distance of 50 mm. The mean consecutive difference (MCD) of <5 µs was obtained after a median of 62 muscle fiber action potentials (MFAPs), confirming a direct muscle fiber activation. The measuring latency was at the median point of ascending depolarizing line of the MFAP. The calculated MFCV from 784 MFAPs was 4.10 ± 0.66 m/s, 3.99 ± 0.57 for female subjects (95%, 2.85 to 5.13), and 4.20 ± 0.73 for male subjects (95%, 2.74 to 5.67). The MFCV was 5.22% faster in male subjects. The calculated fast-to-slow MFCV ratio (F/S ratio) was 1.47 for female subjects (95%, 1.27 to 2.54) and 1.67 for male subjects (95%, 1.31 to 3.74). Aging significantly increased the F/S ratio. As the MFCVs mainly depend on the muscle diameter, their assessment is a quick and helpful tool for estimating it. Its variability by the F/S ratio is also a powerful tool in the follow-up of some neuromuscular disorders.

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Calculating the reference values for jitter parameters utilizing a disposable concentric needle have been already done for the most often tested muscles. Jitter, expressed as the mean consecutive difference (MCD), was measured in the Tibialis Anterior (TA), not routinely tested muscle. Jitter measurement was taken using the intramuscular microaxonal stimulation technique in 32 healthy subjects. The mean MCD and the mean MCD of the 27th value from the 32 subjects had a normal distribution and were 19.79 ± 2.72 µs and 26.88 ± 3.56 µs, respectively. The suggested limit for the mean MCD is ≥ 26 µs and for the individual values is > 34 µs.

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Objectives: To estimate the jitter parameters (single-fiber electromyography) in myasthenia gravis patients mostly by electrical activation in Frontalis, Orbicularis Oculi, and Extensor Digitorum muscles using a concentric needle electrode. Methods: Between 2009 and 2019, a total of 97 myasthenia gravis patients, 52 male, and mean age 54 years were included. Results: Any abnormal jitter parameter in individual muscles was 90.5% (Frontalis), 88.5% (Orbicularis Oculi), and 86.6% (Extensor Digitorum). Any jitter parameter combining Orbicularis Oculi and Frontalis muscle was abnormal in 100% for the ocular, and in 92.9% for the generalized myasthenia gravis. The most abnormal muscle was Orbicularis Oculi for the generalized, and Frontalis for the ocular myasthenia gravis. The decrement was abnormal in 78.4%, 85.9% for the generalized, and 25% for the ocular myasthenia gravis. The mean jitter ranged from 14.2 to 86 µs (mean 33.3 µs) for the ocular myasthenia gravis and from 14.4 to 220.4 µs (mean 66.3 µs) for the generalized myasthenia gravis. The antibody titers tested positive in 86.6%, 91.8% for the generalized, and 50% for the ocular myasthenia gravis. Thymectomy was done in 48.5%, thymoma was found in 19.6%, and myasthenic crisis occurred by 21.6%. Conclusion: The jitter parameters achieved a 100% abnormality in ocular myasthenia gravis if both the Orbicularis Oculi and Frontalis muscles were tested. There was a high jitter abnormality in generalized myasthenia gravis cases with one muscle tested, with about a 2% increase in sensitivity when a second is added. Concentric needle electrode jitter had high sensitivity similar to the single fiber electrode (93.8%), followed by antibody titers (86.6%), and abnormal decrement (78.4%).

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This study was designed to analyze the sensitivity, specificity, and accuracy of jitter parameters combined with repetitive nerve stimulation (RNS) in congenital myasthenic syndrome (CMS), chronic progressive external ophthalmoplegia (CPEO), and congenital myopathies (CM). Jitter was obtained with a concentric needle electrode during voluntary activation of the Orbicularis Oculi muscle in CMS (n = 21), CPEO (n = 20), and CM (n = 18) patients and in controls (n = 14). RNS (3 Hz) was performed in six different muscles for all patients (Abductor Digiti Minimi, Tibialis Anterior, upper Trapezius, Deltoideus, Orbicularis Oculi, and Nasalis). RNS was abnormal in 90.5% of CMS patients and in only one CM patient. Jitter was abnormal in 95.2% of CMS, 20% of CPEO, and 11.1% of CM patients. No patient with CPEO or CM presented a mean jitter higher than 53.6 µs or more than 30% abnormal individual jitter (> 45 µs). No patient with CPEO or CM and mild abnormal jitter values presented an abnormal decrement. Jitter and RNS assessment are valuable tools for diagnosing neuromuscular transmission abnormalities in CMS patients. A mean jitter value above 53.6 µs or the presence of more than 30% abnormal individual jitter (> 45 µs) strongly suggests CMS compared with CPEO and CM.

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OBJECTIVE: To measure the jitter parameters in muscles with denervation/reinnervation in 32 chronic radiculopathy cases. METHODS: Measurements were done in chronic denervated muscles by voluntary and electrical activation using a concentric needle electrode. RESULTS: Mean jitter was abnormal in 87.5% (mean 49.2 µs) and 81.25% (mean 36.8 µs), for voluntary and electrical activation. In muscles with fibrillation potentials (FPs), the mean jitter was abnormal in all cases, and impulse blocking was frequent (53.4-92.3%). In muscles without FPs, the mean jitter was abnormal in 78.9% for voluntary activation and 68.4% for electrical activation. No correlation was found between jitter and motor unit action potential amplitude. CONCLUSION: The muscles with FPs were associated with the immature spread of acetylcholine receptors (AChRs) throughout the muscle membrane. Conversely, the neuromuscular junctions (NMJs) assemble may be repressed by the already reinnervated muscles. For those, higher jitter may be due to the persistence of atrophic fibers expressing neonatal myosin heavy chain (MHC) and immaturity of NMJ composting instead of the overspread of immature AChRs. SIGNIFICANCE: Jitter measurement must be avoided in chronic denervated muscles, regardless of FPs' presence. The activity of reinnervated muscle could maintain neonatal MHC and repress new NMJs development.

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To study the jitter parameters in the distant (DM) and the adjacent muscle (AM) after botulinum neurotoxin type A (BoNT/A) injection in 78 patients, jitter was measured by voluntary activation in DM (n = 43), and in AM (n = 35). Patients were receiving BoNT/A injections as a treatment for movement disorders. Mean age 65.1 years (DM) and 61.9 years (AM). The mean jitter was abnormal in 13.9% (maximum 41.4 µs) of DM, and 40% (maximum 43.7 µs) of AM. Impulse blocking was sparse. We found no correlation of the mean jitter to age, BoNT/A most recent injection (days/units), number of muscles injected, total BoNT/A units summated, number of total BoNT/A sessions, beta-blockers/calcium channel blockers use, and cases with local spread symptoms such as eyelid drop/difficulty swallowing. Maximum mean jitter (41.4/43.7 µs) for DM/AM occurred 61 and 131 days since the most recent BoNT/A, respectively. The far abnormal mean jitter (32.6/36.9 µs) occurred 229 and 313 days since the most recent BoNT/A. We suggested that jitter measurement can be done after BoNT/A in a given muscle other than the injected one, after 8 (DM) and 11 (AM) months, with reference >33 µs and >37 µs, respectively.

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Objective: To estimate jitter parameters in myasthenia gravis in stimulated frontalis and extensor digitorum muscles using the concentric needle electrode. Methods: Forty-two confirmed myasthenia gravis patients, being 22 males (aged 45.6±17.2 years-old) were studied. Jitter was expressed as the mean consecutive difference (MCD). Results: MCD in extensor digitorum was 61.6 µs (abnormal in 85.7%) and in frontalis 57.3 µs (abnormal in 88.1%). Outliers represented 90.5% for extensor digitorum and 88.1% for frontalis. At least one jitter parameter was abnormal in 90.5% of the combined studies. Acetylcholine receptor antibody was abnormal in 85.7% of the cases. Conclusions: Stimulated jitter recordings measured from muscles using concentric needle electrode can be used for myasthenia gravis diagnosis with high sensitivity. Extensive normative studies are still lacking and, therefore, borderline findings should be judged with great caution. .

Objetivo: Mensurar os valores do jitter em pacientes com miastenia gravis nos músculos frontalis e extensor digitorum pela técnica estimulada, utilizando-se eletrodo de agulha concêntrica. Métodos: Foram estudados 42 pacientes, sendo 22 homens (idade 45,6±17,2 anos), com miastenia gravis confirmada. O jitter foi expresso como a média das diferenças consecutivas (MDC). Resultados: A MDC para o extensor digitorum foi 61,6 µs (anormal em 85,7%) e para o frontalis 57,3 µs (anormal em 88,1%). Outliers representaram 90,5% para o extensor digitorum e 88,1% para o frontalis. Pelo menos um parâmetro do jitter foi anormal em 90,5% dos estudos combinados. Anticorpo receptor de acetilcolina estava anormal em 85,7% dos casos. Conclusões: Jitter estimulado mensurado por meio de eletrodo de agulha concêntrica pode ser utilizado para diagnóstico de miastenia gravis com elevada sensibilidade. Estudos normativos mais amplos ainda são necessários e, portanto, valores limítrofes devem ser avaliados com cautela. .

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