Clinical and neurophysiological characteristics of congenital myasthenic syndromes presenting in early infancy

Challenges in Diagnosing and Treating Myasthenia Gravis in Infants and Children with Presentation of Cases

Myasthenia gravis (MG) is a rare condition that impairs function at the neuromuscular junction of skeletal muscles, seen less commonly in children. Causes include autoimmune MG, congenital myasthenic syndromes, and transient neonatal myasthenia gravis. Symptoms of weakness, hypotonia, and fatigability can be reasonably explained by more common causes, thus children with MG disorders commonly experience delays in treatment with severe consequences. This leads to the progression of disease and serious complications including myasthenic crises and exacerbations. We describe 5 cases of MG, which illustrate clinical and genetic challenges in establishing diagnosis and subsequent consequences of delayed diagnosis.

Vocal Cord Paralysis and Feeding Difficulties as Early Diagnostic Clues of Congenital Myasthenic Syndrome with Neonatal Onset: A Case Report and Review of Literature

Herein, we present a newborn female with congenital vocal cord paralysis who required a tracheostomy in the neonatal period. She also presented with feeding difficulties. She was later diagnosed with a clinical picture of congenital myasthenia, associated with three variants of the MUSK gene: the 27-month follow-up was described. In particular, the c.565C>T variant is novel and has never been described in the literature; it causes the insertion of a premature stop codon (p.Arg189Ter) likely leading to a consequent formation of a truncated nonfunctioning protein. We also systematically collected and summarized information on patients' characteristics of previous cases of congenital myasthenia with neonatal onset reported in the literature to date, and we compared them to our case. The literature reported 155 neonatal cases before our case, from 1980 to March 2022. Of 156 neonates with CMS, nine (5.8%) had vocal cord paralysis, whereas 111 (71.2%) had feeding difficulties. Ocular features were evident in 99 infants (63.5%), whereas facial-bulbar symptoms were found in 115 infants (73.7%). In one hundred sixteen infants (74.4%), limbs were involved. Respiratory problems were displayed by 97 infants (62.2%). The combination of congenital stridor, particularly in the presence of an apparently idiopathic bilateral vocal cord paralysis, and poor coordination between sucking and swallowing may indicate an underlying congenital myasthenic syndrome (CMS). Therefore, we suggest testing infants with vocal cord paralysis and feeding difficulties for MUSK and related genes to avoid a late diagnosis of CMS and improve outcomes.

Single fiber electromyography and measuring jitter with concentric needle electrodes

This monograph contains descriptions of the single fiber electromyography (SFEMG) method and of the more recently implemented method of recording jitter with concentric needle electrodes (CNEs). SFEMG records action potentials from single muscle fibers (SFAPs), which permits measuring fiber density (FD), a sensitive measure of reinnervation, and jitter, a sensitive measure of abnormal neuromuscular transmission (NMT). With voluntary activation, jitter is measured between two SFAPs with acceptable amplitude and rise time. With activation by axon stimulation, jitter is measured between the stimulus and individual SFAPs. Pitfalls due to unstable triggers and inconstant firing rates during voluntary activation and subliminal stimulation during axon stimulation should be identified and avoided. In CNE recordings, spikes with shoulders or rising phases that are not parallel are produced by summation of SFAPS; these should be excluded and reference values for CNE jitter should be used. CNE and SFEMG have similar and very high sensitivity in detecting increased jitter, as in myasthenia gravis and other myasthenic conditions. However, jitter is also seen in ongoing reinnervation and some myopathic conditions. With SFEMG, these can be identified by increased FD; however, FD cannot be measured with CNE, and conventional electromyography should be performed in muscles with increased jitter to detect neurogenic or myogenic abnormalities. Jitter is abnormal after injections of botulinum toxin, even in muscles remote from the injection site, and can persist for 6 mo or more. This can complicate the detection or exclusion of abnormal NMT.

Single fiber EMG and measuring jitter with concentric needle electrodes

This monograph contains descriptions of the single-fiber electromyography (SFEMG) method and of the more recently implemented method of recording jitter with concentric needle electrodes (CNE). SFEMG records action potentials from single muscle fibers (SFAPs), which permits measuring fiber density (FD), a sensitive measure of reinnervation, and jitter, a sensitive measure of abnormal neuromuscular transmission (NMT). With voluntary activation, jitter is measured between two SFAPs with acceptable amplitude and rise time. With activation by axon stimulation, jitter is measured between the stimulus and individual SFAPs. Pitfalls due to unstable triggers and inconstant firing rates during voluntary activation and subliminal stimulation during axon stimulation should be identified and avoided. In CNE recordings, spikes with shoulders or rising phases that are not parallel are produced by summation of SFAPS; these should be excluded and reference values for CNE jitter should be used. CNE and SFEMG have similar and very high sensitivity in detecting increased jitter, as in myasthenia gravis and other myasthenic conditions. However, jitter is also seen in ongoing reinnervation and some myopathic conditions. With SFEMG, these can be identified by increased FD; however, FD cannot be measured with CNE, and conventional EMG should be performed in muscles with increased jitter to detect neurogenic or myogenic abnormalities. Jitter is abnormal after injections of botulinum toxin, even in muscles remote from the injection site, and can persist for 6 mo or more. This can complicate the detection or exclusion of abnormal NMT.

Bedside and laboratory diagnostic testing in myasthenia

Myasthenia gravis (MG) and congenital myasthenic syndromes (CMS) are a group of disorders with a well characterised autoimmune or genetic and neurophysiological basis. We reviewed the literature from the last 20 years assessing the utility of various neurophysiological, immunological, provocative and genetic tests in MG and CMS. Diagnostic sensitivity of repetitive nerve stimulation test ranges between 14 and 94% and specificity between 73 and 100%; sensitivity of single-fibre EMG (SFEMG) test ranges between 64 and 100% and specificity between 22 and 100%; anti-acetylcholine receptor (AChR) antibody sensitivity ranges from 13 to 97% and specificity ranges from 95 to 100%. Overall, SFEMG has the highest sensitivity while positive anti-AChR antibodies have the highest specificity. Newer testing strategies that have been investigated over the last couple of decades include ocular vestibular-evoked myogenic potentials, otoacoustic emissions and disease-specific circulating miRNAs in serum for autoimmune myasthenia, as well as next-generation sequencing for genetic testing of CMS. While there has been significant progress in developing newer testing strategies for diagnosing MG and CMS over the last couple of decades, more research is needed to assess the utility of these newer tools regarding their sensitivity and specificity.

Electrophysiological study of neuromuscular junction in congenital myasthenic syndromes, congenital myopathies, and chronic progressive external ophthalmoplegia

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.

Utility and practice of electrodiagnostic testing in the pediatric population: An AANEM consensus statement

Nerve conduction studies and needle electromyography, collectively known as electrodiagnostic (EDX) studies, have been available for pediatric patients for decades, but the accessibility of this diagnostic modality and the approach to testing vary significantly depending on the physician and institution. The maturation of molecular diagnostic approaches and other diagnostic technologies such as neuromuscular ultrasound indicate that an analysis of current needs and practices for EDX studies in the pediatric population is warranted. The American Association of Neuromuscular & Electrodiagnostic Medicine convened a consensus panel to perform literature searches, share collective experiences, and develop a consensus statement. The panel found that electrodiagnostic studies continue to have high utility for the diagnosis of numerous childhood neuromuscular disorders, and that standardized approaches along with the use of high-quality reference values are important to maximize the diagnostic yield of these tests in infants, children, and adolescents.

Pulmonary Manifestations of Neuromuscular Diseases

Neuromuscular disorders comprise a phenotypically diverse group of inherited and acquired diseases; however, they share common pathophysiologic mechanisms which produce significant respiratory complications. Respiratory and bulbar muscle weakness gives rise to ineffective cough, swallowing dysfunction results in aspiration-related lung disease, and abnormal muscle tone produces chest wall deformities - all of which ultimately leads to repeated chest infections and chronic respiratory failure. In this article, the authors describe the respiratory manifestations of neuromuscular disease and their underlying pathophysiological mechanisms. This review also highlights the diagnostic and management tools recommended for acute and chronic care. Aggressive pulmonary management is the most impactful at reducing the overall morbidity and improving the quality of life of children with neuromuscular disorders, at least while definitive molecular and gene replacement therapies remain elusive.

Congenital myotubular myopathy with a novel MTM1 gene mutation in a premature infant presenting with ventilator dependency and intrahepatic cholestasis

Myotubular myopathy is a rare congenital disease characterized by hypotonia and respiratory compromise at birth in affected males. It causes high neonatal mortality. Most surviving newborns need prolonged ventilation and have significantly delayed motor development. Although all patients with congenital myotubular myopathy have respiratory problems such as atelectasis and recurrent lung infections, concurrent neonatal intrahepatic cholestasis is rare. We report a newborn with a myotubular myopathy, ventilator dependency, recurrent lung infections and pleural effusion, facial diplegia, ophthalmoplegia, and progressive intrahepatic cholestasis. A genetic study showed a novel mutation of the MTM1gene: c.1142 G>A (R381Q). We suggest that physicians consider probable concurrent disorders of other organs in neonates with congenital myotubular myopathy.

Congenital myasthenic syndromes

Congenital myasthenic syndromes (CMS) are a heterogeneous group of uncommon, inherited disorders affecting the neuromuscular junction. The defects interfere with presynaptic, synaptic, or postsynaptic function and compromise neuromuscular transmission. Most patients with CMS have similar clinical features regardless of the underlying defect, but attention to clinical and electrodiagnostic parameters can narrow the diagnostic spectrum. Recent advances in our understanding of the cellular mechanisms underlying specific syndromes allow DNA testing for some forms of CMS. Diagnosis of CMS enables a rationale for management to be developed. Two cases of genetically determined CMS as well as an undiagnosed infant are presented to highlight the clinical and electrophysiological difficulties associated with the diagnosis and management of such syndromes.

Neurophysiological strategies for the diagnosis of disorders of the neuromuscular junction in children

The disorders of the neuromuscular junction seen in children, the congenital myasthenic syndromes and autoimmune myasthenia gravis, are very rare. Their clinical symptoms and signs may be variable, most notably in the neonate and infant. They should enter the differential diagnosis of many different clinical presentations, such as 'floppy infant' or attacks of episodic apnoea. Many molecular genetic abnormalities have been discovered in congenital myasthenic syndromes, but screening for these is impracticable without confirmation of a disorder of the neuromuscular junction. In this review, the different neurophysiological tests that can be used are discussed, with their merits and contraindications in children. After consideration of the acceptability of the different techniques, it has been found that stimulation single-fibre electromyography (stimSFEMG) of orbicularis oculi seems to be the best choice in children. In most instances it can be performed on the conscious child and it has a good specificity and high sensitivity. A protocol for using stimSFEMG is described.

A case of congenital bilateral absence of elbow flexor muscles: review of differential diagnosis and treatment

A 1-year-old boy presented to us with congenital inability to flex his elbow. He had bilaterally absent biceps brachii and brachialis muscles, a rare condition. We performed pedicle latissimus dorsi musculocutaneous flaps to the left and right volar upper arm at 21 and 24 months of age, respectively, to create elbow flexors. By 4 years of age, he had excellent elbow flexion bilaterally with strength grade in excess of 4.5. In addition to discussing our patient's treatment options, we discuss other potential causes of weak elbow flexion when faced with this clinical dilemma.

A new analytical method to diagnose congenital myasthenia with stimulated single-fiber electromyography

Stimulated single-fiber electromyography (SSF-EMG) is useful to assess neuromuscular junction (NMJ) abnormalities in children. Conventionally mean consecutive difference (MCD) analysis measures the jitter for each muscle-fiber potential. We present a new algorithm that analyzes the entire SSF-EMG waveform. Cross-correlational coefficients (between 0-1.0) are calculated for consecutive pairs of 100 SSF-EMG waveforms obtained at each needle position in orbicularis oculi, and averaged. A lower normal limit (0.722, mean -3 SD) was established from 123 SSF-EMG samples in 10 adult control subjects, and applied to SSF-EMG data from 23 children referred for a suspected myasthenic syndrome. Results were compared with MCD analysis and related to the final clinical diagnosis. Our results showed that compared with conventional MCD measurement, the new algorithm had better specificity (87% vs. 53%) but similar sensitivity (88% for both). These findings indicate that the cross-correlational method is a useful predictor of NMJ dysfunction in children.