Neonatal Congenital Myasthenic Syndrome Linked to Gene Variants: A Case Report and Treatment Insights

Introduction

Congenital myasthenic syndrome (CMS) is a group of rare inherited neuromuscular disorders characterized by muscle weakness that usually present from birth or early childhood [1]. The cause is usually genetic mutations that affect the neuromuscular junction, where nerve cells communicate with muscle cells to stimulate muscle contraction [1]. The estimated incidence of CMS is 1.5 to 9 per million [2].

The neuromuscular junction can be affected in various ways depending on the location of the mutation [1,2], and CMS is classified into presynaptic, synaptic, and postsynaptic types [3,4]. In presynaptic CMS, the nerve cells initiating the impulse are affected; in synaptic CMS, the space between nerve and muscle cells is affected; and in postsynaptic CMS, the muscle cells receiving the impulse are affected. Most CMS cases are postsynaptic, although presynaptic and synaptic mutations also occur [1,2]. CMS has been linked to 35 genes, each affecting proteins essential for the functioning of the neuromuscular junction, such as ion channels, structural proteins, and signaling molecules [5–7]. For instance, mutations in the CHAT gene, found in 4% to 5% of CMS cases, disrupt acetylcholine synthesis, which is essential for muscle contraction. This CMS subtype can improve with age [7].

Diagnosing CMS in newborns is challenging, as symptoms can resemble more common conditions, such as prematurity, birth asphyxia, and genetic syndromes affecting muscle tone and respiratory function, which can also cause weak muscles and poor feeding [7]. Symptoms in newborns include hypotonia and respiratory failure at birth, with sudden episodes of apnea and cyanosis [1,2]. In the neonatal period, patients with CMS can show difficulties when eating, with poor suck and choking spells. Infants can exhibit eyelid ptosis and facial, bulbar, and generalized weakness of striated muscles [8]. These symptoms can be triggered or exacerbated by fever, infection, stress, and vigorous physical activity. Arthrogryposis multiplex congenita and dysmorphic features can also be part of the syndrome [8]. However, diagnosing CMS in the neonatal period is challenging because prematurity, birth asphyxia, and other syndromes can exhibit similar symptoms and are much more common [8].

Diagnosing CMS is crucial and typically involves genetic testing to identify the specific mutation, which determines the appropriate treatment and management [7]. Treatment strategies can include medications to improve neuromuscular transmission, physical therapy, and sometimes surgical interventions [5–7]. CMS caused by mutations in the CHAT gene has been reported to be treatable with pyridostigmine [7].

In this report, we present a rare and severe case of a newborn girl with autosomal recessive CMS caused by compound heterozygosity for 2 pathogenic variants in the CHAT gene, illustrating the importance of early genetic diagnosis and tailored treatment.

Case Report

A 27-year-old pregnant woman at 31 weeks and 1 day gestation presented to the emergency department at 22: 00 with acute epigastric pain, diarrhea, vaginal spotting, and cramping. Over the preceding two hours, she had experienced increasing Braxton Hicks contractions. Blood tests and vital signs were normal, and an ultrasound revealed a live fetus in cephalic presentation with normal activity and amniotic fluid volume. She was admitted that evening for suspected imminent preterm labor. Apart from a prior ultrasound for suspected pes cavus, which she declined to follow up, her pregnancy had been uncomplicated.

To manage the risk of preterm delivery, she was treated with atosiban, betamethasone, and magnesium sulfate. However, cardiotocography revealed abnormal pre-terminal patterns, prompting discontinuation of atosiban and an emergency cesarean delivery at 03: 39 AM. The procedure was uneventful, revealing a heart-shaped uterus and a short umbilical cord, with approximately 100 mL of blood loss.

At birth, the newborn girl had a normal heart rate but no spontaneous movements or respiratory efforts, with an Apgar score of 2 at 1 min. She was edematous, with pitting edema, but had no dysmorphic features. Mask ventilation with the Neo-Puff improved her oxygen saturation and color, but she remained without respiratory effort. Her Apgar score was 4 at 5 min.

At 15 min after birth, she was nasally intubated with a 3.0 Blueline tube. Her oxygen saturation was maintained at 90%, requiring an oxygen fraction of 60% to 70%. Suspecting respiratory distress syndrome, we treated with 200 mg of endotracheal surfactant (Curosurf), which had no apparent effect. Umbilical venous access was established, and she received penicillin V (160 mg) and gentamicin (7.5 mg) empirically.

Umbilical cord blood gas samples were not analyzed, but capillary blood gases at 50 min after birth showed a pH of 7.05, pCO2 of 11.8 kPa, and a base excess of −7.8 mmol/L, which normalized an hour later (pH 7.31, pCO2 6.9, base excess −1.3). She was transferred to the Neonatal Intensive Care Unit for further evaluation. Upon arrival, she remained intubated and mechanically ventilated, with oxygen requirements maintained at 60% to 70%. A chest X-ray confirmed proper endotracheal tube placement and ruled out pneumothorax or other explanations for her oxygen needs. A second dose of surfactant yielded no significant improvement. Her blood pressure (mean 32 mmHg) and rectal temperature (36.3°C) were normal.

Approximately 1 h postpartum, she began to exhibit weak movements and signs of discomfort. She was treated with fentanyl (5+10 μg) and rocuronium (2 mg) to optimize ventilation compliance. At 2 h of age, she was transferred to the University Hospital in Odense, 70 km away.

Differential diagnoses, including asphyxia, intracranial hemorrhage, infection, and rare congenital disorders, were considered. Over the first 2 days, comprehensive investigations were conducted. There were no signs of infection, and metabolic urine analysis results were normal. Imaging studies, including cranial ultrasound and magnetic resonance imaging, were unremarkable. An ophthalmologic examination revealed spontaneously dilated pupils that later normalized. An electroencephalogram showed age-appropriate suppression patterns, with no abnormalities. Chest X-ray and echocardiography were also normal for her gestational age.

Genetic testing was initiated on the first day of life, to investigate the unusual presentation. Trio-genome sequencing and chromosomal microarray analysis were performed, with a 10-day turnaround time. Additional targeted tests for Prader-Willi and Temple syndromes were negative. On day 6, trio-genome sequencing identified 2 variants in the CHAT gene, encoding choline acetyltransferase (CHAT), a critical enzyme for acetylcholine synthesis.

DNA from peripheral blood was sequenced using the Illumina DNA PCR-Free Library Prep Kit on a NovaSeq 6000 platform, with data mapped to the GRCh38 reference genome. Variants were annotated and filtered using VarSeq software.

The first variant, CHAT c.1679A>G, is a maternally inherited missense mutation in exon 12. It has a minor allele frequency of 8.9e-05 in the gnomAD database and is classified in ClinVar as likely pathogenic. Bioinformatic analysis predicts it to be damaging, and prior functional studies demonstrated significantly reduced enzymatic activity.

The second variant, CHAT c.287-1G>C, is a paternally inherited splice-site mutation before exon 2, absent from gnomAD and ClinVar. Bioinformatic predictions suggest it disrupts the splice acceptor site, producing a non-functional protein. Pathogenic splice variants in CHAT have been linked to CMS.

Based on the clinical presentation and genetic findings, these 2 variants were determined to cause autosomal recessive CMS. The family was counseled about a 25% recurrence risk, with options for preimplantation genetic testing or prenatal diagnostics.

At 7 days, pyridostigmine was started at 1 mg 6 times daily (0.5 mg/kg). This improved feeding tolerance, allowed spontaneous movements closer to age-appropriate levels, and justified extubation. She was briefly extubated at 10 and 13 days but required re-intubation, due to persistent weakness. Pyridostigmine was increased to 32 mg/day but later reduced to 16 mg due to adverse effects.

At 4 weeks, intravenous neostigmine replaced pyridostigmine at an initial dose of 0.25 mg 8 times daily. She remained mechanically ventilated, experiencing occasional respiratory arrest episodes due to laryngomalacia, which was managed with high-dose caffeine. Neostigmine frequency increased to 12 times daily, and she continued to gain weight and develop comparably to other premature infants.

At 38 weeks +1 day postmenstrual age, she was extubated and required only nasal continuous positive airway pressure briefly. Within a week, high-flow oxygen was limited to nights, and respiratory support ceased entirely. Respiratory arrest episodes resolved, and salbutamol inhalations were discontinued.

Neostigmine transitioned to continuous subcutaneous administration via an Insuflon catheter at 0.09 mg/h (0.7 mg/kg/day). The parents were trained to manage the device. After 2.5 months, she was discharged and was bottle-fed, with no signs of club-foot. Neonatal physiotherapist evaluations were normal.

At corrected age 2 months, her neostigmine dose was adjusted to 0.6 mg/kg/day. At the time of this report, she was thriving, tolerated colds well, and continued physiotherapy twice weekly. Her Achilles tendons appeared short and require treatment.

Discussion

This case report highlights several important aspects of diagnosing and managing CMS, particularly in neonates. From this case, it can be learned that CMS can present with symptoms that overlap with more common neonatal conditions, making early diagnosis challenging. This underscores the importance of genetic testing, especially when symptoms of hypotonia, respiratory failure, and feeding difficulties are present, as they can point to a rare but treatable genetic disorder, like CMS. Moreover, the report emphasizes the significance of understanding the genetic underpinnings of CMS, to guide treatment strategies effectively.

CMS is a diverse group of inherited disorders caused by defects in neuromuscular transmission. Among these, mutations in the CHAT gene are significant, as they disrupt the synthesis of acetylcholine, a neurotransmitter crucial for muscle contraction [7,9–15]. Establishing the exact genetic diagnosis is important, because targeted and effective treatment is sometimes possible.

This report describes a rare case of a newborn girl with CMS due to compound heterozygosity for 2 pathogenic variants in the CHAT gene: a maternally inherited missense variant (CHAT c.1679A>G) and a paternally inherited splice variant (CHAT c.287-1G>C). The clinical presentation included severe hypotonia, respiratory failure at birth, and lack of spontaneous movements, consistent with CMS symptoms. However, making the diagnosis was complicated because the girl was born premature at gestational age 31 weeks +1 day, with a low Apgar score, making prematurity and birth asphyxia more likely diagnoses. Genetic analysis revealed the presence of the 2 pathogenic variants, both of which have been predicted to be damaging and likely lead to non-functional protein products.

The identified missense variant CHAT c.1679A>G has been previously reported and shown to reduce the enzymatic activity of the CHAT enzyme significantly. The splice variant CHAT c.287-1G>C, although not previously reported, is predicted to disrupt normal splicing and likely result in a truncated, nonfunctional protein.

The compound heterozygosity of the 2 variants led to the patient’s diagnosis of CMS, which explains the observed severe phenotype.

The clinical presentation of our patient exhibits a very classical case of CMS, with lack of movement due to weakness of striated muscles, apnea, and poor suck in the neonatal period. The hypotonia, episodic apnea, and difficulties with eating are all very well described in patients with CMS due to CHAT variants [8]. Although very common in patients with CMS [14,15], eyelid ptosis is not described in our patient.

Respiratory failure at birth, with episodes of apnea and cyanosis, is a cardinal symptom of CMS [8]. Although very frequent in patients with CMS due to pathogenic CHAT variants, it is not restricted to this subgroup of CMS [8,14,15]. The significant overlap in symptoms and clinical findings between the different subgroups of CMS and between CMS and other rare congenital diseases makes the clinical presentation inadequate to solely distinguish and diagnose the patients. Trio-genome sequencing is our standard first tier approach when the parents are available for testing. The limitations of trio-genome sequencing have to be considered, which is the reason the analysis for Prader-Willi and Temple syndrome was also initiated in this patient. Capalbo et al [9] previously classified this variant as pathogenic, and Ohno et al [10] performed a functional assay demonstrating that the CHAT enzyme harboring this variant exhibits significantly less enzymatic activity than the wild-type enzyme.

Early genetic analysis to conclude the subtype of CMS, and thereby the possibility to initiate early treatment, is therefore crucial to reduce morbidity and mortality.

The paternally inherited variant CHAT c.287-1G>C is a splice variant located in the acceptor site before exon 2 of the gene. This variant has not been reported in any of the databases GnomAD, ClinVar, or the Human Gene Mutation Database (HGMD; a curated database of reported genetic variants and publications describing their association [or lack thereof] to disease). Four out of five bioinformatic tools predict that this variant will remove the acceptor site and thereby likely result in a non-functional protein product. Two other splice variants in the CHAT gene have been reported as pathogenic in HGMD [11,12], supporting the notion that disrupted splicing is a mechanism of pathogenicity in the CHAT gene. Other pathogenic splice variants in the CHAT gene have been described, including that by Schwartz et al [11], who described a patient with CMS caused by a paternal variant abolishing the acceptor site before exon 10 of the CHAT gene, combined with a maternally inherited deletion of several genes on chromosome 10.

The management of CMS involves improving neuromuscular transmission through the use of acetylcholinesterase inhibitors, such as pyridostigmine and neostigmine. In the present case, the initial treatment with oral pyridostigmine showed some but still insufficient effect, and due to adverse effects, the regimen was adjusted to intravenous neostigmine, followed by continuous subcutaneous administration. This approach effectively managed the girl’s symptoms, allowing for gradual improvement in muscle strength, respiratory function, eating, and general development. The comparison with other CMS mutations, such as those involving acetylcholinesterase (ACHE) or RAPSN mutations, suggests that the response to acetylcholinesterase inhibitors can vary widely depending on the exact genetic defect. For example, patients with ACHE mutations can demonstrate a milder phenotype and can be more responsive to treatment than are those with mutations such as CHAT or RAPSN, which often result in more severe neuromuscular impairment.

Despite the severe initial presentation, our patient showed significant improvement with appropriate treatment. She was able to be weaned off mechanical ventilation, and her respiratory support requirements decreased over time. Continuous subcutaneous neostigmine administration was well-tolerated and effective. The girl’s development and thriving are closely monitored, with ongoing physiotherapy to support motor function. This suggests that with early diagnosis and aggressive treatment, patients with severe forms of CMS like this one can experience a significant improvement in outcomes. This emphasizes the importance of early genetic diagnosis to tailor therapy effectively, which is becoming an increasing trend in managing CMS.

The recurrence risk for autosomal recessive CMS in future pregnancies is 25%. Therefore, genetic counseling of the family is crucial before future pregnancies, and options such as preimplantation or prenatal genetic testing has been discussed with the parents. Additionally, understanding the evolution of treatment protocols for CMS, especially with evolving drug formulations (like subcutaneous neostigmine), helps ensure that families are well-informed about the potential for improving outcomes, even in severe cases. Moreover, comparing phenotype and genotype data across CMS mutations can assist clinicians in predicting the likely clinical course and therapeutic response.

Conclusions

This case underscores the critical role of genetic testing in diagnosing CMS, particularly when symptoms overlap with more common conditions. Trio-genome sequencing facilitated early genetic diagnosis, enabling the timely initiation of tailored treatment and significantly improving the patient’s clinical outcome. The use of acetylcholinesterase inhibitors, such as neostigmine, led to substantial improvement in this infant’s respiratory and motor function, even in the presence of severe CMS due to CHAT mutations. This report contributes to the growing knowledge of CMS management and highlights the potential for positive outcomes with early genetic diagnosis and targeted treatment, providing a reference for similar future cases.