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To report SUSPECTED ADVERSE REACTIONS, contact argenx at 1-833-argx411 or FDA at 1-800-FDA-1088 or www.fda.gov/medwatch
To report SUSPECTED ADVERSE REACTIONS, contact argenx at 1-833-argx411 or FDA at 1-800-FDA-1088 or www.fda.gov/medwatch
Congenital Myasthenic Syndromes (CMS)
CMS education hub providing an overview of CMS, their key clinical features, and common misdiagnoses
What are congenital myasthenic syndromes (CMS)?
CMS are a group of rare, heterogeneous genetic disorders of neuromuscular transmission that cause fatigable muscle weakness in children and adults1–6
Clinical presentation is variable and depends on the genetic subtype.1 However, clinical exam findings could include a child or adult with:1,5,7,8
- Fatigable muscle weakness
- Affecting extraocular, facial, bulbar, respiratory, trunk, and/or limb muscles
- No autoantibodiesa
- Decremental response on repetitive nerve stimulation
- Normal or mildly elevated creatine kinase levelsb
- Normal or non-specific muscle biopsy findings
If it could be CMS, order a free genetic test to diagnose, or refer to a specialist for support
If it is CMS, consider referring your patient for enrollment into a clinical study to improve understanding of the disease
Learn more:
Free genetic testing
Referring to a specialist
Diagnosing
CMS
Educational events
CMS clinical studies
CMS are often mistaken for other neuromuscular disorders, most commonly myasthenia gravis (MG), congenital myopathy, and muscular dystrophy6,9,10
Delayed diagnosis or misdiagnosis can result in patients receiving inappropriate interventions and can delay access to treatments suitable for their specific CMS subtype6,7,11
Signs that indicate a child or adult may have CMS, rather than another neuromuscular condition:
Myasthenia Gravis
The following should raise suspicion of CMS, rather than MG:1,5,8,10,12-14
- No autoantibodiesa
- No response to immunotherapy
- Fatigable weakness that is limb-girdle predominate
- Symptoms that began in infancy or early childhood
Congenital Myopathy
The following should raise suspicion of CMS, rather than congenital myopathy:1,5,8,15,16
- Fatigable or fluctuating muscle weakness
- Normal or non-specific muscle biopsy findings
- Decrement on repetitive nerve stimulation
Muscular Dystrophy
The following should raise suspicion of CMS, rather than muscular dystrophy:1,5,8,17–19
- Fatigable or fluctuating muscle weakness
- Normal or mildly elevated creatine kinase levelsb
- Normal or non-specific muscle biopsy findings
- Decrement on repetitive nerve stimulation
Additional CMS information
Diagnosis and referral resources
Explore CMS diagnosis, specialist support options, and current research opportunities to help improve outcomes for people living with CMS
Educational events
Find information on live and virtual events designed to enhance your understanding of CMS and their diagnosis
aSpecifically anti‑AChR, anti‑MuSK, and anti‑LRP4 autoantibodies; however, a lack of autoantibodies does not exclude seronegative MG.
bNormal or slightly elevated (usually ≤10-fold normal value) CK levels.
1. Finsterer J. Orphanet J Rare Dis. 2019;14(1):57; 2. Rodríguez Cruz PM, et al. Int J Mol Sci. 2018;19(6):1677; 3. Ramdas S, et al. Curr Opin Neurol. 2024;37(5):493–501; 4. Ohno K, et al. J Hum Genet. 2025. https://doi.org/10.1038/s10038-025-01355-9; 5. Abicht A, et al. GeneReviews. https://www.ncbi.nlm.nih.gov/books/NBK1168/. 2003 May 9 [updated 2021 Dec 23]. Accessed 02 February 2026; 6. Theuriet J, et al. Brain. 2024;147(11):3849–62; 7. Practical Neurology. Iyadurai SJP. Congenital Myasthenic Syndrome Treatment. www.practicalneurology.com/diseases-diagnoses/neuromuscular/congenital-myasthenic-syndrome-treatment/31555/ July 2019. Accessed March 2026; 8. Maggi L, et al. Neurol Sci. 2019;40(3):457–68; 9. Kinali M, et al. J Neuroimmunol. 2008;201–202:6–12; 10. Kao J, et al. Neurology. 2018; 6;91(19):e1770–7; 11. Della Marina A, et al. Front Hum Neurosci. 2020;14:560860; 12. Vinciguerra C, et al. Brain Sci. 2023;13(9):1286; 13. Alshehri AM, et al. Ther Adv Neurol Disord. 2025;18:17562864251346333; 14. Wiendl H, et al. Ther Adv Neurol Disord. 2023;16:17562864231213240; 15. North KN, et al. Neuromuscul Disord. 2014;24(2):97–116; 16. Claeys KG. Dev Med Child Neurol. 2020;62(3):297–302;
17. Attarian S, et al. J Neurol. 2024;271(9):5778–803; 18. Kang PB, et al. Neurology. 2015;84(13):1369–78; 19. Straub V, et al. Neuromuscul Disord. 2018;28(8):702–10.
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