What is the prevalence of Electromyography (EMG) mutations?

Prevalence of EMG Mutations

There is no such entity as an "EMG mutation"—electromyography (EMG) is a diagnostic test, not a genetic condition. You may be asking about mutations in genes that cause neuromuscular disorders detectable by EMG, or about the prevalence of specific genetic conditions diagnosed using EMG testing.

Clarifying the Question

EMG is an electrodiagnostic technique used to evaluate muscle and nerve function, not a gene or mutation 1, 2, 3. However, several genetic neuromuscular conditions are diagnosed with the help of EMG findings:

Most Common Genetic Conditions Diagnosed with EMG Support

Myotonic Dystrophy Type 1 (DM1):

• Prevalence varies by population, but classic DM1 affects approximately 1 in 8,000 individuals 4
• Caused by CTG trinucleotide repeat expansion in the DMPK gene, with pathogenic alleles having ≥50 repeats 4
• EMG shows characteristic myotonic discharges with myopathic motor unit action potentials 4
• Cardiac manifestations occur in approximately 80% of patients and are a major cause of mortality 5, 4

Myotonic Dystrophy Type 2 (DM2):

• Caused by CCTG repeat expansion in the CNBP gene 5, 4
• Cardiac problems are less frequent (10-20%) and typically less severe than DM1 5, 4
• More variable clinical manifestations with later onset (20-70 years) 5

Myotonia Congenita:

• Caused by mutations in the CLCN1 gene encoding chloride channel protein 6, 7
• Myotonic discharges are recorded in 95.8% of examined muscles on EMG 7
• Can present as "EMG disease"—diffusely increased insertional activity in otherwise asymptomatic individuals 6
• Specific prevalence data not provided in available guidelines

Charcot-Marie-Tooth Disease (CMT):

• CMT1A (PMP22 duplication) is the most common hereditary neuropathy, accounting for 70% of demyelinating CMT cases 5
• When testing is restricted to patients with clinically probable CMT1, the yield is 54-80% 5
• MFN2 mutations cause approximately 33% of axonal CMT2 cases 5
• Cx32 (GJB1) mutations account for approximately 12% of all CMT cases 5

Congenital Myopathies:

• Overall prevalence estimated at 1 in 26,000 to 28,000 5
• RYR1 mutations are most prevalent at approximately 1 in 90,000 5
• Cardiac involvement is rare but can include dilated cardiomyopathy and sudden death 5

Clinical Pitfalls to Avoid

• Do not confuse "EMG disease" (a descriptive term for unexpected EMG findings) with an actual genetic diagnosis—these patients may have subclinical myotonic disorders like asymptomatic myotonia congenita with CLCN1 mutations 6
• EMG is an extension of clinical examination, not a replacement—it must be interpreted in the context of clinical findings 1
• Do not delay genetic testing when hereditary neuropathy is suspected—a stepwise approach based on inheritance pattern and EMG findings (demyelinating vs. axonal) improves diagnostic yield 5

Diagnostic Algorithm for Suspected Genetic Neuromuscular Disease

If myotonic discharges are present on EMG:

• Consider genetic testing for DMPK (DM1) and CNBP (DM2) genes first 4
• If negative and patient is asymptomatic, test for CLCN1 mutations (myotonia congenita) 6, 7
• Obtain cardiac evaluation before anesthesia or sedation due to high cardiac risk in myotonic dystrophy 4

If demyelinating neuropathy pattern on EMG with positive family history:

• Test for PMP22 duplication first (70% yield in autosomal dominant demyelinating CMT) 5
• If negative, test for MPZ mutations (5% of CMT) and GJB1 mutations (12% of all CMT) 5

If axonal neuropathy pattern on EMG:

• Test for MFN2 mutations first (33% of CMT2 cases) 5
• Consider GJB1 testing if inheritance pattern is unclear 5