Confirming the Diagnosis: Exams, EMG, and Genetics

The path to a diagnosis for myotonia congenita involves a combination of your personal history, a physical examination, and specialized technical tests. Because muscle stiffness can be a symptom of several different conditions, doctors use a “staged” approach to move from clinical suspicion to a definitive answer ^[1][2].

The Clinical Consultation

The first step is a detailed review of your symptoms and family history. Your doctor will look for the hallmark “warm-up phenomenon”—stiffness that is worst after rest but improves with repeated movement ^[3]. During the exam, they may perform a percussion test, where they tap a muscle (often the thumb or tongue) with a reflex hammer to see if it dimples or stays contracted, which is a physical sign of myotonia ^[1].

They will also look for hypertrophy, which is the presence of unusually large, well-defined muscles that occur because the muscle is constantly “working” even when you think it is resting ^[4].

Needle Electromyography (EMG)

An Electromyography (EMG) test is often used to help confirm that your stiffness is caused by electrical “hyperexcitability” in the muscle fibers ^[2].

• The Procedure: A tiny, thin needle is inserted into various muscles to listen to their electrical activity.
• Myotonic Discharges: If you have myotonia congenita, the EMG will record “myotonic discharges.” These are bursts of electrical signals that rapidly change in pitch and volume ^[5].
• The “Dive-Bomber” Sound: To a neurologist, these signals sound like a WWII dive-bomber plane or a revving motorcycle ^[4]. While this confirms your muscles are hyperexcitable, it doesn’t name the specific gene responsible, as other conditions can also produce this sound ^[2].

Blood Tests and CK Levels

Your doctor may order a blood test to check your Creatine Kinase (CK) levels. CK is an enzyme that leaks into the blood when muscle fibers are stressed or damaged. In myotonia congenita, CK levels are typically normal or only mildly elevated (often about 2 to 4 times the normal limit) ^[3][6]. Very high CK levels are unusual for this condition and might suggest a different type of muscle disease ^[7].

Genetic Testing

While clinical findings and an EMG can support a diagnosis, genetic testing is the standard way to definitively confirm a molecular diagnosis and help distinguish between the Thomsen (dominant) and Becker (recessive) forms ^[1].

• CLCN1 Sequencing: This test looks for “spelling errors” in the CLCN1 gene.
• Multigene Panels: Often, doctors use a panel that tests several genes at once, including CLCN1, SCN4A (linked to sodium-channel myotonia), and genes related to myotonic dystrophy ^[8]. This is helpful because the symptoms of these conditions can look very similar ^[9].
• Deletion/Duplication Analysis (MLPA): Sometimes standard sequencing misses a “missing page” in the gene. A test called MLPA may be needed if doctors strongly suspect the Becker form but can only find one mutation instead of two ^[10].
• Variant of Uncertain Significance (VUS): A VUS means a gene change was found, but it is not known if it causes the disease. A VUS is not diagnostic. Your doctor may recommend testing family members to help interpret it ^[11].

Differentiating Similar Conditions

A key goal of testing is to rule out other conditions that cause myotonia:

1. Sodium-Channel Myotonias (SCN4A): These are often triggered by eating potassium-rich foods or by cold, and they may affect the face more than the legs ^[12]. Paramyotonia congenita typically worsens with continued movement, rather than warming up.
2. Myotonic Dystrophy (DM1/DM2): These are systemic diseases that cause muscle wasting, cataracts, and heart issues ^[2]. Genetic testing can help tell these apart from myotonia congenita ^[13]. Importantly, some repeat expansions in DM1/DM2 require dedicated testing and might be missed by an ordinary multigene sequencing panel.

Components of a Typical Diagnostic Evaluation

When you receive your results, ensure the following have been addressed to have a complete picture of your diagnosis:

• [ ] Clinical Exam: Confirmation of the “warm-up” phenomenon and presence of percussion myotonia ^[1].
• [ ] EMG Report: Documentation of myotonic discharges in various muscle groups ^[2].
• [ ] Genetic Evaluation: Identification of specific mutations in the CLCN1 gene ^[1].
• [ ] Inheritance Pattern: Clarity on whether the variants support a dominant or recessive pattern ^[14].