Myotonia congenita

What it is, symptoms and effects

From: MedlinePlus Genetics, National Library of Medicine

Myotonia congenita is a disorder that affects muscles used for movement (skeletal muscles). Beginning in childhood, people with this condition experience bouts of sustained muscle tensing (myotonia) that prevent muscles from relaxing normally. Although myotonia can affect any skeletal muscles, including muscles of the face and tongue, it occurs most often in the legs. Myotonia causes muscle stiffness that can interfere with movement. In some people the stiffness is very mild, while in other cases it may be severe enough to interfere with walking, running, and other activities of daily life. These muscle problems are particularly noticeable during movement following a period of rest. Many affected individuals find that repeated movements can temporarily alleviate their muscle stiffness, a phenomenon known as the warm-up effect.

The two major types of myotonia congenita are known as Thomsen disease and Becker disease. These conditions are distinguished by the severity of their symptoms and their patterns of inheritance. Becker disease usually appears later in childhood than Thomsen disease and causes more severe muscle stiffness, particularly in males. People with Becker disease often experience temporary attacks of muscle weakness, particularly in the arms and hands, brought on by movement after periods of rest. They may also develop mild, permanent muscle weakness over time. This muscle weakness is not seen in people with Thomsen disease.

Causes and biological mechanisms

From: MedlinePlus Genetics, National Library of Medicine

Mutations in the CLCN1 gene cause myotonia congenita. The CLCN1 gene provides instructions for making a protein that is critical for the normal function of skeletal muscle cells. For the body to move normally, skeletal muscles must tense (contract) and relax in a coordinated way. Muscle contraction and relaxation are controlled by the flow of charged atoms (ions) into and out of muscle cells. Specifically, the protein produced from the CLCN1 gene forms a channel that controls the flow of negatively charged chlorine atoms (chloride ions) into these cells. The main function of this channel is to stabilize the cells' electrical charge, which prevents muscles from contracting abnormally.

Mutations in the CLCN1 gene alter the usual structure or function of chloride channels. The altered channels cannot properly regulate ion flow, reducing the movement of chloride ions into skeletal muscle cells. This disruption in chloride ion flow triggers prolonged muscle contractions, which are the hallmark of myotonia.

Inheritance and family implications

From: MedlinePlus Genetics, National Library of Medicine

The two forms of myotonia congenita have different patterns of inheritance. Thomsen disease is inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder. In most cases, an affected person has one parent with the condition.

Becker disease is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. Most often, the parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but do not show signs and symptoms of the condition.

Because several CLCN1 mutations can cause either Becker disease or Thomsen disease, doctors usually rely on characteristic signs and symptoms to distinguish the two forms of myotonia congenita.

How common is it?

From: MedlinePlus Genetics, National Library of Medicine

Myotonia congenita is estimated to affect 1 in 100,000 people worldwide. This condition is more common in northern Scandinavia, where it occurs in approximately 1 in 10,000 people.

Reported clinical features and what the terms mean

The following findings are associated with this condition in Orphanet. They are not a checklist for diagnosing yourself, and they do not all occur in every affected person. Some are examination, imaging or laboratory findings that cannot be recognised at home.

The frequency labels describe how often a finding was reported among people with the condition in the source. They do not give the chance that a person with that symptom has the condition. Definitions below reproduce HPO terminology; they explain the term, not the likely severity in an individual.

EMG abnormality · Very frequent (99-80%)

Abnormal results of investigations using electromyography (EMG).

EMG: myotonic discharges · Very frequent (99-80%)

High frequency discharges in electromyography (EMG) that vary in amplitude and frequency, waxing and waning continuously with firing frequencies ranging from 150/second down to 20/second and producing a sound that has been referred to as a dive bomber sound.

Muscle stiffness · Very frequent (99-80%)

A condition in which muscles cannot be moved quickly without accompanying pain or spasm.

Myotonia · Very frequent (99-80%)

An involuntary and painless delay in the relaxation of skeletal muscle following contraction or electrical stimulation.

Myotonia with warm-up phenomenon · Very frequent (99-80%)

Myotonia that occurs after a period of rest and decreases with continuing exercise.

Myalgia · Frequent (79-30%)

Pain in muscle.

Skeletal muscle hypertrophy · Frequent (79-30%)

Abnormal increase in muscle size and mass not due to training.

Clumsiness · Occasional (29-5%)

Lack of physical coordination resulting in an abnormal tendency to drop items or bump into objects.

Other findings in the same source

From: Orphanet

Additional reported features include EMG: myopathic abnormalities (Occasional (29-5%)); Feeding difficulties in infancy (Occasional (29-5%)); Muscle spasm (Occasional (29-5%)); Progressive distal muscle weakness (Occasional (29-5%)); Arrhythmia (Very rare (<4-1%)); Cardiac conduction abnormality (Very rare (<4-1%)); Dysphagia (Very rare (<4-1%)). This is a selected summary, not a complete description of the condition.

Which doctor should you see?

The suggested department for discussing Myotonia congenita is Neurology, with a neurologist as the relevant type of clinician. General physician / Family Medicine; paediatrician for children. Referral depends on symptoms.

Additional services that may be relevant, depending on the findings, include: Clinical Genetics.

This is an editorial referral starting point. The appropriate clinic depends on the person’s age, symptoms, previous diagnosis and local services. The first clinician can decide whether another specialty or a team is needed; a department label does not confirm the diagnosis.

How to prepare for an assessment

Bring a short timeline of the main symptoms: when they first appeared, whether they are constant or episodic, what seems to change them, and how they affect daily activities. Include previous reports, discharge summaries, current medicines and supplements, allergies, and any relevant family history. A dated record is more useful than trying to match every feature in an online article.

Ask the clinician what is already established and what remains uncertain. If a test is suggested, ask what question it answers, what its limitations are and how the result would change the next step. The information here is not an instruction to arrange every possible test. In children, bring growth, developmental and school information if it is relevant to the concern.

• Which nervous-system findings help explain the symptoms?
• Would an assessment of walking, communication or daily function be helpful?
• Are rehabilitation or other specialist services relevant?

Treatment discussions and follow-up

The material gathered for this draft does not provide a complete condition-specific treatment pathway for Myotonia congenita. That gap does not mean that treatment is unavailable. A clinician needs to establish the diagnosis and review current guidance before recommending medicines, procedures, rehabilitation or other support.

Before leaving the appointment, clarify the next review date, who will communicate results, and whom to contact if the situation changes. Discuss difficulties with sleep, work, school, mobility, eating or emotional wellbeing when these are relevant. Practical support may require coordination between the treating clinician and other services.

The collected references do not establish a complete prevention or long-term outlook section for this entry. Missing information should not be interpreted as proof that prevention is impossible or that a particular outcome is inevitable. Ask what is known for the exact subtype, stage and personal circumstances, and which uncertainties remain.

When to seek emergency help

Severe breathing difficulty, collapse, new stroke-like symptoms, a seizure that is prolonged or repeated without recovery, uncontrolled major bleeding, or an immediate risk of self-harm require emergency help. In India, call 112 or reach the nearest emergency department. This is a general, non-exhaustive warning list; it is not a condition-specific triage tool.