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Myotonia congenita (Thomsen, dominant)
This form of myotonia primarily affects skeletal muscles, leading to stiffness that can improve with repeated movement. It is caused by genetic changes in the CLCN1 gene and typically appears in childhood.
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Overview
Myotonia congenita, also known as Thomsen disease when inherited in a dominant manner, is a genetic disorder affecting muscle function. Individuals with this condition experience myotonia, which is the delayed relaxation of muscles after voluntary contraction [PMID:31542159]. This means that after performing a movement, such as gripping an object or standing up, the muscles may remain contracted for a short period, leading to stiffness. The symptoms often improve with repeated activity, a phenomenon sometimes referred to as the 'warm-up' effect.
Symptoms & clinical features
The primary symptom of myotonia congenita (Thomsen type) is muscle stiffness, which can range from mild to more severe. This stiffness is often most noticeable after a period of rest and can improve as muscles are used repeatedly [PMID:20301389]. Common situations where stiffness is experienced include standing up from a seated position, starting to walk, or initiating movements after sleep. Some individuals might find eye closure difficult or notice stiffness in their hands or legs. Unlike some other muscle conditions, individuals with Thomsen myotonia congenita typically do not experience progressive muscle weakness or muscle wasting.
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Affected organs
Myotonia congenita (Thomsen type) primarily affects skeletal muscles throughout the body. These are the muscles responsible for voluntary movement, such as those in the limbs, trunk, and face. The heart muscle (cardiac muscle) and the muscles of internal organs (smooth muscle) are generally not affected by this condition.
Risks & severity
The severity of myotonia in Thomsen disease can vary significantly among affected individuals, even within the same family. Symptoms typically begin in childhood, often before the age of 10 [PMID:20301389]. While the muscle stiffness can impact daily activities, it does not usually lead to significant disability or reduce life expectancy. Most individuals learn to manage their symptoms, and the 'warm-up' phenomenon often helps to alleviate stiffness during activity. The condition is not known to be progressive in terms of muscle weakness.
Genetic causes
Myotonia congenita (Thomsen type) is caused by pathogenic variants in the CLCN1 gene. This gene provides instructions for making a protein called ClC-1, which functions as a chloride channel. Chloride channels are essential for regulating the electrical excitability of muscle cells. In skeletal muscle, the ClC-1 channel plays a crucial role in stabilising the electrical charge across the muscle cell membrane, which is vital for normal muscle relaxation after contraction [PMID:31542159]. When the CLCN1 gene has a pathogenic variant, the ClC-1 channel may not function correctly, leading to an imbalance in muscle cell excitability. This can result in prolonged muscle contraction and the characteristic stiffness seen in myotonia.
- CLCN1 chloride voltage-gated channel 1The CLCN1 gene provides instructions for creating chloride channels, crucial for the proper electrical signalling and relaxation of skeletal muscles.
Inheritance pattern
Myotonia congenita (Thomsen type) follows an autosomal dominant inheritance pattern. This means that a person only needs to inherit one copy of the altered CLCN1 gene from one parent to develop the condition. If a parent has the condition, there is a 50% chance with each pregnancy that their child will inherit the pathogenic variant and develop the condition. This inheritance pattern affects both males and females equally. Unaffected family members typically do not carry the pathogenic variant and therefore cannot pass it on.
Each child has a 50% chance of inheriting the pathogenic variant, regardless of sex.
Diagnosis & testing
Diagnosis of myotonia congenita (Thomsen type) is often suspected based on the characteristic clinical symptoms, such as muscle stiffness that improves with exercise. A specialist clinician, such as a neurologist, may conduct a physical examination and take a detailed medical history. Electromyography (EMG), which measures the electrical activity of muscles, can help confirm the presence of myotonia. Definitive diagnosis is made through genetic testing of the CLCN1 gene, which can identify the specific pathogenic variant. In the UK, such testing may be accessed via the NHS Genomic Medicine Service, often following referral from a neurologist or clinical geneticist. Relevant testing falls under specific R-codes within the NHS Genomic Test Directory.
Management & lifestyle
Management for myotonia congenita (Thomsen type) is primarily focused on symptom relief and improving quality of life. For many, the symptoms are mild and do not require specific medical intervention, with individuals learning to manage their stiffness through strategies like gradual warm-up before activity. In cases where symptoms are more bothersome, certain medications may be considered to help reduce muscle stiffness. These decisions are made by a neurologist, who can provide personalised advice. Physical therapy and occupational therapy can also be beneficial in developing strategies to cope with daily challenges posed by muscle stiffness. Regular follow-up with a specialist is often recommended.
UK care pathway
In the UK, individuals suspected of having an inherited muscle condition like myotonia congenita can be referred to specialist services within the NHS. This typically involves seeing a neurologist or a clinical geneticist. If a genetic condition is suspected, they can refer for genetic testing through the NHS Genomic Medicine Service, following established R-codes for neuromuscular disorders. Genetic counsellors are also available to provide information and support to individuals and families, helping them understand the condition, its inheritance pattern, and the implications of a genetic diagnosis.
Frequently asked questions
Will myotonia congenita (Thomsen type) get worse over time?
Myotonia congenita (Thomsen type) is generally considered a non-progressive condition. While the severity of muscle stiffness can vary, it typically does not lead to increasing muscle weakness or significant disability as a person ages.
Can myotonia congenita (Thomsen type) be cured?
Currently, there is no cure for myotonia congenita (Thomsen type). However, symptoms can often be managed effectively through lifestyle adjustments, physical therapy, and in some cases, medication to help reduce muscle stiffness and improve daily function.
What should I do if I think my child has myotonia congenita?
If you suspect your child has myotonia congenita or is experiencing unusual muscle stiffness, you should consult your GP. They can refer you to a paediatrician or a neurologist for further assessment and potential genetic testing to confirm a diagnosis.
Are there any specific exercises or activities I should avoid?
Generally, individuals with myotonia congenita are encouraged to stay active. While sudden, strenuous movements might trigger stiffness initially, regular, gradual warm-up can often help. A physiotherapist can provide tailored advice on safe and beneficial exercises, and there are typically no specific activities that must be avoided, unless advised by a specialist.
How does this condition affect pregnancy?
Women with myotonia congenita (Thomsen type) can typically have healthy pregnancies. However, some women may experience changes in symptoms during pregnancy or after childbirth due to hormonal fluctuations. It is important to discuss your condition with your obstetrician and neurologist to ensure appropriate monitoring and management throughout pregnancy.