Non-dystrophic myotonias (NDM)

Muscle channelopathies are a group of conditions that affect how the muscles work. They are divided into two types: the periodic paralyses (PP) and the non-dystrophic myotonias, depending on their primary symptom. Non-dystrophic myotonias (NDM) are rare genetic conditions that cause episodes of muscle stiffness, known as myotonia. Some people with NDM will also experience muscle weakness.

There are two main types of NDM:

• Myotonia congenita
• Paramyotonia congenita

Symptoms of NDM usually start in early to late childhood or the teenage years. The main symptom is episodes of myotonia – when it’s hard to relax muscles after they’ve been used. This can feel like muscle stiffness, cramps, locking, or tightness. For some people, this can be painful and cause fatigue. Myotonia can happen in different muscles of the body, including those in the legs, arms, hands, abdomen, back, and face (including the tongue). This might cause difficulty walking, using stairs, standing from a seated position, releasing grip, or problems with chewing and swallowing. Episodes of myotonia can last for several seconds or minutes. They can be triggered by various factors, including temperature, movement after rest, or with exercise.

Myotonia congenita

In myotonia congenita, myotonia tends to affect the leg muscles the most but can happen in other muscles too. Triggers include long periods of rest or sitting, extreme cold or hot environments, and stress. Myotonia can usually be improved by using and activating the muscle repeatedly, a process known as the ‘warm-up phenomenon’.  This can be felt when standing up and when beginning to walk after sitting for a while. The first few steps can feel stiff and difficult but become easier as the movement continues.

Paramyotonia congenita

In paramyotonia congenita, myotonia affects the hands and face muscles the most, but can also happen in the legs and other muscles. Triggers include cold environments, exercise, long periods of rest or sitting, and stress. Unlike myotonia congenita, myotonia gets worse when repeatedly using the muscles. Some people with paramyotonia congenita can also experience episodes of muscle weakness or paralysis. This might last from a few minutes to hours.

Genetic changes

NDM is caused by genetic changes that affect ion channels in muscle cells. Ion channels control the flow of electrical signals that tell muscles to contract and relax. In myotonia, the muscles receive repeated signals instead of just one. These repeated signals cause the muscles to remain contracted for longer and take longer to relax.

• Myotonia congenita: Changes in the CLCN1 gene occur and affect the muscle chloride channels.
• Paramyotonia congenita: Changes in the SCN4A gene occur and affect the muscle sodium channels.

Inheritance

How NDM is inherited depends on the specific genetic change.

• Myotonia congenita can be inherited in two ways: autosomal dominant (Thomsen type) or autosomal recessive (Becker type).
• Paramyotonia congenita is inherited in an autosomal dominant pattern.

For more information, see our inheritance and genetics page.

NDMs can be hard to diagnose because they are rare, and symptoms can vary a lot between people, or be similar to other conditions.

A specialist may use a combination of the following to diagnose NDM:

• History of symptoms – what happens and when
• Physical examination
• Blood tests – to measure creatine kinase (CK) levels
• Nerve conduction study and electromyography (EMG) – to test the function of nerves and muscles and look for myotonia

Genetic testing, using a blood sample, can confirm a suspected diagnosis by identifying changes in the genes known to cause NDM.

For more information, see our diagnosis page.

People with NDM should have access to a specialist and healthcare team who have knowledge of NDM and can review their condition. All types of NDM can be managed with medication, alongside changes in diet and activity. If you do not have contact with a specialist doctor, speak to you GP about getting access to one.

All patients in England have access to the highly specialised muscle channelopathy service based in London. This service offers specialist diagnosis and treatment for patients with non-dystrophic myotonia or periodic paralysis. Patients can be referred to the specialist service by their GP or hospital specialist. Video appointments can be offered to those that cannot travel to London. For more information, visit The NHS Channelopathy Service.

Medication

There are two main medications used to treat myotonia: mexiletine and lamotrigine.  Mexiletine can be taken regularly or when needed to manage muscle stiffness. Before starting mexiletine, a heart test called an electrocardiogram (ECG) is needed, and sometimes another test called an echocardiogram (ECHO). The prescribing doctor will decide the right dose. Lamotrigine can also be used but needs to be taken regularly to be effective.

Exercise and physiotherapy

Exercise and staying active is important for everyone and can help maintain muscle strength and mobility. Because exercise can sometimes trigger symptoms, people with NDM should speak to a physiotherapist for advice before starting exercise. A physiotherapist is a healthcare professional who helps manage symptoms through movement, exercise, and manual therapy. They can put together a suitable exercise plan and discuss methods to help prevent falls and reduce stiffness.

Anaesthesia stops a person feeling pain during a procedure or surgery. People with NDM are at a higher risk of complications when using general anaesthesia. It’s essential that the anaesthetist and surgical team are aware of a diagnosis of NDM beforehand, if possible. They should carry out a pre-operative assessment.

Local or regional anaesthesia is preferred over general anaesthesia. If using general anaesthesia is needed, avoid depolarising agents like suxamethonium, as they can trigger a myotonic crisis with severe muscle stiffness. If a myotonic crisis occurs, it must be treated immediately with intravenous sodium channel blockers such as lidocaine.