Centronuclear myopathy

What it is, symptoms and effects

From: MedlinePlus Genetics, National Library of Medicine

Centronuclear myopathy is a condition characterized by muscle weakness (myopathy) and wasting (atrophy) in the skeletal muscles, which are the muscles used for movement. The severity of centronuclear myopathy varies among affected individuals, even among members of the same family.

People with centronuclear myopathy begin experiencing muscle weakness at any time from birth to early adulthood. The muscle weakness slowly worsens over time and can lead to delayed development of motor skills, such as crawling or walking; muscle pain during exercise; and difficulty walking. Some affected individuals may need wheelchair assistance as the muscles atrophy and weakness becomes more severe. In rare instances, the muscle weakness improves over time.

Some people with centronuclear myopathy experience mild to severe breathing problems related to the weakness of muscles needed for breathing. People with centronuclear myopathy may have droopy eyelids (ptosis) and weakness in other facial muscles, including the muscles that control eye movement. People with this condition may also have foot abnormalities, a high arch in the roof of the mouth (high-arched palate), and abnormal side-to-side curvature of the spine (scoliosis). Rarely, individuals with centronuclear myopathy have a weakened heart muscle (cardiomyopathy), disturbances in nerve function (neuropathy), or intellectual disability.

A key feature of centronuclear myopathy is the displacement of the nucleus in muscle cells, which can be viewed under a microscope. Normally the nucleus is found at the edges of the rod-shaped muscle cells, but in people with centronuclear myopathy the nucleus is located in the center of these cells. How the change in location of the nucleus affects muscle cell function is unknown.

Causes and biological mechanisms

From: MedlinePlus Genetics, National Library of Medicine

Centronuclear myopathy is most often caused by mutations in the DNM2, BIN1, or TTN gene. The proteins produced from the DNM2 and BIN1 genes are involved in endocytosis, a process that brings substances into the cell. The protein produced from the BIN1 gene plays an additional role in the formation of tube-like structures called transverse tubules (or T tubules), which are found within the membrane of muscle fibers. These tubules help transmit the electrical impulses necessary for normal muscle tensing (contraction) and relaxation. The protein produced from the DNM2 gene also regulates the actin cytoskeleton, which makes up the muscle fiber's structural framework. DNM2 and BIN1 gene mutations lead to abnormal muscle fibers that cannot contract and relax normally, resulting in muscle weakness.

The TTN gene provides instructions for making a protein called titin that is an essential component of muscle fiber structures called sarcomeres. Sarcomeres are the basic units of muscle contraction; they are made of proteins that generate the mechanical force needed for muscles to contract. TTN gene mutations decrease or alter titin's activity in muscle fibers. It is unclear how these mutations lead to centronuclear myopathy, but it is likely that the altered protein cannot interact with other proteins in the sarcomere, leading to dysfunction of the sarcomere. Abnormal sarcomeres prevent muscle fibers from contracting and relaxing normally, resulting in muscle weakness.

Some people with centronuclear myopathy do not have identified mutations in the DNM2, BIN1, or TTN genes. Mutations in other genes associated with this condition are found in a small percentage of cases. Some males with signs and symptoms of severe centronuclear myopathy may have a condition called X-linked myotubular myopathy, which is similar to centronuclear myopathy, and is often considered a subtype of the condition, but has a different genetic cause. In some people with centronuclear myopathy, the cause of the disorder is unknown. Researchers are looking for additional genes that are associated with centronuclear myopathy.

Inheritance and family implications

From: MedlinePlus Genetics, National Library of Medicine

When centronuclear myopathy is caused by mutations in the DNM2 gene, it is inherited in an autosomal dominant pattern, which means one copy of the altered DNM2 gene in each cell is sufficient to cause the disorder. Rarely, BIN1 gene mutations that are inherited in an autosomal dominant pattern can cause centronuclear myopathy.

Centronuclear myopathy caused by TTN gene mutations and most cases caused by BIN1 gene mutations are inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.

Other cases of centronuclear myopathy that are not caused by these genes are typically inherited in an autosomal recessive manner, although some follow an autosomal dominant pattern.

How common is it?

From: MedlinePlus Genetics, National Library of Medicine

Centronuclear myopathy is a rare condition; its exact prevalence is unknown.

Which doctor should you see?

The suggested department for discussing Centronuclear myopathy is Clinical Genetics, with a clinical geneticist as the relevant type of clinician. Paediatrician (children) or physician (adults), with clinical geneticist referral.

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.

• Does the exact genetic or chromosome finding explain the observed features?
• Would a genetic counsellor help the family understand the result?
• Which organ-specific assessments are appropriate for this particular diagnosis?

Treatment discussions and follow-up

The material gathered for this draft does not provide a complete condition-specific treatment pathway for Centronuclear myopathy. 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.