Introduction

Centronucleomyopathy, or CNM, is an uncommon neuromuscular condition that is marked by the abnormal position of the nucleus in muscle fibre cells, resulting in muscular weakness and compromised motor functions (the body's ability to coordinate movements).¹ CNM is a complicated and clinically diverse disorder that requires comprehensive treatment plans spanning various medical specialities.² A multidisciplinary approach is an integral component in managing CNM effectively, incorporating expertise from neurology, pulmonology, physical therapy, genetics, and other fields, which can provide combined yet uniform results by addressing the wide range of symptoms and complications associated with the disease.³

Given the exclusiveness and variability of centronuclear myopathy, it is highly important to practice updated standards of care in order to boost patient outcomes and implement evidence-based treatment plans. This blog focuses on the current standards of care in treating centronuclear myopathy.⁴

Understanding centronuclear myopathy (CNM)

Muscles are made up of individual muscle fibres that combine together into bundles and then form muscle mass. These individual muscle fibres are basic muscle cells with a cell membrane (sarcomere), cytoplasm (sarcoplasm), and nucleus (myonuclei). In centronuclear myopathy, the nucleus is located in the centre of the muscle fibre instead of its usual peripheral (outer) position. This structural change at the cellular level differentiates CNM from other diseases with varied genetic backgrounds.^3,4

As CNM is a congenital muscle disorder, the symptoms usually appear at birth or during the first year of life. There are a few cases that are associated with slow progression and can manifest symptoms in childhood or even in adulthood. The typical symptoms of CNM are muscle weakness, generalised hypotonia (reduced muscle tone), and respiratory difficulties due to the compromised capability of respiratory muscles. The severity and the age of onset are highly dependent on the type of genetic mutation involved.³

Molecular and genetic basis of centronuclear myopathy

The appearance of CNM may be associated with abnormalities in the following genes. These genes are responsible for performing key functions in the muscle fibres and maintaining the overall health of muscles.⁵

• The MTM1 gene synthesises myotubularin protein, which keeps the cell intact by regulating the molecular balance through homeostasis and retaining the structure of the cell membrane. If the mutation occurs in this gene, it leads to a severe form of CNM, accompanied by extreme muscle weakness and hypotonia⁶
• The DNM2 gene plays a role in cell membrane remodelling (a dynamic cellular process involving continuous changes in cellular membranes, their composition, structure, and morphology) and manages cell endocytosis. The mutations here typically affect the cell's cytoskeleton (network of tubes that keep the cell shape and framework intact) and cellular transportation of biomolecules (membrane trafficking). The type of CNM caused by this mutation is mild and progresses slowly. The symptoms do not appear early and may develop later on in childhood or adulthood^7,8
• The BN1 gene is responsible for the formation of key structures in the muscle fibres called the T-tubules. These are the continuations of cell membranes that penetrate deep into the muscle fibre (cytoplasm and nucleus). They transmit electrical signals from the outer surface of the cell to the inner cell contents, and actively trigger the release of calcium ions and ultimately muscle contraction. Genetic alterations in this gene disrupt the normal development of T-tubules and affect the active contraction and relaxation mechanism of muscle and various forms of CNM^{3 9}
• Other genes, namely the RYR1, TTN, and sometimes SPEG, can also cause CNM, highlighting the variety and complexity of the disease^{10, 11}

In summary, CNM results from genetic defects primarily by targeting parallel proteins that are responsible for intricate regulation of membrane dynamics and structural organisation in muscle fibres, resulting in the appearance of pathological and clinical features of the CNM. ¹²

How is centronuclear myopathy diagnosed?

CNM is typically diagnosed by a combination of clinical assessment, including physical signs and symptoms, electromyography and imaging studies (MRI), genetic testing (through blood samples), and biopsy (a small sample of muscle tissue is surgically removed, usually from the deltoid muscle or quadriceps muscle).¹³ If parents are carriers, the clinical assessment and genetic testing are performed during pregnancy.¹⁴ The doctor performs a detailed clinical assessment and points out all the key symptoms of CNM, including muscle weakness and reduced muscle tone, and then genetic testing is performed to find out the specific gene mutation involved.¹⁵ Finally, a biopsy is performed to confirm the abnormal positioning of the nucleus within the cell fibres.¹⁶ 

Current standards of care in centronuclear myopathy

The current standards of care for centronuclear myopathy are well-established and research-based management techniques for treating centronuclear myopathy. These standards include the incorporation of a combined approach from various healthcare disciplines and the provision of holistic care for improved patient outcomes, enhanced quality of life, and physical functioning.^17,5

Neuromuscular assessment

A neuromuscular specialist plays a central role in the ongoing management of CNM. They help in keeping the record of disease progression by carefully performing standardised muscle tests, namely CHOP INTEND (Children’s Hospital of Philadelphia Infant Test of Neuromuscular Disorders) and the Medical Research Council (MRC) muscle grading scale. A neuromuscular specialist will monitor any supplementary signs of pain and fatigue that can negatively influence quality of life.^18,19,20

Respiratory care

Routine pulmonary assessment is vital to keep a regular check on lung capacity and the strength of the respiratory muscles.³ Nocturnal hypoventilation (reduced number of breaths during sleep) is common during sleep-related breathing disorders in CNM that require constant attention.¹⁹ To carefully implement home-based ventilation systems, it is crucial to thoroughly train caregiver for their operation and utilisation. Additionally, preventive vaccinations such as influenza and pneumococcal can benefit individuals with a compromised respiratory system.²¹

Physical therapy and rehabilitation 

Physical therapy focuses on stretching exercises and orthoses (braces) to prevent joint contractures and maintain muscle function. Assistive devices such as wheelchairs and walkers can be adapted according to individual patients and can help in preventing injury and elevating the quality of life. Hydrotherapy (due to reduced gravitational stress on muscles and joints) facilitates muscle movement without adding any unwanted strain. Occupational therapy helps in developing skills and tactics to encourage independence in work and everyday life.^22,3

Nutritional assessment

In extreme cases, aspiration pneumonia can occur due to paralysed swallowing muscles; some severe cases may require using feeding tubes to ensure adequate nutrition. Diets rich in calories and nutrients are essential to meet the extra metabolic needs and support overall health. Nutritional assessments play a significant role in curating dietary patterns to keep up with the disease progression.²³

Surgical intervention 

Muscular imbalances can cause skeletal deformities, especially scoliosis, that can directly affect respiratory capacity and quality of life. Muscles support the bones and joints and keep them in place, and because the muscles are directly affected in CNM, it can lead to severe consequences, namely hip joint instability, drooped head syndrome, facial bone deformities, joint contractures, and foot deformities. All these extreme deformities can be life-threatening and require surgical intervention.^10,24,21

Psychological and social support

CNM can have long-lasting effects on the psychological health of patients and caregivers. Access to patient counselling, CNM support groups, and other mental health services is essential to maintain the mental well-being of patients. Connecting individuals and caregivers to organisations and support groups can provide valuable resources for education and treatment. Moreover, it can open the doors to social communication and networking with affected individuals and families.^{25, 26,27,5}

Cardiac care

The human heart is a pumping organ that works tirelessly. It is made up of cardiac muscles, and these muscles can be affected by disorders such as CNM. Some types of CNM, specifically associated with a mutation in the BN1 gene. This mutation can directly impact the T cell function and cause disruptions in the cardiac muscle contraction and relaxation mechanism (the heart's pumping). Individuals with CNM can develop cardiac arrhythmias (irregular heartbeats) and structural heart disorders (heart abnormalities at birth).²⁸ Vigilant cardiac care and consistent monitoring are crucial for early detection of any heart-related abnormality.²⁹

Summary

Centronuclear myopathy is a hereditary condition caused by mutations in certain genes. In most cases, CNM can be detected and diagnosed at birth. However, there are types of CNM that develop slowly and show symptoms in childhood or even adulthood. CNM is a progressive disorder that affects almost all the muscles of the human body, and because of this, it is paramount to define the standards of its care that highlight the incorporation of several healthcare disciplines and professionals to reach a common goal of improved patient outcome and quality of life.  

FAQs

How rare is centronuclear myopathy?

Based on data from recent evidence, CNM affects approximately 0.08 to 0.44 per 100,000 (0.00044% to 0.00088%) individuals in the general population. This is an extremely rare disease and affects fewer than 1 person per 100,000 individuals.³⁰

What is the life expectancy of people diagnosed with centronuclear myopathy? 

The life expectancy depends on the individual patient's condition and the rate of disease progression. In cases when the symptoms are severe, like respiratory complications, life expectancy is low. However, if disease progression is slow and symptoms are mild, a patient may lead a healthy life with a nearly normal life expectancy.