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Nemaline myopathy
This condition primarily affects the skeletal muscles, which are responsible for voluntary movement. Symptoms often appear in infancy or childhood, leading to difficulties with feeding, breathing, and movement. Its severity can vary widely among individuals.
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Overview
Nemaline myopathy is a term for a group of genetic conditions that primarily affect the skeletal muscles, which are the muscles we can consciously control. The name 'nemaline' comes from the Greek word 'nema', meaning thread, referring to the characteristic rod-like inclusions found in muscle fibres when viewed under a microscope [PMID:16538576]. These 'nemaline bodies' are a key diagnostic feature, although their exact role in causing muscle weakness is still being researched.
The condition can manifest with varying degrees of severity, from mild forms with minimal impact on daily life to severe forms that can be life-threatening. While often apparent from birth or early infancy, some milder forms may present later in childhood or even adulthood. Nemaline myopathy is relatively rare, affecting approximately 1 in 50,000 live births.
Symptoms & clinical features
The main symptom of nemaline myopathy is muscle weakness, which can affect many different parts of the body. In infants, this often appears as hypotonia, also known as 'floppy baby syndrome', where muscles feel unusually relaxed or 'floppy'. This can lead to difficulties with feeding, such as problems with sucking and swallowing, and sometimes requires feeding support [PMID:16538576].
Weakness in the facial muscles may cause a distinctive facial appearance, and weakness in the muscles controlling breathing can be a significant concern, sometimes necessitating respiratory support. Motor milestones, like sitting, crawling, and walking, may be delayed. Older children and adults might experience difficulty with activities requiring muscle strength, such as climbing stairs or lifting objects. The severity of these symptoms can differ greatly, even within the same family.
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Affected organs
Nemaline myopathy predominantly affects the skeletal muscles throughout the body. These include muscles involved in movement of the limbs, trunk, and neck. Muscles responsible for breathing (such as the diaphragm and intercostal muscles) and those controlling facial expressions and swallowing are also frequently impacted. While the primary pathology is in the muscles, the heart muscle (cardiac muscle) is generally spared, meaning heart problems are not typically a direct feature of nemaline myopathy. The brain and nervous system are also usually unaffected.
Risks & severity
The severity of nemaline myopathy can range from mild to severe. The most severe forms can be present from birth, leading to significant muscle weakness, feeding difficulties, and respiratory compromise, potentially requiring extensive medical intervention and sometimes having a poor prognosis [PMID:16538576]. Milder forms might present later with less profound muscle weakness, and individuals can have a near-normal lifespan with appropriate management.
Risk factors for more severe outcomes typically involve early onset of symptoms and significant respiratory involvement. The specific gene involved and the type of genetic change can influence the predicted severity, though predicting an individual's exact course remains challenging. Regular monitoring of respiratory and nutritional status is crucial for all affected individuals.
Genetic causes
Nemaline myopathy is caused by changes (variants) in specific genes that are essential for normal muscle function. Two important genes associated with nemaline myopathy in our reference dataset are ACTA1 and NEB. The ACTA1 gene provides instructions for making alpha-actin, a key protein in skeletal muscle fibres that helps muscles contract. Pathogenic variants in ACTA1 can disrupt the normal structure and function of these muscle fibres, leading to weakness [PMID:16538576].
The NEB gene provides instructions for producing nebulin, a large protein that plays a crucial role in maintaining the structural integrity and proper functioning of skeletal muscles. Nebulin acts as a 'molecular ruler' within muscle fibres, helping to regulate the length of actin filaments. Pathogenic changes in NEB can interfere with this structural support, resulting in muscle weakness and the characteristic nemaline bodies observed in muscle biopsies.
- ACTA1 actin alpha 1, skeletal muscle
- NEB nebulinThe NEB gene encodes the nebulin protein, a critical component of skeletal muscle sarcomeres essential for muscle contraction and force generation.
Inheritance pattern
Nemaline myopathy is most commonly inherited in an autosomal recessive pattern. This means that an individual must inherit two altered copies of the same gene - one from each parent - to develop the condition. Parents who carry one altered copy of the gene are known as carriers; they typically do not show symptoms themselves because their other working copy of the gene compensates.
If both parents are carriers of a pathogenic variant in the same gene, there is a 25% chance with each pregnancy that their child will inherit two altered copies and develop nemaline myopathy. There is also a 50% chance their child will be a carrier like them, and a 25% chance their child will inherit two working copies of the gene. In some cases, nemaline myopathy can also be inherited in an autosomal dominant pattern or result from a new (de novo) genetic change that occurred in the affected individual, but the autosomal recessive form is more frequent.
When both parents are carriers, each child has a 25% chance of being affected, 50% of being a carrier, and 25% of being unaffected.
Diagnosis & testing
Diagnosis of nemaline myopathy typically begins with a clinical evaluation based on a person's symptoms and a physical examination. If nemaline myopathy is suspected, a muscle biopsy is often performed. This involves taking a small sample of muscle tissue to be examined under a microscope, where the characteristic nemaline bodies can be observed, aiding diagnosis [PMID:16538576].
Genetic testing is crucial for confirming the diagnosis and identifying the specific gene responsible. This often involves looking for pathogenic variants in genes like ACTA1 and NEB. In the UK, genetic testing for muscle weakness conditions, including nemaline myopathy, falls under the NHS Genomic Medicine Service (GMS) pathways. Referrals for genetic testing are typically made by a paediatrician, neurologist, or clinical geneticist. Relevant NHS R-codes for genetic testing may apply depending on the clinical presentation.
Management & lifestyle
Management of nemaline myopathy is supportive and aims to address the specific symptoms an individual experiences. This involves a multidisciplinary team approach, often including paediatricians, neurologists, respiratory specialists, physiotherapists, occupational therapists, and dietitians. Physical therapy can help maintain muscle strength and flexibility, while occupational therapy assists with adapting daily activities.
For individuals with feeding difficulties, nutritional support may be necessary, sometimes requiring tube feeding. Respiratory support, such as non-invasive ventilation, can be vital for those with breathing problems. Regular monitoring of muscle strength, respiratory function, and nutritional status is essential to adapt management strategies as needed. Genetic counsellors can provide information about the condition, inheritance patterns, and reproductive options.
UK care pathway
In the UK, individuals with suspected or confirmed nemaline myopathy are typically managed through the NHS Genomic Medicine Service (GMS). This pathway ensures access to specialist care, including diagnostic genetic testing. Patients are usually referred to clinical genetics services by their GP or another specialist, such as a neurologist or paediatrician. Genetic counsellors play a key role in explaining genetic test results, discussing the implications for the individual and their family, and outlining inheritance patterns.
Frequently asked questions
What are nemaline bodies?
Nemaline bodies are distinctive rod-like structures that can be seen in muscle fibres when a muscle biopsy sample is viewed under a microscope. Their presence is a characteristic feature of nemaline myopathy, giving the condition its name. These structures are thought to be made up of muscle proteins, but their exact role in causing muscle weakness is still being investigated.
Can nemaline myopathy be cured?
Currently, there is no cure for nemaline myopathy. Management focuses on supportive care, aiming to alleviate symptoms and improve quality of life. This includes therapies like physiotherapy, occupational therapy, and respiratory support, tailored to the individual's specific needs.
How does nemaline myopathy affect breathing?
Weakness in the muscles responsible for breathing, such as the diaphragm and intercostal muscles, can be a significant challenge in nemaline myopathy. This can lead to shallow breathing, recurrent chest infections, and sometimes requires respiratory support, such as non-invasive ventilation, especially during sleep.
What does 'autosomal recessive' inheritance mean for my family?
Autosomal recessive inheritance means that both parents are carriers of a pathogenic gene variant, typically showing no symptoms themselves. For each child they have, there is a 1 in 4 chance the child will inherit two copies of the altered gene and develop nemaline myopathy. Genetic counselling can provide a more detailed understanding of risks for your family.
Is nemaline myopathy always severe?
No, the severity of nemaline myopathy can vary widely. Some individuals experience severe symptoms from birth, while others have a much milder form with less significant muscle weakness that may appear later in life. The specific gene involved and the type of genetic change can influence the typical presentation, but individual outcomes can still differ.
References
- Nicolau S, Milone M. Sporadic Late-Onset Nemaline Myopathy: Current Landscape. Current neurology and neuroscience reports. 2023. PMID: 37856049
- Younger DS. Congenital myopathies. Handbook of clinical neurology. 2023. PMID: 37562885
- Bruno C, Minetti C. Congenital myopathies. Current neurology and neuroscience reports. 2004. PMID: 14683632
- Laitila J, Wallgren-Pettersson C. Recent advances in nemaline myopathy. Neuromuscular disorders : NMD. 2021. PMID: 34561123
- Goebel HH. Congenital myopathies. Seminars in pediatric neurology. 1996. PMID: 8795848
- de Winter JM, Ottenheijm CAC. Sarcomere Dysfunction in Nemaline Myopathy. Journal of neuromuscular diseases. 2017. PMID: 28436394
- Riggs JE, Bodensteiner JB, Schochet SS Jr. Congenital myopathies/dystrophies. Neurologic clinics. 2003. PMID: 14743649
- Shammas I, Alhammad R, Naddaf E. Filamin C-Associated Nemaline Myopathy. Neurology. 2024. PMID: 38657199