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Congenital myasthenic syndrome (RAPSN)
This rare neuromuscular disorder typically presents at birth or in early childhood. It leads to muscle weakness and fatigue due to problems at the neuromuscular junction, where nerve signals are transmitted to muscles.
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Overview
Congenital myasthenic syndromes (CMS) are a group of inherited conditions that disrupt the communication between nerves and muscles, a process that normally allows muscles to contract. This communication happens at a specialised junction called the neuromuscular junction. When this process is affected, it results in muscle weakness and fatigue. CMS caused by variants in the RAPSN gene is one specific form of this syndrome.
Individuals with RAPSN-related CMS typically experience symptoms from birth or early childhood. The severity of symptoms can vary widely among affected individuals. Because it is a congenital condition, meaning it is present from birth, it is usually recognised early in life.
Symptoms & clinical features
The symptoms of RAPSN-related CMS often include generalised muscle weakness, which can worsen with physical activity or during periods of stress or infection. Common early signs in infants may include poor feeding, difficulties with sucking and swallowing, and a weak cry. Affected babies might also show hypotonia, which is reduced muscle tone, making them appear 'floppy'.
As children grow, symptoms can include drooping eyelids (ptosis), double vision (diplopia), and weakness of facial muscles. Weakness in the neck and limb muscles can affect mobility and lead to delays in motor milestones, such as sitting or walking. In severe cases, weakness in the muscles used for breathing can occur, which may be life-threatening [PMID:18274158]. The fatigue and weakness tend to fluctuate and are often worse after periods of activity.
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Affected organs
RAPSN-related CMS primarily affects the skeletal muscles throughout the body. These are the voluntary muscles responsible for movement, including those controlling eye movements, facial expressions, speech, swallowing, limb movement, and breathing. While the condition doesn't directly affect organs like the heart or brain, the sustained weakening of muscles can have secondary impacts on overall health and development, particularly for breathing and feeding functions.
Risks & severity
The severity of RAPSN-related CMS can range from mild, with noticeable symptoms only during exertion, to severe, requiring significant medical support. Symptoms typically manifest at birth or in early childhood. Lifelong muscle weakness and fatigue are characteristic, but their impact can vary. Some individuals may experience episodes of worsening weakness, known as 'myasthenic crises', which can be triggered by infections or certain medications and may require urgent medical attention, especially if breathing muscles are affected [PMID:24190820]. The long-term outlook depends on the specific symptoms and their management.
Genetic causes
Congenital myasthenic syndrome caused by RAPSN variants occurs when there are pathogenic changes in both copies of the RAPSN gene. This gene provides instructions for making a protein called rapsyn. Rapsyn plays a crucial role at the neuromuscular junction, specifically in organising and anchoring acetylcholine receptors on the muscle cell surface [PMID:11342603]. Acetylcholine receptors are essential for receiving chemical signals from nerve cells, which then trigger muscle contraction.
When variants in RAPSN lead to a faulty or absent rapsyn protein, the acetylcholine receptors are not properly positioned or functional at the neuromuscular junction. This disrupts the efficient transmission of nerve signals to the muscle, leading to the characteristic muscle weakness and fatigue seen in this condition. The specific type of variant in the RAPSN gene can influence the severity of the condition.
- RAPSN receptor associated protein of the synapse
Inheritance pattern
RAPSN-related CMS is inherited in an autosomal recessive pattern. This means that an individual must inherit two altered copies of the RAPSN gene - one from each parent - to develop the condition. People who have only one altered copy of the RAPSN gene are called carriers. Carriers typically do not show symptoms of the condition themselves because their other unaffected copy of the gene can still produce enough functional rapsyn protein.
If both parents are carriers of a RAPSN variant, there is a 1 in 4 (25%) chance with each pregnancy that their child will inherit two altered copies of the gene and develop RAPSN-related CMS. There is also a 2 in 4 (50%) chance that their child will be a carrier like them, and a 1 in 4 (25%) chance that their child will inherit two unaffected copies of the gene and not be a carrier or have the condition.
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
Diagnosing RAPSN-related CMS typically involves a combination of clinical assessment and specialised tests. Neurologists will evaluate the individual's symptoms, medical history, and conduct a physical examination focused on muscle strength and function. Electromyography (EMG) and nerve conduction studies (NCS) are often performed to assess nerve and muscle electrical activity. Specific tests, such as repetitive nerve stimulation, may show a characteristic decrease in muscle response, indicating an issue at the neuromuscular junction.
Genetic testing is crucial for confirming a diagnosis of RAPSN-related CMS. This involves analysing the RAPSN gene to identify pathogenic variants. In the UK, genetic testing for suspected congenital myasthenic syndromes is available through the NHS Genomic Medicine Service (GMS) and can be accessed via an R-code, such as R315 Congenital Myasthenic Syndrome. Referrals for genetic testing are typically made by paediatricians or neurologists, often after consultation with a clinical genetics specialist who can also provide genetic counselling.
Management & lifestyle
Management for RAPSN-related CMS is supportive and aims to improve muscle strength and reduce fatigue, often involving a multidisciplinary team. Treatment typically includes medications that enhance the effect of acetylcholine at the neuromuscular junction, such as pyridostigmine. The dosage and type of medication are tailored to the individual's specific needs and response. Careful monitoring for side effects and effectiveness is essential.
Physiotherapy and occupational therapy can help maintain muscle strength, improve mobility, and assist with daily activities. For individuals with feeding difficulties, nutritional support may be necessary. Respiratory support might be required during times of severe weakness or myasthenic crises. Regular follow-up with a neurologist and other specialists is important to adjust treatment as needed and manage any complications. Genetic counselling is also recommended for families to understand the inheritance pattern and reproductive options.
UK care pathway
In the UK, individuals suspected of having a congenital myasthenic syndrome like RAPSN-related CMS are typically referred to specialist paediatric or adult neurology services. If the clinical picture is suggestive, genetic testing will be considered through the NHS Genomic Medicine Service (GMS). This often involves a referral to a clinical genetics service or a specialist regional genetics centre where genetic counselling can also be accessed. Genetic testing is typically requested using established R-codes on the GMS testing directory, such as R315 for Congenital Myasthenic Syndrome, to confirm the diagnosis and guide management.
Frequently asked questions
What is the likelihood of my future children having RAPSN-related CMS if I am a carrier?
If you are a carrier of a RAPSN variant, and your partner is also a carrier, there is a 1 in 4 (25%) chance with each pregnancy that your child will inherit two altered gene copies and develop the condition. If your partner is not a carrier, the risk is much lower, but it is always best to discuss this with a genetic counsellor to understand your specific situation.
Are there medications to treat RAPSN-related CMS?
Yes, medications such as pyridostigmine are often used to help improve muscle strength and reduce fatigue by enhancing communication at the neuromuscular junction. The choice and dosage of medication are carefully determined by a specialist based on the individual's symptoms and response.
Will my child's symptoms get worse over time?
The course of RAPSN-related CMS can vary, with some individuals experiencing stable symptoms and others having fluctuating periods of weakness. Symptoms can be managed with treatment, but they are generally lifelong. Regular medical follow-up is important to monitor and adjust care as needed.
Can certain activities or medications make the symptoms worse?
Yes, physical exertion, infections, stress, and certain medications (like some antibiotics or muscle relaxants) are known to potentially worsen muscle weakness and fatigue in individuals with CMS. It is important to discuss all medications and activities with your medical team.
Where can I get more information or support for RAPSN-related CMS in the UK?
Your clinical genetics team or neurologist is the best source for personalised medical information. Patient support groups for congenital myasthenic syndromes can also offer valuable resources and connection with other families. Organisations like Myasthenia Gravis UK may also provide relevant information and support.