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Health condition

Congenital myasthenic syndrome, a genetic disorder of the neuromuscular junction.

Not autoimmune myasthenia gravis. CMS is genetic, subtype-specific and needs gene-directed treatment through a specialist neuromuscular centre.

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Why trust this guide

  • 01

    Editorial team

    Written by our editorial team from the UK sources listed on each page.

  • 02

    Sourced from guidance

    Checked against UK specialist neuromuscular centre pathways, ENMC consensus and peer-reviewed sources.

  • 03

    Current for 2026

    Reflects modern UK practice including expanded gene panels, subtype-directed therapy and emerging gene-based options.

Key facts

CMS at a glance.

The essentials in plain English - what CMS is, why it isn’t myasthenia gravis, and how UK specialist centres approach it.

  • What it is

    A rare heterogeneous group of genetic disorders of the neuromuscular junction. Distinct from autoimmune myasthenia gravis - no autoantibodies.

  • How rare

    Prevalence around one in 500,000. Onset can be from birth to adulthood, but classically infancy or childhood.

  • Genetics

    Mutations in more than 30 genes across presynaptic, synaptic, postsynaptic and glycosylation pathways. AChR subunits and RAPSN are among the most common.

  • Hallmark feature

    Fatigable muscle weakness that worsens with activity and improves with rest. Ptosis, ophthalmoparesis and limb-girdle weakness are common.

  • Diagnosis

    Clinical pattern plus EMG (repetitive nerve stimulation, single-fibre EMG), negative myasthenia antibodies and confirmatory genetic testing.

  • Treatment principle

    Subtype and gene-directed - the right drug for one CMS can worsen another. Specialist neuromuscular input is essential.

Why this guide matters

The right gene guides the right drug.

CMS is rare, heterogeneous and often misdiagnosed as muscular dystrophy or myasthenia gravis. The three points below shape everything else on this page.

  • CMS is not autoimmune

    Antibody tests are negative. CMS is genetic - immunotherapy does not help and can distract from the real diagnosis.

  • Genes drive treatment choice

    Pyridostigmine helps AChR-deficiency and RAPSN, but worsens slow-channel, COLQ and DOK7 CMS. Salbutamol is now first-line for DOK7.

  • Specialist care is essential

    UK neuromuscular centres coordinate diagnosis, medication, respiratory support and family counselling - and can advise on trials.

How the diagnosis is made

From first symptoms to a confirmed gene.

The steps a UK neurologist or paediatric neuromuscular team will normally follow, in order - so you know what to expect.

  1. 01

    Assessing

    History and family pattern

    Fatigable weakness, floppy infant, delayed milestones or limb-girdle pattern. Family history and consanguinity matter.

  2. 02

    Assessing

    Neuromuscular examination

    Ptosis, ophthalmoparesis, bulbar signs, fatigability on repeated effort, limb-girdle weakness and respiratory reserve.

  3. 03

    Assessing

    Rule out myasthenia gravis

    AChR, MuSK and LRP4 antibodies are negative in CMS. This is a key distinction from autoimmune myasthenia.

  4. 04

    Confirming

    EMG and repetitive stimulation

    Decremental response at low-frequency stimulation, occasional increment at high frequency, and increased jitter or blocking on single-fibre EMG.

  5. 05

    Confirming

    Edrophonium or neostigmine trial

    Some subtypes improve, others do not respond or may worsen. Interpretation is subtype-specific and specialist-led.

  6. 06

    Genetics

    Genetic panel and sequencing

    Targeted CMS panel first, then whole-exome or whole-genome sequencing to confirm the gene and guide therapy.

  7. 07

    Genetics

    Specialist neuromuscular MDT

    Referral to a UK neuromuscular centre - Queen Square, Newcastle, Oxford, Great Ormond Street, Alder Hey, Sheffield, Cambridge or Manchester.

Typical timeline: from first neurology visit to a confirmed genetic diagnosis can take months, but it changes treatment.

Symptoms

What CMS actually looks like.

Fatigable weakness is the hallmark. The pattern varies by subtype - from floppy babies with feeding difficulty to adults with limb-girdle weakness.

  • Fatigable weakness

    Weakness that worsens with activity and improves with rest - the clinical hallmark across CMS subtypes.

  • Ptosis and ophthalmoparesis

    Drooping eyelids and reduced eye movements, often fluctuating through the day and worse when tired.

  • Bulbar features

    Dysphagia, dysarthria and a weak cry or cough. Poor feeding is common in infancy.

  • Floppy baby presentation

    Hypotonia, respiratory difficulty and delayed motor milestones in neonates and infants.

  • Limb-girdle weakness

    Waddling gait and proximal weakness that can mimic muscular dystrophy - typical of DOK7-CMS and LG-CMS glycosylation subtypes.

  • Distal weakness (slow-channel)

    Early wrist and finger extensor weakness suggests slow-channel CMS - treatment is different and stimulants can harm.

  • Adult-onset fatigue

    Some subtypes present in adolescence or adulthood with exercise intolerance, diplopia or limb-girdle weakness.

  • Red flag - respiratory crisis

    Episodic apnoea, worsening bulbar or breathing symptoms need urgent hospital assessment and specialist neuromuscular input.

Treatment

How CMS is treated in the UK.

Subtype and gene-directed. The wrong drug can worsen weakness, so specialist neuromuscular oversight is essential.

  • Pyridostigmine (Mestinon)

    An acetylcholinesterase inhibitor that helps postsynaptic AChR-deficiency and RAPSN CMS. Worsens COLQ, DOK7 and slow-channel subtypes.

  • Salbutamol and ephedrine

    Sympathomimetics that improve DOK7-CMS, COLQ and several other subtypes. Now first-line for DOK7 in specialist practice.

  • 3,4-diaminopyridine (amifampridine)

    Increases acetylcholine release. Effective in fast-channel, AChR-deficiency and some others. Ineffective or harmful in slow-channel and DOK7.

  • Quinidine or fluoxetine

    Open-channel blockers used in slow-channel CMS. Pyridostigmine and 3,4-DAP worsen this subtype and must be avoided.

  • Respiratory support

    Non-invasive ventilation for hypoventilation or apnoea. See our guide to non-invasive ventilation (NIV).

  • Feeding and swallow care

    Speech and language therapy input, thickened feeds or PEG feeding when bulbar weakness is significant.

  • Physiotherapy and rehabilitation

    Graded activity, orthotics, mobility aids and occupational therapy to protect function and prevent contractures.

  • Genetic counselling

    Confirms inheritance, supports family cascade testing and offers reproductive options including PGT for future pregnancies.

What this guide is based on

The sources behind every claim on this page.

UK specialist neuromuscular pathways, ENMC consensus and patient-organisation guidance, current at last review.

Key references

Guidelines and standards we relied on.

A quiet reminder

This guide is for information, not medical advice.

Your specialist neuromuscular team knows your subtype and history and can tell you which parts apply to you. If in doubt, get seen.

  • European Neuromuscular Centre (ENMC). Consensus workshops on congenital myasthenic syndromes.

  • Muscular Dystrophy UK. Congenital myasthenic syndromes information for families and clinicians.

  • Myaware. Patient information on CMS and myasthenia.

  • GeneReviews. Congenital Myasthenic Syndromes - clinical and genetic overview.

  • NHS England. Specialised neuromuscular service specifications.

Red flags

When CMS needs urgent attention.

Most day-to-day management sits in specialist clinics. These situations need urgent input or forward planning.

  • Respiratory crisis

    Increasing breathlessness, weak cough or apnoea needs urgent hospital assessment and specialist neuromuscular input.

  • Wrong-drug worsening

    Pyridostigmine in slow-channel, COLQ or DOK7 CMS can precipitate weakness. Never start empirically without a confirmed gene.

  • Medications that harm NMJ transmission

    Aminoglycosides, fluoroquinolones, macrolides, magnesium, neuromuscular blockers, beta-blockers and some statins can worsen weakness. Carry a MedicAlert.

  • Peri-operative risk

    Anaesthesia and neuromuscular blockers can trigger prolonged paralysis. Elective surgery needs specialist neuromuscular liaison in advance.

  • Sudden infant apnoea

    CHAT and some presynaptic CMS can cause episodic apnoea in infancy. Any unexplained apnoea needs urgent paediatric review.

  • Progressive bulbar weakness

    Worsening swallow or aspiration needs speech and language therapy plus consideration of PEG feeding.

  • Scoliosis and contractures

    Long-standing weakness can cause spinal and joint problems - orthopaedic and rehabilitation input matters.

  • Mental-health impact

    Living with a rare progressive condition carries a real emotional load. Ask for psychological support if you need it.

  • Pregnancy planning

    Some CMS medications need review before pregnancy. Genetic counselling and obstetric-neuromuscular liaison are important.

Living with it

A rare condition, with a clear team around you.

Four things that make the biggest difference day to day - pacing, medication awareness, specialist review and community.

A quiet reminder

Small, steady adjustments beat heroic weeks.

Sustained pacing, sensible medication choices and staying linked to your specialist team make the biggest long-term difference.

  1. 01 Pacing

    Work with the fatigue, not against it

    Rest breaks, energy conservation and graded activity make a real difference. Physiotherapy input helps.

  2. 02 Medicines

    Carry an up-to-date medication list

    Keep a MedicAlert or wallet card listing your CMS subtype and the drugs to avoid. Show it to any new clinician.

  3. 03 Team

    Stay linked to a specialist centre

    Annual review at a UK neuromuscular centre keeps your plan current as evidence and options change.

  4. 04 Community

    Connect with patient support

    Muscular Dystrophy UK and Myaware run peer support, information days and family networks for CMS.

Frequently asked

Everything we get asked about CMS.

Quick answers on genetics, EMG, medication choice and what to avoid.

  • What is congenital myasthenic syndrome?

    CMS is a rare group of genetic disorders that affect the neuromuscular junction - the point where nerve meets muscle. It causes fatigable weakness that worsens with activity and improves with rest. Unlike myasthenia gravis, CMS is not autoimmune and antibody tests are negative.

  • How is CMS different from myasthenia gravis?

    Myasthenia gravis is an autoimmune condition with antibodies to AChR, MuSK or LRP4, usually starting in adulthood. CMS is genetic, often starts in infancy or childhood, and antibodies are negative. Treatment for the two is very different, so getting the diagnosis right matters.

  • How is CMS diagnosed?

    Diagnosis combines the clinical pattern of fatigable weakness, EMG findings (a decrement on repetitive nerve stimulation and increased jitter on single-fibre EMG), negative myasthenia antibodies and confirmatory genetic testing through a specialist neuromuscular centre.

  • Are all CMS treated the same way?

    No - and this is critical. Pyridostigmine helps AChR-deficiency and RAPSN CMS but worsens slow-channel, COLQ and DOK7 subtypes. Salbutamol is now first-line for DOK7. Slow-channel CMS needs channel blockers like fluoxetine or quinidine. Treatment must be gene-directed.

  • Which medicines should be avoided in CMS?

    Aminoglycosides, fluoroquinolones, macrolides, magnesium, neuromuscular blockers, beta-blockers and some statins can worsen weakness. Anyone with CMS should carry a MedicAlert and ensure their GP, dentist and anaesthetist know before any procedure.

  • Is there gene therapy for CMS?

    Not in routine practice yet. Research into gene therapy and antisense oligonucleotides for some CMS subtypes is progressing, and specialist centres can advise on relevant trials. Standard care remains subtype-directed medication and multidisciplinary support.

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