Myasthenic crisis

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Myasthenia gravis reduces the safety margin of neuromuscular transmission through antibodies most commonly directed against the acetylcholine receptor or muscle-specific kinase. Respiratory infection or another physiological stress can turn fluctuating ocular, bulbar, limb and neck weakness into inadequate ventilation. The bedside danger is that the patient stays alert with reasonable saturations while respiratory muscles tire. Dysarthria after prolonged speaking, a wet voice, head drop, weak cough and difficulty handling saliva may signal crisis before routine observations do.

Respiratory assessment is a trajectory, not one threshold. Forced vital capacity, single-breath count, respiratory rate, cough, bulbar function and blood gases must be repeated by a team able to act on deterioration. Negative inspiratory force may add information where equipment and expertise are available. Facial weakness can make spirometry seals unreliable, and a deceptively preserved vital capacity does not neutralise aspiration or upper-airway risk. Non-invasive ventilation suits selected cooperative patients without severe bulbar dysfunction; delayed intubation after exhaustion is hazardous.

Acute therapy combines ventilation, trigger treatment and rapid reduction of pathogenic antibody. Plasma exchange often acts quickly but needs access and haemodynamic stability. Intravenous immunoglobulin is easier to deliver in some settings but carries thrombosis, renal and volume risks. Corticosteroid and longer-term immunosuppression control the underlying disease but do not replace rescue therapy. Exact ventilator, immunoglobulin, exchange and cholinesterase-inhibitor decisions belong to critical-care and neuromuscular specialists using current local protocols.

Key points

Myasthenic crisis is severe myasthenia gravis causing respiratory failure or unsafe bulbar function; it may be the first presentation rather than a complication of known disease.
Infection is a frequent precipitant, but surgery, pregnancy, aspiration, undertreatment, abrupt immunotherapy change and medicines that impair neuromuscular transmission must also be sought.
Ask the patient to count aloud on one breath and assess speech, swallowing, neck flexion, cough and accessory-muscle use, then obtain serial forced vital capacity through experienced respiratory staff.
A normal oxygen saturation does not exclude ventilatory failure because neuromuscular weakness first causes falling tidal volume and carbon-dioxide retention rather than primary gas-exchange failure.
Arterial or venous blood gas showing hypercapnia is a late and worrying sign; do not postpone airway planning until carbon dioxide rises or the patient becomes drowsy.
Bulbar weakness, copious secretions and inability to protect the airway may require intubation even when the measured vital capacity is not yet profoundly reduced.
Treat the trigger and give plasma exchange or intravenous immunoglobulin through a specialist plan; onset, vascular access, haemodynamic risk and local availability influence the choice.
Pyridostigmine is often withheld after intubation because secretions complicate ventilation, then reintroduced before extubation under neurological and critical-care direction.
Avoid or carefully review magnesium, aminoglycosides, fluoroquinolones, macrolides, beta-blockers, sedatives and neuromuscular blockers; the clinical need and alternatives determine action rather than an unqualified blacklist.
Steroids can transiently worsen weakness when introduced or escalated, so immunosuppression changes during crisis require monitored specialist planning.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Intercurrent infection

Respiratory and other infections commonly increase weakness through systemic stress, fever, secretions and reduced neuromuscular transmission reserve.

Medicine and procedure effects

Magnesium, selected antibiotics, beta blockers, sedatives, anaesthetic agents and surgery can impair transmission or ventilation in susceptible patients.

Treatment disruption

Missed pyridostigmine, abrupt immunotherapy change, undertreated disease or early corticosteroid worsening may permit rapid generalisation during instability.

Pregnancy, aspiration and physiological stress

Pregnancy-related change, aspiration, pain and other acute illness can precipitate crisis even when limb weakness initially appears stable.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Transmission reserve collapse
Autoimmune loss of functional postsynaptic receptors leaves insufficient safety margin to activate bulbar and respiratory muscle fibres reliably.
2
Bulbar protection failure
Weak palate, pharynx and cough cause secretion pooling, aspiration and inability to maintain a safe airway.
3
Ventilatory muscle failure
Diaphragmatic and accessory-muscle weakness reduces tidal volume and cough before hypoxaemia, eventually causing carbon-dioxide retention and fatigue.
4
Self-reinforcing deterioration
Aspiration, infection and exhaustion further impair transmission, creating rapid decline unless ventilation and immune rescue interrupt the cycle.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Bulbar fatigue

Speech becomes nasal or slurred, chewing slows, swallowing fails and secretions pool, often worsening with repeated testing and improving after rest.

Respiratory muscle weakness

Tachypnoea, shallow breathing, orthopnoea, paradoxical abdominal movement, weak cough and inability to complete sentences indicate ventilatory compromise.

Neck and limb pattern

Head drop and proximal limb weakness accompany ocular or bulbar features without sensory loss, sphincter disturbance or upper motor-neurone signs.

Precipitating infection

Fever may be absent under immunosuppression, so cough, dysuria, aspiration, infiltrates and inflammatory changes require active investigation.

Impending arrestRed flag

Exhaustion, drowsiness, silent weak cough, rising carbon dioxide or inability to clear secretions mandates immediate definitive airway action.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Serial forced vital capacityFirst step
Why
Measure the direction and severity of respiratory-muscle decline.
Interpretation and limitations
Use absolute and weight-related values as part of the whole bedside picture; a poor lip seal, fatigue and bulbar risk can make a single number falsely reassuring.
02
Blood gas
Why
Detect carbon-dioxide retention, acidosis and gas-exchange complications.
Interpretation and limitations
Hypercapnia indicates advanced ventilatory failure and should accelerate airway support; normal early gases do not exclude rapidly evolving weakness.
03
Chest radiograph and respiratory microbiology
Why
Identify pneumonia, aspiration, collapse or another precipitant.
Interpretation and limitations
Select cultures and viral testing by context, starting sepsis treatment without waiting when infection is clinically likely.
04
Full blood count, CRP, renal, liver and electrolytes
Why
Assess infection, treatment suitability and reversible metabolic aggravators.
Interpretation and limitations
Correct potassium, phosphate and other abnormalities carefully; renal impairment affects immunoglobulin risk and several antimicrobial choices.
05
ECG and continuous cardiac monitoring
Why
Detect arrhythmia and support safe use of interacting or haemodynamically active treatment.
Interpretation and limitations
Bradycardia with muscarinic symptoms suggests cholinergic excess, while tachyarrhythmia may reflect sepsis, hypoxia or treatment complications.
06
Myasthenia antibody and neurophysiology review
Why
Clarify diagnosis if crisis is the first presentation or phenotype is atypical.
Interpretation and limitations
Acetylcholine-receptor, MuSK or selected other antibodies and repetitive stimulation support classification, but rescue treatment must not await results.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Cholinergic excess

Diarrhoea, sweating, salivation, miosis and fasciculation with weakness suggest excess acetylcholinesterase inhibition, though this is less common than myasthenic deterioration.

Guillain–Barré syndrome or botulism

Areflexia and sensory symptoms support GBS, while fixed pupils and descending paralysis favour botulism; both can cause respiratory failure.

Pneumonia or pulmonary embolism

Primary gas-exchange failure can cause breathlessness and hypoxaemia without proportional fatigable weakness, although it may simultaneously trigger crisis.

Functional breathlessness

Positive inconsistency may support functional symptoms only after objective bulbar, ventilatory and cardiopulmonary emergencies are assessed.

Additional chapter-specific clues

Cholinergic alternative

Excess cholinesterase inhibitor can cause diarrhoea, sweating, salivation, bradycardia, miosis and fasciculation, but true cholinergic crisis is now uncommon.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01First minutesEscalate before decompensationFirst stepEscalationA patient with known or possible myasthenia develops breathlessness, dysphagia or rapidly worsening weakness.

1Call critical care and neurology, position safely, give suction and oxygen only as needed, and assess cough, secretions, speech, swallow, neck flexion and breathing pattern.
2Measure forced vital capacity serially and obtain a blood gas without allowing either result to delay intubation when exhaustion or airway protection is failing.
3Prepare difficult-airway and aspiration precautions, choosing non-invasive ventilation only for an alert, cooperative patient with manageable secretions and close monitoring.

02Trigger controlFind and reverse the precipitantRespiratory or bulbar decline has been stabilised sufficiently for parallel investigation.

1Look for chest, urinary and systemic infection, aspiration, recent surgery, pregnancy, missed medicines and abrupt changes to corticosteroid or immunosuppression.
2Reconcile every prescription and over-the-counter exposure for magnesium, certain antibiotics, beta-blockade, sedatives and other agents that can worsen transmission.
3Treat infection and metabolic abnormalities using alternatives with the lowest neuromuscular risk, involving microbiology and pharmacy when a concerning medicine remains essential.

03Rapid immunotherapyReduce pathogenic antibody burdenCrisis or a severe impending crisis is confirmed by the multidisciplinary team.

1Use plasma exchange first where it is available promptly and clinically suitable under current NHS England criteria; select intravenous immunoglobulin when exchange is unavailable, contraindicated or otherwise inappropriate after specialist review.
2Start the selected complete course with procedure-specific observations and laboratory monitoring, reviewing response daily rather than switching after one slow day.
3Plan corticosteroid and steroid-sparing therapy with neurology, anticipating possible early steroid-related worsening and infection prophylaxis where relevant.

04Ventilator liberationRestore treatment and function safelyStrength and the precipitating illness improve after rescue therapy.

1Demonstrate sustained cough, secretion control, adequate respiratory measures and improving bulbar function before an extubation trial, not merely a good oxygen saturation.
2Reintroduce pyridostigmine at a specialist-selected point before extubation when secretions are controlled, checking heart rate and gastrointestinal effects.
3After discharge revise the crisis plan, vaccinations, thymus and long-term treatment, and give the patient a medicine-warning and emergency-contact record.

Key medicines and prescribing safety3 treatments · regimens, roles and cautions

Provides rapid immunomodulation for myasthenic crisis when selected instead of plasma exchange.

Intravenous immunoglobulin

For life-threatening myasthenic crisis with intensive-care respiratory or bulbar failure, current NHS England criteria specify a total 2 g/kg over 2–5 days using the commissioned weight and infusion policy.

Assess thrombosis, renal impairment, fluid load, haemolysis, headache and aseptic meningitis; record product and batch and slow or stop infusion for significant reaction. The 1 g/kg-first regimen applies to a hospitalised non-life-threatening exacerbation, not to myasthenic crisis.

Improves neuromuscular transmission and symptomatic strength but does not remove the pathogenic antibody driving crisis.

Pyridostigmine

Usual oral dosing is individualised through the current BNF; during intubated crisis it may be withheld then cautiously reintroduced by neurology before extubation.

Excess causes salivation, diarrhoea, abdominal cramp and bradycardia, while secretions impede ventilation; renal impairment may increase exposure.

Provides longer-term immunosuppression after immediate ventilatory and antibody-removal treatment.

Methylprednisolone or prednisolone

Initiation and escalation follow a neuromuscular specialist schedule, often alongside rescue therapy rather than as an unsupervised emergency bolus.

Early worsening can occur; monitor infection, glucose, mood, blood pressure, bone and gastrointestinal risk and provide prophylaxis according to dose and duration.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Respiratory arrest and aspiration

Delayed airway support permits hypercapnic arrest, secretion obstruction and aspiration pneumonia without timely airway and ventilatory support.

Ventilator-associated morbidity

Prolonged intubation increases pneumonia, airway trauma, delirium, thrombosis and deconditioning while neuromuscular recovery evolves in critical care.

Cardiovascular instability

Hypoxia, infection, autonomic stress and treatment can provoke arrhythmia, pressure instability and cardiac injury during severe crisis.

Critical-illness weakness

Immobilisation, sepsis and corticosteroid exposure add neuropathy or myopathy that complicates weaning even after junctional transmission improves.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Repeat forced vital capacity, single-breath performance, respiratory rate, cough and bulbar assessment at a frequency matched to the trajectory.
Monitor carbon dioxide and consciousness when work of breathing rises, recognising that pulse oximetry alone cannot detect early hypoventilation.
Measure fluid balance, creatinine, blood count and thrombosis signs during immunoglobulin, and haemodynamics, calcium and access complications during plasma exchange.
Assess sputum burden, chest imaging, inflammatory markers and microbiology response while minimising sedating and transmission-impairing medicines.
Track objective limb, neck and ocular strength daily, avoiding excessive repeated testing that exhausts a critically weak patient.
Before discharge confirm swallow, nutrition, mobility, long-term immunotherapy ownership and written advice about relapse triggers and medicines.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Saturation falls late

Neuromuscular hypoventilation initially preserves lung oxygen transfer; respiratory reserve can disappear while pulse oximetry still appears acceptable.

Bulbar risk outranks volume

A patient who cannot handle saliva may need intubation despite a less alarming vital capacity because aspiration and upper-airway failure are separate threats.

Carbon dioxide means fatigue

Early distressed patients often lower carbon dioxide; a normalising or raised value during deterioration may mark dangerous respiratory-muscle exhaustion.

Rescue choices are contextual

Plasma exchange may act faster, while immunoglobulin can be easier to deliver; vascular access, renal and haemodynamic risk determine the individual balance.

Medicines need nuance

Potentially aggravating drugs are not all absolute prohibitions, but necessity, safer alternatives and intensified observation must be explicitly considered.

11Common pitfallsFrequent interpretation and management errors.

01
Reassuring from normal oxygen saturation in a tiring patient.
02
Waiting for hypercapnia before involving critical care.
03
Using one forced vital capacity as a substitute for clinical trajectory.
04
Ignoring bulbar secretion failure when respiratory numbers look stable.
05
Giving magnesium or a high-risk antibiotic without pharmacy review.
06
Treating with pyridostigmine alone instead of rescue immunotherapy.
07
Extubating before cough, swallow and secretion control recover.