Myasthenic crisis masked as a chest infection
A 67-year-old retired teacher presents with 4 weeks of fluctuating double vision and slurred speech and 1 week of difficulty chewing and swallowing. She has lost 4 kg and now coughs after drinking water. Examination shows fatigable ptosis, a nasal voice, and proximal weakness that worsens with repetition; reflexes are preserved. Sitting vital capacity is 1.1 L (predicted 3.4 L), peak cough flow is low, and she is using accessory muscles. SpO₂ is 93% on room air. She is on azithromycin for 'bronchitis', has just received intravenous magnesium sulphate for leg cramps in the emergency department, and takes atenolol for hypertension. Creatinine is 150 µmol/L (baseline 110), eGFR 35. CT chest shows a 4.5 cm anterior mediastinal mass. Anti-AChR antibodies are pending. The medical team plans to start pyridostigmine and admit her to a general ward for observation.
1. Do you accept the plan for ward admission? Justify your disposition decision using the bedside numbers, and state how you would secure the airway.
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No. This is myasthenic crisis and she needs a high-dependency or intensive-care bed with elective intubation planned.
Reasoning from the numbers:
- Vital capacity 1.1 L is about 32% of predicted and below the usual intubation thresholds (VC <20 mL/kg, or <1 L, or falling rapidly).
- Bulbar failure - coughing after fluids, nasal voice, 4 kg weight loss - means she cannot protect her airway; a low peak cough flow predicts failure of non-invasive ventilation and of extubation.
- She is using accessory muscles with SpO₂ 93%; hypercapnia is likely to follow.
- The 20/30/40 rule (VC <20 mL/kg, inspiratory pressure weaker than −30 cmH₂O, expiratory pressure <40 cmH₂O) is met on the volume criterion alone.
Airway plan: elective intubation in theatre or ICU with anaesthesia present, before decompensation. Avoid succinylcholine (resistance and hyperkalaemia risk); use a non-depolarising agent with caution and monitor afterwards. Non-invasive ventilation may be used briefly in a cooperative patient with intact bulbar function and manageable secretions - that is not this patient.
Ward admission would be unsafe: respiratory failure in myasthenia can progress over hours, and a general ward cannot monitor VC serially. Admit to ICU/HDU, start serial VC and blood gases, and involve neurology and respiratory physiotherapy the same day.
2. Which drugs must be stopped immediately, and how would you choose between IVIG and plasma exchange given her renal function? What about pyridostigmine?
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Stop the precipitants:
- Azithromycin - macrolides impair neuromuscular transmission and are a recognised cause of crisis. Substitute a safer antibiotic for the respiratory infection (e.g. amoxicillin–clavulanate or ceftriaxone, guided by cultures).
- Magnesium sulphate - presynaptic calcium antagonism produces profound weakness; it is contraindicated in myasthenia.
- Atenolol - beta-blockers can worsen myasthenic weakness; stop or replace with an alternative antihypertensive.
- Also avoid fluoroquinolones, aminoglycosides, calcium-channel blockers and, where possible, statins if weakness worsens.
Immunotherapy:
- Plasma exchange (5 exchanges over 7–10 days) works fastest and is not nephrotoxic, but needs central access, causes hypotension and coagulopathy, and removes circulating drug. Given eGFR 35, many would favour PLEX.
- IVIG 2 g/kg over 2–5 days is equivalent in trials and easier to give, but carries a real risk of osmotic nephropathy, thrombosis and volume overload in renal impairment. If used, choose a sucrose-free product, pre-hydrate, infuse slowly, and monitor creatinine daily.
- Do not give IVIG immediately after PLEX (washout); allow at least 48 hours or give IVIG first.
- Check renal function and urine output daily; both options are bridges while immunosuppression (steroids ± azathioprine) takes effect.
Pyridostigmine: in established crisis with bulbar secretions, it is often reduced or temporarily withheld while ventilated (to limit secretions), then restarted cautiously. Starting it alone and observing on a ward is not adequate treatment.
3. What is the significance of the mediastinal mass, and how would you sequence thymectomy, immunosuppression and discharge planning?
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The mass is almost certainly a thymoma, and thymoma is a paraneoplastic driver of myasthenia. All patients with thymoma and myasthenia should undergo thymectomy, but not as an emergency - operating during crisis carries high morbidity.
Sequence:
- Stabilise first: control the airway, remove precipitants, give PLEX or IVIG, and start immunosuppression (prednisolone, with azathioprine or mycophenolate as a steroid-sparing agent).
- Characterise the mass with contrast CT and, if needed, MRI; assess invasion and obtain thoracic surgery/oncology review. Consider whether neoadjuvant therapy is needed for locally invasive disease.
- Thymectomy after stabilisation, usually within weeks to a few months, with pre-operative optimisation (some units give prophylactic PLEX/IVIG). Complete resection is the oncological goal as well as a myasthenia treatment.
- Continue immunosuppression after surgery; do not expect immediate remission - improvement is often delayed by months, and some patients need long-term therapy.
- If AChR antibodies are negative, request anti-MuSK and consider Lambert–Eaton (anti-VGCC, small-cell lung cancer) - relevant with a mediastinal mass and smoking history.
- Discharge planning: a written crisis action plan, avoidance of the drugs listed above, respiratory physiotherapy, vaccination, bone protection on steroids, and an early follow-up with neurology and thoracic surgery. Counsel her that deterioration with infection requires same-day assessment, not a ward referral.