A 28-year-old male with a 3-year history of generalized myasthenia gravis (MG) on stable prednisolone and azathioprine therapy presents with acute onset severe ptosis, bulbar weakness, and respiratory distress over 48 hours. Vital capacity has dropped from 2.8 L to 1.5 L. He denies recent infection or medication non-compliance. What is the most appropriate immediate next step?

Explanation

## Myasthenic Crisis: Recognition and Acute Management This patient has **myasthenic crisis**—acute, life-threatening exacerbation of MG with respiratory muscle involvement. Immediate intervention is required to prevent respiratory failure. ### Definition and Pathophysiology **Key Point:** Myasthenic crisis is a medical emergency characterized by acute worsening of muscle weakness (especially respiratory and bulbar muscles) that may require mechanical ventilation. It results from severe reduction in functional AChR at the neuromuscular junction due to antibody-mediated destruction and complement activation. **High-Yield:** Triggers include infection (UTI, pneumonia), medication non-compliance, stress, surgery, and rarely, immunosuppressive therapy initiation (paradoxical worsening). ### Diagnostic Criteria for Crisis - Acute worsening of weakness over hours to days - **Vital capacity < 2.0 L or < 30 mL/kg** (this patient: 1.5 L) - Bulbar weakness (dysphagia, dysarthria, weak cough) - Respiratory muscle fatigue - Risk of aspiration and airway compromise ### Acute Management Algorithm ```mermaid flowchart TD A[Suspected myasthenic crisis]:::urgent --> B[Assess airway & vital capacity]:::action B --> C{VC < 2.0 L or respiratory distress?}:::decision C -->|Yes| D[Prepare for intubation]:::urgent D --> E[Start plasmapheresis or IVIG]:::action E --> F[ICU monitoring]:::action C -->|No| G[Close monitoring, supportive care]:::action G --> H[Plasmapheresis/IVIG if deterioration]:::action ``` ### Immediate vs. Delayed Interventions | Intervention | Timing | Indication | Mechanism | |---|---|---|---| | **Plasmapheresis** | Immediate (within hours) | Myasthenic crisis with respiratory involvement | Removes pathogenic AChR antibodies; effect in 24–48 hrs | | **IVIG** | Immediate (within hours) | Myasthenic crisis; alternative if plasmapheresis unavailable | Blocks Fc receptors on macrophages; effect in 3–5 days | | **Intubation** | Preemptive if VC < 1.5 L | Respiratory failure imminent | Prevents aspiration; allows mechanical ventilation | | **Increase immunosuppression** | Delayed (after crisis stabilization) | Chronic management adjustment | Prevents recurrence | **Clinical Pearl:** In myasthenic crisis, do NOT increase anticholinesterases (pyridostigmine) acutely—cholinergic toxicity can mimic crisis and worsen respiratory status. Temporary cessation may be necessary. ### Why Plasmapheresis/IVIG Now? 1. **Vital capacity 1.5 L** = critical threshold; respiratory failure imminent 2. **Bulbar + respiratory symptoms** = life-threatening crisis, not stable exacerbation 3. **Rapid onset (48 hrs)** = acute antibody-mediated decompensation requiring urgent antibody removal 4. **Effect timeline**: Plasmapheresis works in 24–48 hours; IVIG in 3–5 days—both must start immediately **Warning:** Delaying plasmapheresis/IVIG in crisis increases mortality and morbidity from respiratory failure and aspiration. [cite:Harrison 21e Ch 382; Robbins 10e Ch 27]