## Clinical Scenario: Myasthenic Crisis This patient is in **myasthenic crisis**—acute severe exacerbation of MG with respiratory compromise (vital capacity <1.5 L indicates impending respiratory failure). The recent initiation of high-dose corticosteroids may have precipitated a "steroid-induced flare." ## Diagnostic Criteria for Myasthenic Crisis **Key Point:** Myasthenic crisis is defined as acute worsening of MG requiring mechanical ventilation or intubation, usually triggered by infection, medication changes, or steroid initiation. ### Clinical Features - Acute onset of severe weakness (hours to days) - Bulbar weakness (dysphagia, dysarthria) - Respiratory muscle weakness (vital capacity <1.5 L) - Risk of aspiration and respiratory failure ## Immediate Management of Myasthenic Crisis ### Step 1: Supportive Care & Monitoring - **Admit to ICU** with continuous cardiorespiratory monitoring - Assess vital capacity and arterial blood gases - Prepare for intubation (but do NOT intubate prophylactically unless vital capacity <1 L or PaCO~2~ rising) - NPO status due to dysphagia ### Step 2: Discontinue Anticholinesterase Agents **High-Yield:** Anticholinesterase agents (pyridostigmine, neostigmine) must be **stopped immediately** in myasthenic crisis because: 1. They can worsen cholinergic toxicity 2. They are ineffective when acetylcholine receptors are severely depleted 3. Excess acetylcholine may cause "cholinergic crisis" (muscle weakness, fasciculations, bronchospasm) **Clinical Pearl:** The distinction between myasthenic crisis and cholinergic crisis is clinical; both present with weakness. Cholinergic crisis has muscarinic signs (bronchospasm, miosis, salivation). Edrophonium test is contraindicated in crisis due to risk of cardiac arrhythmias. ### Step 3: Immunotherapy | Modality | Mechanism | Timing | Efficacy | |---|---|---|---| | **IVIG** | Blocks Fc receptors on macrophages; reduces antibody-mediated complement activation | Within 24–48 hrs of crisis onset | 50–80% response in 3–5 days | | **Plasmapheresis** | Removes pathogenic antibodies and immune complexes | Equally effective as IVIG | 50–80% response in 3–5 days | | **Corticosteroids** | Immunosuppression (slow onset) | Continue at current dose; do NOT increase acutely | Takes weeks to months | **Warning:** Do NOT increase corticosteroid dose acutely in crisis—this may worsen the initial flare. Continue at the current dose; the patient is already on prednisolone 40 mg daily. **Mnemonic: IVIG vs Plasmapheresis in Crisis — "IVIG is IV, Plasmapheresis is Plasma"** - Both are equally effective - IVIG: easier access, no need for central line, but slower onset (3–5 days) - Plasmapheresis: faster onset (24–48 hrs), but requires vascular access and is more invasive - **Choice depends on availability and clinical urgency.** IVIG is preferred in most centers due to ease of administration. ## Management Algorithm for Myasthenic Crisis ```mermaid flowchart TD A[Acute worsening of MG + respiratory compromise]:::urgent --> B[Admit to ICU]:::action B --> C[Assess vital capacity & ABG]:::action C --> D{VC < 1 L or rising PaCO2?}:::decision D -->|Yes| E[Prepare for intubation]:::action D -->|No| F[Continue non-invasive support]:::action E --> G[Discontinue anticholinesterase]:::action F --> G G --> H[Start IVIG or Plasmapheresis]:::action H --> I[Continue corticosteroids at current dose]:::action I --> J[Monitor for improvement over 3-5 days]:::outcome ``` ## Why Each Option is Correct or Incorrect **Correct Answer: Option B (IVIG + ICU + Discontinue Anticholinesterase)** - Addresses the immediate crisis with ICU-level care - IVIG is first-line immunotherapy in most centers (equally effective as plasmapheresis) - Discontinuing anticholinesterase prevents cholinergic toxicity and allows recovery - Vital capacity <1.5 L mandates ICU admission and preparation for mechanical ventilation [cite:Harrison 21e Ch 381]
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