A 52-year-old man with known myasthenia gravis (anti-AChR antibody positive) presents with acute worsening of weakness and respiratory distress. Clinical examination shows severe bulbar weakness and limb paralysis. Serum potassium is 3.2 mEq/L. Which investigation is most appropriate to assess disease severity and guide immediate management?

## Investigation of Choice in Myasthenic Crisis ### Clinical Context: Myasthenic Crisis **Key Point:** Myasthenic crisis is a life-threatening complication characterized by acute respiratory failure due to severe weakness of respiratory and bulbar muscles. Pulmonary function tests (FVC and NIF) are the gold-standard investigations to assess severity and guide intubation decisions. **High-Yield:** In myasthenic crisis: - FVC <15–20 mL/kg or NIF >–20 cm H~2~O predicts high risk of respiratory failure - FVC and NIF are non-invasive, rapid, and repeatable - Serial measurements guide timing of mechanical ventilation - These tests are more useful than electrophysiological studies in acute settings ### Pulmonary Function Tests in MG Crisis | Parameter | Normal Value | Threshold for Intubation | Clinical Significance | |---|---|---|---| | Forced Vital Capacity (FVC) | >80 mL/kg | <15–20 mL/kg | Predicts respiratory failure risk | | Negative Inspiratory Force (NIF) | >–80 cm H~2~O | >–20 cm H~2~O (less negative) | Indicates inspiratory muscle weakness | | Peak Expiratory Flow (PEF) | >400 L/min | <100 L/min | Reflects cough effectiveness | **Clinical Pearl:** Patients with FVC <15 mL/kg, NIF >–20 cm H~2~O, or PEF <100 L/min should be considered for elective intubation to prevent aspiration and respiratory failure. Waiting for blood gas abnormalities is dangerous. ### Why Pulmonary Function Tests Are Preferred in Crisis 1. **Rapid assessment:** Results available within minutes; guides immediate clinical decisions. 2. **Non-invasive:** Can be repeated serially to monitor disease trajectory. 3. **Predictive value:** Strong correlation with need for mechanical ventilation. 4. **Bedside availability:** Can be performed at the patient's bedside in ICU. **Warning:** Do NOT delay intubation waiting for blood gas changes (hypercapnia is a late sign). Elective intubation based on FVC/NIF is safer than emergency intubation after respiratory arrest. ### Why Other Investigations Are Not Appropriate - **RNS/SFEMG:** Useful for diagnosis, not for assessing acute severity or guiding crisis management. - **Edrophonium test:** Historically used for diagnosis but is obsolete, carries risk of cholinergic crisis, and is not available in most centres. [cite:Harrison 21e Ch 378]