Management of Myasthenia Gravis Crisis with Supraventricular Tachycardia
In a patient with myasthenic crisis who develops SVT, immediately assess hemodynamic stability: if hypotensive, altered, or in shock, perform synchronized cardioversion without delay; if stable, attempt modified Valsalva maneuver followed by adenosine 6 mg IV push, but avoid beta-blockers and use calcium-channel blockers with extreme caution due to potential worsening of myasthenic weakness.
Immediate Hemodynamic Assessment
Determine stability first: Hypotension (systolic BP < 90 mmHg), altered mental status, signs of shock, chest pain, or acute heart failure mandate immediate synchronized cardioversion at 50–100 J without attempting vagal maneuvers or pharmacologic therapy 1, 2
Myasthenic crisis itself creates instability: Respiratory insufficiency requiring mechanical ventilation, bulbar weakness with aspiration risk, and flaccid tetraparesis define the crisis state and complicate SVT management 3, 4
Critical distinction: A patient already intubated for myasthenic crisis may appear "stable" from a cardiac standpoint (adequate BP, alert on sedation) but remains critically ill from the neuromuscular perspective 3, 5
Management of Hemodynamically Unstable SVT in Myasthenic Crisis
Proceed directly to synchronized cardioversion at 50–100 J for narrow-complex SVT or 100 J for wide-complex tachycardia; this restores sinus rhythm in nearly 100% of cases and avoids pharmacologic agents that may worsen myasthenic weakness 1, 2
Do not delay cardioversion to attempt vagal maneuvers or drug therapy when the patient exhibits hypotension, altered mental status, or respiratory decompensation 1, 2
Ensure adequate sedation before cardioversion if the patient's hemodynamic status permits, but do not delay the procedure 1, 2
Management of Hemodynamically Stable SVT in Myasthenic Crisis
First-Line: Vagal Maneuvers
Modified Valsalva maneuver (patient supine, bearing down 10–30 seconds to generate 30–40 mmHg intrathoracic pressure) terminates SVT in approximately 43% of attempts and carries no risk of worsening myasthenic weakness 2
Carotid sinus massage (5–10 seconds steady pressure after confirming absence of carotid bruit) achieves an overall vagal success rate of 27–28% 2
Never apply pressure to the eyeball—this technique is dangerous and abandoned 2
Second-Line: Adenosine (Preferred Pharmacologic Agent)
Adenosine is the safest first-line drug for myasthenic crisis patients because it has no effect on neuromuscular transmission and terminates 90–95% of AVNRT and 78–96% of AVRT episodes 2, 6
Dosing protocol: 6 mg rapid IV push (1–2 seconds) through a large proximal vein, followed immediately by 20 mL saline flush; if no conversion in 1–2 minutes, give 12 mg rapid IV push; a third 12 mg dose may be administered (maximum cumulative 30 mg) 2, 6
Absolute contraindication: Active asthma or bronchospasm—many myasthenic crisis patients have concurrent respiratory infections that may include bronchospasm 2, 6
Transient side effects (flushing, dyspnea, chest discomfort) resolve within 60 seconds and do not worsen myasthenic weakness 6
Third-Line: Calcium-Channel Blockers (Use with Extreme Caution)
Diltiazem 15–20 mg IV over 2 minutes achieves 64–98% conversion when adenosine fails or is contraindicated, but carries theoretical risk of worsening neuromuscular blockade 1, 2
Verapamil 2.5–5 mg IV over 2 minutes is an alternative calcium-channel blocker with similar efficacy and similar theoretical concerns 1, 2
Critical safety warning: The 2015 ACC/AHA/HRS guideline recommends reviewing and stopping medications with known risk of worsening myasthenia, including beta-blockers and IV magnesium 1
Calcium-channel blockers are not explicitly listed as contraindicated in myasthenia gravis in the immune-related adverse events guideline, but their negative inotropic effects and potential for hypotension in critically ill patients warrant extreme caution 1
Fourth-Line: Beta-Blockers (Generally Avoided)
Beta-blockers are explicitly listed as medications with known risk of worsening myasthenia and should be reviewed and stopped according to the ASCO guideline for immune-related myasthenia gravis 1
Metoprolol and esmolol are reasonable alternatives for SVT in general populations but should be avoided in myasthenic crisis due to risk of exacerbating neuromuscular weakness 1
If beta-blockers must be used (e.g., adenosine and calcium-channel blockers both contraindicated), use the shortest-acting agent (esmolol) with continuous monitoring for worsening weakness 1
Synchronized Cardioversion as Rescue Therapy
Elective synchronized cardioversion with appropriate sedation achieves 80–98% success in hemodynamically stable patients when pharmacologic therapy fails or is contraindicated 2
This is the safest definitive option for myasthenic crisis patients because it avoids all pharmacologic agents that may worsen neuromuscular transmission 2
Post-Conversion Management
Continuous cardiac monitoring is essential immediately after conversion because premature atrial or ventricular complexes frequently trigger recurrent SVT within seconds to minutes 2, 6
If immediate recurrence occurs, consider repeat cardioversion rather than long-acting AV-nodal blockers (diltiazem, verapamil, beta-blockers) that may worsen myasthenic weakness 2
Investigate and treat triggers: Respiratory infections are the most common precipitant of myasthenic crisis (occurring in 60–70% of cases) and may also trigger SVT through catecholamine release and hypoxia 3, 4
Concurrent Management of Myasthenic Crisis
Do not delay crisis-specific therapy: Plasmapheresis is preferred over IVIG as the treatment of choice for myasthenic crisis and should be initiated urgently regardless of SVT management 7, 4
Maintain mechanical ventilation and airway protection; the median duration of ventilation is 12–14 days under sufficient treatment 3
Pyridostigmine or neostigmine should be continued for symptomatic treatment unless cholinergic crisis is suspected 3, 7
Initiate or optimize immunosuppression with corticosteroids and azathioprine once the acute crisis is stabilized 3
Critical Cardiac Monitoring in Myasthenic Crisis
Severe cardiac arrhythmias occur in 17% of myasthenic crisis patients and are fatal in approximately 10% of all crisis cases (6 of 63 crises in one series) 8
Temporary pacemaker placement should be provided where clinically indicated, as cardiac complications—not the myasthenic weakness itself—are the primary cause of mortality in crisis 8
Continuous telemetry monitoring is mandatory throughout the crisis period, as arrhythmias may develop at any point during the acute illness 8
Medications to Absolutely Avoid
Beta-blockers (metoprolol, esmolol, propranolol)—explicitly listed as worsening myasthenia 1
IV magnesium—explicitly contraindicated in myasthenia gravis 1
Fluoroquinolones, aminoglycosides, and macrolide antibiotics—may precipitate or worsen crisis 1
Calcium-channel blockers in pre-excited atrial fibrillation or suspected VT—may precipitate ventricular fibrillation 1, 2
Algorithm Summary
Assess hemodynamic stability → If unstable (hypotension, altered mental status, shock), proceed directly to synchronized cardioversion at 50–100 J 1, 2
If stable, attempt modified Valsalva maneuver (43% success rate) or carotid massage (27% success rate) 2
If vagal maneuvers fail, give adenosine 6 mg IV push followed by 20 mL saline flush; repeat with 12 mg doses up to maximum 30 mg total 2, 6
If adenosine fails or is contraindicated (asthma), consider diltiazem 15–20 mg IV over 2 minutes with extreme caution and continuous monitoring for worsening myasthenic weakness 1, 2
If all pharmacologic options fail or are contraindicated, perform elective synchronized cardioversion with appropriate sedation 2
Avoid beta-blockers entirely due to explicit contraindication in myasthenia gravis 1
Continue plasmapheresis or IVIG for the underlying myasthenic crisis regardless of SVT management 7, 4