Myasthenia gravis

Myasthenia gravis

Other names	Myasthenia
Anesthetic relevance	Critical
Anesthetic management	Evaluate for acute exacerbation Continue outpatient MG meds 🠕 sensitivity to nondepolarizing NMBs 🠗 resistant to succinylcholine Monitor for cholinergic symptoms May require prolonged ventilatory support
Specialty	Neurology, immunology
Signs and symptoms	Muscle weakness
Diagnosis	Serum antibody level, EMG, nerve conduction studies
Treatment	Acetylcholinesterase inhibitor, steroids
Article quality
Editor rating	Comprehensive
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Myasthenia gravis is an autoimmune disease that produces antibodies against nicotinic acetylcholine receptors in the postsynaptic membrane of the neuromuscular junction, leading to varying degrees of muscle weakness and fatigue and can affect ocular, bulbar (muscles involved in speech, chewing and swallowing), respiratory, and proximal skeletal muscles. Symptoms are worst at the end of the day or after exertion. Classification of myasthenia depends on whether the patient has only ocular or ocular and extraocular weakness.

Anesthetic implications

Preoperative optimization

Elective surgery should be delayed for patients who are exhibiting acutely worsened symptoms (i.e. myasthenic crisis)

Outpatient medications

Patients should take their morning dose of acetylcholinesterase inhibitor and/or steroids.

• Steroid use may indicate more severe disease and/or active symptoms
• Acetylcholinesterase inhibitor use may result in:
  • Altered patient drug requirements following surgery
  • Increased vagal reflexes
  • Possibility of disrupting bowel anastomoses secondary to hyperperistalsis
  • Cholinergic side effects including salivation, miosis, bradycardia
• IV pyridostigmine is 1/30 of the PO dose

Labs and studies

• PFTs may be helpful to establish baseline respiratory function
  • If thymoma present, may be useful to characterize obstruction

Premedication

• Consider pretreatment with metoclopramide or H2 blocker for aspiration prophylaxis
  • Patients with involvement of respiratory or bulbar muscles are at higher risk for aspiration
• Use sedating premedication with caution as patients may be more sensitive to respiratory depressants
• IVIG or plasma exchange can be used to improve symptoms of active flare for emergent cases

Associated comorbidities

• Thymoma
• Diabetes mellitus
• Thyroid disorders
• Systemic lupus erythematosus
• Rheumatoid arthritis

Intraoperative management

Neuromuscular blockade

Patients with myasthenia gravis are:

• More sensitive to nondepolarizing neuromuscular blockers
• Resistant to succinylcholine
  • At increased risk for developing phase II neuromuscular block, particularly with repeat doses of succinylcholine
  • ED95 is 2.6 times normal in these patients.
• In patients taking an acetylcholinesterase inhibitor for myasthenia management, cholinergic crisis is possible if acetylcholinesterase inhibitor is used for neuromuscular blockade reversal

Airway management

• Consider increased aspiration risk if bulbar symptoms are present
• If thymoma present, may cause anterior mediastinal mass effect

Other medications

• Consider stress dose steroids if taking chronically
• Medications which may contribute to muscle weakness:
  • Calcium channel blockers
  • Magnesium
  • Aminoglycoside antibiotics

Postoperative management

• Extubate awake with pulmonary mechanics similar to baseline status
• Risk factors for requiring postoperative ventilation support:
  • Leventhal Criteria
    • Disease duration >6yrs
    • Concomitant pulmonary disease
    • Peak inspiratory pressure <-25 cmH2O
    • Vital capacity <40 mL/kg (or <2.9L)
    • Pyridostigmine dose >750 mg/day
  • Other indicators include preoperative use of steroids, and previous episode of respiratory failure

• Resume outpatient anticholinesterase medication
• Patients should be monitored in either a ICU or step-down unit

Related surgical procedures

• Thymectomy

Pathophysiology

Myasthenia gravis is an autoimmune disease leading to the production of antibodies against nicotinic acetylcholine receptors in the postsynaptic membrane of the neuromuscular junction.

Signs and symptoms

Weakness in one or more of the following muscle groups:

• Ocular
• Bulbar (swallowing, chewing, speech)
• Respiratory
• Proximal skeletal

Diagnosis

Myasthenia gravis can be difficult to diagnose, and is based on a combination of:

• Serum antibody level
• Electromyography
• Nerve conduction studies
• Edrophonium test
• CT/MRI to evaluate for enlarged thymus gland

Not to be confused with Congenital myasthenic syndrome (CMS) or Lambert-Eaton syndrome.

Treatment

Myasthenia gravis is typically medically managed using:

• Acetylcholinesterase inhibitor (e.g. pyridostigmine) to increase the presence of acetylcholine in the neuromuscular junction
• Steroids

Prognosis

Patients with managed myasthenia gravis have a normal life expectancy.