Rosen & Barkin's 5-Minute Emergency Medicine Consult (466 page)

• Pediatric MG is rare and distinct:
  
  • Congenital MG: Genetic defect
    
  • Juvenile MG: Autoimmune disorder
    
  • Transient neonatal MG: Postdelivery complication from placental transfer of maternal antibodies
    
• Adult MG has bimodal distribution:
  
  • 1st peak in 2nd and 3rd decades affecting mostly women
    
  • 2nd peak in 6th and 7th decades affecting men
    

• Antibody-mediated attack on nicotinic acetylcholine receptors
  
• Up to 20% of patients may be acetylcholine receptor antibody (AChR Ab) negative
  
• Penicillamine can cause MG as well as other autoimmune conditions
  
• Many medications may worsen myasthenic weakness:
  
  • Aminoglycosides, macrolides, quinolones, antimalarials
    
  • Local anesthetics
    
  • Antidysrhythmics (propafenone, quinidine, procainamide)
    
  • β-Blockers, calcium-channel blockers
    
  • Anticonvulsants (phenytoin, carbamazepine)
    
  • Antipsychotics (phenothiazine, atypicals)
    
  • Neuromuscular blocking agents
    
  • Iodine-containing radiocontrast
    

DIAGNOSIS

Fluctuating focal weakness

• Symptoms worsen with repeated activity:
  
  • Improve with rest
    
• Ocular weakness:
  
  • Diplopia
    
  • Ptosis while driving or reading
    
• Bulbar and facial muscle weakness:
  
  • Trouble chewing, speaking, swallowing
    
  • Inability to keep jaw closed after chewing
    
  • Slurred, nasal speech
    
• Limb weakness:
  
  • Difficulty climbing stairs, rising from chair, reaching up with arms
    

• Ocular findings:
  
  • Ptosis, diplopia
    
  • Inability to keep eyelid shut against resistance
    
  • No pupillary changes
    
• Bulbar and facial findings:
  
  • Ask patient to count to 100; look for changes in speech.
    
  • Decreased facial expression
    
  • Head droop
    
• Limb findings:
  
  • Repetitive testing of proximal muscles or small muscles of hand results in weakness.
    
  • Reflex and sensory exam are normal.
    

• Assess for respiratory compromise
  
• Search for secondary triggers (e.g., infectious source)
  

• CBC
  
• Electrolytes
  
• LFTs
  
• Thyroid function tests
  
• Anti-AChR Ab:
  
  • Positive in 90% with generalized disease
    
  • Positive in 50% with ocular disease
    
• Other tests for initial diagnosis:
  
  • Antistriated muscle antibody
    
  • Antinuclear antibody
    
  • Rheumatoid factor
    
  • ESR
    

• Head CT or MRI to rule out compressive lesions causing cranial nerve findings
  
• Chest CT with contrast to look for associated thymoma
  
• CXR as needed to evaluate for infectious source
  

• Edrophonium (Tensilon) test:
  
  • Short-acting AChE inhibitor
    
  • A positive test produces rapid, short-lived (2–5 min) improvement in strength
    
  • Sensitivity 95% in generalized MG and 86% in ocular MG
    
  • False positives possible with Lambert–Eaton, Guillain–Barré, MS, botulism, and others
    
  • Keep patient on cardiac monitor during test.
    
  • Atropine at bedside for possible bradycardia
    
  • Suction at bedside for possible increased secretions
    
• Ice test:
  
  • Place ice on eyelid for 2 min.
    
  • Improvement in ptosis suggests MG.
    

• Amyotrophic lateral sclerosis
  
• Botulism
  
• Electrolyte abnormalities
  
• Graves disease
  
• Guillain–Barré syndrome
  
• Hyperthyroidism
  
• Inflammatory muscle disorders
  
• Intracranial mass lesions
  
• Lambert–Eaton syndrome
  
• Multiple sclerosis
  
• Periodic paralysis
  
• Tick paralysis
  

TREATMENT

Attention to airway management

Myasthenic crisis:

• Most important is early intubation and mechanical ventilation.
  
• Signs of impending failure:
  
  • Vital capacity <20 mL/kg
    
  • Negative inspiratory pressure >–30 cm H
    ₂
    O
    
  • Negative expiratory pressure <40 cm H
    ₂
    O
    
• Considerations regarding paralytics:
  
  • Decreased sensitivity to depolarizing agents may necessitate higher dose; consider doubling the usual dose of succinylcholine.
    
  • Nondepolarizing agents can cause extended paralysis; consider halving the usual dose.
    
  • Others recommend midazolam, etomidate, or thiopental instead.
    

• Treat infections aggressively.
  
• Search for and remove triggers.
  
  • Careful medication history.
    
• Myasthenic crisis may require plasmapheresis or IV gamma globulin (IVIG).
  
  • Plasmapheresis: Remove 1–1.5 plasma volume each session × 5 sessions
    
  • IVIG: 0.4 mg/kg/d × 5 days
    
• Initiate high-dose corticosteroids.
  
• Discontinue AChE inhibitors while intubated.
  
• Atropine for AChE inhibitor effects (bradycardia, GI symptoms, increased bronchial or oral secretions)
  

• Edrophonium (Tensilon): 2 mg IV over 15–30 sec; if no effect after 45 sec, can give 2nd dose of 3 mg IV. If still no response, final dose of 5 mg IV can be given (total 10 mg).
  
• Prednisolone 1 mg/kg/d for crisis
  
• Atropine for cholinergic crisis 0.5 mg IV or IM
  

Other medications that may be initiated by neurologist:

• Prednisone, AChE inhibitors, azathioprine, mycophenolate, mofetil, cyclosporine, tacrolimus, rituximab
  

FOLLOW-UP

• New-onset myasthenic symptoms
  
• Diagnosis unclear, but myasthenia a possibility
  
• Myasthenic patients with worsening symptoms
  
• Myasthenic crisis or questionable respiratory status mandates admission to ICU.
  

Myasthenic patients who are improving can be considered for discharge in consultation with neurology.

Any discharged patient should have neurology follow-up arranged.

• Search for signs of myasthenic crisis in any MG patient who presents to the ED.
  
• Search carefully for secondary conditions in patients with worsening MG.
  
• Place patient on cardiac monitor and keep atropine and suction at bedside when performing edrophonium test.
  

• Bershad EM, Feen ES, Suarez JI. Myasthenia gravis crisis.
  South Med J.
  2008;101:63–69.
  
• Conti-Fine BM, Milani M, Kaminski HJ. Myasthenia gravis: Past, present and future.
  J Clin Invest.
  2006;116:2843–2854.
  
• Gajdos P, Chevret S, Toyka KV. Intravenous immunoglobulin for myasthenia gravis.
  Cochrane Database Syst Rev.
  2012;12:CD002277.
  
• Scherer K, Bedlack RS, Simel DL. Does this patient have myasthenia gravis?
  JAMA.
  2005;293:1906–1914.
  
• Thieben MJ, Blacker DJ, Liu PY, et al. Pulmonary function tests and blood gases in worsening myasthenia gravis.
  Muscle Nerve.
  2005;32:664–667.
  

See Also (Topic, Algorithm, Electronic Media Element)

• Amyotrophic Lateral Sclerosis
  
• Botulism
  
• Guillain–Barré Syndrome
  
• Hyperthyroidism
  
• Multiple Sclerosis
  

The author gratefully acknowledges Kelley Ralph’s contribution for the previous edition of this chapter.

CODES