eMedicine Specialties > Emergency Medicine > Neurology

Myasthenia Gravis: Differential Diagnoses & Workup

Author: William D Goldenberg, MD, Clinical Assistant Instructor, Department of Emergency Medicine, Kings County Hospital Center and SUNY Downstate Medical Center
Coauthor(s): Richard H Sinert, DO, Associate Professor of Emergency Medicine, Clinical Assistant Professor of Medicine, Research Director, State University of New York College of Medicine; Consulting Staff, Department of Emergency Medicine, Kings County Hospital Center
Contributor Information and Disclosures

Updated: Jan 30, 2009

Differential Diagnoses

Myocardial Infarction
Pulmonary Embolism

Workup

Laboratory Studies

• No laboratory tests are available in a time frame that is useful to confirm the emergency diagnosis of myasthenia gravis (MG).
• An arterial blood gas determination can help guide respiratory management and should be obtained early. An elevated pCO₂ suggests progressive respiratory failure and may indicate the need for emergency airway management.

Imaging Studies

• Chest radiography is indicated to determine the presence of aspiration or other pneumonias, which commonly occur in patients with myasthenia gravis.
• CT scan or MRI of the chest is highly accurate in detecting thymoma. Every patient with myasthenia gravis should be screened for thymoma. Chest radiography is relatively insensitive in screening for thymoma, as it does not detect up to 30% of cases.

Other Tests

• Tensilon (edrophonium) challenge test is useful in diagnosing myasthenia gravis and in distinguishing myasthenic crisis from cholinergic crisis due to its rapid onset and short duration of action.^5,6 A positive response is not completely specific for myasthenia gravis because several other conditions (eg, amyotrophic lateral sclerosis) may also respond to edrophonium with increased strength. Once the patient's airway and ventilation are secured, an initial test dose of edrophonium is given. Some patients may respond noticeably to a small dose (1 mg). If no adverse reaction occurs following the test dose, another dose (3 mg) of edrophonium should produce noticeable improvement in muscle strength within 1 minute.⁶ If no improvement occurs, an additional dose of 5 mg can be administered to total no more than 10 mg.⁵
  • Patients who respond generally show dramatic improvement in muscle strength, regaining facial expression, posture, and respiratory function within 1 minute.
  • During this procedure, the patient must be monitored carefully because edrophonium can cause significant bradycardia, heart block, and asystole. The risk of serious bradyarrhythmias and syncope was only 0.16%, but atropine still should be available at the bedside.⁵ The return of muscle weakness after edrophonium wears off combined with residual increased oral secretions can exacerbate respiratory distress and the risk of aspiration.
  • Patients with a cholinergic crisis may respond to edrophonium challenge by increasing salivation and bronchopulmonary secretions, diaphoresis, and gastric motility (ie, SLUDGE syndrome).^6,7 These changes should be managed expectantly, as the half-life of edrophonium is short (ie, approximately 10 min).
  • If muscle strength fails to improve following the maximum dose of edrophonium, the patient is having a cholinergic crisis or has another cause of weakness that is unrelated to myasthenia gravis.
  • The effects of edrophonium are brief, and repeated doses may be required before oral anticholinesterase medication can take effect.⁶
  • In patients with less severe exacerbations, the degree of improvement with edrophonium may be subtle. Many authors recommend having several blinded observers assess the patient's response in these cases. Some authors also suggest that a clear endpoint, such as improvement in eyelid ptosis or extraocular movements, since these muscles are independent of voluntary effort.⁵
• Ice pack test
  • Cooling may improve neuromuscular transmission. In a patient with myasthenia gravis who has ptosis, placing ice over an eyelid will lead to cooling of the lid, which leads to improvement of the ptosis.
  • Lightly placing ice that is in a surgical glove or that is wrapped in a towel over the eyelid will cool it within 2 minutes.
  • A positive test is clear resolution of the ptosis.⁵
  • This test has a pooled sensitivity and specificity of 82% and 96%, respectively. However, the literature generally overestimates the usefulness since most of the studies were case-control designs.⁸
• Additional tests (eg, standard electromyography, single-fiber electromyography, repetitive nerve stimulation, assays for acetylcholine receptor antibody [ARA]) are used to confirm the diagnosis of myasthenia gravis, but these tests usually are not available on an emergent basis.⁵ The ARA test offered the highest specificity of these additional tests since these other tests only reveal a disorder of neuromuscular conduction.⁸
• Patients with respiratory distress should have an evaluation of pulmonary function, providing that the patient is not in obvious respiratory failure. 
  • This evaluation includes pulse oximetry, a measure of pulmonary function (ie, peak expiratory flow, forced expiratory volume in 1 second [FEV1]), and ABG sampling to determine PCO₂ level.⁴
  • Evidence of hypoxemia, poor respiratory effort, or CO₂ retention is an indication for intubation and mechanical ventilation.⁴

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