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Myasthenia Gravis (MG) and Lambert-Eaton Myasthenic Syndrome (LEMS)

Background

  • Disorders of the neuromuscular junction

  • MG affects the post-synaptic cleft at the acetylcholine receptor (fatigability worsens with use)

  • LEMS affects the pre-synaptic cleft at the calcium channels (fatigability improves with use) - Many cases are paraneoplastic (classically small cell lung carcinoma)

Presentation

  • Double vision, ptosis, dysarthria, dysphagia

  • Dyspnea looks different than in other conditions: air hunger, usually also with dysphagia

  • Initially, the pt may not look sick or distressed, but may have a short inspiratory time or difficulty speaking in complete sentences due to shallow breathing

  • Most pts have a known history of myasthenia, but up to 20% present initially with crisis

Evaluation

  • Physical exam
    • Look closely for ptosis, nasal speech, weak neck flexion/extension (same nerve roots as diaphragm), interrupted speech to take extra breaths, holding upward gaze
    • These pts do not exhibit “huffing and puffing” like in COPD/asthma exacerbations
      • Pts with NMJ disease can go from talking to intubated within several hours!
    • LEMS: less ocular weakness but does have extremity weakness and absent reflexes that improve with muscle use (facilitation)
    • Pulmonary compromise is very rare in LEMS

  • EMG/NCS

    • MG: decremental response to repetitive stimulation
    • LEMS: increased amplitude in response to repetitive stimulation

  • Labs: myasthenia antibody panels (send prior to IVIG/PLEX being given)

  • Imaging: consider chest CT to look for thymic hyperplasia

Management

  • Monitor NIF (negative inspiratory force) at baseline and Q4H-Q8H

    • Measure of diaphragmatic strength (more negative = more force)
    • Normal is more negative than -60
    • If more positive than -30, consider elective intubation
    • Note that pt effort will affect NIF values

  • IVIG or PLEX

    • Both have similar supportive evidence; IVIG is usually easier to do
    • PLEX has the risks you would expect with dialysis (e.g. fluid shifts) and coagulopathy
    • IVIG -> check IgA levels. Can increase risk of DVT, has risk of aseptic meningitis and provides significant fluid load so not ideal for pts with CHF

  • Steroids

    • Usually up-titrated SLOWLY (by 10-20mg Prednisone daily)
    • Rapid increases in steroids can worsen pts with MG, so talk to Neurology before adjusting

  • Pyridostigmine

    • Typically continue at their home dose
    • Too much pyridostigmine can make pts worse (more secretions), so for those doing poorly on >90mg per dose, consider lowering the dose

  • Treat underlying causes of exacerbations—usually infections or other toxic/metabolic insults

  • Remove/avoid exacerbating medications: Fluoroquinolones, aminoglycosides, beta blockers, Mg. There are many medications to avoid. Please refer to UpToDate for more thorough list.

  • LEMS specific management:

    • 3,4-Diaminopyridine
    • Can respond to IVIG or Pyridostigmine
    • Workup for underlying neoplasm