AMyasthenia 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, diplopia or ptosis with sustained 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 à 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.
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