Myasthenia etc

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

Background

  • Both are disorders at the neuromuscular junction
  • MG affects the post-synaptic cleft at the acetylcholine receptor
  • LEMS affects the pre-synaptic cleft at the calcium channels

 

Presentation

  • Double vision, ptosis, dysarthria, dysphagia
  • Dyspnea looks different than in other conditions: air hunger, usually also with dysphagia
  • Initially, the patient 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

  • Exam: Look closely for ptosis, nasal speech, weak neck flexion/extension (same nerve roots as diaphragm), interrupted speech to take extra breaths
  • These patients do not exhibit “huffing and puffing” work of breathing in the way that pts with COPD/asthma exacerbations or post exercise may look
    • Patients with NMJ disease can go from talking to intubated within several hours!
    • LEMS: less ocular weakness, but do have extremity weakness and absent reflexes. Many cases are paraneoplastic (classically small cell carcinoma of the lung)
    • Pulmonary compromise is very rare in LEMS
  • EMG/NCS - MG will show decremental response to rapid stim, and single fiber EMG (gold standard) can be done. SFEMG cannot be done in ICU settings due to electrical artifact
  • Myasthenia antibody panels can be ordered (send prior to IVIG/PLEX being given)
  • Chest CT -> looking for thymic hyperplasia

 

Management

  • NIF (negative inspiratory force, a measurement of diaphragmatic strength; normal is < -60) at baseline on arrival and then Q4-6H
    • If getting below -30 consider elective intubation, certainly ICU
    • Can see NIF under flowsheets on Epic
    • Some pts’ effort can be impaired resulting in poor 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 -> have to check IgA levels (deficiency is a rare cause of anaphylaxis) and can increase risk of DVT, has risk of aseptic meningitis and provides and significant fluid load so not ideal for pts with CHF
  • Steroids are also usually up-titrated SLOWLY (by 10-20 mg prednisone daily, not much more of an increase) and pyridostigmine is continued
  • Rapid increases in steroids can worsen patients with MG
  • Too much pyridostigmine can make patients worse, so for those doing poorly on >90 mg 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, and magnesium
  • LEMS: 3,4-diaminopyridine and can respond to IVIG or pyridostigmine