Myasthenia Gravis (MG) and Lambert-Eaton Myasthenic Syndrome (LEMS)¶
Background¶
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Disorders of the neuromuscular junction
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MG affects the post-synaptic cleft at the acetylcholine receptor
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LEMS affects the pre-synaptic cleft at the calcium channels
- Many cases are paraneoplastic (classically small cell lung carcinoma)
Presentation¶
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Double vision, ptosis, dysarthria, dysphagia
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Dyspnea looks different than in other conditions: air hunger, usually also with dysphagia
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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
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Most pts have a known history of myasthenia, but up to 20% present initially with crisis
Evaluation¶
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Exam
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Look closely for ptosis, nasal speech, weak neck flexion/extension (same nerve roots as diaphragm), interrupted speech to take extra breaths
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These patients do not exhibit “huffing and puffing” like in COPD/asthma exacerbations
- Patients with NMJ disease can go from talking to intubated within several hours!
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LEMS: less ocular weakness, but do have extremity weakness and absent reflexes.
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Pulmonary compromise is very rare in LEMS
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EMG/NCS
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MG: decremental response to repetitive stimulation
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LEMS: increased amplitude in response to repetitive stimulation
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Labs
- Myasthenia antibody panels (send prior to IVIG/PLEX being given)
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Imaging
- Consider chest CT to look for thymic hyperplasia
Management¶
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Monitor NIF (negative inspiratory force) at baseline and q4h
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Measure of diaphragmatic strength
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Normal is \<-60
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If below -30, consider elective intubation
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Note that patient effort will affect NIF values
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IVIG or PLEX
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Both have similar supportive evidence; IVIG is usually easier to do
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PLEX has the risks you would expect with dialysis (e.g. fluid shifts) and coagulopathy
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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
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Steroids
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Usually up-titrated SLOWLY (by 10-20 mg prednisone daily)
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Rapid increases in steroids can worsen patients with MG**, so talk to neuro before adjusting**
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Pyridostigmine
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Typically continue at their home dose
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Too much pyridostigmine can make patients worse (more secretions), so for those doing poorly on >90 mg per dose, consider lowering the dose
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Treat underlying causes of exacerbations: usually infections or other toxic/metabolic insults
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Remove/avoid exacerbating medications: fluoroquinolones, aminoglycosides, beta blockers, and magnesium
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LEMS specific management:
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3,4-diaminopyridine
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Can respond to IVIG or pyridostigmine
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Workup for underlying neoplasm
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