Myasthenia Gravis
21/2/11
FANZCA Part II Notes
PY Mindmaps
- autoimmune disruption of post-synaptic acetylcholine receptors @ NMJ
- up to 80% of functional receptors loss
- typically young woman
- may have thymus hyperplasia
- prevalence = 14.2 cases per 100,000
HISTORY
- mild ptosis -> bulbar palsy and respiratory failure
- most marked after prolonged exertion
- severity of MG (duration, functional capacity, doses of medications)
- dose of steroid and duration
- may be on immunosuppressive agents, plasmapheresis or immunoglobulin infusion
- bulbar symptoms
- upper airway muscle weakness can produce a myasthenic crisis (airway collapse and obstruction + inability to swallow secretions)
- chewing fatigue
- significant other cardio/respiratory disease – heart failure, COPD, restrictive lung disease, recurrent aspiration pneumonia
EXAMINATION
- swallow
- functional capacity
- effectiveness of cough
- jaw closure often weak and cannot be maintained against resistance
- airway assessment
- focused RESP and CVS examination
- evidence of proximal myopathy and strength
Reasons to present to ICU and requiring MV
- upper airway obstruction
- inability to clear secretions
- pneumonia
- post surgical procedures (including thymetomy resection)
- tapering of immunomodulatory therapy
- pregnancy with disease exacerbation
INVESTIGATIONS
Diagnostic
- edrophonium test (tensilon test)
- electrophysiological studies
- EMG
- Ach receptor and muscle specific receptor tyrosine kinase antibody testing
Respiratory assessment
- spirometry
- PEFR
- CXR
- ABG
MANAGEMENT
ICU Management
Resuscitate
- admit to ICU if VC < 25mL/kg, weak cough, not clearing secretions
- intubate if indicated (airway protection, fatigue, hypercapnic respiratory failure)
- physio
Specific Therapies
- anticholinesterase inhibitors: pyridostigmine, rivastigmine
- plasma exchange
- IVIG
- corticosteroids (treatment resistant MG crises)
- restart oral medications as soon as possible (may need IV neostigmine (30mg pyridostigmine:1mg neostigmine) or hydrocortisone if not able to tolerate PO medications)
- incentive spirometry
- introduction to physiotherapy
- GORD/aspiration prophylaxis: H2 antagonists, Na+ citrate, metoclopramide, appropriate starvation
Underlying Cause
- thymectomy
- good analgesia
Intraoperative
- avoid muscle relaxation if possible (may not be given major abdominal surgery)
- if required use small titrated doses of NDNMB (10mg atracurium IV boluses) – very sensitive
- plasmapheresis depletes plasma choline esterase levels -> prolonged action of sux, miv, remi + ester based LA’s
- sux can be used (dose 1.5mg/kg)
- keep warm
- use PNS
- intubation
- MRSI if indicated
- controlled ventilation
- volatile maintenance
- good analgesia
- intraoperative hydrocortisone/dexamethasone if indicated
- avoid reversal if possible (increased risk of cholinergic crisis) -> if need to reverse use standard doses
- extubate once wide awake and obey commands (able to lift head off pillow for 5 seconds)
- N/G tube may be required so can have regular medication
- discussion with neurology about patient degree of optimisation required for surgery
- plan for post-operative ventilation if required (ICU)
- plan for analgesic technique as indicated
Predictors of Post operative Ventilation
- major body cavity surgery
- duration of disease (> 6 years)
- history of chronic respiratory disease
- dose requirements of >750mg/day
- preoperative VC of <3L
Drugs exacerbating MG
- neuromuscular blocking drugs
- antibiotics: aminoglycosides, macrolides
- CVS drugs: beta-blockers, Ca2+ channel blockers, procainamide, quinidine
- corticosteroids
- Mg
- iodinated contrast
- d-penicillamine
- opioids: morphine and pethidine
Jeremy Fernando (2011)