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)
Jeremy Fernando (2011)
- 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
Jeremy Fernando (2011)
- 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)