Myasthenia Gravis

OVERVIEW

• 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

• cholinesterase 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