FRCA Notes

Lambert Eaton Myasthenic Syndrome

The curriculum asks for "understanding of the principles of anaesthesia for patients with myasthenic syndrome"; there has yet to be an SAQ on the topic.

Resources

  • Lambert-Eaton Myasthenic Syndrome (LEMS) is an immune-related acquired disorder characterised by reduced release of acetylcholine at the motor nerve terminal of the NMJ
  • It is typically associated with malignancy; 50% are associated with small cell carcinoma of the bronchus
  • Ectopic production of IgG auto-antibodies to pre-synaptic voltage-gated calcium channels
  • The lack of calcium influx impairs acetylcholine release into the synaptic cleft

  • Weakness ± tenderness of proximal limb muscles which, unlike MG, tends to improve with exercise
  • Depressed tendon reflexes
  • Relative sparing of cranial nerves (i.e. lacks the bulbar and ocular symptoms of MG)
  • Possibility of respiratory muscle weakness ± respiratory failure

Autonomic features

  • Dysautonomia occurs in most patients due to reduced ACh release at other cholinergic sites
  • Most concerning for anaesthetists are GI slowing and postural hypotension
  • Others include classic anticholinergic features:
    • Dry mouth
    • Urinary retention
    • Constipation
    • Diplopia
    • Erectile dysfunction

Investigation

  • Detection of auto-antibodies
  • Characteristic EMG finding of tetanic stimulation causing incrementally improved CMAPs

Management

  • Treat underlying malignancy

  • In non-malignant LEMS, immunosuppresion with:
    • Steroids
    • Azathioprine

  • Most patients respond to oral 3,4-diaminopyridine (3,4-DAP)
    • It blocks voltage-gated potassium channels at the NMJ
    • This prolongs the action potential and increases Ach release

  • Short-term optimisation with IVIg or plasma exchange

Perioperative management of the patient with Lambert-Eaton syndrome

  • A standard approach to pre-operative management
  • Consider optimisation with pre-admission for IVIg/plasma exchange
  • Consent the patient for prolonged post-operative ventilation

Monitoring

  • AAGBI as standard
  • Arterial line in view of autonomic dysfunction
  • Continuous, quantitative neuromuscular monitoring if NMBA are to be used

Drug choices

  • Be mindful that induction agents and PPV will exacerbate autonomic dysfunction
  • Patients are sensitive to NMBA (both depolarising and non-depolarising) - should be avoided or dose reduced
    • In one case report a patient required over 2hrs to recover 4 twitches following 20mg of atracurium (Anaesthesia Cases, 2016)
  • Avoid the same drugs as in myasthenia gravis

  • A cocktail of papaveretum (10mg), thiopentone (200mg), suxamethonium (50mg) and vecuronium (1mg) with nitrous/isoflurane maintenance (BJA, 1990) is ill-advised...

  • A standard approach of multi-modal analgesia, multi-modal anti-emesis, appropriate VTE and antibiotic prophylaxis and a low threshold for HDU monitoring seems appropriate