Presynaptic neuromuscular block of autoimmune origin. The cause is paraneoplastic (small cell lung cancer in the majority of cases) in about 50% of cases. Muscle weakness is caused by the presence of IgG antibodies directed against the voltage-dependent presynaptic calcium channels, resulting in a decrease in the amount of ACh released with action potentials. This syndrome is generally observed in patients over 40 years of age and is twice as common in men as in women. When this syndrome is of  paraneoplastic origin, it appears by average of at least 10 months  before the diagnosis of the primary tumor is established. Approximately 5% of the cases are children and the cause is either autoimmune or a hematologic cancer

Muscle weakness is characterized by ptosis, diplopia and impairment of proximal muscles, mainly at the level of the lower limbs. There are often signs of autonomic dysfunction: orthostatic hypotension, dry mouth, hypohydrosis.

Clinically, the main difference with myasthenia gravis is the fact that muscle contraction improves transiently after exercise ("Lambert’s sign"), which is confirmed to EMG (posttetanic facilitation). Muscle function generally improves following the administration of diaminopyridine 3-4. If this treatment is not sufficient, corticotherapy and or immunosuppression are added.

Anesthetic implications:

increased sensitivity to depolarizing and non-depolarizing muscle relaxants; risk of hypotension, with a good response to the filling and vasopressors; neostigmine is less effective to antagonise non-depolarizing muscle relaxants: it is better to use  3-4-diaminopyridine. This diagnosis should be kept in mind in case of abnormal duration of a neuromuscular block in a patient older than 50 years.

References:

•        Small S, Ali HHA, Lennon VA, Brown RH, Carr DB, De Armendi A. 
  Anesthesia for an unsuspected Lambert-Eaton myasthenic syndrome with autoantibodies and occult small cell lung carcinoma. 
  Anesthesiology 1993; 76: 142-5.

-        Leonovicz BM, Gordon EA, Wass CT. 
Paraneoplasic syndromes associated with lung cancer: a unique case of concomitant subacute cerebellar degeneration and Lambert-Eaton syndrome. 
Anesth Analg 2001; 93: 1157-9.

-         Weingarten TN, Araka CN, Mogensen ME, Sorenson JP et al. 
Lambert-Eaton myasthenic syndrome during anesthesia: a report of 37 patients. 
J Clin Anesth 2014, 26 : 648-53.

-         Portaro S, Parisi D, Polizzi A, Ruggieri M et al. 
Long-term follow-up in infantile-onset Lambert-Eaton myasthenic syndrome. 
J Child Neurol 2014; 29: NP58-61

Updated: February 2019