Lambert-Eaton myasthenic syndrome

Background

• Autoantibodies against Ca channels of axon nerve terminals responsible for ACh release
• Epidemiology
  • Predominantly a disease a/w older men w/ history of cigarette smoking and lung cancer
    • 50% of pts have concurrent small-cell lung cancer
  • Syndrome can precede detection of malignancy by several years

Clinical Features

• Fluctuating symmetric weakness and fatigue, esp of proximal leg muscles
• Improvement in strength with sustained or repeated exercise (in contrast to MG)
  • Lambert sign: handshake strength increases over several seconds
• Myalgias
• Muscle stiffness (especially in hip and shoulders)
• Paresthesias
• Metallic taste
• Autonomic symptoms (dry mouth, impotence)
• Eye movements are unaffected
• Sensory examination normal

Differential Diagnosis

Weakness

• Neuromuscular weakness
  • Upper motor neuron:
    • CVA
    • Hemorrhagic stroke
    • Multiple sclerosis
    • Amyotrophic Lateral Sclerosis (ALS) (upper and lower motor neuron)
  • Lower motor neuron:
    • Spinal and bulbar muscular atrophy (Kennedy's syndrome)
  • Spinal cord disease:
    • Infection (Epidural abscess)
    • Infarction/ischemia
    • Trauma (Spinal Cord Syndromes)
    • Inflammation (Transverse Myelitis)
      • Acute flaccid myelitis
    • Degenerative (Spinal muscular atrophy)
    • Tumor
  • Peripheral nerve disease:
    • Guillain-Barre syndrome
    • Toxins (Ciguatera)
    • Tick paralysis
    • Diabetic peripheral neuropathy
  • Neuromuscular junction disease:
    • Myasthenia gravis crisis
    • Botulism
    • Organophosphate toxicity
    • Lambert-Eaton myasthenic syndrome
  • Muscle disease:
    • Rhabdomyolysis
    • Dermatomyositis
    • Polymyositis
    • Alcoholic myopathy
• Non-neuromuscular weakness
  • Can't miss diagnoses:
    • ACS
    • Arrhythmia/Syncope
    • Severe infection/Sepsis
    • Hypoglycemia
    • Periodic paralysis (electrolyte disturbance, K, Mg, Ca)
      • Hypokalemic periodic paralysis
      • Thyrotoxic periodic paralysis
    • Respiratory failure
  • Emergent Diagnoses:
    • Symptomatic Anemia
    • Severe dehydration
    • Hypothyroidism
    • Polypharmacy
    • Malignancy
    • Aortic disease - occlusion, stenosis, dissection
  • Other causes of weakness and paralysis
    • Acute intermittent porphyria (ascending weakness)

Treatment

• Supportive (progression to respiratory or bulbar failure is rare)
• Acetylcholinesterase inhibitors (e.g. pyridostigmine) can improve symptoms

Disposition

• Admission required when infectious complications occur or when severe disability requires inpatient immunotherapy

See Also

• Myasthenia Gravis