Snapshot A 40-year-old man presents with increasing muscle weakness. He reports difficulty rising from seated position and difficulty climbing stairs. He denies any blurry vision or any rashes on his body. He has a 100-pack-year history of smoking. Physical exam reveals proximal muscle weakness, decreased deep tendon reflexes, and dry mucous membranes. A bedside edrophonium test is conducted, which is negative. His chest radiography shows a suspicious finding - the round opacity shown in the square. Introduction Progressive weakness caused by autoantibody to presynaptic calcium channel on motor neurons type II hypersensitivity reaction Lambert-Eaton Syndrome (LES) also known as Lambert-Eaton myasthenic syndrome Epidemiology less common than myasthenia gravis Pathogenesis antibodies against presynaptic, voltage-gated Ca2+ channels at neuromuscular junction (see illustration) ↓ acetylcholine (ACh) release Associated conditions small cell (oat cell) carcinoma of lung recall other paraneoplastic secretions may be ACTH or ADH in 40% of patients with LES, cancer is found other autoimmune diseases May be diagnosed with prolonged paralysis following use of neuromuscular blocking agents after surgery Presentation Symptoms/physical exam proximal muscle weakness difficulty rising from a chair difficulty climbing stairs ↓ tendon reflexes autonomic symptoms dry mouth impotence improves temporarily with muscle use (vs myasthenia gravis, where symptoms worsen with muscle use) spares extraocular muscles Evaluation Negative edrophonium test cholinesterase inhibitor allows more ACh to present at the neuromuscular junction test is positive if patients regain strength test is negative in LES and positive in myasthenia gravis Voltage-gated Ca2+ channel antibody assay 75-100% of patients ACh receptor antibody assay more commonly positive in myasthenia gravis found occasionally in low titers in LES Differential Diagnosis Botulism Myasthenia gravis Amyotrophic lateral sclerosis Polymyositis Treatment Treat underlying malignancy Medical therapy for symptomatic treatment pyridostigmine inhibits acetylcholinesterase 3,4-diaminopyridine (DAP) increases the release of ACh Immunotherapy plasma exchange IVIG oral immunosuppressants azathioprine Prognosis, Prevention, and Complications Prognosis if with simultaneous malignancy, prognosis is poorer LES does not affect respiratory system as significantly as myasthenia gravis does Complications impaired quality of life