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Myasthenic Syndrome

For further information, see CMDT Part 24-35: Disorders of Neuromuscular Transmission

Key Features

  • There is defective release of acetylcholine in response to a nerve impulse, leading to weakness, especially of the proximal muscles of the limbs

  • May be associated with small-cell carcinoma, sometimes developing before the tumor is diagnosed, and occasionally occurs with certain autoimmune diseases (eTable 39–1)

eTable 39–1.Paraneoplastic syndromes associated with cancer.
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eTable 39–1. Paraneoplastic syndromes associated with cancer.
Hormone Excess or Syndrome Non–Small Cell Lung Cancer Small Cell Lung Cancer Breast Cancer Renal Cell Carcinoma Adrenal Cancer Hepatocellular Carcinoma Gastrointestinal Cancers Plasma Cell Myeloma Lymphoma Thymoma Prostatic Cancer Ovarian Cancer Choriocarcinoma Germ Cell Cancers
Endocrine
Hypercalcemia ++   ++ ++ ++     ++ +   ++      
Cushing syndrome + ++   + ++         ++ +      
SIADH ++ ++                        
Hypoglycemia         + ++ +              
Gonadotropin secretion + ++   + + + +           ++ ++
Hyperthyroidism                            
Hematologic
Erythrocytosis       ++ + ++                
Pure red cell aplasia                 + ++        
Coagulopathy     ++       ++       ++ +    
Thrombophlebitis     +       ++       ++ +    
Neurologic
Lambert-Eaton myasthenic syndromes + ++ +       +   + +   +    
Subacute cerebellar syndrome   ++ +       +   +     +    
Sensory motor peripheral neuropathy   ++                        
Stiff man syndrome     +                      
Dermatologic
Dermatomyositis ++ ++ +       +         +    
Acanthosis nigricans +   +       ++       +      
Fever       ++   ++     ++ +        
Hypertrophic osteoarthropathy ++                          

+, reported associated; ++, strong association.

SIADH, syndrome of inappropriate antidiuretic hormone.

Clinical Findings

  • Variable weakness, typically improving with activity

  • Dysautonomic symptoms may also be present

  • A history of malignant disease may be obtained

  • Unlike myasthenia gravis, power steadily increases with sustained contraction

Diagnosis

  • Electrophysiologic diagnosis: the muscle response to stimulation of its motor nerve increases remarkably after exercise or if ...

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