AAEM minimonograph #44: diseases associated with excess motor unit activity

Muscle Nerve. 1994 Nov;17(11):1250-63. doi: 10.1002/mus.880171103.


Stiff-man syndrome is due to hyperexcitability of anterior horn cells, possibly related to interference with the synthesis or action of gamma-aminobutyric acid. Unexpected acoustic and exteroceptive stimuli produce exaggerated muscle responses. Needle electrode examination of involved muscles yields nonspecific findings and demonstrates involuntary motor unit activity. The appearance and firing pattern of motor units are normal except that agonist and antagonist muscles may contract concurrently. Continuous muscle fiber activity (Isaacs' syndrome) comprises a heterogeneous group of hereditary and acquired disorders that cause hyperexcitability of peripheral nerves. Some are associated with electrophysiologic evidence of peripheral neuropathy and some are not. Repetitive afterdischarges often follow the M-, H-, and F-waves. Needle electrode examination reveals an abnormal pattern of motor unit firing, consisting of myokymic discharges, doublets and multiplets, neuromyotonic discharges, and fasciculations. These abnormalities may occur alone or in combination.

Publication types

  • Review

MeSH terms

  • Electrophysiology
  • Humans
  • Motor Neurons
  • Neural Conduction
  • Neuromuscular Diseases / drug therapy
  • Neuromuscular Diseases / physiopathology*
  • Peripheral Nervous System Diseases / classification
  • Peripheral Nervous System Diseases / complications
  • Peripheral Nervous System Diseases / genetics
  • Peripheral Nervous System Diseases / physiopathology*
  • Peripheral Nervous System Diseases / therapy
  • Stiff-Person Syndrome / complications
  • Stiff-Person Syndrome / physiopathology
  • Tetanus / physiopathology