performing the test
NORMAL
CMUAP'S
(Compound Motor Unit Action Potentials)
LEFT TRICEP, LATERAL HEAD. RADIAL NERVE. C-7 NERVE ROOT.
When a muscle fiber loses its nerve supply, it exhhibits a characteristic irritability manifested as spontaneous discharges at rest. Singe muscle discharge, called FIBRILLATIONS have a short duration (.5 to 1.5 msec), low amplitude (50-300 microvolts) and a REGULAR rhtyhm. They are usually positive (downward) in their initial deflection.
Positive Sharp Waves
(negative is up)
Positive sharp waves and fibrillations are abnormal but aren't characteristic of any single disease. They may be seen whenever a muscle cell membrane becomes hyper-irritable. If they are widespread in all four extremities and consistent with the clinical history, the dreaded Amyotrophic Lateral Sclerosis (or Lou Gherig's Disease) must be suspected. However, there is a great deal of subjectivity in the interpretation of an EMG and, as Dr. Forbes Norris used to remind: "One swallow does not a summer make."
NEUROLOGY ROUNDS
1971
Forbes (Ted) Norris on the left
Polyphasicity, (more than 5 baseline crossings on an EMG) and increased serration may be seen in re-inervated muscles or primary disorders of muscle called MYOPATHIES. In most myopathies, however, amplitude is reduced and the action potential prolonged.
Above we see a famous pattern, almost pathognomonic for MYOTONIC DYSTROPHY by the unmistakeable "dive bomber" sound.
EMG's are invasive and should only be done by physicians and then only when indicated by the clinical history.
Questions? Comments?
Write me at DrJohn@idiom.com
