Which factors increase the difficulty of detecting abnormal spontaneous potentials on anal sphincter electromyography (EMG)?

Updated: Aug 06, 2019
  • Author: Jasvinder Chawla, MD, MBA; Chief Editor: David C Spencer, MD  more...
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The presence of physiologic tone without volitional effort in the anal sphincter, in contrast to peripheral skeletal muscles, makes study of this muscle very challenging for an electromyographer. The subject at rest maintains sustained firing of isolated motor unit potentials at a low rate. This activity varies considerably with changes in subject position. The activity continues during sleep, although the discharge rate drops substantially compared with that during wakefulness. Sphincter activity ceases completely only during attempted defecation. The presence of physiologic tonic activity at rest makes detection of abnormal spontaneous potentials difficult in a partially denervated muscle. In contrast, the paretic sphincter may reveal abundant fibrillation potentials, positive sharp waves, and complex repetitive discharges, as in any denervated limb muscle.

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