What anatomy knowledge is needed to perform 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 anal sphincter is innervated by the pudendal nerve, which derives from the anterior division of the S3, S4, and, occasionally, also S2 spinal nerves. The pudendal nerve, which innervates the external anal sphincter (EAS), is formed primarily by the ventral rami of the second and third sacral spinal nerves, with a frequent contribution from the first sacral spinal nerve. [2] Although the superficial EAS is circular in nature, it should be considered to be 2 separate muscles. The EAS is divided into 3 striated subcutaneous, superficial, and deep muscles. The superficial EAS muscle, only 2-5 millimeters below the skin surface just outside the anal canal (usually 10-12 mm from the anal orifice), has the highest diagnostic yield. [2, 22] The superficial EAS is usually more severely affected by proximal nerve lesions than the deeper EAS muscles. [22]

Interdigiitation of muscle fascicles across the midline results in substantial overlap of innervation between the two sides. This enables partial reinnervation from the contralateral side after unilateral pudendal neurectomy. [23] The anal sphincter, which normally is under volitional control, shares similar physiologic properties with the skeletal muscles of the limbs.

The superficial EAS can be localized on needle EMG by its higher number of low-threshold motor units that fire continuously during relaxation. [2] It can be reached with a sharp angle of insertion relative to the mucosal surface. [2] The role of the superficial EAS is to protect the anal orifice. The deeper muscles serve intrinsic sphincter functions. [22]

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