Sophisticated diagnostic testing is currently being used in clinical research and in anorectal physiology laboratories to quantify the function of the colon and anorectum. Following is a description of these techniques as they relate to the management of fecal incontinence and disordered defecation.
Figure 28.1 Bruel-Kjaer (Copenhagen, Denmark) ultrasound probe (type 1850) with a 7.0 MHz transducer (focal length, 2 to 5 cm) housed with a plastic cone.
Endoanal ultrasonography permits accurate imaging of both the internal and external anal sphincters. It can assess the continuity and thickness of the muscle and currently is considered the single best method for detecting anal sphincter defects. Endoanal ultrasonography is performed using a Bruel-Kjaer (Copenhagen, Denmark) ultrasound scanner with a 360-degree rectal endoprobe (type 1850) with a 7.0 MHz transducer (focal length, 2–5 cm) housed within a plastic cone (Fig. 28.1). The normal IAS is a continuous hypoechoic band of smooth muscle surrounded by the thick echogenic layer of the striated EAS. A sphincter defect occurs when there is disruption in these muscle bands. Location and severity of the defect can be described by circumferential distance in degrees, percentage of thickness, and distance from the anal verge (Fig. 28.2). Measurements are usually taken in the proximal, middle, and distal anal canal. It is important to recognize the physiologic split in the proximal EAS as it merges with the puborectalis muscle of the levator ani. Misinterpretation of this finding as a sphincter defect can result in an increased prevalence of reported defects. The puborectalis muscle appears as a U-shaped or V-shaped thick echogenic layer outside the IAS in the proximal anal canal. Magnetic resonance imaging (MRI) may be equally as effective or better at diagnosing sphincter defects, especially with the use of a vaginal or rectal coil. For this purpose, MRI is more expensive, and currently its use is largely investigational. It may be beneficial in cases in which endoanal ultrasonography results are inconclusive or the quality of the study is poor.
Electromyography Electromyography (EMG) is used to evaluate neuromuscular integrity of the EAS following a traumatic injury such as childbirth, as well as to document the presence of pelvic ﬂoor neuropathy (86). This technique measures the electrical activity arising in muscle fibers during contraction and at rest. Different types of electrodes may be employed, including surface electrodes, concentric needle electrodes, and single-fiber electrodes. Surface electrodes are less invasive because they are applied near or within the anal canal, but they are capable only of recording basic anal sphincter activity. This technique often is used in conjunction with biofeedback therapy. Concentric needle electrodes are most commonly used in anorectal physiology laboratories to selectively survey an individual muscle’s activity. Insertion of the thin needle-like cannulas containing steel wire electrodes can be painful. Even smaller single-fiber EMG electrodes are used to record the activity of single muscle fibers, which can be quantified to calculate fiber density. Following denervation injury, increased muscle fiber density occurs during reinnervation. Thus, single-fiber EMG can provide indirect evidence of neurologic injury by mapping the EAS and identifying injured areas. This technique is used rarely in clinical practice. Endoanal ultrasonography offers increased patient comfort and more reliable results than EMG and has replaced this technique for the detection of EAS disruption because of increased patient comfort and more reliable results.
Motor nerve conduction studies provide another means of measuring pelvic ﬂoor neuropathy. The axon of a nerve is stimulated, and the time it takes the action potential to reach the muscle supplied by the nerve is recorded. The delay between stimulation and the muscle