Figure 28.2 A: Endoanal ultrasound image from the distal anal canal demonstrating defects in the internal sphincter from 10 to 3 o’clock and the external sphincter from 10 to 2 o’clock. B: Endoanal ultrasound image from the middle anal canal demonstrating defects in the internal sphincter from 12 to 2 o’clock and the external sphincter from 10 to 1 o’clock. C: Endoanal ultrasound image from the proximal anal canal demonstrating an intact IAS and a normal physiologic split in the external sphincter.
Figure 28.3 St. Mark’s electrode used for measuring pudendal nerve motor terminal latency. The stimulating electrode is on the fingertip, and the receiving electrode is on the proximal finger near the knuckle.
response is called the nerve latency. Pudendal nerve terminal motor latency (PNTML) can be determined by transrectal stimulation of the pudendal nerve using a St. Mark’s electrode (87). A nerve stimulator is mounted on an examination glove at the fingertip (Fig. 28.3) and positioned transrectally over each ischial spine. A stimulus of up to 50 mV for 0.1 milliseconds is applied, and the latency of the EAS muscle contraction is measured. A value of 2.2 milliseconds or less is considered normal. A recent study evaluating normative values for pudendal and perineal nerve latencies observed increased latencies with increased age (88). Prolongation of the PNTML is indicative of damage to that nerve or the presence of a demyelinating condition. Pudendal nerve function has prognostic value in the surgical repair of traumatic sphincter injuries and is useful in preoperative counseling (89).
Anal Manometry Anal manometry is used to quantify function of the anal sphincter mechanism. Water-perfused manometry catheters or water-filled balloons are most often used to measure anal canal pressures. Resting anal canal pressures reﬂect IAS function, and pressures in the lower anal canal during maximal voluntary contraction reﬂect EAS function. Vector analysis can be used to detect asymmetry within the anal sphincter. Anal manometry provides indirect evidence of sphincter injury; low resting tone indicates IAS injury, and decreased maximum squeeze pressure indicates EAS injury. Anal pressures are inﬂuenced by a variety of factors, including tissue compliance and muscular tone. Consequently, anal manometry results are difficult to interpret and correlate poorly with the specific anatomic defect. Interpretation is further complicated by the wide variation of normal pressure values that change with age and parity. Significant overlaps occur between manometric values for incontinent patients and those without incontinence. Thus, anal manometry may be of limited value in the evaluation and treatment of anal sphincter defects and fecal incontinence.