Variations in motor evoked potential latencies in the anal sphincter system with sacral magnetic stimulation

The anal sphincter system of the skeletal musculature consists of various parts but functions as a single unit. The motor latencies measured at the puborectalis, deep external anal sphincter, superficial external anal sphincter, and subcutaneous external anal sphincter have yet to be clearly elucidated. This study was designed to measure the motor latencies of these sphincters separately and to investigate the electrophysiologic heterogeneity of the anal sphincter system.

In 20 preoperative patients with colon cancer without either neurologic or anorectal diseases, motor evoked potentials were taken separately bilaterally at levels measuring approximately 5, 3.8, 2.6, and 1.5 cm from the perineal skin using a concentric needle electrode after magnetic stimulation on the sacral region. These latencies were considered to be separately measured at the puborectalis, deep, superficial, and subcutaneous external anal sphincter.

The motor latencies (median (95 percent confidence interval)) were 4.33 (4.1-4.93; right) and 4.24 (3.97-5.22; left) ms at the 5 cm level; 5.96 (5.45-7.32; right) and 6.44 (5.49-8.20; left) ms at the 3.8 cm level; 9.28 (7.99-11.21; right) and 8.88 (7.68-10.93; left) ms at the 2.6 cm level; and 11.06 (9.69-14.13; right) and 9.18 (8.09-10.31; left) ms at the 1.5 cm level. Significant differences were seen in the latencies between the 5 cm level and the other levels and between the 3.8 cm level and the other levels.

The motor latencies in the anal sphincter system significantly varied depending on the distance from the skin. The latency may be shortest in the puborectalis and increasingly longer in the deep, superficial, and subcutaneous EAS, in that order.