Quantitative electromyographic analysis of levator ani and external anal sphincter muscles of nulliparous women

OBJECTIVES: Our aims were to introduce a method of digital quantitative ele ctromyography of the levator ani and external anal sphincter muscles and to establish reference values. STUDY DESIGN: Fifteen nulliparous, symptom-free women underwent concentric needle electromyographic examination of the levator ani and external anal s phincter. We sampled the levator ani transvaginally at 4 sites and the exte rnal anal sphincter at 2 sites. The signal was filtered and amplified, and digital recordings were made at 3 levels of voluntary activation at each si te. Analyses of motor unit action potentials and interference patterns were performed with the use of these taped signals. Normal ranges were generate d and compared with those established for other striated muscles. RESULTS: The mean age of the subjects was 28.7 +/- 7.5 years. A median of 2 4 motor unit action potentials was recorded in each levator ani, and a medi an of 6 was recorded in each external anal sphincter. Parameters of the lev ator ani action potentials were significantly greater than those of the ext ernal anal sphincter in amplitude (0.48 vs 0.37 mV; P = .001), duration (10 .40 vs 8.27 ms; P = .002), number of turns per second (2.80 vs 2.28; P < .0 01), and area (0.65 vs 0.36; P < .001). Parameters of the interference patt erns were significantly greater in the levator ani than in the external ana l sphincter in number of turns per second (241.6 vs 183.9; P = .015), ampli tude (302.7 vs 225.3 muV; P < .0001), activity (95.6 vs 61.2; P = .004), en velope size (861.1 vs 567.6 <mu>V; P < .0001), and number of small segments (105.8 vs 81.4; P = .047). There were no significant differences between l evator ani, external anal sphincter, and published parameters from the bice ps muscle with regard to amplitude and duration of motor unit action potent ials. CONCLUSIONS: Electromyography of the levator ani and external anal sphincte r is feasible and well tolerated. Our findings confirm that the levator ani muscle has larger, more readily recruited motor units than does the extern al anal sphincter. Ranges for important quantitative electromyographic para meters for these muscles are similar to those published for the biceps.