Striated urethral sphincter activity does not alter urethral pressure during filling cystometry

Objective The purpose of this study was to determine the relationship between urethral pressure and the neuromuscular activity of the urethral sphincter with the use of quantitative electromyography during bladder filling. Study design Women who underwent multichannel urodynamic testing with concentric needle electromyography of the striated urethral sphincter between December 2000 and February 2002 were studied. Raw electromyography signals were processed by a electromyography instrument that was equipped with automated motor unit analysis software programs. Quantitative electromyography software was used to analyze the electrical activity of the urethral sphincter during filling cystometry. Results One hundred women (mean age, 60 years [range, 22-82 years]; median parity, 3 children [range, 0-8 children]) were studied. Most women (79%) were postmenopausal, and 68% of those women were receiving hormone replacement therapy. Quantitative electromyography values increased significantly at 300 mL and maximum cystometric capacity; however, there was no significant increase in urethral pressure. The median change in urethral pressure at 300 mL and maximum cystometric capacity were 4 cm water (interquartile range, 0-8 cm) and 0 cm water (interquartile range, −4-8 cm), respectively (P = .229). The median change in quantitative electromyography at 300 mL and maximum cystometric capacity were 9 μV (range, 5-14 μV) and 10 μV (range, 7-19 μV), respectively (P< .0005). There was no correlation between change in urethral pressure and motor unit activation on quantitative electromyography at 300 mL or maximum cystometric capacity. Fifty-six women had no change or a decreased urethral pressure at maximum cystometric capacity, yet all but 1 of these women had increased motor unit activation on quantitative electromyography. Conclusion Urethral pressure does not increase during filling cystometry, despite increased activity of the striated urethral sphincter, which suggests that urethral pressure change does not reflect the integrity of the striated urethral sphincter.