Aims of Study:
Recently, several studies have reported evidences of partial denervation in the pelvic floor muscles in Stress incontinence(SI) patients, using electrophysiological and histomorphological methods. These results suggested that the denervation in the pelvic floor muscles possibly caused by childbirth vaginally is one of the etiological factors of type 2 SI accompanied with urethral hypermobility. Whereas, an etiology of another type of SI, that is type 3 SI considered due to intrinsic sphincteric deficiency remains unclear. We determined electromyographic features of the striated urethral sphincter in type 3 SI patients and evaluated the correlation of electromyographic changes with the clinical findings.

Methods:
We performed electromyography(EMG) of the striated urethral sphincter muscle and urodynamic studies including Valsalva leak point pressure measurement in a total of 51 women, 41 female patients with type 3 SI due to intrinsic sphincteric deficiency(ISD) and 10 women with normal urinary control(NUC). EMG parameters evaluated were duration, amplitude, and number of phases in individual motor unit potentials(MUP) of the striated urethral sphincter at rest, and also an interference pattern at maximal voluntary contraction.

Results:
In patients with SI, mean values of MUP parametersü}SD were duration 4.2ü}0.9 msec., amplitude 299.9ü}112.0 mV. and number of phases 4.1ü}0.7. The values in women with NUC were 5.2ü}0.3 msec., 428.2ü}25.6 mV., and 3.4ü}0.4, respectively. The MUP of SI patients showed significantly shorter duration (p=0.0014) , lower amplitude (p=0.0008) and larger number of phases (p=0.0022). 30 (73%) of the SI patients showed an obvious low amplitude (<350 mV)/short duration(<4.5 msec.)/polyphasic pattern and early recruitment of interference activity with low amplitude at voluntary contraction of the striated sphincter, both indicating existence of myogenic damages. These patients showing myogenic damages had significantly lower Valsalva leakpoint pressure (p<0.0001) and more leakage in pad weigh test (p=0.0101), compared with the SI patients without myogenic damage findings.

Conclusions:
These results suggested that myogenic-dominant damages of the striated urethral sphincter may contribute to the etiology of ISD in the majority of type 3 SI.