Vaginal pressure-pain thresholds: initial validation and reliability assessment in healthy women

Pelvic muscle pain syndromes are poorly understood and lack reliable diagnostic criteria. Furthermore, animal models suggest that somatic pain can influence the development of pelvic visceral pain dysfunction. To develop psychophysical measures to facilitate diagnosis of pelvic floor myofascial pain syndromes, this pilot was designed to preliminarily test the feasibility, reliability, and validity of pressure-pain thresholds (PPTs) to identify and quantify pelvic floor pain sensitivity.

We conducted a cross-sectional study of pelvic floor PPTs using 19 healthy women. Using a prototype vaginal pressure algometer, we measured PPTs and calculated intraclass correlations for short-term and intermediate-term reliability. Validity was assessed by correlating numerical rating scores for pain (0 to 100) at standard pressure levels applied to the right iliococcygeus muscle.

The mean PPT of all pelvic floor sites was 1.52 kg/cm (SD=0.62), whereas thresholds of nonmuscle vaginal sites (anterior and posterior wall) were 1.65 kg/cm (SD=0.64). Pain numerical rating scores were positively correlated with stimulus intensity at the right iliococcygeus (Pearson r=0.61). Intraclass correlation demonstrated good short-term reliability at this same site for the first versus second, and second versus third measurements (0.75, 0.64); 1-week repeat reliability was also good for the right pubococcygeus, iliococcygeus, and obturator (0.69, 0.84, and 0.61, respectively), and both nonmuscle vaginal sites.

These data suggest that PPTs may prove to be valid and reliable measures of pelvic floor somatic pain sensitivity in healthy women. Broader studies including a pelvic pain cohort should be conducted to corroborate these results and determine the technique's external validity and clinical relevance.