Validity and reliability of an instrumented speculum designed to minimize the effect of intra-abdominal pressure on the measurement of pelvic floor muscle strength

- We developed an improved instrumented speculum to measure vaginal closure force.
- The improved speculum measurements were unconfounded by intra-abdominal pressure.
- We compared speculum measurements to subjective measures of pelvic muscle strength.
- The improved speculum had acceptable discriminant validity and repeatability.

BackgroundMeasurements of pelvic floor muscle strength are contaminated by crosstalk from intra-abdominal pressure. We tested an improved instrumented speculum designed to minimize this crosstalk. The hypotheses were that the speculum yields: 1) maximum vaginal closure forces unrelated to intra-abdominal pressure, 2) discriminatory validity between women with strong vs. weak pelvic floor muscles, and 3) acceptable test-retest reliability.MethodsMaximum voluntary vaginal closure force was measured in 40 incontinent women (20-77 years) on two visits spaced one month apart. At the baseline visit, intra-abdominal pressure was also estimated via intra-vesical catheterization during the vaginal closure force measurement. Subjective estimate of pelvic floor muscle strength was also assessed using digital palpation by a skilled examiner to determine group placement as “strong” (n = 31) or “weak” (n = 9).FindingsVaginal closure force was not significantly correlated with intra-abdominal pressure (r = − .26, P = .109). The groups with subjectively scored strong and weak pelvic floor muscles differed significantly by mean [SD] maximum vaginal closure force (3.8 [1.7] vs. 1.9 [0.8] N respectively, P < .01.) Across both time points the mean vaginal closure force was 3.42 [1.67] N with a range of .68 to 9.05 N. Mean Visit 1 and Visit 2 vaginal closure force scores did not differ (3.41 [1.8] and 3.42 [1.6] N, respectively). The vaginal closure force repeatability coefficient was 3.1 N.InterpretationThe improved speculum measured maximum vaginal closure force without evidence of crosstalk from intra-abdominal pressure, while retaining acceptable discriminant validity and repeatability.