Measuring length-tension function of the anal sphincters and puborectalis muscle using the functional luminal imaging probe

The functional luminal imaging probe (FLIP) has been used to measure the distensibility of the anal canal. We hypothesized that with increasing distension of the anal canal with FLIP there will be an increase in length of the anal sphincter muscle allowing measurement of the length-tension function of anal sphincter and puborectalis muscles (PRM). We studied 14 healthy nulliparous women. A custom-designed FLIP bag (30-mm diameter) was placed in the vagina and then in the anal canal, distended in 10-ml steps with volumes ranging from 30 to 90 ml. At each volume, subject performed maximal voluntary squeezes. Length-tension measurements were also made with a manometric probe system. Tension was calculated (pressure × radius) in Newtons per meter using a custom software program. Peak tensions at different FLIP volumes were compared with the manometric data. No change in the luminal CSA was noted at low fluid volumes; the sphincter muscles were able to fully collapse the FLIP bag within the anal canal/vagina even at rest. At larger volumes, with each squeeze there was an increase in the bag pressure and reduction in the cross-sectional area, which represents concentric contraction of the muscle. Both rest and squeeze tension increased with the increase in volume in the anal as well as vaginal canal indicating that the external anal sphincter and puborectalis muscles produce more tension when lengthened. FLIP device, which has been used to describe the distensibility of the anal canal can also provide information on the length-tension function of the anal sphincters and PRM. NEW & NOTEWORTHY The functional luminal imaging probe (FLIP) has been used to describe the distensibility of the anal canal. This report is the first to describe the use of the FLIP in the vaginal canal and the anal canal to provide information on the length-tension function of the anal sphincter and puborectalis muscles, which may provide clinicians with additional information regarding the active components of muscle contraction involved in the anal closure function.