Viscoelastic assessment of anal canal function using acoustic reflectometry: a clinically useful technique.

Background: Anal acoustic reflectometry is a new reproducible technique that allows a viscoelastic assessment of anal canal function. Five new variables reflecting anal canal function are measured: the opening and closing pressure, opening and closing elastance, and hysteresis.

Objective: The aim of this study was to assess whether the parameters measured in anal acoustic reflectometry are clinically valid between continent and fecally incontinent subjects.

Design: This was an age- and sex-matched study of continent and incontinent women.

Setting: The study was conducted at a university teaching hospital.

Patients: One hundred women (50 with fecal incontinence and 50 with normal bowel control) were included in the study. Subjects were age matched to within 5 years.

Main Outcome Measures: Parameters measured with anal acoustic reflectometry and manometry were compared between incontinent and continent groups using a paired t test. Diagnostic accuracy was assessed by the use of receiver operator characteristic curves.

Results: Four of the 5 anal acoustic reflectometry parameters at rest were significantly different between continent and incontinent women (eg, opening pressure in fecally incontinent subjects was 31.6 vs 51.5 cm H2O in continent subjects, p = 0.0001). Both anal acoustic reflectometry parameters of squeeze opening pressure and squeeze opening elastance were significantly reduced in the incontinent women compared with continent women (50 vs 99.1 cm H2O, p = 0.0001 and 1.48 vs 1.83 cm H2O/mm, p = 0.012). In terms of diagnostic accuracy, opening pressure at rest measured by reflectometry was significantly superior in discriminating between continent and incontinent women in comparison with resting pressure measured with manometry (p = 0.009).

Conclusions: Anal acoustic reflectometry is a new, clinically valid technique in the assessment of continent and incontinent subjects. This technique, which assesses the response of the anal canal to distension and relaxation, provides a detailed viscoelastic assessment of anal canal function. This technique may not only aid the investigation of fecally incontinent subjects, but it may also improve our understanding of anal canal physiology during both the process of defecation and maintenance of continence.