Sampling reflex: pathogenic role in functional defecation disorder

The sampling reflex is necessary to begin defecation or flatulence. It consists of a simultaneous rectoanal inhibitory reflex (RAIR) mediated by relaxation of the internal anal sphincter and rectoanal excitatory reflex (RAER) mediated by contraction of the external anal sphincter. The aim of this study was to evaluate the sampling reflex in patients with functional defecation disorder (FDD).

A prospective cohort study was conducted on 58 obstructed defecation syndrome (ODS) patients with FDD. All 58 patients and 20 controls were evaluated with anorectal manometry to study the sampling reflex. Quantitative RAIR (total duration of reflex; maximal amplitude of relaxation; residual pressure at the lowest point of the RAIR) and RAER data (maximal amplitude of contraction; duration) were obtained. The straining test on manometry was considered positive for FDD if there was a muscle contraction/lack of relaxation or an insufficient pressure gradient for the passage of feces. Defecography was performed on all the patients with assessment of the anorectal angle and persistence or increase of puborectalis indentation.

Fifty (86.2%) FDD patients had an altered sampling reflex, showing incomplete/short duration of RAIR and excessive contraction/duration of RAER. More specifically, there was a correlation between a positive straining test and a short total duration of RAIR (ρ 0.92) as well as with excessive duration of RAER (ρ 0.89). There was also a correlation between lack of muscle relaxation on defecography and short total duration of RAIR ((ρ 0.79) and between lack of muscle relaxation on defecography and excessive duration of RAER (ρ 0.83). Altered maximal amplitude relaxation had the highest sensitivity in detecting impairment of RAIR (87.9) while maximal amplitude contraction had the highest sensitivity in detecting impairment of RAER (89.6). High residual pressure at the lowest point of RAIR had the highest specificity in detecting impairment of RAIR (80.0) while RAER duration had the highest specificity in detecting impairment of RAER (77.7).

The sampling reflex is impaired in patients with FDD. This finding provides an important insight into the pathogenesis of obstructed functional defecation.