Defecation 2: Internal anorectal resistance is a critical factor in defecatory disorders.

BACKGROUND

The aim of this study was to test our hypothesis that the reason why imaging is of little assistance in diagnosing "constipation" causes may be related to the high sensitivity of internal anorectal flow resistance in defecation to small changes in geometry. We applied a mathematical model to describe the effects on flow mechanics of observed changes in the shape of the rectum and anus during defecation.

METHODS

Three groups of patients were studied with video proctograms. Group 1 comprised 4 patients with normal defecation studied with video proctography or magnetic resonance imaging (MRI). Group 2 comprised 8 patients with fecal incontinence, studied by video X-ray electromyography. Group 3 comprised 8 patients with constipation evaluated by video MRI.

RESULTS

Three muscle vectors open the anorectal angle prior to defecation, causing the anorectal luminal diameter to increase to approximately twice its resting size. These vectors are forwards (anterior wall), backwards and downwards (posterior wall). Resistance to passage of a fecal bolus through the anorectum is determined by viscous friction against the anorectal wall and by the energy required to deform the bolus as it flows. The observed changes in anorectal geometry serve to reduce both the viscous friction in the anus and the deformation of the bolus, which reduces the force required to facilitate its passage through the anus. For example, if the effective diameter of the anus is doubled during defecation, the frictional resistance is reduced by a factor of 8.

CONCLUSIONS

The sensitivity of flow resistance to geometry explains why MRI or computed tomography (CT) scans taken during defecation are not often helpful in diagnosing causation. Small changes in geometry can have a disproportionate affect on flow resistance. Combining accurate directional measurements during dynamic MRI or CT scans taken during defecation with observations of bolus deformation, and if possible, simultaneous anorectal manometry, may provide clinically helpful insights on patients with anorectal evacuation disorders.