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Defecation 2: Internal anorectal resistance is a critical factor in defecatory disorders
1School of Mechanical and Chemical Engineering, The University of Western, Perth, Australia. mark.bush@uwa.edu.au
Techniques in Coloproctology
|July 25, 2012
Summary
Imaging is limited in diagnosing constipation because anorectal flow resistance is highly sensitive to small changes in rectal and anal geometry during defecation. Mathematical modeling reveals how geometry significantly impacts flow dynamics and diagnostic accuracy.
Area of Science:
- Physiology
- Biomechanical Engineering
- Medical Imaging
Background:
- Diagnosing constipation causes is challenging, with imaging often providing limited assistance.
- The study hypothesizes that high sensitivity of anorectal flow resistance to geometric changes during defecation contributes to this diagnostic limitation.
Purpose of the Study:
- To investigate the relationship between anorectal geometry changes during defecation and internal anorectal flow resistance.
- To apply a mathematical model to understand the impact of altered rectal and anal shapes on defecation mechanics.
Main Methods:
- Video proctography and magnetic resonance imaging (MRI) were used to study three patient groups: normal defecation (n=4), fecal incontinence (n=8), and constipation (n=8).
- Dynamic imaging techniques captured changes in anorectal geometry during defecation.
Main Results:
- Muscle vectors (anterior, posterior, and downward) expand the anorectal angle and double the luminal diameter during defecation.
- Anorectal flow resistance is influenced by viscous friction and bolus deformation.
- Geometric changes significantly reduce frictional resistance; doubling anal diameter decreases resistance by a factor of 8.
Conclusions:
- The high sensitivity of flow resistance to anorectal geometry explains the limited utility of standard MRI or CT scans in diagnosing constipation causation.
- Small geometric alterations have a disproportionately large effect on flow resistance.
- Integrating dynamic imaging with bolus deformation analysis and manometry may enhance diagnosis of anorectal evacuation disorders.
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