The association between constipation and urinary retention is well known (1,2).
Several mechanisms may explain this relationship, including sharing of the innervations of the internal anal and urinary sphincters via S2-S4 nerve roots, and the presence of impacted stool in the rectum leading to invaginations in the posterior wall of the bladder and urethral obstruction (1,2).
Interestingly, in laboratory experiments involving rats, rectal distention with a balloon diminished bladder contractility (3). So, along with many other factors, constipation should routinely be considered a potential cause of acute urinary retention.
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1. Selius BA, Subedi R. Urinary retention in adults: diagnosis and initial management. Am Fam Physician 2008;77:643-650. https://www.aafp.org/afp/2008/0301/p643.html
2. Ariza Traslavina, Del Ciampo LA, Ferraz IS. Acute urinary retention in a pre-school girl with constipation. Rev Paul Pediatr 2015;33:488-492. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4685571/
3. Miyazato M, Sugaya K, Nishijima S, et al. Rectal distention inhibits the spinal micturition reflex via glycinergic or GABAergic mechanisms in rats with spinal cord injury. Urol Int 2005;74:160-65. https://www.ncbi.nlm.nih.gov/pubmed/15756069