Thickening of the internal anal sphincter: correlation with external rectal intussusception and associated features at MR proctography and anal manometry.

OBJECTIVES: To determine if internal anal sphincter (IAS) thickness can be reproducibly measured with endoanal MRI in conjunction with MR defecography, and if IAS thickening is associated with external rectal intussusception (i.e., external rectal prolapse) at MR defecography or high-resolution anal manometry (HRM) measurements. METHODS: In this case-control study, 119 patients underwent clinical endoanal MRI, dynamic MR defecography, and HRM. Patients with intra-anal or external rectal intussusception (cases) were matched to those who had neither (controls). Five radiologists, blinded to all clinical information and MR defecography images, measured the maximum lateral IAS thickness on endoanal MR images (≤ 3 mm considered normal). Intraclass correlation coefficient (ICC) estimated inter-rater reliability. Between-group differences and associations among IAS thickness, external rectal intussusception, and anorectal pressures at HRM were analyzed. RESULTS: Among 52 cases, 40 had external and 12 had intra-anal rectal intussusception. Median IAS thickness was 2.5 mm (1.1-7.3) with good/moderate inter-rater agreement/reliability (ICC = 0.623). IAS thickening was more frequent in patients with external rectal intussusception (18/40; 45%) than in patients with intra-anal, intra-rectal, or no rectal intussusception (25%, 17%, 10%, respectively; p < 0.002). External rectal intussusception was associated with lower mean anal resting pressure (48 ± 29 mmHg), compared to controls and patients with other degrees of intussusception (75-81 mm Hg, p < 0.004). Anal resting pressure was also lower in patients with thickened IAS (53.5 mmHg vs. 73.8 mmHg; p =0.003). In multivariable analysis, only external rectal intussusception independently predicted anal resting pressure (β-coefficient = -28.9 mmHg, P < 0.0001). IAS thickening was associated with increased anterior-posterior separation of the sphincters and circumferential morphology of external rectal intussusception (P[Formula: see text] 0.015). CONCLUSION: IAS thickening can be reliably measured on endoanal MRI and is strongly associated with external rectal intussusception on MR defecography and reduced anal resting pressure at HRM.