Endocoil magnetic resonance imaging quantification of external anal sphincter atrophy

Abstract Background Anal function depends on the integrity and quality of the sphincter muscles. The diagnosis of external anal sphincter atrophy on endocoil magnetic resonography has been associated with poor outcome from sphincter repair, although the imaging criteria for atrophy remain unclear. Methods Women with intact sphincters on endosonography and either normal (more than 60 cmH2O) (n = 9) or low (n = 16) squeeze pressures had endocoil magnetic resonography and electromyography. The area and fat content of the external anal sphincter and puborectalis were measured on mid-coronal magnetic resonography and images were graded as showing normal, intermediate or advanced atrophy. The definition of the external anal sphincter on endosonography and the thickness of the internal anal sphincter were also assessed. Results Women with a normal anal squeeze pressure had a larger external anal sphincter cross-sectional area (mean(s.d.) 240(56) versus 193(62) mm2; P = 0·01) with a lower mean fat content (mean(s.d.) 23(4) versus 30(6) per cent; P < 0·001) than those with low squeeze pressures. There was an overall correlation between squeeze pressure, cross-sectional area (r = 0·32, P = 0·02) and fat content (r = − 0·51, P < 0·001). Patients with a thin (less than 2 mm) internal anal sphincter and/or a poorly defined external sphincter on endosonography were more likely to have atrophy (positive predictive value 74 per cent). Conclusion Potential endosonographic markers for external anal sphincter atrophy are suggested, and a visual scale for endocoil magnetic resonographic assessment has been validated.