Comparison between anal endosonography and digital examination in the evaluation of anal fistulae

A prospective trial was performed comparing the accuracy of digital examination and anal endosonography in defining the anatomy of anal fistulae. Before operation 38 consecutive patients were assessed by the consultant in charge of the case, by a research fellow and by anal endosonography involving two radiologists. These findings were compared with the operative findings. Consultants correctly identified 26 of 33 internal openings, 29 of 34 primary tracks and 15 of 21 secondary tracks. The research fellow correctly identified 26 internal openings, 24 primary tracks and 10 secondary tracks. There was no significant difference between the accuracy of consultants and the research fellow. Anal endosonography identified 10 internal openings based on initial criteria. This rose to 24 when revised ultrasonographic criteria were applied. There was no statistical difference between consultant assessment and anal ultrasonography in correctly identifying intersphincteric and transphincteric tracks. Ultrasonography is unable to assess primary superficial, suprasphincteric and extrasphincteric tracks or secondary supralevator and infralevator tracks. Consultant assessment of secondary supralevator and infralevator tracks was correct in 78 per cent of cases.