11 - Ultrasound and diagnosis of obstetric anal sphincter injuries

Summary

Introduction

Anal endosonography is a relatively recent development in the evolution of diagnostic ultrasound. Following the original description of the technique by Bartram in 1989, Sultan et al. clarified and validated the interpretation of images. Subsequently, other approaches were described such as transvaginal and transperineal, allowing imaging of the undisturbed anal sphincter. It is now also possible to obtain three-dimensional ultrasound images as well as magnetic resonance images. Anal endosonography is now regarded as the gold standard investigation in patients presenting with faecal incontinence. It is also useful in the diagnosis of anal pain, anorectal tumours, fistulae, abscesses and anismus. The advent of anal endosonography has enabled considerable research into obstetric related anal sphincter trauma, the major aetiological factor in the development of anal incontinence.

Anatomy of the anorectum

Before any attempt is made to image the anal sphincter it is important to have a clear understanding of the anatomy and normal variation of the anorectal musculature to avoid misinterpretation. However, the precise anatomy of the anal sphincter mechanism has remained controversial and there is still disagreement as to whether the external anal sphincter is composed of one, two or three parts. The three-part classification of the external anal sphincter into deep, superficial and subcutaneous components (Figure 11.1) remains the most popular. These subdivisions are difficult to demonstrate during cadaveric dissections, however, and they are certainly not identifiable during surgery.