1% of all vaginal deliveries are at risk of sustaining an anal sphincter injury. Obstetric anal sphincter injury encompasses both third- and fourth-degree perineal tears.
A third-degree perineal tear is defined as a partial or complete disruption of the anal sphincter muscles, which may involve either the external (EAS) and internal anal sphincter (IAS) muscles, or both.
A fourth-degree tear is defined as a disruption of the anal sphincter muscles with a breach of the rectal mucosa. There is a suggestion that there has been an increase in obstetric anal sphincter injury (OASI) over the past 10 years. This may be due to increased detection or to increased occurrence with more older mothers, higher BMI and higher birth weights of babies. In collaboration with others, the RCOG developed a care bundle for OASI in 2017 to try to reduce these potentially life-changing injuries.
Anatomy of the anal sphincter
The anal sphincter complex includes the EAS and the IAS.
The EAS is composed of striated muscle and contributes to 25% of the resting anal sphincter tone. It is under voluntary control and is innervated by the pudendal nerve. It is the anterior portion of this muscle (i.e. the part in the 12 o'clock position) that is most often torn in sphincter injuries.
The IAS is the thickened continuation of circular smooth muscle of distal colon. Its appearance is described as 'pale, white and rubbery'. It is under autonomic control and is responsible for 70% of the resting tone. It plays a crucial role in maintenance of continence. Damage to the IAS is more predictive of symptoms of anal incontinence than damage to EAS.
|Classification of third- and fourth-degree tears|
Injury to the perineum involving the anal sphincter complex:
|Fourth-degree||Injury to the perineum involving the anal sphincter complex (EAS and IAS) and anal epithelium|
When the tear involves anal mucosa but the anal sphincter complex is still intact ('button hole' tear), it needs to be classified as a separate entity.