Following a difficult vaginal delivery, examination reveals a perineal tear extending through the perineal muscles and the entire anal sphincter complex (both external and internal sphincters), and continuing through into the anal epithelium. How is this tear classified?

Answer: D. Fourth degree

Fourth-degree perineal tear

A fourth-degree tear involves the anal sphincter complex AND the anal epithelium - the deepest grade of perineal trauma.

The progression

1. 1st - vaginal epithelium/skin only.
2. 2nd - perineal muscles, sphincter intact.
3. 3rd - anal sphincter complex (3a <50% EAS, 3b >50% EAS, 3c IAS torn).
4. 4th - extends through the sphincter into the anal epithelium (mucosa).

Why this matters

A fourth-degree tear is a major OASIS, carrying the highest risk of faecal incontinence and complications. It requires repair in theatre by a trained operator, repairing the anal epithelium, then the internal and external sphincters, then the perineal muscles and skin in layers, with antibiotics and laxatives afterwards.

Common exam contrast

A second-degree tear extends into the perineal body/muscles but does not involve the anal sphincter or anal mucosa - involvement of the sphincter is 3rd degree and of the anal mucosa is 4th degree.