Above the arcuate line the sheath is complete; below it all aponeuroses pass in front
Mnemonic
“Above the arcuate line the sheath is complete; below it, everything goes in front”:
- Above the arcuate line: the internal oblique aponeurosis splits, so external oblique and half of internal oblique pass in front, while half of internal oblique and transversus pass behind
- Below the arcuate line: all three aponeuroses pass anteriorly, leaving only transversalis fascia behind the muscle
The arcuate line lies about midway between the umbilicus and the pubic symphysis, at the level where the inferior epigastric vessels enter the sheath.
“Below the line there is no posterior sheath”, which is why a rectus sheath haematoma below the arcuate line can spread freely into the extraperitoneal space and present with shock, whereas above it the haematoma is contained and stays localised.
Fothergill’s sign distinguishes it from an intra-abdominal mass: a rectus sheath haematoma remains palpable and does not cross the midline when the muscle is tensed.
Expansion
Above the arcuate line
- Anterior wall - external oblique aponeurosis + anterior leaf of internal oblique
- Posterior wall - posterior leaf of internal oblique + transversus abdominis aponeurosis
Below the arcuate line
- Anterior wall - all three aponeuroses
- Posterior wall - none; only transversalis fascia and peritoneum
The inferior epigastric vessels enter the sheath at the arcuate line, which is why this is the level at which they become vulnerable and where a rectus sheath haematoma below the line can spread freely and be large.
The contents of the sheath are rectus abdominis, pyramidalis, the superior and inferior epigastric vessels, and the terminal parts of the lower intercostal nerves.