The rectus sheath encloses the rectus abdominis and pyramidalis muscles along with the vessels that supply the rectus abdominis muscle. The arrangement of the sheath above and below the arcuate line is a common clinical point in exams. In this article we will discuss the gross and functional anatomy of the rectus sheath. We will also discuss the clinical relevance of the structure, and provide a summary of key points at the end of the article.
The rectus abdominis muscle lies within the rectus sheath, the sheath is formed by the merging of the aponeurosis of transversus abdominis, external and internal oblique abdominal muscles. The external oblique is the most superficial muscle of the anterior abdominal wall. The transversus abdominus is the deepest of the three muscles and its fibers run in a horizontal direction.
The part of the rectus sheath above the costal margin is incomplete, anterior layer is made by the aponeurosis of external oblique muscle while posterior layer is deficient. Below the rib margin down to the level below the umbilicus rectus sheath is complete. The anterior layer is made by the aponeurosis of external oblique and one of the two layers of internal oblique aponeurosis. The internal oblique aponeurosis divides into two. The anterior part of the internal oblique aponeurosis passes in front of the rectus abdominis muscle with the external oblique aponeurosis. The posterior division of the internal oblique aponeurosis passes behind the rectus abdominis muscle with the transversus abdominis aponeurosis. However, below the arcuate line (which lies midway from the umbilicus to the pubis), all three of the muscular aponeuroses pass anterior to the rectus abdominus, and the posterior surface of the muscle is covered only by transversalis fascia, and parietal peritoneum.
Below the arcuate line, the posterior surface of the rectus sheath lies directly against the transversalis fascia. It encloses the rectus abdominus muscle and pyramidalis muscle. The pyramidalis muscle is a small anterior muscle that lies within the rectus sheath. It inserts onto the pubic symphysis and pubic crest and some of its fibers arise from the pubis and anterior pubic ligament. The muscle passes superiorly to insert into the linea alba, midway between the umbilicus and pubis. The muscle is innervated by the anterior portion of T12, and is supplied by the inferior and superior epigastric arteries. At the midline both anterior and posterior layers decussate to the opposite side forming a continuous aponeurosis. The dense fibrous line formed by these decussation is called linea alba.
The other contents of the rectus sheath along with two muscles described above are superior and inferior epigastric vessels, termination of lower five intercostal and 12th thoracic nerves and accompanying vessels.
Superior epigastric artery anastomose with inferior epigastric at the level of umbilicus after entering into the rectus sheath behind rectus abdominis muscle. Branches supply rectus muscle and some perforate the anterior sheath to supply the skin. At the upper part of the rectus sheath a branch is given off which passes anterior to the xiphoid process and anastomose with the contralateral branch.
The nerves that enter the rectus sheath pierce the posterior wall near the lateral margin and supply the muscles.
Rectus sheath haematoma- This is a condition where a haematoma forms within the rectus sheath, around the rectus abdominis muscle, it is usually due to damage to the superior and inferior epigastric arteries or their branches. They occur more commonly in women, and most commonly present in the lower segment of the sheath. The condition is usually self-limiting, and is associated with trauma, coagulopathies and vascular diseases. Diagnosis is made via CT or ultrasound scan.
Divarication of the rectus abdominis- This is separation of the two halves of the muscle i.e. widening of the linea alba. It often occurs in pregnant women, postoperatively, in athletes or the obese.