Rectus SheathEdit
The rectus sheath is a key fibrous enclosure of the abdominal wall that surrounds the rectus abdominis muscle. It is formed by the aponeuroses of the flat abdominal muscles—the external oblique, internal oblique, and transversus abdominis—and it serves to stabilize the abdominal contents, aid in movement and respiration, and provide a conduit for nerves and vessels. Its structure is central to both normal physiology and a range of clinical conditions.
The sheath extends from the costal margin to the pubic region and plays a crucial role in maintaining intra-abdominal pressure and supporting the posterior wall of the abdominal cavity. Its architecture, which differs above and below a key boundary known as the arcuate line (linea arcuata), has important implications for surgical approaches and the risk profile of various abdominal procedures. In the upper abdomen, the sheath has a more divided arrangement that leaves a posterior layer behind the rectus muscle, whereas in the lower abdomen the aponeuroses pass predominantly in front of the rectus abdominis, leaving a thinner posterior boundary.
Anatomy
Composition and architecture
The rectus sheath is formed by the combined aponeuroses of three lateral abdominal muscles: - external oblique - internal oblique - transversus abdominis
This composite sheath encapsulates the rectus abdominis muscle along its length. The precise arrangement of these aponeuroses differs relative to the arcuate line: - Above the arcuate line, the anterior layer consists of the external oblique aponeurosis plus part of the internal oblique aponeurosis, while the posterior layer is formed by the remaining internal oblique aponeurosis, the transversus abdominis aponeurosis, and the transversalis fascia. - Below the arcuate line, all three lateral aponeuroses pass anterior to the rectus abdominis, and only the transversalis fascia remains as the posterior boundary.
The rectus sheath also interacts with the semilunar line (Linea semilunaris), where the lateral boundary of the rectus muscle is defined. The apex of the sheath is contiguous with the linea alba at the midline.
Blood supply and innervation
Inside the rectus sheath, the vascular and neural supplies are critical for function and sensation: - Blood supply comes from the superior epigastric arteries (continuation of the internal thoracic arteries) and the inferior epigastric arteries (branch of the external iliac arteries). These vessels form an anastomotic network that helps nourish the rectus muscle and the overlying tissues. - Innervation is provided primarily by the lower six intercostal nerves and the subcostal nerve, collectively referred to as the thoracoabdominal nerves. These nerves traverse the abdominal wall and branch within the sheath to supply motor function to the abdominal muscles and sensory information from the overlying skin.
Relations and boundaries
The rectus sheath lies between the linea alba medially and the semilunar line laterally, with the rectus abdominis muscle occupying the interior of the sheath. The anterior and posterior leaves of the sheath provide structural integrity that is crucial during movements that increase intra-abdominal pressure, such as coughing, heavy lifting, and Valsalva maneuvers.
Development and variation
During embryologic development, the aponeuroses of the lateral abdominal muscles migrate and fuse to form the composite sheath. Variations in the relative contributions of each muscle’s aponeurosis can occur, which may impact the exact posterior or anterior composition of the sheath in a given individual. Such variations can be clinically relevant in the planning of incisions or the interpretation of imaging.
Clinical significance
Rectus sheath hematoma
A rectus sheath hematoma is a hemorrhage within the sheath, typically arising from tearing of the epigastric vessels or from muscle fibers themselves. It presents with sudden, localized abdominal pain and may produce a palpable or visible abdominal mass. Risk factors include anticoagulation, pregnancy, heavy coughing, strenuous exertion, and hypertension. Diagnosis is usually by imaging, with CT scans offering precise localization and assessment of ongoing bleeding. Management ranges from conservative observation to interventional radiology or surgical control in more severe cases.
Surgical incisions and Abdominal wall surgery
Knowledge of the rectus sheath enhances surgical planning for abdominal procedures. Midline incisions, often through the linea alba, exploit the relatively avascular plane and preserve the rectus sheath layers; they minimize tissue disruption and postoperative pain. In selected cases, lateral or paramedian approaches may penetrate the rectus sheath to access the peritoneal cavity, emphasizing the importance of preserving motor nerves and preserving vascular integrity to reduce complications and hernia risk. The arrangement of the aponeuroses below the arcuate line is particularly relevant for surgeons performing extraperitoneal or anterior approaches.
Hernias and other pathologies
Defects in the rectus sheath can contribute to herniation along the abdominal wall. Epigastric hernias may occur through defects in the linea semilunaris or within the anterior portion of the sheath, whereas Spigelian hernias involve herniation through the aponeurotic layer adjacent to the rectus sheath. Although rare, rectus sheath–related hernias require imaging and may necessitate surgical repair. Awareness of the sheath’s boundaries helps distinguish these conditions from intra-abdominal processes.
Imaging and diagnosis
Ultrasound and computed tomography (CT) are common tools for evaluating the rectus sheath, its integrity, and related vascular structures. Imaging helps distinguish hematomas, hernias, and inflammatory or neoplastic processes that involve the abdominal wall.