Answer : Spigelian Hernia Discussion A spigelian hernia occurs along the semilunar line, which traverses a vertical space along the lateral rectus border from the costal margin to the pubic symphysis. Adriaan van der Spieghel described the spigelian fascia as the aponeurotic structure between the transversus abdominis muscle laterally and the posterior rectus sheath medially. This fascia is what makes up the semilunar line, and it is through this fascial layer that a spigelian hernia forms. The spigelian fascia varies in width along the semilunar line, and it gets wider as it approaches the umbilicus. The widest portion of the spigelian fascia is the area where the semilunar line intersects the arcuate line of Douglas (the linea semicircularis). It is in this region, between the umbilicus and the arcuate line, where more than 90% of spigelian hernias are found.It is thought that since the spigelian fascia is widest at this point, it is also weakest in this region. Below the arcuate line, all of the transversus abdominis aponeurotic fibers pass anterior to the rectus muscle to contribute to the anterior rectus sheath, and there is no posterior component of the rectus sheath. The rearrangement of muscle and fascial fibers at the intersection of the arcuate and semilunar lines is thought to cause an area of functional weakness that is predisposed to hernia formation. As the hernia develops, preperitoneal fat emerges through the defect in the spigelian fascia bringing an extension of the peritoneum with it. The hernia usually meets resistance from the external oblique aponeurosis, which is intact and does not undergo rearrangement of its aponeurotic fibers at the arcuate line. For this reason, almost all spigelian hernias are interparietal in nature, and only rarely will the hernia sac lie in the subcutaneous tissues anterior to the external oblique fascia. This fact makes the accurate diagnosis of spigelian hernias more challenging. The hernia also cannot develop medially due to resistance from the intact rectus muscle and sheath. Therefore, a large spigelian hernia is most often found lateral and inferior to its defect in the space directly posterior to the external oblique muscle. Clinical Manifestations and Diagnosis The patient most often presents with a swelling in the middle to lower abdomen just lateral to the rectus muscle. The patient may complain of a sharp pain or tenderness at this site. The hernia is usually reducible in the supine position. However, up to 20% of spigelian hernias will present incarcerated, and for this reason operative repair is mandatory once the hernia is confirmed on diagnosis. The reducible mass may be palpable, even if it sits below the external oblique musculature. When the diagnosis is unclear, radiologic imaging may be necessary. Ultrasound examination has been shown to be the most reliable and easiest method to assist in the diagnostic work-up. Testa and colleagues found that abdominal wall ultrasonography was accurate in 86% of cases of spigelian hernia. If the hernia is fully reduced during examination and no mass is palpable, ultrasound evaluation can show a break in the echogenic shadow of the semilunar line associated with the fascial defect. Ultrasound can also identify the nonreduced hernia sac passing through the defect in the spigelian fascia. Computed tomographic scanning of the abdomen will also confirm the presence of a spigelian hernia. Treatment The treatment for spigelian hernia is operative repair once the diagnosis has been confirmed, given the risk for incarceration. This is usually performed under general anesthesia given the need for splitting of the external oblique muscle. ref - Skandalakis' Surgical Anatomy > Chapter 9. Abdominal Wall and Hernias Related Self Assessment Question
