This middle aged lady has presented with chronic lower abdominal pain - a clinical entity with a differential diagnosis encompassing the urinary, gastrointestinal and reproductive systems, as well as the abdominal wall. Considering her age and the location of the pain, intermittent diverticulitis is a possibility, as is renal colic. In addition, the possibility of an inguinal or femoral hernia should not be neglected. However, the worsening of the pain upon standing and walking provides an important clue here - this is suggestive of a pain source within the abdominal wall. The chronic, non progressive nature of the pain is further supportive of this. Her examination provides an additional clue - tenderness over a single discrete location in the left lower abdomen, when the abdomen is tensed. Consideration of the relevant surface anatomy shows that this is directly over the left lateral border of the rectus muscle. Could this be a Spigelian hernia? Unfortunately, in the absence of a palpable lump, clinical examination alone cannot determine this. Ultrasonography of the abdomen is sensitive for the detection of Spigelian hernias - and in this patient, confirms the clinical diagnosis. Note that a follow up CT scan is only necessary in patients in whom the diagnosis is in doubt. As Spigelian hernias possess a high risk for obstruction and strangulation, surgical repair is essential. As her hernia is not acutely complicated, surgery may be performed electively, using either open or laparoscopic techniques. Note that colonoscopy and urinalysis are not indicated in this patient; nor are laxatives, antibiotics or a high-fibre diet.
Spigelian hernias are rare, accounting for only 0.12% of all abdominal wall hernias. They are most commonly encountered in the fifth and sixth decades of life, but can occur at any age. There is a slightly higher incidence in women, with a male:female ratio of 1:1.4. To understand the mechanism of herniation, one must appreciate the anatomy of the semilunar line, rectus sheath, and semicircular line. The semilunar line is the boundary between the muscle body and the anterior aponeurosis of the transversus abdominis muscle. Note that although the term 'line' is used, this is more like a wide band of tissue. The rectus sheath is formed from the fused aponeuroses of the transversus abdominis, external oblique and internal oblique muscles; it covers the rectus abdominis. Above the level of the umbilicus, the rectus sheath passes both in front and behind the rectus abdominis; however, below the umbilicus, the sheath only passes in front of the rectus. The level at which the posterior rectus sheath disappears is called the semicircular line. Note that the point where the vertical semilunar and horizontal semicircular lines intersect is a potential weak point for herniation. This likelihood is increased by the passage of the inferior epigastric artery through this area. In addition, the fibres of the semilunar line run at angles to each other above the semicircular line, but run in parallel below this level. This further predisposes to herniation below the umbilicus. In practice, most Spigelian hernias lie in a transverse, 6-cm-wide zone above the interspinal plane. This is often called the "spigelian hernia belt". Risk factors for herniation include stretching in the abdominal wall caused by obesity, multiple pregnancies, previous surgery or scarring. Most Spigelian hernias are simply protrusions of preperitoneal fat. However, the greater omentum, small intestine or colon may also herniate. A few patients present with a visible or palpable mass along the Spigelian aponeurosis; in these cases the diagnosis is obvious. In other patients the initial presentation may be obstruction or frank strangulation. However, many patients present with abdominal pain only, which is classically provoked or aggravated by maneuvers that increase intra abdominal pressure and relieved by rest. During palpation, these patients should be asked to alternately tense and relax the abdominal muscles. The presence of an area of tenderness over the semilunar line when the abdomen is tensed is strongly suggestive (but not pathgnomonic) of a Spigelian hernia. This occurs because the ring of the hernial orifice becomes firm when the abdomen is tensed; palpation subsequently presses it against the hernia, resulting in tenderness. Note that important differential diagnoses to consider include tumors of the abdominal wall, and spontaneous hematomas of the rectus sheath. If a Spigelian hernia is suspected, ultrasonography of the abdomen is recommended as the first line investigation. CT scanning is only required if the diagnosis is in doubt. Note that plain abdominal x-rays are insensitive for the diagnosis. Note also that a Spigelian hernia can be confused with a lipoma or a parietal abscess on sonography. As there is a high risk of obstruction and strangulation, surgery is advised in all patients. This may be performed via open techniques or laparoscopically. Note that regardless of the technique employed, placement of a mesh is essential to minimize recurrence.