Etiology
Abdominal wall hernias, or external hernias (where abdominal contents protrude beyond the abdominal cavity), include inguinal, femoral, umbilical, incisional, spigelian, epigastric, lumbar, and obturator hernias. All abdominal wall hernias consist of a peritoneal sac that protrudes through a weakness or defect in the muscular layers of the abdomen. The defect may be congenital or acquired. Weakness of the transversalis fascia, which is the layer immediately outside peritoneum, is the main cause of abdominal wall hernias, especially in the groin.
Prevalence and Epidemiology
Abdominal wall hernias are a common clinical problem, especially in elderly patients because of the weakness of the abdominal wall and conditions that increase intra-abdominal pressure. Abdominal wall hernias represent a frequent imaging finding in the abdomen; thus, their actual prevalence and distribution are probably underestimated in the published literature.
Prevalence and Epidemiology
Abdominal wall hernias are a common clinical problem, especially in elderly patients because of the weakness of the abdominal wall and conditions that increase intra-abdominal pressure. Abdominal wall hernias represent a frequent imaging finding in the abdomen; thus, their actual prevalence and distribution are probably underestimated in the published literature.
Clinical Presentation
Most abdominal wall hernias are asymptomatic; however, in the United States, surgery for complications of an external hernia is one of the most common emergent procedures in patients older than 50 years of age. Between 4% and 6% of all diagnosed hernias require emergent surgical repair and are commonly associated with older age, femoral or scrotal location, greater 30-day reoperation risk, and decrease overall survival. Prompt diagnosis is desirable, because delayed treatment is associated with greater morbidity.
Pathophysiology
Hernias can occur in any portion of the abdominal wall; however, inguinal, umbilical, epigastric, para-umbilical, incisional, and femoral hernias were the most commonly reported hernias in a recent review by Dabbas and colleagues ( Figure 84-1 ). They usually manifest at points of weakness of the abdominal wall, where no muscle is present, along the midline, in the linea semilunaris on each side, and in the inferior lumbar space.
Abdominal wall hernias may be acquired or congenital. Acquired hernias are more frequently seen in patients who are obese or elderly or in those with previous trauma or surgery. Congenital hernias include indirect inguinal hernias and gastroschisis and omphalocele, occurring lateral to or at the umbilicus, respectively.
Imaging
In the past, the diagnosis of a hernia was made clinically or by means of plain radiographs or barium studies. Increasingly, diagnosis is made by computed tomography (CT) or ultrasonography. Cross-sectional imaging is required when the clinical presentation is misleading or inconclusive or in the presurgical assessment of the contents of an incarcerated hernia.
Radiography
Small, noncomplicated abdominal wall hernias are not seen on conventional radiographs. In some cases, such as in patients with a large wall defect or large hernia sac, the hernia may be visible. In these cases, air-, fluid-, or stool-filled loops are seen in abnormal locations in the abdomen, most frequently in the inguinal or umbilical area ( Figure 84-2 ).
In some cases of complicated abdominal wall hernia, such as bowel incarceration or strangulation, conventional radiographs allow detection of signs of mechanical ileus, bowel loop enlargement, thickening of intestinal folds and air/fluid levels.
In the past, the diagnosis of abdominal wall hernias was confirmed with barium studies or intraperitoneal administration of iodinated contrast agents. In current clinical practice, conventional radiography, barium studies, and herniography no longer play a pivotal role in the diagnosis of abdominal wall hernias.
Computed Tomography
For abdominal wall hernias, CT shows the specific anatomic site of the hernial sac; its shape, connections, and contents; and characteristics of the wall defect. Hernias can be distinguished from masses of the abdominal wall (e.g., tumor, hematoma, abscess, undescended testis, and aneurysm). CT also allows assessment for the presence of complications (e.g., incarceration, bowel obstruction, volvulus, and strangulation) ( Figure 84-3 ).
Most CT protocols for hernia evaluation include oral and intravenous contrast agent administration. However, owing to the high resolution of modern CT scanners, intravenous use of a contrast agent may not be necessary, especially in patients with abnormal renal function. Some authors advocate image acquisition while the patient performs the Valsalva maneuver to increase intra-abdominal pressure, which may aid in demonstrating some hernias, particularly those involving the ventral abdominal wall.
Multidetector row CT (MDCT) is now widely available and is pivotal in assessing patients with suspected abdominal wall hernia. It has a short acquisition time, good coverage, and excellent resolution. Moreover, three-dimensional information and multiplanar reformatted images provide excellent anatomic depiction of abdominal wall anatomy and useful information for surgical planning. MDCT is considered the first imaging choice when complications of abdominal wall hernias are suggested.
Magnetic Resonance Imaging
The role of magnetic resonance imaging (MRI) in assessing the abdominal wall is evolving. MRI can detect and characterize hernias with a sensitivity of 91%, specificity of 92%, positive predictive value of 97%, and negative predictive value of 79% considering laparoscopy as the gold standard. Useful sequences include coronal three-dimensional T1-weighted images without fat saturation during Valsalva maneuver and, at rest, axial turbo spin echo T2-weighted and axial short tau inversion recovery (STIR) comparing both groins.
Ultrasonography
Ultrasonography is a relatively inexpensive, noninvasive, and widely available modality that plays a pivotal role in evaluating patients with suspected abdominal wall hernias. No patient preparation is required. Ultrasonography allows dynamic evaluation (e.g., during Valsalva maneuver) to confirm herniation of intra-abdominal contents through a wall defect. However, when necessary, it requires flotation pads to achieve best resolution and avoid the “bang effect” of direct transducer placement on the skin.
Imaging Algorithm
Ultrasonography should be used as the primary imaging modality in asymptomatic patients with suspected abdominal wall hernia on physical examination and in children and women of child-bearing age. Ultrasonography is particularly useful in thin patients and may add information regarding complications in patients with acute presentation. However, its utility in obese patients or in patients with abdominal wall scarring may be limited.
MDCT provides excellent anatomic detail and is particularly useful in patients who are obese or who have significant abdominal wall scarring and when physical examination is limited. In addition, because the entire abdomen is visualized, MDCT may detect subtle signs of complications such as bowel obstruction, incarceration, and strangulation. Hence, MDCT is the imaging modality of choice in patients with suspected complicated abdominal wall hernia. ( Tables 84-1 and 84-2 ).
| Modality | Accuracy | Limitations | Pitfalls |
|---|---|---|---|
| Radiography | There is limited availability of source literature for comparing accuracy of different imaging modalities for detecting abdominal wall hernias. | Not useful for small defects | |
| CT | Ionizing radiation | ||
| MRI | Expensive and not widely available Limited spatial resolution Limited on postoperative patients Patient-limiting factors (e.g., claustrophobia, pacemaker) | ||
| Ultrasonography | Body habitus, significant scarring of overlying tissues, patients in distress Operator dependent Requires high-frequency transducer | Lipoma of the spermatic cord or abdominal wall |
| Hernia Type | Imaging Features | Preferred Modality | Clinical Significance |
|---|---|---|---|
| INGUINAL HERNIAS | |||
| Indirect | Lateral to epigastric vessels | US or MDCT | Common in men; may pass into the scrotum |
| Direct | Medial to epigastric vessels | US or MDCT | Often bilateral; do not pass into the scrotum or labia majora |
| Femoral | Medial to femoral vessels | MDCT | High tendency to incarcerate |
| VENTRAL HERNIAS | |||
| Umbilical | Around the umbilicus | US or MDCT | Prone to incarceration and strangulation |
| Hypogastric | Below the umbilicus—midline | US or MDCT | |
| Epigastric | Above the umbilicus—midline | US or MDCT | |
| Paraumbilical | Lateral to the umbilicus | MDCT | Related to diastasis of rectus abdominis muscles |
| Spigelian | Linea semilunaris | MDCT | High frequency of incarceration |
| LUMBAR HERNIAS | |||
| Superior | Below the 12th rib | MDCT | Asymptomatic; strangulation is uncommon |
| Inferior | Above iliac crest | MDCT | |
| INCISIONAL HERNIAS | |||
| Parastomal | Lateral to stoma site | MDCT | May cause bowel obstruction |
| PELVIC HERNIAS | |||
| Obturator | Through obturator foramen | MDCT | Prone to incarceration and strangulation |
| Sciatic | Through sciatic foramen | MDCT | |
| Perineal | Through pelvic floor | MDCT | Most frequent; no emergent treatment required |
| OTHER HERNIAS | |||
| Richter | Herniation of antimesenteric wall of the bowel—usually femoral hernia | MDCT | Prone to incarceration and strangulation |
| Littre | Herniation of Meckel’s diverticulum—inguinal hernias | MDCT | |
| Interparietal | Between fascial planes of abdominal wall | MDCT | |
Imaging of Specific Lesions
Inguinal Hernias
Inguinal hernias represent the most common type of abdominal wall hernias. They may occur in children (indirect most common) and adults (direct and indirect) and manifest medial (direct) or lateral (indirect) to the inferior epigastric vessels. Regardless of age, inguinal hernias are more common in males than in females. In children, most inguinal hernias develop because the peritoneal extension accompanying the testis fails to obliterate. In adults, they are caused by acquired weakness and dilation of the internal inguinal ring, which is a defect in the fascia transversalis.
Indirect inguinal hernias are more commonly seen in men, where the hernia sac passes through the internal or deep inguinal ring into the scrotum, anteromedially to the spermatic cord, and lateral to the epigastric vessels ( Figure 84-4 ). In females, an indirect inguinal hernia follows the round ligament into the labia majora. In some cases, indirect herniated content may pass all the way into the scrotum (known as a complete hernia) and may contain intestine (small bowel or colon), mesenteric fat, the appendix, foreign bodies, bladder, ureter, or any peritoneal cavity content (fluid, air).
Direct inguinal hernias are located medial to the inferior epigastric vessels, are thought to be acquired, appear between 30 and 40 years of age, and are often bilateral. They run lateral to the remains of the obliterated umbilical artery and are contained by the aponeurosis of the external oblique muscle ( Figure 84-5 ). Unlike the indirect inguinal hernias, the direct hernia sac lies behind the spermatic cord and is unlikely to reach the scrotum.
Radiography
In some cases, including those with large wall defects or large hernia sacs, inguinal hernias may be visible as air-, fluid- or stool-filled bowel loops in the inguinal area.
Computed Tomography
CT allows visualization of the following:
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Abdominal wall defect
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Hernia sac, medial (direct inguinal hernia [ Figure 84-6 ]) or lateral (indirect inguinal hernia [see Figure 84-3 ]) to the epigastric vessels in the inguinal region
Figure 84-6
Axial unenhanced computed tomography image demonstrating herniation of mesenteric fat through the internal inguinal ring (arrowhead).
- •
Hernia content, which may include intra-abdominal fat, bowel loops, appendix, foreign bodies, bladder, and ascites
In cases in which the hernia sac contains bowel loops, signs of incarceration, obstruction, or strangulation should be sought.
Magnetic Resonance Imaging
Visualization of abdominal wall discontinuity and the hernia sac with intra-abdominal contents may be seen on MRI.
Ultrasonography
Ultrasonography allows visualization of abdominal wall discontinuity and protrusion of intra-abdominal contents into the subcutaneous tissue. Assessment should be made of hernial sac content and, in males, the degree of extension into the scrotum. Dynamic evaluation should be performed during Valsalva’s maneuver, in which the hernia sac should become larger, unless the contents are incarcerated. Because the superficial and inferior epigastric vessels are not easily seen on ultrasonography, distinction between direct and indirect inguinal hernias may not be possible.
Femoral Hernias
Femoral hernias are far less frequent than inguinal hernias and are especially rare in children. They occur more commonly in women and, for unclear reasons, have a tendency to be right sided. They arise from a defect in the attachment of the transversalis fascia to the pubis and thus occur medial to the femoral vein and posterior to the inguinal ligament ( Figure 84-7 ). They are difficult to differentiate clinically from inguinal hernias and have a tendency to incarcerate.
