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MUSCULOSKELETAL ULTRASOUND SYMPOSIUM |
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| Year : 2007 | Volume
: 17
| Issue : 4 | Page : 290-298 |
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| High resolution ultrasonography of the anterior abdominal wall |
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Sudheer Gokhale
Ultrasonography Clinic. 569, MG Road and Choithram Hospital and Research Centre, Indore, India
Click here for correspondence address and email
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Keywords: Abdominal wall; cold abscess; hernia.
How to cite this article:
Gokhale S. High resolution ultrasonography of the anterior abdominal wall. Indian J Radiol Imaging 2007;17:290-8 |
 Introduction | |  |
With the introduction of high-frequency, high-resolution probes, detailed examination and recognition of different layers of the abdominal wall is now possible on USG examinations. A high-resolution examination is capable of deciding whether an abnormality is in the abdominal wall or inside the abdominal cavity. Physical findings in abdominal wall pathologies have low specificity and often a clinically suspected intra-abdominal lump proves to be in the abdominal wall. Typically when Carnett's sign is positive, a USG examination of the abdominal wall is advised. [1],[2]
| Technique | | |
High-frequency linear array transducers, exquisitely show the anatomy of the abdominal wall layers. Examination of the skin, however requires very high-frequency probes or the use of some sort of stand-off device. All patients were scanned with a Voluson 730 Expert scanner (GE Medical Systems, Bangalore, India) or an HDI 5000 scanner (Philips Medical Systems, Mumbai, India), using high-frequency (6-12 mHz) linear transducers. Extended or panoramic views were often recorded to demonstrate the lesion in relation to the adjacent structures in the abdominal wall.
There is no need for abdominal preparation. Abdominal wounds, if any should be cleaned and if possible covered with a thin, sterile, plastic adhesive membrane; more simply, the probe may be covered with sterile plastic or a glove.
| Understanding the Layered Structure of the Anterior Abdominal Wall | | |
The abdominal wall is a layered structure, consisting of skin, superficial fascia, subcutaneous fat, the muscle layer, the transversalis fascia and a layer of extra-peritoneal fat [Figure - 1]. The most important layer to be appreciated is the muscle layer and is composed of paired midline rectus muscles and anterolaterally situated internal and external oblique and the transverse abdominis muscles. The aponeurosis of the external and internal oblique and transverse abdominis muscles continues medially as the rectus sheath. Two layers of the rectus sheath enclose the rectus muscles. The posterior rectus sheath is absent below the arcuate line. In the midline, the two recti are separated by fusion of the anterior and posterior layers of the rectus sheath to form the linea alba.
The skin is echogenic. The subcutaneous fat is usually hypoechoic. [3] The muscles reveal medium-level echoes with a lamellar pattern of the muscle fibers.
| Abdominal Wall Muscles and Low Back Pain | | |
Previous research has shown that the transverse abdominis muscle is part of the support and protection mechanism of the spine. The twin transverse abdominis muscles support the lumbar girdle. One of the most important actions of the transverse abdominis, is to draw-in the abdominal wall and this action has been shown to stiffen the sacroiliac joints. Each muscle belly thickens as well as shortens in length during this action, to form a deep musculo-fascial corset supporting the lumbopelvic region. People with deficient contraction of the transverse abdominis muscle, respond poorly to low back pain treatment measures [4],[5] [Figure - 2].
Real-time USG can reliably assess the changes in the thickness of the abdominal muscles, when they contract. [6],[7],[8] Hides et al, have found good correlation between USG and MRI measurements of the thickness of the transversus abdominis and the internal oblique muscles, as well as the fascial slide. [9]
| Hernias | | |
Hernias are common abdominal wall lumps. They are classified into different categories, depending on their location and cause. With high-resolution scanning, the fascial defect underlying the hernia, can be visualized. The appearance of the herniated bowel loops varies depending on their air/fluid content and the presence/absence of obstruction. Real-time examination can demonstrate induction and reduction of hernias. When the patient is asked to cough or perform a Valsalva maneuver, the increase in the intra-abdominal pressure causes herniation of peritoneal contents.
Inguinal Hernia
Inguinal hernias are the commonest of all hernias. An indirect internal hernia occurs when there is protrusion of the peritoneal sac contents through the internal inguinal ring into the inguinal canal. [10] The internal inguinal ring is a defect in the transversalis fascia. It is located anterior to the femoral vessels, lateral to the inferior epigastric artery and above the inguinal ligament. The superficial inguinal ring is a defect in the external oblique aponeurosis. A direct inguinal hernia occurs when the peritoneal sac contents protrude directly through the weakened inguinal canal floor, medial to the inferior epigastric artery. Identification of the inferior epigastric artery helps differentiate a direct from an indirect inguinal hernia [11],[12] . [Figure - 3].
The USG appearance of an inguinal hernia depends upon its contents [Figure - 4], which commonly include fat or air or fluid-containing bowel loops. When there is bowel obstruction within a hernia, the bowel loop becomes distended, edematous and aperistaltic [Figure - 4]c. In old people, especially with a coexisting bladder outflow obstruction, the urinary bladder may sometimes extend into the hernia [Figure - 5]. In female neonates and infants, the ovaries may prolapse into the hernia [Figure - 6].
Femoral Hernia
A femoral hernia occurs when there is herniation of peritoneal sac contents through the femoral canal [Figure - 7]a. On transverse scanning, the herniation is seen to lie medial to the femoral vein [13] [Figure - 7]b. Typically, the patients are elderly and obese and complain of abdominal or groin pain, with or without a palpable mass.
Ventral Hernia
They occur typically where there is no muscle support along the linea alba in the midline in the epigastrium [Figure - 8] or periumbilical region [Figure - 9]. The hernia typically has a button mushroom-like appearance. The defect in the rectus sheath can be very well seen on a 3D coronal projection [Figure - 9]b. The hernia very often contains only fatty tissue, but at times may be large and contain bowel loops also. [14],[15]
Spigelia More Detailsn Hernia
A Spigelian hernia occurs through a defect in the aponeurosis of the transverse abdominis muscle and the rectus sheath. The commonest site is the point where the linea semilunaris crosses the arcuate line. [16],[17] The hernia may sometimes extend laterally and present as a flank lump [Figure - 10].
Incisional Hernia
A focal defect in an incisional scar may lead to an incisional hernia. It usually develops 4-8 months after surgery, but may go unnoticed by the patient, till it attains a significant size. It can even develop after laparoscopic procedures[Figure - 11]. [10],[18],[19] USG is very useful in screening for hernias at surgical sites and in the follow-up of the integrity of wire-mesh implants [Figure - 12]a, b.
| Fluid Collections | | |
Localized fluid collections in the abdominal wall may be due to seromas, abscesses or liquefying hematomas.
Non-infected seromas following surgery are usually anechoic collections. Fluid collections complicated by infection or hemorrhage appear more complex with variable internal echoes, layering and/or septae [Figure - 13]. [3]
A cold abscess secondary to tuberculosis of the ribs may present as a lump. USG reveals pockets of fluid, along the costal margin [Figure - 14]A, with internal echoes. Destruction of the corresponding rib may be seen [20],[21] [Figure - 14]b.
Suture abscesses are seen as irregular collections within the layers of the abdominal wall, around infected sutures; fragments of suture material may sometimes be seen within these collections [Figure - 15].
Hematomas commonly occur in the rectus sheath, in patients who are on anticoagulant therapy or in those who suffer from some coagulation disorder, especially following violent muscle contraction e.g. after a bout of coughing or seizures. The shape of the hematomas depends on their location and follows the limits of the rectus sheath. Above the arcuate line, hematomas are usually ovoid in shape, with a supero-inferior long axis, typically seen on one side. Below the arcuate line, they can extend across the midline, as there is no midline aponeurosis [Figure - 16]. [27],[28],[29] In neonates, an abdominal wall hematoma may be limited to the subcutaneous layer and may be spread transversely in the supra-umbilical region. In post-operative patients, large hematomas may often be seen, in the vicinity of the surgical scar [Figure - 17].
| Miscellaneous Lesions | | |
Everted Xiphisternum
Rarely, an everted xiphisternum may present as a hard, painless mass in the epigastrium [Figure - 18].
Divarication of Recti
The weak and stretched aponeurosis at the linea alba may bulge out when the patient strains, simulating a large hernia [Figure - 19].
Urachal Cyst
A urachal cyst develops from urachal remnants. It is usually situated between the umbilicus and the urinary bladder. USG reveals a clear cyst or with faint internal echoes [22],[23] [Figure - 20].
Endometriosis
Endometriosis of the abdominal wall usually develops as complication of uterine surgery, due to seeding of endometrial tissue. Typically it is seen as a focal mass at the scar of previous surgery. The size and the associated pain may vary cyclically [Figure - 21]. [24],[25],[26]
| Neoplasms | | |
Lipomas are the commonest abdominal wall neoplasm. They are well-defined, ovoid or pad-like masses [Figure - 22]. Most of them reveal an iso to slightly hyperechoic texture as compared to the muscles, along with a thin echogenic capsule. [3],[30]
A desmoid tumor arises from the fascia or the muscle aponeurosis . It often develops at the site of a previous surgical scar and is more common in women than men [31],[32] [Figure - 23].
Metastatic melanoma is probably the commonest malignant tumor to occur in the abdominal wall. Less commonly, metastases from lymphoma, lung, breast, ovary and colon may be seen [Figure - 24]
| Inguinal Masses | | |
Lymph Nodes
Lymph nodes have a characteristic USG appearance with an echogenic central area and a hypoechoic peripheral zone [30] [Figure - 25].
Undescended Testis
An undescended testis may get arrested anywhere between the scrotum and the renal hilum. By far the commonest site, is the inguinal canal. A low, undescended testis is usually smaller, ovoid and reveals a homogenous hypoechoic structure. The absence of an echogenic hilum distinguishes a testis from a lymph node [33],[34],[35] [Figure - 26].
Lipoma of the Spermatic Cord and Inguinal Canal
The commonest tumor of the spermatic cord is a lipoma, which occurs commonly in the inguinal canal. Though it is a poorly recognized entity, it is commonly seen as a spindle-shaped, echogenic mass along the inguinal canal or in the upper part of scrotum [Figure - 27]. Clinically it is indistinguishable from an inguinal hernia. [36],[37].
Pseudoaneurysm
Pseudoaneurysms develop due to local trauma to the vessel wall, especially after catheterization procedures. They are extra-axial cystic structures, which may reveal pulsations and swirling of echogenic blood within the cystic fluid-filled cavity on USG. Color Doppler reveals "to and fro" flow, with a typical "yin-yang" sign [38],[39] [Figure - 28].
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Correspondence Address:
Sudheer Gokhale
53 Wasudevnagar, Indore - 452 004
India
 Source of Support: None, Conflict of Interest: None  | Check |
DOI: 10.4103/0971-3026.36880
[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6], [Figure - 7], [Figure - 8], [Figure - 9], [Figure - 10], [Figure - 11], [Figure - 12], [Figure - 13], [Figure - 14], [Figure - 15], [Figure - 16], [Figure - 17], [Figure - 18], [Figure - 19], [Figure - 20], [Figure - 21], [Figure - 22], [Figure - 23], [Figure - 24], [Figure - 25], [Figure - 26], [Figure - 27], [Figure - 28] |
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