The Clinical Problem

The retroperitoneum is a clinically silent area where large fluid collections that cannot be readily diagnosed by conventional radiographic techniques (plain films) may accumulate. Such collections include hematomas (trauma, anticoagulation, surgery), abscesses (infection), lymphoceles (surgery), and urinomas (trauma, surgery). Common sites of bleeding include the psoas muscle and the perinephric space. Computed tomography (CT) is generally preferable to ultrasound because it is more sensitive and specific in identifying the presence and extent of the collection, but ultrasound may be used for diagnosis when CT is not available or applicable (remote areas, patients too sick to go to CT). Acute retroperitoneal hemorrhage is a potentially lethal condition that may have a variety of sonographic characteristics, ranging from complex fluid to mimicking a focal mass.

HEMATOMAS

An unexplained hematocrit drop may indicate that a patient has bled internally. Often the site of the bleed is unclear to the clinician. Patients at risk for unexplained hematocrit drop are those who (1) have recently undergone an operation or medical procedure; (2) are taking anticoagulants (e.g., warafin, heparin, aspirin); (3) have had a recent injury such as a car or bicycle accident, or trauma such as stabbing; (4) have bleeding or clotting problems, such as hemophiliacs or leukemics; or (5) have spontaneous or traumatic vascular rupture (ruptured abdominal aortic aneurysm, bleeding tumor).

The following are the most likely sites of asymptomatic hematomas:

1. In the abdominal wall around an incision

 2. Deep to an incision

 3. In a site where fluid collects adjacent to a surgical site (e.g., in the cul-de-sac, paracolic gutters, or subhepatic space)

 4. Around the spleen (perisplenic), liver (perihepatic), or kidney (perinephric)

 5. In the retroperitoneum (this site is particularly likely in patients with no previous injury, such as those taking anticoagulants or with bleeding problems)

 6. In the iliopsoas or rectus muscles, particularly in hemophiliacs

Hematomas may develop into abscesses; they are good culture media for bacteria. Expansion of a hematoma on subsequent sonograms suggests that the lesion is infected or that there has been rebleeding. Normal hematomas slowly retract.

URINOMAS

A urinoma is a walled-off collection of extravasated urine. Urine usually collects around the kidney or under the ureter in the perinephric space. Urinomas develop mainly in patients who have had trauma, who have passed a renal stone, or who have undergone operations such as a renal transplant. Urinomas may be asymptomatic and may be found years after the original process that caused them occurred. It is useful to follow the progress of urinomas that occur after an operation because they usually resolve spontaneously. Generally, the collection is anechoic unless it has become infected.

LYMPHOCELES

Lymphoceles are collections of lymph fluid and are common after surgical procedures. Most are small, develop within 10 to 21 days after surgery, and resolve spontaneously. Treatment of larger lesions includes surgery, sclerosis, or percutaneous drainage. Most are anechoic and resemble simple cysts. Lymphoceles frequently are septated and occasionally contain debris or loculations, making them difficult to differentiate from a hematoma, urinoma, or abscess.

ABSCESSES

Abscesses are pockets of purulent fluid and commonly occur in the retroperitoneum. They may be relatively asymptomatic, particularly in the psoas muscle, presenting with fever rather than with localized symptoms. These infections may be a primary complication of surgery, septicemia, or trauma, or may be caused by spread from an adjacent organ, such as the kidney, bowel, or spine. A preexisting fluid collection such as a pancreatic pseudocyst may also become secondarily infected. Percutaneous ultrasound-guided aspiration is an important diagnostic test, as are clinical symptoms such as elevated white count, fever, chills, localized tenderness, and increasing size. The appearance on ultrasound is rarely specific, so distinguishing abscesses from other collections or masses is challenging and usually a needle aspiration is an integral part of the examination.

VASCULAR ABNORMALITIES

Vascular pathologic conditions can develop after surgery, trauma, or a medical condition. They include arteriovenous fistulas, aneurysms, and varices. They can be confused with simple fluid collections. Color flow imaging and Doppler spectral analysis can define whether the abnormality is vascular or not and are essential before inserting a needle into any “fluid” collection to prevent inadvertent puncture of a vascular abnormality.

OTHER MASSES AND PSEUDOMASSES

Benign masses and pseudomasses—such as horseshoe kidneys, ptotic kidneys, dromedary hump, double collecting system, bowel duplication cysts, and adenopathy—should be considered as possible explanations for a retroperitoneal mass. Perhaps the most frequent problem occurs when a loop of aperistaltic bowel mimics a true mass or fluid collection. Changing the patient’s position during the examination and watching the area while the patient drinks can usually make the distinction.

Anatomy

RETROPERITONEUM

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