Obstructive Uropathy Assessment Using Bedside Ultrasonography

Obstructive uropathy is a blockage of normal urinary flow. The backup of urine into the unilateral or bilateral kidneys causes hydronephrosis. Back pressure of urine into the collecting system of the kidneys may produce dilatation within the tract; as the kidneys filtration system becomes affected, it becomes the primary cause of obstructive nephropathy. ^[1]  Patients who present to the emergency department (ED) or to another ambulatory care setting commonly report abdominal and flank pain. Unfortunately, patient history and physical examination often lack the necessary sensitivity and specificity for accurate diagnosis of underlying etiologies without further testing. Focused bedside ultrasonography is a valuable diagnostic tool that often facilitates timely diagnosis for these patients. ^{[2, 3, 4, 5, 6, 7]}

Ultrasonography is especially important in determining the cause of sudden onset of abdominal or flank pain, because patients with symptomatic abdominal aneurysms can be difficult to distinguish from those with renal colic. In fact, making an accurate diagnosis of ureteronephrolithiasis is especially important, because renal colic is the most common misdiagnosis in patients with missed abdominal aortic aneurysms.

Ultrasonography is the first-line imaging modality used in diagnostics of the urinary system. It provides valuable morphologic information, but its usefulness in assessment of function of the renal parenchyma is limited. Dynamic renal scintigraphy provides much more accurate information about the parenchymal function of kidneys and about urinary outflow. ^[8]

Functional renal imaging, most commonly with MAG3 nuclear medicine renal scan, is recommended in evaluation of children with urinary tract dilation (UTD) suspicious for obstructive uropathy. Alternatively, renal function can be evaluated via functional magnetic resonance urography (fMRU), which provides superior anatomic detail. However, data comparing equivalency of these methods are sparse. Authors of a retrospective review found that differential renal function determined by MAG3 and fMRU in children was not statistically different and concluded that these methods are similar and potentially equivalent. However, MAG3 does not clearly differentiate the dilated collecting system from functional parenchymal tissue, although fMRU does. ^[9]

Benefits of focused emergency renal sonography (FERS) include the following:

• Decreases time to diagnosis of obstructive uropathy

• Facilitates accurate diagnosis of obstructive uropathy ^[10]

• Enhances assessment of the degree of obstruction in renal colic

• Helps rule out other, more dangerous, pathology (eg, symptomatic aortic aneurysm) ^[11]

• Aids identification of obstructive causes of renal insufficiency

• Is safe for pregnant patients and children and requires less radiation than either intravenous pyelography (IVP) or helical computed tomography (CT) ^{[12, 13, 14, 15]}

Performance of FERS should not delay initiation of emergent treatments such as use of intravenous fluids or pressors, when indicated. Although the need for ongoing resuscitation and a state of extremis are not contraindications, FERS can be difficult to perform in such situations.

Patients should be evaluated in the supine position but can be moved to posterior oblique and lateral decubitus positions for improved visualization. The entire abdomen of male patients should be exposed for the examination. Special care should be taken to minimize exposure of sensitive areas in female patients.

Indications for FERS include the following:

• Sudden onset of abdominal pain

• Colicky flank pain that radiates to the groin

• Hematuria

• Acute renal insufficiency or failure

• Urinary retention

• Pyelonephritis

• Trauma

• Need to evaluate recurrent symptoms of stone disease ^[16]

Although renal masses and cysts are often identified by FERS, these are outside of the scope of emergency ultrasonography. Patients with suspected renal masses or cysts should be referred to a radiologist for further evaluation.

Relevant anatomy

The kidneys are retroperitoneal structures; the right kidney is more caudal than the left. The right kidney is posteroinferior to the liver and the gallbladder, and the left kidney is inferomedial to the spleen. The bladder is located in the pelvis.

Components of examination

Focused emergency renal sonography (FERS) should include transverse and longitudinal views of both kidneys and the bladder. Visualization of the kidneys should reveal clear anatomic relationships to liver or spleen for unambiguous identification.

The video below depicts a demonstration of evaluation for renal uropathy.

Procedures

In visualizing the right kidney, one should place the transducer-probe over the right flank, lateral to the liver. This allows the liver to be used as an acoustic window and avoids interference from air-filled bowel. In addition, the probe can be placed posterior to the liver for improved visualization of the kidney.

Relieving obstruction and protecting renal function are the main therapeutic purposes for obstructive uropathy and often involve surgical treatment; the ureter catheter is one of the instruments commonly used in surgery. Innovative application of ureteral catheters in the operation for obstructive uropathy can realize real-time monitoring of intraoperative renal pelvis pressure, can reduce the incidence of lithotripsy postoperative complications, and can expand the indications for balloon dilatation in ureteral stricture, which has certain clinical significance. ^[17]

(See images below.)

In visualizing the left kidney, one should place the transducer-probe over the left flank, lateral and posterior to the spleen. This allows the spleen to be used as an acoustic window and avoids interference from air-filled bowel.

(See images below.)

The capsule of the kidneys should appear smooth and echogenic, forming clear borders for the kidneys. A normal-appearing kidney should be approximately 9 to 13 cm long, 5 cm wide, and 3 cm deep. ^[18]

The cortex of the kidneys should be gray but less echogenic than either liver or spleen. Thus, hydronephrosis seen in central areas should appear anechoic. Ultrasonography has been found to be 73-100% sensitive for picking up collecting system obstruction. ^[19]

Hydronephrosis is graded as follows:

• Mild: Minimal separation of the central sinus echo complex by tubular anechoic urine-filled renal pelvis and calyces

• Moderate: Definitive anechoic separation of the entire renal sinus

• Severe: Marked dilation of the renal pelvis with cortical thinning

(See images below.)

In visualizing the bladder, one should place the transducer-probe over the pubic symphysis and direct it inferiorly into the pelvis. A full bladder provides a better acoustic window than a bladder that is not full.

False-positives are seen in the following situations ^[20] :

• Dilated renal vasculature

• Renal sinus cysts

• Bladder distention

• Medullary pyramids (in young patients)

Causes of false-negatives include the following ^[20] :

• Dehydration

• Stones smaller than 3 mm

Important considerations include the following:

• Scan both kidneys. This allows for more accurate assessment of hydronephrosis and avoids the pitfall of misdiagnosing prominent renal pyramids as hydronephrosis. ^[21]

• Remember that cysts can be mistaken for hydronephrosis. However, cysts tend to be peripheral, and hydronephrosis should always be central.

• If the kidneys are difficult to visualize, reposition the patient in the posterior oblique or lateral decubitus position.

• Note that the presence of hydronephrosis does not rule out other pathology, such as an abdominal aortic aneurysm.

• Absence of hydronephrosis does not rule out urolithiasis.

• Consider nonstone causes of hydronephrosis, such as urinary retention, pregnancy, and neoplasm.

• Refer the patient to a radiologist for further evaluation when cysts or masses are identified.

In a study seeking to evaluate the usefulness of contrast-enhanced ultrasonography in renal perfusion disorders caused by obstructive uropathy, Ustyniak and associates found that both Doppler ultrasonography and contrast-enhanced ultrasonography may be useful for monitoring perfusion disorders in the course of obstructive uropathy, provided that appropriate methods are used to compare blood flow in both kidneys. ^[22]

Stevens-Johnson syndrome (SJS) is an acute mucocutaneous eruption with blisters of the skin and hemorrhagic erosions of mucous membranes. One case report describes air-leak syndrome and obstructive uropathy occurring simultaneously in a teenage patient affected by SJS. Despite the paucity of cases in adult literature, post-renal causes for acute kidney injury must be considered in SJS, especially in the setting of gross hematuria. Bedside point-of-care ultrasonography may be a useful tool for excluding obstructive uropathy. Pneumothorax is a rare but documented complication of SJS in pediatric cases and, to a lesser extent, in adult patients. Extra care should be exercised when one is caring for mechanically ventilated patients with a diagnosis of SJS. ^[23]