Obstructive Uropathy Assessment using Bedside Ultrasonography

Updated: Jun 09, 2016
  • Author: Timothy Jang, MD; Chief Editor: Caroline R Taylor, MD  more...
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Overview

Overview

Patients who present to the emergency department (ED) or other ambulatory care setting commonly report abdominal and flank pain. Unfortunately, patient history and physical examination often lack the necessary sensitivity and specificity to accurately diagnose underlying etiologies without further testing. Focused bedside ultrasonography is a valuable diagnostic tool that can often facilitate a timely diagnosis for these patients. [1, 2, 3, 4, 5, 6]

Ultrasonography is especially important in determining the cause of the sudden onset of abdominal or flank pain, since 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, since renal colic is the most common misdiagnosis in patients with missed abdominal aortic aneurysms.

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

  • Decreases the time to diagnosis for obstructive uropathy
  • Helps accurately diagnose obstructive uropathy [7]
  • Helps assess the degree of obstruction in renal colic
  • Helps rule out other, more dangerous, pathology (eg, symptomatic aortic aneurysm) [8]
  • Helps identify obstructive causes of renal insufficiency
  • Is safe in pregnant patients and children and requires less radiation than either intravenous pyelography (IVP) or helical CT [9, 10, 11, 12]
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Indications

Indications for focused emergency renal sonography (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
  • Trauma
  • Evaluating recurrent symptoms of stone disease [13]

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

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Contraindications

Performance of focused emergency renal sonography (FERS) should not delay the initiation of emergent treatments such as intravenous fluids or pressors, when indicated. Although ongoing resuscitation and extremis are not contraindications, FERS can be difficult to perform in such situations.

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Positioning

Patients should be evaluated in the supine position but can be moved to the posterior oblique and lateral decubitus positions for improved visualization. Male patients should have the entire abdomen exposed for the examination. Take special care with female patients to minimize the exposure of sensitive areas.

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Technique

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 gallbladder, while 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 include clear anatomical relationship to the liver or spleen for unambiguous identification.

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

Demonstration of evaluation for renal uropathy. Video courtesy of Meghan Kelly Herbst, MD. Also courtesy of Yale School of Medicine, Emergency Medicine.

Technique

To visualize the right kidney, the transducer-probe should be placed 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. See images below.

Probe placement for longitudinal view of the right Probe placement for longitudinal view of the right kidney.
Probe placement for transverse view of the right k Probe placement for transverse view of the right kidney.
Longitudinal view of the right kidney. Longitudinal view of the right kidney.

To visualize the left kidney, the transducer-probe should be placed over the left flank, lateral and posterior to the spleen. This allows the spleen to be used as an acoustic window and also avoids interference from air-filled bowel. See images below.

Probe placement for longitudinal view of the left Probe placement for longitudinal view of the left kidney.
Probe placement for anterior approach to the left Probe placement for anterior approach to the left kidney.
Longitudinal view of the left kidney. Longitudinal view of the left kidney.

The capsule of the kidneys should appear smooth and echogenic, forming clear borders to the kidneys. A normal-appearing kidney should measure approximately 9-13 cm long, 5 cm wide, and 3 cm deep. [14]

The cortex of the kidneys should be gray but less echogenic than either the liver or spleen. Thus, hydronephrosis seen in the central areas should appear anechoic. Ultrasound has been found to be 73-100% sensitive for picking up collecting system obstruction. [15]

Hydronephrosis is graded as follows:

  • Mild - Minimal separation of the central sinus echo complex by tubular anechoic urine-filled renal pelvis and calyces
  • Moderate - Definite anechoic separation of the entire renal sinus
  • Severe - Marked dilation of the renal pelvis with cortical thinning

See images below.

Hydronephrosis. Hydronephrosis.
Hydronephrosis. Hydronephrosis.
Cine loop depicting renal hydronephrosis. Video courtesy of Meghan Kelly Herbst, MD. Also courtesy of Yale School of Medicine, Emergency Medicine.

To visualize the bladder, the transducer-probe should be placed over the pubic symphysis and directed inferiorly into the pelvis. A full bladder provides a better acoustic window.

Causes of false positives are as follows [16] :

  • Dilated renal vasculature
  • Renal sinus cysts
  • Bladder distention
  • In young patients, medullary pyramids

Causes of false negatives are as follows [16] :

  • Dehydration
  • Stones smaller than 3 mm
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Pearls

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. [17]
  • 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.
  • The presence of hydronephrosis does not rule out other pathology, such as an abdominal aortic aneurysm.
  • The absence of hydronephrosis does not rule out urolithiasis.
  • Consider nonstone causes of hydronephrosis, such as urinary retention, pregnancy, and neoplasm.
  • Patients in whom cysts or masses are identified should be referred to a radiologist for further evaluation.
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Complications

Typically, no complications are associated with this procedure.

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