Hydronephrosis is the dilation of the renal collecting system (renal pelvis and calyces) due to obstruction of urinary flow. POCUS allows for rapid bedside assessment prior to a formal ultrasound, helping to identify hydronephrosis in patients with acute kidney injury, flank pain, and/or urinary retention.
Indicator: Toward the patients head for longitudinal view
Steps for Image Acquisition:
Identify the Kidneys: Place the probe at the mid-axillary line near the costal margin. Often need to slide more posterior to identify it. Visualize the entire kidney.
Optimize Image: Sweep through the kidney to avoid missing partial obstruction. Adjust depth to center the kidney and gain to clearly visualize the kidney and collecting ducts.
Identify Hydronephrosis:
Normally, the collecting duct is collapsed/barely visible
Dilated collecting ducts will be anechoic (black). Look for fluid in the renal pelvic and calyces
Assess both kidneys - it can be unilateral or bilateral
Grading Hydronephrosis
None: No visible collecting duct dilation
Mild: Minimal separation of the renal pelvis
Moderate: Dilation of the pelvis and major calyces
Severe: Extensive calyceal dilation with cortical thinning
Note; The video shown has no features of hydronephrosis
The Bear Paw sign is a characteristic ultrasound finding in significant hydronephrosis. It is produced by dilation of the renal calyces and pelvis. Anechoic, dilated calyces spread around the echogenic renal sinus, resembling the spread toes of a bear’s paw.
False Positives
Full bladder: A distended bladder can cause transient collecting system distension. Have the patient void and repeat the scan to see if the dilation resolves.
Extrarenal pelvis: A normal anatomical variant where the renal pelvis extends beyond the hilum, appearing prominent on ultrasound. It can mimic hydronephrosis but does not indicate true obstruction.
Overhydration: High urine output states can cause mild, non-obstructive dilation of the collecting system, especially after large fluid resuscitation.