Hydronephrosis
- 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.
- Hydronephrosis can be caused by:
- External compression (e.g., retroperitoneal fibrosis, masses)
- Ureteral obstruction (stones, tumors, strictures).
- Bladder outlet obstruction (e.g., BPH, neurogenic bladder, stricture).
- Pregnancy (physiologic hydronephrosis).
Ultrasound Setup
Probe: Curvilinear
Preset: Abdominal
Position: Supine
Indicator: Toward the patients head for longitudinal view
Pitfalls: Mistaking renal cysts for hydronephrosis, not looking at both kidneys, misinterpreting extra-renal pelvis, ignoring hydration status (ie very dehydrated = may not see hydro).
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
- Note; The video shown has no features of hydronephrosis
- Check out UBC's video of severe hydronephrosis: https://www.ubcimpocus.com/all-images?tag=Severe+hydronephrosis
The Bear Paw Sign
- 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.
POCUS 101.com
Grading Hydronephrosis
- Grade 0: No visible collecting duct dilation
- Grade 1 (mild): Minimal dilation of renal pelvis
- Grade 2: (mild-moderate) Dilation of pelvis + major calyces
- Grade 3: (moderate) Dilation of pelvis + major/minor calyces
- Grade 4: (severe) dilation + cortical thinning.