Hydronephrosis suggests obstructive nephrolithiasis in the context of colicky flank pain, particularly in a patient with gross or microscopic hematuria. Hydronephrosis may also be seen in an overdistended bladder, which is seen in patients with urinary retention. For example, patients with an enlarged prostate may develop hydronephrosis due to postobstructive nephropathy. It is also a common variant in pregnancy due to an enlarging gravid uterus compressing on the lateral ureters.

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Considerations in Renal US

Patients who are severely dehydrated may have a falsely negative ultrasound for hydronephrosis, so intravenous hydration may be required for accurate evaluation of the collecting system.²

One must also consider that the actual renal calculus is often not seen on ultrasonography. For this reason, spiral CT (computed tomography) continues to be the modality of choice for visualizing renal calculi. In fact, Sheafor et al. found that spiral CT was more sensitive (96%) compared with ultrasound (sensitivity, 61%) in the detection of renal calculi.³

However, renal ultrasonography allows for early diagnosis and therefore treatment of renal calculi, prior to CT. It may also serve to reduce radiation from repeated CTs in the setting of recurrent episodes of renal colic. Patients with known renal calculi, without urinary tract infection, and with evidence of hydronephrosis on ultrasound may not require spiral CT. These patients may be discharged from the emergency department with a urine strainer, pain medications, and close urology follow-up.

Clinical Indications for Performing Bladder Imaging

Some of the main indications for bedside bladder ultrasound examination are to obtain images that are important to assess for specific conditions:

• Infection – bladder wall thickening.
• Obstruction – bladder calculi, visualization of the vesico-ureteral junctions.
• Retention – pre- and postvoid residual volumes.

Performing a Bladder Ultrasound

Patient Positioning

The patient should be placed in a supine position and the bladder should be moderately to completely full at the time of assessment, which provides an acoustic window. This also aids in the visualization of the prostate or uterus and ovaries.