Ultrasound Of The Urinary Tract
Ultrasound Of The Urinary Tract
Ultrasound of the renal and urinary system is easily performed and is a core application of emergency ultrasound.1 It is fundamental to the evaluation of patients who present to the ED with undifferentiated flank or abdominal pain and is an integral component of the FAST examination. (See Figure 18) Although not typically a life-saving examination, renal and urinary tract ultrasound can expedite patient care and avoid exposing patients to unnecessary ionizing radiation. The scan focuses on identifying hydronephrosis, assessing for ureteral jets, and determining bladder volume. Possible additional findings on urinary tract ultrasound include renal cysts or masses and bladder masses as well as abdominal aortic aneurysms, which often present similarly.
Assessing For Hydronephrosis
Hydronephrosis is noted on ultrasound when dilatation of the calyces and pelvis result in anechoic areas within the central collecting system. (See Figure 19) Use of color-flow Doppler can help differentiate between renal vasculature and hydronephrosis. When hydronephrosis is noted, it is important to determine whether the obstruction is unilateral or bilateral and, if possible, intrinsic or extrinsic. Hydronephrosis is usually graded as follows: mild (defined by prominent calyces and mild splaying of the renal pelvis), moderate (characterized by a bearclaw appearance), or severe (defined by the presence of cortical thinning). Of note, hydronephrosis may be challenging to appreciate in volume-depleted patients; such patients may need to be hydrated first.
When combined with urine studies and clinical examination, BUS helps the emergency clinician better differentiate the diagnosis in patients who present with acute flank pain. Studies show that the sensitivity of emergency physician–performed urinary tract ultrasound for the detection of hydronephrosis approaches that reported in the radiology literature. Rosen et al examined the use of ultrasound by emergency physicians to detect hydronephrosis in patients with ureteral colic and reported the following test characteristics: sensitivity of 72%, specificity of 73%, positive predictive value of 85%, and negative predictive value of 54%.160 Gaspari and Horst assessed the value of BUS in determining the cause of renal colic and found its overall sensitivity and specificity for detecting hydronephrosis to be 87% and 82%, respectively.161 Stratification of the renal ultrasound data according to the results on urinalysis revealed a similar sensitivity (88%) and improved specificity (85%).
It has been suggested that the presence or absence and frequency of ureteral jets (assessed with colorflow or power Doppler) correlate with the presence or absence and degree of ureteral obstruction. The value of Doppler visualization of ureteral jets in hydronephrosis has been studied in both children and adults, and results are encouraging in terms of differentiating severe from nonsignificant obstruction.162,163
Bladder Ultrasound Prior To Urethral Catheterization In Young Children
Bladder catheterization is the method of choice for obtaining appropriate urine samples in young children. The minimum volume of urine necessary for accurate urinalysis and culture is 2 mL,164 yet it is often unclear whether or not the patient has a sufficient amount of urine in the bladder for the test to be of value. Urethral catheterization is uncomfortable and should not be repeated, to avoid urethral trauma and additional pain.
Bedside ultrasound is a simple, noninvasive modality that can assess bladder volume and has been shown to reduce the number of unsuccessful urethral catheterization attempts because of an empty bladder.164-171 Chen et al performed a prospective, 2-phase study comparing success rates between an observational period and an intervention phase in which rapid BUS was performed prior to urethral catheterization. They noted that 24% of the patients were initially identified as having insufficient urine and that the overall initial urethral catheterization success rate was higher with the use of ultrasound (96%) than without (72%).169 Milling et al likewise performed a prospective study on the use of ultrasound in infants undergoing urinary catheterization.170 Although they observed a similar urinary catheterization failure rate (16%), they developed a urinary bladder index (ie, the product of anteroposterior and transverse diameters expressed in centimeters squared) and determined its sensitivity and specificity. Milling et al found that the sensitivity of this index in predicting the failure to obtain 2 mL of urine was 100% and that its specificity was 97%.170 Several other studies have demonstrated the value of performing BUS prior to suprapubic bladder aspiration.172-175 Both Gochman et al172 and Munir et al173 showed that ultrasound significantly improved suprapubic aspiration success rates and decreased the number of failed attempts.
Renal ultrasound has been shown to be useful in the management of patients who present to the ED with undifferentiated flank or abdominal pain. Renal and urinary tract ultrasound may obviate CT in a subset of ED patients and, in doing so, decrease ED length of stay and avoid unnecessary exposure to ionizing radiation. Bladder ultrasound is simple and noninvasive and should be performed in order to decrease the number of unsuccessful urethral catheterization attempts in young children.
James Q. Hwang; Heidi Harbison Kimberly; Andrew S. Liteplo; Dana Sajed
March 2, 2011