What is the role of CT scanning in identifying secondary signs of obstruction in nephrolithiasis?

Updated: Jan 13, 2020
  • Author: Chirag N Dave, MD; Chief Editor: Bradley Fields Schwartz, DO, FACS  more...
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Secondary signs of obstruction may be visible only on CT scans. In some cases, if a stone was passed shortly before the study, these signs may be the only evidence that the patient has or ever had a stone. These secondary signs include ureteral dilation with hydronephrosis, renal enlargement from interstitial edema (nephromegaly), and inflammatory changes, such as stranding or streaking in the perinephric fatty tissue.

In a 1996 study of 54 ureteral stone patients reported by Katz et al, hydronephrosis was present in 69%, proximal ureteral dilation was found in 67%, and perinephric stranding was detected in 65%. The other secondary signs had a similar frequency in adults and children. In the study, only 2 of the patients with ureteral calculi did not demonstrate any of the secondary signs of obstruction. The other secondary signs had a similar frequency in adults and children. [37]

A similar 1996 study by Smith et al involving 220 patients found an even higher correlation between these secondary signs of obstruction and the presence of a ureteral calculus. In particular, the combination of collecting system dilation and perinephric stranding had a positive predictive value of 98%, while the absence of both of these secondary signs had a negative predictive value of 91%. [38]

However, perinephric stranding was found less often in children with ureteral calculi than in adults in a 2001 study by Smergel and associates; therefore, this secondary sign, at least in the pediatric population, may be less reliable. [39]

An additional secondary sign of acute renal obstruction on noncontrast CT scans has been reported by investigators from Johns Hopkins University. This sign is defined as a reduction in renal parenchymal attenuation (radiologic density) on the nonenhanced CT scan of the acutely obstructed renal unit compared with the normal unobstructed contralateral kidney. The difference in density is at least 2 standard deviations. This sign was identified in 95% of patients with acute ureteral obstruction, which suggests it is a reliable indicator.

Rarely, in indeterminate cases in which the secondary signs are negative and a stone is strongly suspected clinically but not clearly visible on the unenhanced CT scan, IV contrast can be used to help visualize the ureter. Repeat scanning after contrast infusion allows for improved visualization of the ureters. This allows physicians to make direct comparisons with the earlier studies to help make the correct diagnosis. Flat abdominal radiograph films taken after the contrast provide information similar to IVP, but delayed films or scans are likely to be needed.

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