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1
Department of Radiology, Weill Medical College of Cornell University, Box 141,
New York Presbyterian Hospital, 525 E. 68th St., New York, NY 10021.
2
Department of Diagnostic Radiology, Yale University School of Medicine, 333
Cedar St., New Haven, CT 06510.
Received February 5, 2001;
accepted after revision July 25, 2001.
Address correspondence to R. C. Smith.
Abstract
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MATERIALS AND METHODS. Over a 29-month period, 850 patients with acute flank pain were evaluated with unenhanced CT. Confirmation of the CT diagnosis was obtained retrospectively for 172 patients with ureteral stones: 115 stones passed spontaneously and 57 required intervention. Stone size was defined as the maximum diameter within the plane of the axial CT section. Stone location was classified as proximal ureter (above the sacroiliac joints), mid ureter (overlying the sacroiliac joints), distal ureter (below the sacroiliac joints), and ureterovesical junction.
RESULTS. The spontaneous passage rate for stones 1 mm in diameter was 87%; for stones 2-4 mm, 76%; for stones 5-7 mm, 60%; for stones 7-9 mm, 48%; and for stones larger than 9 mm, 25%. Spontaneous passage rate as a function of stone location was 48% for stones in the proximal ureter, 60% for mid ureteral stones, 75% for distal stones, and 79% for ureterovesical junction stones.
CONCLUSION. The rate of spontaneous passage of ureteral stones does vary with stone size and location as determined by CT. These rates are similar to those previously published based on radiography.
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Unenhanced helical CT is now used almost exclusively for the diagnosis and treatment of patients with acute ureterolithiasis [5]. CT will reveal virtually all stones regardless of composition, including uric acid stones that are typically radiolucent on radiography. Uric acid stones account for 5-10% of all urinary calculi [6]. The only stones known to be radiolucent on CT are those composed of pure protease inhibitors such as indinavir [7].
Several factors might result in a difference between CT and urography in determining stone size, including magnification on radiography, error and variability in CT measurements, the ability of CT to reveal tiny stones that would never be visible on radiography, and the ability of CT to visualize stones that are radiolucent on radiography (e.g., uric acid stones). No prior study has examined the relationships between the size and location of ureteral stones as determined by unenhanced CT and their rate of spontaneous passage. Given the new role of unenhanced CT in the diagnosis and treatment of patients with acute ureterolithiasis, it is important to determine this relationship.
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For the purposes of this study, we defined spontaneous passage as occurring if no intervention was performed. This means that spontaneous passage was patient-dependent, because many factors, such as pain tolerance and the presence of infection, determine the need for intervention independent of stone size and location. In our study population of 172 patients, each with a solitary stone, 115 stones passed spontaneously; 57 patients required interventional therapy, including ureteroscopy (n = 26), percutaneous nephrostomy (n = 18), and extracorporeal shock wave lithotripsy (n = 13).
All CT examinations were performed with a HiSpeed Advantage CT scanner (General Electric Medical Systems, Milwaukee, WI). Axial images were obtained from the top of the kidneys to the base of the bladder using a 5-mm slice thickness, a pitch of 1, and a reconstruction interval of 5 mm. No oral or IV contrast material was administered.
The CT images were interpreted together by one senior genitourinary radiologist and one senior radiology resident. Stone size was measured at the maximal diameter within the plane of the axial CT image using standard soft-tissue window and level settings. For those patients in whom the course of the ureter was readily apparent in the plane of the CT section, the measurement was taken perpendicular to the course of the ureter. Stone location was defined as proximal (above the sacroiliac joints), mid (overlying the sacroiliac joints), distal (below the sacroiliac joints), and at the ureterovesical junction.
Frequency of spontaneous passage was calculated independently for stone size and stone location. For each stone location, frequencies were also calculated as a function of stone size. All frequency comparisons were made using a chi-square test.
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When we compare the overall frequency of spontaneous passage among the three groups of stones (measuring 1-4 mm, 5-7 mm, or 8 mm and larger), the differences are statistically significant (p < 0.001).
Table 2 shows the relationship of stone location to the overall frequency of spontaneous passage as well as the frequency at each location as a function of size. The overall frequency of spontaneous passage was 48% for proximal stones, 60% for mid ureteral stones, 75% for distal stones, and 79% for stones located at the ureterovesical junction. These differences in overall frequency are statistically significant for stones in the proximal ureter versus stones in the distal ureter (p < 0.001) and for stones located at the ureterovesical junction (p < 0.002). In addition, for each location except the ureterovesical junction, no statistically significant differences were noted in frequency of spontaneous passage based on size. For stones at the ureterovesical junction, the differences in frequency of spontaneous passage based on size were statistically significant (p < 0.02). Two of the seven stones at the ureterovesical junction that failed to pass spontaneously measured 2 mm in diameter.
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Our study showed that CT measurements of ureteral stone size have a nearly linear relationship with the frequency of spontaneous passage. Our results are very similar to those of studies reported in the literature that used radiography to measure stone size. We found that stones measuring 4 mm or smaller will usually pass spontaneously (frequency of spontaneous passage = 78%); stones measuring 5-7 mm frequently pass spontaneously (frequency of spontaneous passage = 60%); and stones measuring 8 mm or larger usually will not pass spontaneously (frequency of spontaneous passage = 39%). In our study, no stones larger than 10 mm passed spontaneously.
With regard to stone location, our data indicate that if a stone is present in the proximal ureter at the time of diagnosis, the overall frequency of spontaneous passage is less than 50%, even for smaller stones (Table 2). The frequency of spontaneous passage of stones in the mid and distal ureters was significantly higher than that of stones in the proximal ureter, but passage rates were independent of stone size. We found that the frequency of spontaneous passage of stones lodged at the ureterovesical junction at the time of initial diagnosis was strongly determined by stone size and varied from 33% to 92% (Table 2). However, two stones lodged at the ureterovesical junction that measured 2 mm did not pass spontaneously. Thus even tiny stones at the ureterovesical junction may require follow-up imaging, depending on the clinical circumstances.
Prior studies using radiography have examined the relationship between the size and location of ureteral stones at the time of initial diagnosis and their subsequent frequency of spontaneous passage [1,2,3]. These studies could not take into account radiolucent stones such as those composed of uric acid, xanthine, or mucoprotein matrix. Uric acid stones are by far the most common radiolucent stone and account for 5-10% of all urinary calculi [6]. In addition, radiography is unable to detect many calcium-containing stones for a variety of technical reasons, including small size, faint radiopacity, and obscuration by overlapping structures.
Despite the limitations of radiography for making these determinations, the size and location of ureteral calculi are considered the most important prognostic factors for the treatment of patients with acute ureterolithiasis. The initial work on this topic was published by Sandegard in 1956 and 1958 [1, 2]. The latter study included 122 confirmed cases, and stones were categorized into three groups: small (< 4 mm), medium (4-6 mm) and large (> 6 mm). Stones were also characterized as being present in the upper or lower half of the ureter. Sandegard found that small stones in the lower half of the ureter passed spontaneously in 93% of patients, whereas small stones in the upper half of the ureter passed spontaneously in 81% of patients. Sandegard also reported that medium-sized stones in the lower half of the ureter passed spontaneously in 53% of cases but large or medium-sized stones in the upper half of the ureter rarely passed spontaneously.
In 1977, Ueno et al. [8] evaluated 520 patients and calculated the rate of spontaneous passage of ureteral calculi as a function of stone width and length (in 1-mm increments). Their study found that the degree of obstruction was more directly related to the width rather than the length of the stone and concluded that the width was the critical measurement. They reported passage rates of 100%, 93%, 87%, and 78% for stones measuring 1, 2, 3, and 4 mm in width, respectively. The rate of spontaneous passage dropped to 57% for stones 5 mm in width, 35% for 6-mm stones, 28% for 7-mm stones, and 14% for 8-mm stones. No stones exceeding 8 mm in width passed spontaneously.
On the basis of a recent meta-analysis of the literature, the American Urological Association published guidelines for the treatment of ureteral stones [4]. This analysis reported an overall spontaneous passage rate of 71-98% for stones in the distal ureter that are 5 mm or smaller and a spontaneous passage rate of 29-98% for stones in the proximal ureter that are 5 mm or smaller. These guidelines do not specify a definition of stone size.
Our review of the literature revealed no accepted standard technique to measure stone size on radiographs. Measurements can be taken perpendicular (width), parallel (length), or even oblique (e.g., greatest dimension) to the expected course of the ureter. It would seem most important to determine the greatest dimension of the stone perpendicular to the true long axis of the ureter. In real practice, however, the orientation of the greatest length of a given stone will be unknown. Even under ideal circumstances, given the finite focal-spot size and the divergence of the X-ray beam, some magnification will occur on radiographs. In fact, using a standard anode-to-film distance of 40 inches and assuming a distance of 5 inches from the stone to the cassette, an error of slightly less than 10% is to be expected for a stone whose greatest dimension is perpendicular to the X-ray beam. Otnes and Sandnes [9] compared stone size measured on radiographs to the size of the recovered stone. They found that overestimation of size was more common than underestimation. Most radiographic measurements were within ± 25% of the actual measurement; the maximal overestimation was 4 mm.
In 1991, Morse and Resnick [10] determined the frequency of spontaneous passage for ureteral stones in a series of 378 patients. They reported an overall frequency of spontaneous passage of 60%. The frequency was related to stone location: 22% for proximal stones, 46% for mid ureteral stones, and 71% for distal ureteral stones. A review of the literature published by Hubner et al. [11] in 1993 included 2,704 cases derived from six studies; they reported frequencies of spontaneous passage of 12% for proximal ureteral stones, 22% for mid ureteral stones, and 45% for distal ureteral stones. Neither Morse and Resnick nor Hubner et al. distinguished between the distal ureter and the ureterovesical junction.
A study by Kinder et al. [12] of 134 patients with confirmed ureteral stones divided stone location into upper third of the ureter, middle third, lower third, and ureterovesical junction. They reported a frequency of spontaneous passage of 94% for ureterovesical junction stones that were less than or equal to 5 mm; stones greater than 5 mm had a frequency of spontaneous passage of only 45%.
In conclusion, the rate of spontaneous passage of ureteral stones does vary with stone size and location as determined by CT. These rates are very similar to those previously published based on radiography.
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