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1
Department of Diagnostic Radiology, Yale University School of Medicine, 333
Cedar St., New Haven, CT 06510.
2
Department of Radiology, Cornell University Medical College, Box 141, New York
Presbyterian Hospital, 525 E. 68th St., New York, NY 10021.
3
Department of Radiology, Rush-Presbyterian Medical Center, 1650 W. Harrison
St., Chicago, IL 60612.
Received December 18, 2000;
accepted after revision February 1, 2001.
Address correspondence to R. C. Smith.
Abstract
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SUBJECTS AND METHODS. The duration of flank pain was prospectively determined in 227 consecutive patients diagnosed with acute ureterolithiasis on unenhanced helical CT. These CT studies were evaluated for the presence or absence of perinephric stranding, ureteral dilatation, perinephric fluid, collecting system dilatation, periureteral stranding, and nephromegaly. The frequency of each sign was determined as a function of the duration of pain.
RESULTS. The frequency of moderate or severe perinephric stranding increased from 5% at 1-2 hr to 51% at 7-8 hr (p < 0.001); ureteral dilatation increased from 84% at 1-2 hr to 97% at more than 8 hr (p < 0.03); moderate or severe perinephric fluid increased from 0% at 1-2 hr to 22% at 3-4 hr (p < 0.03); collecting system dilatation increased from 68% at 1-2 hr to 89% at 7-8 hr (p < 0.03); periureteral stranding increased from 35% at 1-2 hr to 76% at 7-8 hr (p < 0.004); and nephromegaly increased from 40% at 1-2 hr to 54% at 7-8 hr (p < 0.36).
CONCLUSION. All CT secondary signs of ureteral obstruction except nephromegaly showed a significant increase in frequency as duration of flank pain increased. This observation may explain why the CT studies of some patients with acute ureterolithiasis show negative findings for some or all CT secondary signs of obstruction. Therefore, knowledge of the duration of pain is important when interpreting unenhanced CT studies in patients with acute ureterolithiasis.
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All CT examinations were performed using a Hi-Speed Advantage CT scanner (General Electric Medical Systems, Milwaukee, WI). Patients were scanned in the prone position from the top of the kidneys to the bladder base using a helical data acquisition with 5-mm-thick sections and a pitch of 1.0. Scans were acquired using 280-350 mAs at 120 kVp. Neither oral nor IV contrast material was administered.
The CT scans were reviewed retrospectively by two of the authors who did not have knowledge of the duration of the patient's symptoms. The presence or absence of the following were noted on the symptomatic side: perinephric stranding, ureteral dilatation, perinephric fluid, collecting system dilatation, periureteral stranding, and nephromegaly. When present, perinephric stranding and perinephric fluid were subjectively graded as mild, moderate, or severe. Examples are shown in Figures 1,2,3. We defined ureteral dilatation to be present when the diameter of the ureter was 2 mm or greater. Nephromegaly and collecting system dilatation were assessed subjectively by comparing the two sides in accordance with prior publications [1,5,8].
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In addition, we measured the size of each stone (in millimeters) as the greatest dimension within the axial plane of the CT section. Stone location was classified as proximal (if above the level of the top of the sacrum), mid (if at the level of the sacrum), distal (if below the sacrum and above the ureterovesical junction), at the ureterovesical junction, or within the urinary bladder. For those stones located in the proximal, mid, and distal ureter, we also determined the presence or absence of a rim sign.
For purposes of data analysis, we partitioned patient records into groups based on the duration of pain at the time of imaging in 2-hr intervals up to 8 hr and a group of those whose duration of pain exceeded 8 hr, as follows: 1-2 hr duration (n = 20), 3-4 hr (n = 55), 5-6 hr (n = 34), 7-8 hr (n = 35), and 8 hr or more (n = 83). If we had used 1-hr intervals, some groups would have contained insufficient numbers of patients to perfrom meaningful statistical analysis. Within each of these groups, the frequency of patients who had each obstructive sign (and its severity, if applicable) was calculated. These frequencies were compared using a chi-square test.
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Ureteral Dilatation
Using 2 mm or greater as the definition of ureteral dilatation, 84% of
patients whose duration of pain was 2 hr or less had ureteral dilatation. This
frequency increased to 96% at 3-4 hr and reached a maximum of 97% at more than
8 hr (Fig. 5). These
differences were statistically significant (p < 0.03).
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Perinephric Fluid
None of the 20 patients whose duration of pain was 1-2 hr had moderate or
severe perinephric fluid, and only two had mild perinephric fluid. The
frequency of moderate or severe perinephric fluid reached a maximum of 22% in
patients whose duration of pain was 7-8 hr
(Fig. 6). These differences
were statistically significant (p < 0.03).
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Collecting System Dilatation
Only 68% of patients whose duration of pain was 1-2 hr had collecting
system dilatation, and this increased to a maximum of 89% in patients whose
duration of pain was more than 8 hr (Fig.
7). These differences were statistically significant (p
< 0.03).
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Periureteral Stranding
Seven (35%) of the 20 patients whose duration of pain was 1-2 hr had
periureteral stranding. The frequency of periureteral stranding increased to a
maximum of 76% in patients whose duration of pain was 7-8 hr
(Fig. 8). These differences
were statistically significant (p < 0.004).
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Nephromegaly
Eight (40%) of the 20 patients whose duration of pain was 1-2 hr had
unilateral nephromegaly. The frequency of nephromegaly increased to a maximum
of 54% in patients whose duration of pain was 7-8 hr and then decreased over
time (Fig. 9). These
differences were not statistically significant (p < 0.36).
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Ureteral Rim Sign
Sixty-five stones were present at the ureterovesical junction or in the
bladder and were therefore excluded from this analysis
[9]. Of the remaining 162
stones, the frequency of the ureteral rim sign varied from 60% in patients
whose duration of pain was 5-6 hr to 73% at 7-8 hr
(Fig. 10). These differences
were not statistically significant.
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Stone Size
The average stone size was 2.3 mm in patients whose duration of pain was 2
hr or less and reached a maximum of 4.1 mm at more than 8 hr (p <
0.007). When partitioned by ranges of size, 90% of stones detected at 1-2 hr
were 4 mm or smaller in diameter, and this percentage decreased to 66% at more
than 8 hr (Fig. 11). This
difference was statistically significant (p < 0.04). The
percentage of stones 8 mm or greater in diameter increased steadily from 0% at
1-2 hr to 12% at more than 8 hr. This latter difference was not statistically
significant.
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Stone Location
There was no clear relationship between stone location and duration of
pain. For example, the frequency of stones located in the distal ureter, at
the ureterovesical junction, or within the bladder varied from 76% at 1-2 hr,
to 90% at 5-6 hr, to 81% at more than 8 hr
(Fig. 12).
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Experiments in dogs performed by Rose and Gillenwater [10] showed that complete occlusion of the ureter results in rapid elevation of ureteral pressure as well as a rapid increase in ureteral diameter from approximately 2 mm at baseline to approximately 5 mm after as few as 10 min of complete obstruction. Similar changes have been reported in human subjects [11, 12]. No published studies evaluate the normal range of ureteral diameter on unenhanced CT, but we have anecdotally noted that the normal mean ureteral diameter is approximately 1-2 mm. Excluding the most proximal portion of the ureter and using a definition of ureteral dilatation of 2 mm or greater, we found that ureteral dilatation as revealed on unenhanced CT occurs rapidly. It was present in 84% of patients whose duration of pain was 2 hr or less (Fig. 5) and increased to 97% at more than 8 hr. The temporal relationship between duration of pain and ureteral dilatation likely accounts for CT sensitivity of ureteral dilatation of only 90% in patients with acute ureterolithiasis that have been previously reported [8].
Even with complete ureteral obstruction, renal blood flow remains at or above baseline levels for up to 90 min, and it may remain at 50% or more of baseline values for up to 72 hr [13,14,15]. On the other hand, if obstruction is partial or intermittent, normal renal blood flow will be maintained for prolonged periods of time. Whatever the degree of obstruction, fluid will accumulate in the renal interstitium over time. As renal edema develops, excess fluid is drained by renal lymphatics. These lymphatics arise in either a subcapsular location or deep within the renal parenchyma [16,17,18]. The deep lymphatics form larger trunks that follow the main renal vein to the paraaortic lymph nodes. As these trunks exit the renal hilum, tributaries of the subcapsular network join them [19]. Intrarenal communication exists between the deep and subcapsular lymphatics, with preferential flow directed via valves from the deep to the subcapsular system. In addition, lymphatics in the perinephric space freely communicate with the subcapsular lymphatics. These perinephric lymphatics eventually drain into paraaortic lymph nodes.
Two studies have shown that the role of the renal lymphatics during obstruction is to drain excess fluid that accumulates in the renal interstitium [18, 20]. These studies postulate that, initially, there is increased lymphatic flow into the deep system and toward the renal hilum. If complete or highgrade obstruction persists, there will be progressive dilatation of the renal pelvis, with compression of the hilar lymphatics and diversion of lymph flow to the subcapsular and perinephric lymphatics. If obstruction is partial or intermittent, there may not be a significant diversion of renal lymphatic drainage to the subcapsular and perinephric lymphatics. The perinephric lymphatics run either in or immediately adjacent to the fibrous septa of the perinephric space. Thickening of the perinephric septa and enlargement of the perinephric lymphatic channels as a result of ureteral obstruction are seen on CT images as linear or curvilinear perinephric stranding. In addition, discrete foci of perinephric fluid can often be identified. This fluid presumably collects in, between, or along the septa of the perinephric space.
For purposes of data analysis, we classified perinephric stranding on CT as absent, mild, moderate, or severe. We found that perinephric stranding was present early in the course of obstruction and that its overall frequency remained nearly constant over time at about 75%. This is somewhat lower than previously published sensitivities of perinephric stranding in patients with acute ureterolithiasis [4, 8]. The lack of universal presence of perinephric stranding likely reflects those patients with mild or intermittent obstruction in whom lymphatic drainage is predominantly toward the renal hilum. On the other hand, we found that in those patients who manifested perinephric stranding, the severity of perinephric stranding increased significantly over time. Perinephric stranding was classified as moderate or severe in only 5% of patients whose duration of pain was 2 hr or less and reached a maximum of 51% at 7-8 hr.
Renal edema results in unilateral enlargement of the obstructed kidney. On CT studies, renal enlargement is usually assessed subjectively. It is expected that nephromegaly would develop more slowly over time than ureteral dilatation. In fact, we found nephromegaly to be present in only 39% of patients whose duration of pain was 2 hr or less, and it reached a peak of 65% at 5-6 hr. This latter value is similar to sensitivities of nephromegaly for diagnosing ureteral obstruction that have been previously reported [4, 13]. Nephromegaly would be expected to decrease over time in some patients as a result of decreased renal blood flow and increased lymphatic drainage.
The ureter has lymphatics present in the submucous, intramuscular, and adventitial layers, all of which freely communicate [19]. Periureteral stranding presumably results from increased drainage into the periureteral lymphatics and fluid leakage into the surrounding tissues. Because intraureteral pressure rises almost immediately after obstruction occurs, periureteral stranding should be seen early in the course of obstruction and should increase over time similar to perinephric stranding. We found that periureteral stranding was present in 35% of patients whose duration of pain was 2 hr or less and reached a peak of 76% at 7-8 hr. Periureteral stranding may be seen less frequently than perinephric stranding because it is distributed over a long length of the ureter, depending on stone location. In fact, periureteral stranding was present in 78% of patients with proximal stones, in 85% of patients with mid-ureteral stones, but in only 54% of patients with more distal stones. These differences are statistically significant (p < 0.001). We performed a similar analysis that revealed that perinephric stranding was present in 77% of patients with proximal stones, in 77% of patients with midureteral stones, and in 69% of patients with distal stones, but these differences were not statistically significant (p < 0.41).
Because the ureter and the renal collecting system are continuous, it might be expected that collecting system dilatation would parallel ureteral dilatation. In fact, previous studies have shown collecting system dilatation to have a frequency of approximately 83% in the patients with acute ureterolithiasis, which is similar to the frequency of ureteral dilatation [8]. However, we found that collecting system dilatation develops more slowly than ureteral dilatation, and, therefore, its frequency is strongly determined by the duration of pain at the time of the CT examination. Ureteral dilatation was present in 82% of patients whose duration of pain was 2 hr or less. On the other hand, collecting system dilatation was present in only 68% of patients whose duration of pain was 2 hr or less. This discrepancy may have occurred because collecting system dilatation can easily be confused with normal prominence of the renal pelvis or an extrarenal pelvis. For this reason, we always assess for collecting system dilatation by using the upper or lower poles of the kidneys.
It seems plausible that variations in pain tolerance account for the variability in time at which patients with renal colic seek medical attention. Patients with small stones that pass spontaneously may not seek medical attention during a brief episode of renal colic. On the other hand, patients with renal colic whose symptoms persist over time are more likely to seek medical attention and are also more likely to have larger stones that do not pass spontaneously. We found that the average size of ureteral stones had a direct correlation with the duration of pain. Average stone size was 2.3 mm in patients who sought medical attention after 2 hr or less of pain, whereas average stone size was 4.1 mm in patients who presented after 8 hr or more of pain.
The ureteral rim sign refers to a rim of soft-tissue attenuation that is often seen surrounding ureteral stones. The rim sign is thought to represent edema of the ureteral wall at the level of obstruction and would therefore be expected to increase in frequency for stones that did not move down the ureter over time. Our data reveal that the frequency of the rim sign did not change significantly over time. This may reflect the fact that most stones are only partially obstructing and do move down the ureter at a rapid rate.
The frequency of all CT secondary signs of ureteral obstruction peaks at approximately 6-8 hr of pain duration. It therefore seems reasonable to obtain scans in patients with flank pain and suspected renal colic within this window of time to maximize findings of secondary signs of obstruction. Similarly, if scanning is performed much earlier and the CT findings are equivocal (or entirely negative) but clinical symptoms persist, repeated scans should be obtained at 6-8 hr of pain duration.
In conclusion, all CT secondary signs of ureteral obstruction except nephromegaly showed a significant increase in frequency as duration of flank pain increased. This may explain why the CT studies of some patients with acute ureterolithiasis show negative findings for some or all CT secondary signs of obstruction. Therefore, knowledge of the duration of pain is important when interpreting unenhanced CT studies in patients with acute ureterolithiasis.
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