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Oleg Rud, Department of Urology, St Elisabeth Klinikum Straubing, St Elisabeth Street 23, 94315 Straubing, Germany. e-mail: firstname.lastname@example.org
Study Type – Diagnosis (reliability)
Level of Evidence 2b
What's known on the subject? and What does the study add?
Conventional sonography is the standard imaging technique used in patients with acute renal colic. A decreasing hydronephrosis index (HI) is associated with an increase in obstructive uropathy. Obstructive uropathy leads to an increase in the renal resistive index (RRI).
The present study shows that both the RRI and HI methods are easily practicable in patients presenting with stone-related renal colic. The grade of hydronephrosis correlates positively with the HI but not with the RI.
• To confirm the reliability of assessements of the renal resistive index (RRI) and the hydronephrosis index (HI) comprising two sonographic techniques providing additional information in patients with acute renal colic.
PATIENTS AND METHODS
• Sonographic measurement of hydronephrosis and assessment of common clinical criteria was performed in 22 consecutive patients presenting with unilateral stone-related renal colic. RRI and HI were separately recorded by two investigators within a prospective study.
• Interobserver agreement and comparison of sonographic with computed tomography (CT) findings were assessed with the Cohen's kappa statistic (κ) for attributive ordinal characteristics and Spearman's rank correlation/rho (ρ) for attributive metric characteristics.
• There was a significant correlation between HI and the sonographically-evaluated grade of hydronephrosis, although not between RRI and the grade of hydronephrosis.
• For all procedures (RRI, HI, sonography and CT), significant differences between the symptomatic and the asymptomatic kidney were assessed.
• Interobserver agreement was excellent for the grade assessment of hydronephrosis by conventional sonography (κ= 0.82; P < 0.001), good to very good for HI (ρ= 0.60; P= 0.003) and acceptable to good for RRI (ρ= 0.49; P= 0.021).
• The RRI and HI methods are both easily practicable as stageless examination methods in patients presenting with stone-related renal colic, and both also reliably distinguish between obstruction and non-obstruction.
• Exact thresholds for both methods must still be defined based on further successive studies.
• Additionally, changes of values under medical expulsive therapy and correlation with the functional status of the obstructed kidney remain to be examined.
Assessments of the renal resistive index (RRI) and the hydronephrosis index (HI) comprise two sonographic techniques that can generate useful additional information in patients with acute renal colic in both a primary care setting and during the course of medical expulsive therapy (MET). The RRI represents the reduction in diastolic blood flow speed in relation to systolic flow speed and can be measured, for example, in the Arteriae arcuatae during an obstruction of the pelvicaliceal system [1–23]. The HI represents the relationship between the entire area of the kidney to the area of the pelvicaliceal system, with increasing obstruction resulting in a reduced HI value [24,25].
For a new examination method to become established medical practice, it has to primarily fulfil two criteria, First, the technique should display and correlate with a clinically relevant endpoint. The detection of an increase in any obstruction or a worsening in kidney function are clinically relevant criteria in cases of stone-related renal colic. Second, the technique must exhibit a high level of reproducibility. This in turn requires studies that can prove good intra-observer agreement (i.e. one investigator observes the same measurements at different points in time), good interobserver agreement (i.e. different investigators observe the same measurements congruently) and good equipment agreement (i.e. one or more investigators observe the same measurements using different examination equipment). The RRI and HI are two diagnostic techniques with high potential. Until now, they have taken no place in the routine evaluation of patients with acute renal colic. Investigations into the value of these techniques in the diagnosis of acute renal colic have previously only given contradictory results for the RRI [1–23] and remain totally absent for the HI. Studies on the reproducibility of both examination methods in this indication have equally not yet been published.
The present prospective study aimed to compare RRI, HI, sonography and native helical CT in the initial diagnostic of acute stone-related renal colic and to investigate the interobsever agreement of the sonography-based techniques.
PATIENTS AND METHODS
The prospective study design, approved by the local ethics committee, was based on the data obtained from all consecutive patients with acute renal colic during the period from January to May 2010 at St Elisabeth-Hospital in Straubing, Germany. Patients were examined ≤48 h within the onset of colic pain. Exclusion criteria were the absence of stones in the helical native CT; the presence of only one kidney; evidence of renal co-pathology (known renovascular and/or renoparenchymal diseases, amongst others) ; the unavailability of one of the predetermined observers (two urologists with renosonographic experience, see below); and patient age <18 years. A run-in phase was conducted before the study, with five patients not being included in the analysis. During this time, both investigators (O.R. and P.W.) agreed on standardized measuring conditions in conjunction with a DEGUM (German Society of Ultrasound in Medicine) certified urologist who was experienced with colour-coded duplex Doppler sonography (M.M.).
Standardized examinations were conducted according to the above listed inclusion and exclusion criteria. To provide comparable conditions for sonography and to render colour-coded duplex Doppler sonography reliably interpretable in the first place, patients had to be low pain (or pain-free). This was normally achieved through the use of metamizole (in a few cases with low-potency opiates). A sonographic examination was conducted once by each investigator within a maximum of 12 min after examination before the other one, and with neither knowing the results of the other's examination. Both investigators then subsequently reached a consensus on the conventional sonographic grade of hydronephrosis. A colour-duplex capable sonographic device with integrated pulse-wave-doppler (Pro Fokus 2202; B-K Medical, Herlev, Denmark) with a 3.5-MHz convex head was used for all scans. A native helical CT scan was performed either immediately before or directly after the examination. CT scans were performed using a 64-line multidetector scanner (Brilliance, Phillips, Eindhoven, the Netherlands). Alongside general patient criteria (age, gender, height, weight, body mass index, first or recurring stone, creatinine serum value, side of renal colic) the CT gave both stone localization and stone size.
Conventional sonographic and CT graduation of hydronephrosis was conducted separately according to colic side and contralateral kidney using a five-point classification based on the criteria of the Society for Foetal Urology: grade 0 = no hydronephrosis; grade 1 = isolated distension of the renal pelvis; grade 2 = clear distension of renal pelvis with calyx neck recognizable; grade 3 = massive renal pelvic distension with ectasia of neck of the calyx without reduction of the renal parenchyma; grade 4 = as grade 3 but with a reduction of the renal parenchyma .
RRI calculations were based on the results of colour-coded duplex Doppler sonographic examinations. Accordingly, the Arteriae arcuatae were identified at the lower, mid- and upper poles of the renal pelvis. During the measurement, the patient was told to hold his/her breath. The RRI was only calculated if four consecutive Doppler curves could be recorded (Fig. 1).
The RRI is calculated according to the formula: (maximum systolic flow speed – end-diastolic flow speed)/maximum systolic flow speed. A mean value was calculated from the three RRI values per side (lower pole, upper pole and mid-pole) for the corresponding kidney [1–23]. An RRI ≥ 0.70 and a difference in the RRI values (ΔRRI) between the symptomatic and contralateral kidney >0.07 are given in the literature as being suggestive of an obstruction [1–3,5–7,11,12,15,16,20,23].
The HI was established both by sonography and CT for the affected and contralateral kidney. Measurements were based on a longitudinal section of the kidney at its largest extension. The outer contours of the kidney were outlined with the trackball and the sonography device or CT scanner software automatically calculated the kidney area (cm2). This was then repeated for the intrarenal pelvis together with the calyx groups (Figs 2 and 3). The HI (as a percentage) was calculated using the formula: 100× (total kidney area – intrarenal pyelon and calyx area)/(total kidney area) [24,25].
Normally distributed continual variables are given as the mean (sd) and non-parametric continual variables are given as the median (interquartile range). The distribution of results between the affected and unaffected kidneys was calculated through statistical analysis of the data using the Student's t-test (or the Mann–Whitney–Wilcoxon U-test for non-normal distribution) and the chi-squared test (categorial distinction). The agreement of both investigators with regard to the evaluation of hydronephrosis using the three sonographic techniques (conventional sonography, RRI and HI) and a comparison of these values with the CT measurements were verified using Cohen's kappa statistic (κ) for attributive-ordinal characteristics and Spearman's rank/rho correlation (ρ) (linear association) for attributive-metric characteristics. The coefficients were each in the range between –1 and +1, where <0.4 = bad to moderate agreement; 0.40–0.59 = acceptable to good; 0.60–0.80 = very good; and >0.80 = excellent agreement. All statistical tests were two-sided. P≤ 0.05 was considered statistically significant. All analyses were performed using SPSS, version 17.0 (SPSS Inc., Chicago, IL, USA).
The descriptive data from the study group are displayed in Table 1. The mean (sd) age of patients was 50.9 (15.2) years, and patients were predominantly men (82%). Side distribution was equal, with predominantly solitary stones in the lower ureter (55%). Mean (sd) stone size was 5.6 (2.7) mm. In total, five (23%) patients passed their stones spontaneously after the study investigations were performed. All patients were confirmed (both sonographically and by CT) to have an asymptomatic contralateral kidney, free from obstruction or other pathology. The separate evaluations of the colic kidney as carried out by both investigators are detailed in Table 1. The joint consensus regarding sonographic grade was identical to the CT evaluation. There were three (14%) patients who showed no sonographic or CT obstruction; however, an obstruction was subsequently found when an intervention became necessary; these obstructions could not be displayed image-morphologically as a result of a dichotomic pelvicaliceal system. The descriptive RRI, ΔRRI and HI evaluations of both sonographic investigators and the CT HI are shown in Tables 2 and 3. No significant correlation was found between RRI, ΔRRI, HI, sonographic obstruction degree and CT obstruction degree with clinical criteria (stone localization, stone size, creatinine level, age, gender, side, body mass index, duration of colic and recurrent stone disease) (all P > 0.05). There was a significant correlation between HI and sonographic consensus grade of hydronephrosis (=degree of obstruction in CT analysis) at the colic side (Investigator 1: ρ=−0.47, P= 0.029; Investigator 2: ρ=−0.56, P= 0.007), although not between RRI and grade of hydronephrosis (Investigator 1: ρ=–0.03, P= 0.896; Investigator 2: ρ=–0.15, P= 0.514).
Table 1. Examination criteria
Distribution within the study group (n= 22)
Age (years), mean (sd)
Women, n (%)
BMI (kg/m2), mean (sd)
Right stone localization, n (%)
Recurrent stone, n (%)
Colic duration (h), mean (sd)
Stone localization, n (%)
Many obstructions in various localization, n (%)
Stone size (imm), mean (sd)
Active stone removal, n (%)
Creatine (mg/dL), mean (sd)
Sonography – colic side (Investigator 1), n (%)
Sonography – colic side (Investigator 2), n (%)
CT – colic side (=I1 & I2 consensus sonography), n (%)
Table 2. Differences in the distribution of renal resistive index (RRI), ΔRRI and hydronephrosis index (HI) for both investigators, as well as between the CT calculated HI at the different sonographic consensus hydronephrosis grades (colic side)
Study group (n= 22)
Grade 0 (n= 3)
Grade 1 (n= 15)
Grade 2 (n= 2)
Grade 3(n= 2)
Data are given as the mean (sd).
RRI – Investigator 1
RRI – Investigator 2
ΔRRI – Investigator 1
ΔRRI – Investigator 2
HI (sonography) – Investigator 1
HI (sonography) – Investigator 2
Table 3. Differences in the distribution of renal resistive index (RRI), ΔRRI, hydronephrosis index (HI) (sonography) and HI (CT) between the colic and contralateral unaffected kidney
Data are given as the mean (sd).
RRI – Investigator 1
RRI – Investigator 2
HI (sonography) – Investigator 1
HI (sonography) – Investigator 2
Degree of hydronephrosis (sonography) – Investigator 1
Degree of hydronephrosis (sonography) – Investigator 2
Degree of hydronephrosis (sonography – consensus, CT)
Table 2 shows the different examination methods (RRI, ΔRRI and HI) together with the degree of obstruction in the sonographic consensus evaluation. Significant differences between the various grades of hydronephrosis were seen in the sonographic HI evaluation (Investigator 2, P= 0.007) and the CT HI (P < 0.001). Indeed, there was a significant positive correlation between the HI value from Investigator 2 and the HI value given in the CT for the colic side (ρ= 0.49; P= 0.021).
Table 3 sets out the differences for the respective examination methods between the unaffected and the colic kidney. All methods (RRI, HI, sonographic and CT degree of hydronephrosis) gave significant differences between both kidneys. Equally, there were differences between the RRI and HI of the colic side compared to the unaffected side in patients without sonographic hydronephrosis (Grade 0, n= 3).
Interobserver agreement (i.e. a comparison of the results obtained by Investigators 1 and 2) over the degrees of hydronephrosis on the colic side was excellent in the conventional sonographic evaluation (κ= 0.82; P < 0.001); good to very good (ρ= 0.60; P= 0.003) using the HI; and acceptable to good using the RRI (ρ= 0.49; P= 0.021). Only 23% of patients (Investigator 1) or 64% of patients (Investigator 2) showed an RRI ≥ 0.7, and ΔRRI was found to be >0.07 on the colic side in 59% (Investigator 1) and 55% (Investigator 2) of patients (Table 4). Agreement between the two observers was insufficient at these threshold levels (κ= 0.29 and κ=–0.39, respectively).
Table 4. Interobserver agreement (Investigator 1 versus Investigator 2) for different diagnostic procedures in the evaluation of hydronephrosis
The goal of imaging in patients with acute renal colic is to confirm initial diagnosis, determine stone size and localization, and to precisely quantify the degree of hydronephrosis, as well as evaluate potential complications . Conventional sonography may be the standard imaging technique used in kidney examinations and is also ubiquitously available; however, native (contrast-medium free) helical CT has become the gold standard [26–29]. Native helical CT fulfils the previously listed requirements for a reliable mainstream clinical diagnostic; nevertheless, four limitations of this technique must also be noted. First, any determination of the extent of hydronephrosis is imprecise and does not allow reliable differentiation between obstructive and non-obstructive expansion of the pelvicaliceal system or between chronic and acute forms. Second, limits in kidney function as a result of the presence of a stone-related obstruction cannot be deduced from the results of the CT scan itself. Third, the imaging technique cannot be freely repeated because of the associated radiation dose, which is particularly important for patients with recurrent stones as a result of the cumulative dose. For this reason, CT imaging lends itself only to initial diagnosis and should not be performed as a part of the ongoing evaluation of patients who could spontaneously pass their kidney stones (MET). Fourth, CT imaging is not available in all primary or outpatient care settings at all times (e.g. especially not if a patient becomes symptomatic during the night).
The use of colour-coded duplex Doppler sonography to determine the RRI was introduced into obstructive uropathic diagnostics by Platt et al. [6,7]. This stemmed from the observation that an immediate change in renal haemodynamics occurs during acute ureteric obstruction. This change is caused (amongst other things) by the vasoactive substances thromboxan A2, angiotensin 2 and endothelin . This in turn leads to an increase in intrarenal resistance, which can be determined by colour-coded duplex Doppler sonography of the Arteriae arcuatae. A RRI value of 0.7 shows a 70% reduction in the sonographically-obtained end-diastolic flow speed relative to systolic flow speed. In this respect, the RRI correlates with the extent of urinary obstruction, even if this same effect can be observed during renovascular or parenchymal disease and the index can show age-related changes [1–3,5].
The studies available in the literature have not yet defined uniform threshold values for the RRI or ΔRRI results above which a relevant obstructive uropathy can be assumed. The thresholds on which the present study are based (≥0.7 and >0.07 respectively) are the values recommended in the literature [1–3,5–7,11,12,15,16,20,23], despite these not being uniformly identified in all studies [19,22]. Furthermore, it remains unclear whether the RRI correlates with kidney function and, as such, whether it can be used as a clinical parameter during an expulsive therapy course. To date, indications to abort MET have been based on the development of clinical or laboratorial infection, an increase in creatinine level or analgesia-refractive renal colic . A non-invasive and readily available indicator that can be calculated with colour-coded duplex Doppler sonography and anatomical–morphological data such as the RRI is highly desirable. However, in addition, any association between RRI and the functional state of the kidney remains to be proven, and this test could still only be sensibly employed if it was shown to exhibit a high level of intra- and interobserver agreement. The available studies on the value of the RRI for stone-related renal colic are summarized in Table 5[1–23].
Table 5. Narrative literature review of the available studies on patients with acute hydronephrosis to determine the diagnostic benefit of renal resistive index (RRI) and ΔRRI
The HI was first described by Shaprio et al.  as a non-invasive, objective and stepless (dimensionless) longitudinal progress monitor in a study group of 46 children (aged 0–13 years) with hydronephrosis of various causes . An HI value approaching 100% indicates normal and obstruction-free urinary function, with a decreasing HI being associated with an increase in obstructive uropathy. According to the definition of HI given by Shapiro et al. , measurements should concentrate exclusively on intrarenal distension of the pelvicaliceal system because of the lower significance of extrarenal system blockage. In another study, Venkatesan et al.  showed a narrow correlation between HI and the sonographically-established grade of hydronephrosis (graded according to the Society of Foetal Urology) using data from 1207 examinations on pelvi-ureteric junction obstruction. Studies of patients with stone-related acute obstruction and investigations into the reproducibility of the HI results remain absent from the literature. The RRI and HI do not compete with the helical native CT in terms of being the standard reference technique in the first diagnostics of acute renal colic; rather, these two sonographic techniques are to be considered as complimentary to the CT and to comprise follow-up care diagnostics during MET.
In any case, the present study should be considered as the first of many investigations in this area and it is not completely devoid of limitations, which must be considered when interpreting the results. The study group was relatively small and, in this respect, further studies on the interobserver variability of RRI and HI should incorporate a much larger number of patients. Also, the present study did not generate data on intra-observer variability or equipment variability. However, the collection of such data is not without its problems because the close time-spacing required between evaluations could lead to an investigator being influenced by the initial findings, and a study design needs to be devised whereby this potential bias could be excluded. The RRI can be affected by various renovascular or renoparenchymal pathologies, and it is also possible that the type of analgesic therapy affects the values returned by this index, which was not considered in the framework of the present study. Nevertheless, the RRI (as indicated by the records obtained in the present study) only delivers reliable results for low-symptom (symptomless) patients, which meant that regular analgesic medication was necessary. A most important question in this area, namely how RRI and HI progress over the course of acute obstructive uropathy (e.g. during MET), was not addressed by the present study design. Furthermore, it would be equally valuable to establish a correlation between RRI and HI using kidney function data (diuretic dimercaptosuccinic acid scintigraphy/DMSA), ideally obtained over a period of time.
In summary, the present prospective study shows acceptable to good interobserver agreement for the RRI and good to very good interobserver agreement for the HI in patients with stone-related obstruction of the pelvicaliceal system. Both stepless procedures are simple to perform during the initial sonographic diagnostics and also allow the distinction between an obstructed and obstruction-free pelvicaliceal system, including in those patients with evident obstruction, as well as in those without conventional sonographic kidney obstruction. Because of the reduced interobserver agreement of the RRI in comparison with HI and the inadequate sensitivity of the thresholds for RRI in the present study (RRI ≥ 0.7, ΔRRI > 0.7), as well as several other threshold values described in the literature (Table 5), in our opinion, HI rather than RRI should be explored further. The data obtained from the small patient group in the present study are to be considered primarily as a starting point for further studies. Indeed, further studies should aim to define thresholds for these techniques (inclusive of RRI), as well as investigate index value changes and their correlation with obstructed kidney function over the course of MET.
Critical revision: M. Burger, S. Brookman-May, C. Gilfrich, H. Häuser, H.-M. Fritsche, W. F. Wieland, A. M. Ahmed and S. Brookman-May. Study concept and design: S. Brookman-May. Acquisition of data: O. Rud, P. Waliszewski, J. Moersler and S. Brookman-May. Drafting of the manuscript: S. Brookman-May, O. Rud, J. Moersler, J. Peter and H.-M. Fritsche. Statistical analysis: S. Brookman-May and O. Rud.
CONFLICT OF INTEREST
The authors declare that there are no conflicts of interest.