Physiologic hydronephrosis as a cause of emergent right abdominal pain in pregnancy-truth or myth?

Acute abdominal pain (AAP) in pregnant patients poses a unique diagnostic challenge, as there are multiple confounding factors. In pregnant women with acute right abdominal pain, when the appendix cannot be visualized on ultrasound (US), magnetic resonance imaging (MRI) is commonly performed to evaluate the appendix, and exclude other non-obstetric causes for abdominal pain ( 1 ). Physiologic dilatation of the renal collecting system is a common phenomenon during pregnancy reportedly occurring in up to 90% of pregnant patients ( 2 , 3 ), likely caused by the gravid uterus causing extrinsic compression on the ureters. It is more common and severe on the right side ( 2 - 6 ), with positive correlation to gestation week ( 5 ). Most cases of physiologic hydronephrosis are considered asymptomatic, for example Ferguson et al. report a 90% incidence of asymptomatic hydronephrosis ( 7 ). In cases where the workup of right abdominal pain yields only right sided hydronephrosis with no obstructing stone and no other identifiable cause for pain, symptomatic hydronephrosis may be considered the cause for abdominal pain ( 8 ). The definitions of symptomatic hydronephrosis vary, some authors include criteria such as fever or laboratory findings indicating urinary tract infection (UTI) ( 9 , 10 ), while others mention only pain and hydronephrosis ( 5 , 11 ). Symptomatic hydronephrosis without an obstructing stone is reported to occur in up to 4.7% of pregnant women and 16.6% of patients with physiologic hydronephrosis ( 5 ). We found no literature attempting to validate physiologic hydronephrosis as a possible cause of acute right abdominal pain in pregnant patients referred to the emergency room. We aimed to evaluate whether the incidence of hydronephrosis is higher in pregnant women with right AAP (AAP group) and no other identifiable cause, as compared with the incidence in asymptomatic pregnant women (APW group). We present this article in accordance with the STROBE reporting checklist (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-563/rc ).

The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by institutional ethics board of Rabin Medical Center (No. RMC-0789-21) and individual consent for this retrospective analysis was waived. Following Institutional Review Board (IRB) approval, a retrospective review was performed including all consecutive abdominal MRI scans referred for AAP and suspected acute appendicitis in pregnant patients between 2014 and 2021. The control cohort included MRI scans performed in APW for fetal evaluation since the initiation of the fetal MRI program in Rabin Medical Center, between the years 2020 and 2022. Fetal MRI was performed as early as 24 weeks of gestation, therefore all scans performed in gestational weeks 1–23 in the AAP group were excluded ( Figure 1 ). Study flow chart. AAP, acute abdominal pain; APW, asymptomatic pregnant women; MRI, magnetic resonance imaging. AAP MRI protocol included T2 weighted (T2W) coronal, axial and sagittal planes 4 mm thick scans, T2 fat suppression coronal and axial planes 4 mm scans, and a T1 weighted (T1W) axial 3 mm axial scan. The APW fetal MRI protocol consisted of three-plane single-shot T2-weighted scans, three-plane steady-state free precession images, at least one plane of T1W images, diffusion-weighted imaging (DWI), and T2*/fast field echo (FFE) sequences. Since scan protocols differ between the AAP and APW, the radiologist could not be blinded to the indication. Medical records of patients in the AAP group were reviewed for clinical and pathological outcomes. Records were not reviewed for patients in the APW group and the reason for fetal screening was not documented. These patients were assumed to lack AAP requiring emergency department evaluation as they were scanned in an ambulatory pre-appointed setting. Inclusion criteria included 18 years of age and above, gestational age 24 weeks and above, available clinical data, and available MRI images considered to be of good quality be the reviewing radiologist. Informed consent was waived by the IRB. Patients with a diagnosis of appendicitis or any other identifiable cause for abdominal pain were excluded from the study. Appendicitis was defined as pathologically proven appendiceal inflammation, resected due to suspicious findings on MRI, or clinical suspicion even when the MRI report was negative for appendicitis. Other causes for abdominal pain were defined as conditions indicated on laboratory results or on MRI, either prospectively at the time of diagnosis or retrospectively during the review for this study. Specifically, cases with laboratory exams suggestive of a UTI and/or pyelonephritis, as well as patients with demonstration of an obstructing stone on MRI, were excluded from the study. A fellowship trained abdominal radiologist with 13 years of experience (A.A., R1) reviewed the MRI images and evaluated for qualitative and quantitative parameters. Qualitative parameters included findings suspicious of appendicitis (including appendiceal maximal diameter, intraluminal content hyperintense on T2W images, peri-appendiceal fat stranding, free fluid), findings suggestive of an alternative diagnosis (including gynecological, hepato-biliary, intestinal and other causes), and the presence and location of a urinary stone. Evaluation of hydronephrosis and hydroureters was performed on axial and coronal T2 images available in both groups, and the presence of urinary tract dilatation including the right ureter, right kidney, left ureter and left kidney was documented, qualitatively categorized as none, mild, moderate or severe ( Figure 2 ). Hydronephrosis and ureteronephrosis grades. (A) Normal kidney; (B) mild hydronephrosis; (C) moderate hydronephrosis; (D) severe hydronephrosis; (E) normal ureter; (F) mild ureteronephrosis; (G) moderate ureteronephrosis; (H) severe ureteronephrosis. Quantitative measurement of the maximal anteroposterior diameter of ureters was performed on axial T2W images, between the inner anterior and outer posterior wall at the widest part of the ureter ( Figure 3 ), a method adopted from measurements of blood vessels on US ( 12 ). The renal pelvis was not measured quantitatively since extrarenal pelvises may cause significant variability and anatomical bias, however qualitative evaluation as mentioned above did include presence of hydronephrosis involving the kidney, separately from hydroureter. Quantitative measurement of the maximal ureteral diameter (indicated by arrow). A radiology resident (D.H., R2) with 4 years of experience separately reviewed the images for quantitative and qualitative evaluation of the urinary system, to validate the results and assess inter-rater agreement. Prior to the separate review by R2, the two readers reviewed random cases of each dilatation grade to ensure consistency in the reviewing and evaluation process. Statistical analysis was performed using SPSS (IBM, Armonk, NY, USA, 27 th edition). Absolute interclass correlation coefficients were used to determine inter-rater agreement. Continuous parameters were compared using analysis of variance (ANOVA) test. For a categorical assessment of hydronephrosis, mild, moderate and severe scores were aggregated and defined as hydronephrosis or hydroureter, while no dilatation scores were defined as negative hydronephrosis and hydroureter. Categorical variables were compared using Chi-squared test. Multivariate analysis was performed using multivariate analysis of variance (MANOVA) test. Statistical significance was set at P<0.05.

During the study period, MRI scans were performed in 140 patients with AAP, of whom 85 patients were excluded due to gestational age earlier than 24 weeks. Of the remaining 55 patients, nine patients were operated on due to suspected appendicitis, 2 of whom had a normal appendix, and 7 patients had confirmed appendicitis, thus excluded from the study. Patients were also excluded from the study due to appendagitis (n=1), endometriosis (n=1), pyelonephritis (n=4), duodenitis (n=1), complicated ovarian teratoma (n=1) and premature contractions (n=1), number of patients in parentheses. APW scans were performed in 165 patients, 3 of whom did not have data regarding gestational age, thus excluded from the study. The final AAP group included 39 patients and APW group included 162 patients, with median age of 31 and 32 years, respectively (P=0.431) and median gestational age of 28 and 32 weeks, respectively (P<0.001). Quantitative and qualitative measurements of hydronephrosis factors showed no statistically significant differences between the two groups ( Table 1 ). Maximal diameters of the right ureter were similar between the groups for both readers (mean 4.6 and 5.2 mm in the AAP group vs. 5.3 and 5.5 mm in the APW group, P=0.219 and P=0.572, R1 and R2 respectively). Maximal diameters of the left ureter were also similar (data in Table 1 ). AAP, acute abdominal pain; APW, asymptomatic pregnant women; SD, standard deviation; R1, first reader; R2, second reader. Scoring for hydronephrosis involving the kidney and hydroureter was also similar between the groups. The mean and median scores are detailed in Table S1 . Overall hydronephrosis of any degree on either side was more common on the right side than the left, and the kidney was involved more commonly in dilatation than the ureter ( Table 1 ). Right renal hydronephrosis was found in 33.3% and 53.8% of the AAP group versus 43.8% and 45.1% of the APW group (P=0.280 and P=0.324, for R1 and R2 respectively). Right hydroureter was found in 28.2% and 33.3% in the AAP group versus 28.4% and 30.9% in the APW group (P=0.946 and P=0.765, for R1 and R2 respectively). There were no significant differences in the left side as well, as shown in Table 1 . Inter-reader agreement showed good to excellent reliability in qualitative as well as quantitative measurements. Intraclass correlation coefficients (ICCs) of qualitative evaluation in a scale of 1 (no dilatation) to 4 (sever dilatation) were 0.899 for right renal hydronephrosis, 0.913 for right hydroureter, 0.781 for left renal hydronephrosis and 0.737 for left hydroureter. Intraclass correlation of quantitative measurements of the ureters measured 0.945 for the right ureter and 0.822 for the left ureter. Average measures and confidence intervals (CIs) presented in Table 2 . CI, confidence interval. Multivariate analysis was performed using data from the more experienced reader (R1). Gestational age remained different between AAP and APW groups (P<0.001), while age and maximal diameter of the right and left ureters remained similar between the groups (P=0.431, P=0.155 and P=0.477, respectively, Table 3 ). AAP, acute abdominal pain; APW, asymptomatic pregnant women; SD, standard deviation.

Emergent right abdominal pain in pregnant patients with no appendicitis or any other identifiable cause on MRI, was not related to a higher incidence of hydronephrosis or hydroureter, or to an increased diameter of the renal ureter, as compared to asymptomatic ambulatory pregnant patients. These findings suggest that acute right abdominal pain, even in the presence of a dilated right-sided collecting system, is not necessarily attributable to physiologic hydronephrosis and alternative diagnosis should be carefully considered. Overall hydronephrosis of any degree on either side was found in 43%/48% of the patients (R1/R2 respectively), in keeping with previous reports ranging from 29% ( 5 ) to 90% ( 2 , 3 ). Right sided dilatation of the urinary system during pregnancy was more common than dilatation on the left side, the more experienced reader found any degree of right sided hydronephrosis in 42% and hydroureter in 28.5% of overall cases, as compared to left sided hydronephrosis in 12.5% and hydroureter in 8% of overall patients. Similar differences were found by the less experienced reader (47% and 31% on the right versus 15% and 8.5% on the left, kidney and ureter, respectively). These results are consistent with previous reports ( 2 , 3 , 6 ). There is variability in the definition of symptomatic hydronephrosis in pregnancy, since different criteria are used. Chitale and Chitale used hydronephrosis with flank pain, fever and leukocyturia and/or leukocytosis, found in 7% of pregnancies, of which 88% were on the right ( 13 ). Bayraktar et al. used hydronephrosis and loin pain, but with no laboratory evidence of incipient UTI, found in 4.7% of pregnant patients ( 5 ). Ercil et al. defined symptomatic hydronephrosis as flank pain and/or fever ( 9 ), Fainaru et al. specified flank and loin pain, pyelonephritis or urinary obstruction ( 10 ), and some papers do not specify the exact definition ( 14 ). In our study, we excluded patients with laboratory evidence of UTI and pyelonephritis, aiming at a hypothetical definition of physiological hydronephrosis causing pain, similar to Spencer and colleagues who defined symptomatic hydronephrosis as patients with pain, and no hematuria or signs of UTI ( 11 ). The results show no significant differences in urinary tract dilatation between the symptomatic group suffering from pain, and the asymptomatic group. This finding suggests that attributing the pain in pregnant patients with no laboratory evidence of UTI is not necessarily attributable to hydronephrosis. This study has several limitations. It is a retrospective study performed in a single tertiary referral center. Qualitative evaluations are a significant portion of the study, and a different classification system may have shown slightly different results, however the ICC showed good to excellent reliability and quantitative measurements were also available, showing similar trends as the qualitative factors. Another limitation is the lack of clinical and pathological data for patients in the APW group, which may have influenced both the univariate and multivariate analyses. However, as these patients were referred for fetal MRI in an outpatient setting, under scheduled, non-urgent circumstances, and were able to tolerate prolonged scan times, it is likely that they were indeed asymptomatic. While this cannot be confirmed with complete certainty, we believe this limitation is unlikely to have introduced significant bias.

acute right abdominal pain, even in the presence of a dilated collecting system, in the absence of laboratory exams supporting UTI, is not necessarily attributable to physiologic hydronephrosis and alternative diagnoses should be carefully considered. Further research is needed to define and validate the criteria for symptomatic hydronephrosis.

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