UUT stones are among the most common diseases in urology, which often cause hydronephrosis or even renal pyonephrosis. And calculi-related obstructive pyonephrosis (COP) occurs in about 3.2% of patients with UUT stone (1). COP is considered a urological emergency, and it can cause a rapid loss of renal function and can quickly progress to sepsis and even septic shock (2). In recent years, the incidence of sepsis caused by UUT stone has increased, and the associated mortality rate is still high, although the management of the patients has improved (2, 3). So early diagnosis and treatment of COP are particularly important to protect renal function and prevent septic shock.
The evolution of renal pyonephrosis has not been exactly investigated. Obstruction caused by UUT stone and infection is two vital etiological mechanisms of COP (4, 5). This study found that patients with renal pyonephrosis had a higher degree of hydronephrosis than those without pyonephrosis. And the grade of hydronephrosis is an independent predictor of COP in multivariate logistic analysis. Luca Boeri et al. (6) analysed a cohort of 46 patients with renal pyonephrosis and 76 individuals without renal pyonephrosis treated at a single centre. These authors also found that the grade of hydronephrosis is an independent predictor of COP, and they think patients with grade III-IV hydronephrosis are three times more at risk of renal pyonephrosis than patients with grade I-II. Hydronephrosis increases pressure in the renal pelvis and reduces urine formation, so the risk of retrograde infection by bacteria is significantly increased.
Escherichia coli is the most common infecting organism in urinary tract infection (UTI) due to retrograde infections (6, 7, 8). In our study, E. coli was also the most common infecting organism (32.5% of cases). So, the preoperative urine leukocyte count and WBBC were higher in patients with pyonephrosis than in the hydronephrosis group. But the positive rate of mid-stream urine culture (MUC) was only 36.4% in patients with pyonephrosis, and 33.3% of patients’ renal pelvis urine cultures were positive, and preoperative mid-stream urine cultures were negative in patients with pyonephrosis. Similar results were also found in a recent study; Madhusudan et al. (9) found the positive rate of midstream urine culture below 50% in patients with pyonephrosis. Liu et al. (10) also found preoperative urine culture played a role in predicting SIRS after PCNL, but it was unable to prevent the occurrence of SIRS. Because of the urinary obstruction, the infection may persist in the upper system when the MUC is negative. Although far higher than that in patients with hydronephrosis, we believe the MUC cannot be a good predictor of COP because of the low positive rate.
Previous studies have observed that UTIs are more common in women than men, and males are more likely to suffer from urolithiasis (4, 10, 11). In this study, there were twice as many men as women; 24 of 49 patients with pyonephrosis are women. Therefore, we propose that female patients with UUT are more likely to suffer from renal pyonephrosis because of the high risk of UTI. In addition, 15 of 24 female patients with pyonephrosis developed urogenic sepsis, and only three males developed urogenic sepsis. We believe that the difference between genders is significant, especially in those who are older and with low serum albumin; female patients with renal pyonephrosis caused by UUT stones are more likely to develop urogenic sepsis.
Primary drainage, including percutaneous nephrostomy or retrograde ureteral intubation, is the principle for diagnosing and treating pyonephrosis (9, 12, 20). A percutaneous nephrostomy or retrograde ureteral intubation is a necessary and effective surgical procedure, but they are invasive. So, we are looking for a non-invasive method to identify pyonephrosis from hydronephrosis.
Radiological examinations, such as ultrasound, magnetic resonance imaging (MRI), and computed tomography (CT), are important in the diagnosis of pyonephrosis because they can achieve a rapid result and help determine the status of renal infection. Ultrasound is a conventional diagnostic method for upper urinary calculi. It has been reported that renal pyonephrosis may be suggested when strong echo points, uneven density, and blurred images occur in the renal collecting system (12). However, ultrasound is highly subjective and has no quantitative indicators, limiting the value of renal pyonephrosis diagnosis. Magnetic resonance (MR) is a reliable tool to differentiate pyonephrosis from hydronephrosis but is not widely used in the diagnosis of upper urinary calculi due to its low efficiency in diagnosing calculi (13, 14). CT is by far the most commonly used method for diagnosing upper urinary calculi, and the diagnostic rate of calculi is as high as 95–100%. Meanwhile, it can clearly show the renal parenchyma, renal collecting system, and perirenal conditions(15).
The HU is the scale of tissue density at standard temperature and pressure, related to two predefined values: air defined as -1000 HU and water 0 HU (23). Physicians routinely use the HU attenuation value measured by computed tomography to assess the hardness of urinary calculi, helping to determine the stone treatment and define the nature of the kidney masses (16, 17, 21, 22).
Stunell et al. (18) reported no non-specific findings, such as thickening wall of the renal pelvis and blurring of perirenal fat, in computed tomography in patients with pyelonephritis. Also, the HU value of the renal collecting system was not measured in their study. However, Yuruk et al. (19) conducted a retrospective study of 105 patients with calculi-related obstructive hydronephrosis and revealed that the HU value of the renal collecting system in patients with pyonephrosis was significantly higher than in patients with hydronephrosis. The best cut-off value for predicting pyonephrosis in the renal collecting system was 9.21 (65.9% sensitivity and 87. 9% specificity). We also report similar results; HU values of the dilated renal collecting system of patients with pyonephrosis were higher than patients with hydronephrosis (p = 0. 02). The ROC curve analysis revealed that the HU value of 9.5 was the best cut-off to predict calculi-related obstructive pyonephrosis (71.4% sensitivity and 70. 2% specificity).
In a recent study, 11 patients with obstructive UTI and 22 patients with Calculi-related hydronephrosis were retrospectively evaluated, and percutaneous nephrostomy was performed in all patients; these authors report that patients in obstructed urinary systems with renal pelvis urine culture positivity had a higher HU value than negative patients (20). In another study, they found the HU values of the dilated renal collecting system can differentiate COP from COH and predict septic complications after surgery. The best cut-off value to predict septic complications in the renal collecting system was 7.3. Patients with HU values of more than 7.3 are 8-times more likely to progress to sepsis postoperative (6). In our study, we did not detect a was a significant difference between the patients with or without sepsis in HU value (p = 0. 11), and we found that there is a significant difference between the patients with or without sepsis in age, sex, and the level of serum albumin. We believe older, low serum albumin female patients with renal pyonephrosis caused by UUT stones are more likely to develop urogenic sepsis.