Impact of Obesity on Outcomes of Supine Percutaneous Nephrolithotomy

Introduction

Percutaneous nephrolithotomy (PCNL) has become the gold standard treatment for large renal stones, universally endorsed by American, European, and other national guidelines. ^{1 –3} Nephrolithiasis has been linked with obesity in several epidemiological studies, and reports have demonstrated that overweight patients may have multiple urinary abnormalities that increase their risk of stone formation. ^4,5

Obesity poses several problems in the management of stone disease from diagnosis and imaging through anesthesia and surgery itself. Obesity has generally been considered a factor that affects the outcomes of PCNL procedures due to anesthesia-related issues (i.e., challenges in ventilation), imaging issues (i.e., poor visualization of stone on fluoroscopy or ultrasonography), and technical aspects of endoscopic stone removal (i.e., difficulty using conventional length instruments and in positioning instruments properly for effective stone removal).

Several studies have evaluated the impact of body mass index (BMI) on PCNL outcomes, but in the majority of these studies, PCNL has been performed in the prone position. ⁶ Very few studies have looked into the effect of BMI on the outcomes or complications of PCNL in the supine position. The purpose of the current study was to evaluate the impact of obesity on the outcomes of complete supine PCNL in a single tertiary center. To the best of our knowledge, this is the first study on this topic.

Patients and Methods

We retrospectively evaluated our prospectively collected database of all patients who underwent PCNL from June 2009 to June 2014 at our center. For this study, we selected only adult patients who underwent PCNL in the complete supine decubitus position. The patients were stratified into two groups according to their BMI at the time of surgery: <30 kg/m² (group 1, nonobese) and ≥30 kg/m² (group 2, obese). All patients underwent noncontrast CT at least 6 months before and on the first postoperative day (POD1). Preoperative data, namely age, sex, and BMI, were also recorded. Guy's stone score (GSS) was used to classify stone complexity. ⁷

The following intraoperative and postoperative data were collected: operative time (from cystoscopy until kidney drainage), fluoroscopy time, transfusion rate (intra and until discharge), tubeless approach (yes/no), nephrostomy indwelling time, complication rate, and length of hospital stay. The immediate success rate was defined as no residual fragments >4 mm on the CT scans at POD1. Complications were classified according to the Clavien-modified system ⁸ ; complications scored at ≥3 points or higher were considered major complications.

Patients with sterile preoperative urinalysis received oral 100 mg nitrofurantoin b.i.d. for 7 days before surgery and third-generation cephalosporin during the induction of anesthesia, whereas patients with a history of positive urine culture were given oral culture-specific antibiotics (for 7 days before surgery and also during induction of anesthesia). All patients received general anesthesia during the procedure.

The surgical technique was similar for all cases. Patients were positioned in a completely supine position, as previously described. ⁹ Briefly, the edge of the body was placed just outside the border of the operating table, and the ipsilateral arm was positioned hanging over the thorax. No pads were used. In some cases, to facilitate the puncture, adhesive tape was used to draw abdominal fat tissue to the contralateral side. Initially, cystoscopy was performed at the top of a 6F open-ended ureteral catheter in the upper ureter or kidney. Subsequently, retrograde pyelography and puncture were performed under fluoroscopic guidance. The puncture was achieved using an 18-gauge needle guided by fluoroscopy. A hydrophilic guidewire was inserted and passed into the ureter.

In cases where multiple tracts were planned, all punctures and guidewire placements were performed before tract dilation. Tract dilation was accomplished with sequential fascial dilators, and a 30F Amplatz sheath was used to perform the procedure. The stones were broken using an ultrasonic lithotripter. An 18F nephrostomy tube was placed at the end of the procedure in cases of bleeding, residual stones, renal pelvis perforation, or multiple accesses; in the absence of these findings, a Double-J stent was placed and left indwelling for 2 weeks. Operation time was recorded from the beginning of cystoscopy to the end of nephrostomy tube placement or stent placement.

Statistical analysis was conducted using SPSS^® version 20 (SPSS, Inc., Chicago, IL). Student’ s t-test and chi-square tests were used to compare continuous and categorical variables, respectively, between the groups. Statistical significance was set at p-value of <0.05.

Results

In all, 401 patients met the inclusion criteria of the study. Group 1 (nonobese) comprised 307 patients (76.5% of the total cohort), and group 2 (obese) 94 patients (23.5% of the total cohort). The demographic characteristics are shown in Table 1. Group 2 (BMI ≥30 kg/m²) had a higher percentage of female patients (67.3% vs 50.4%, p = 0.002), higher mean BMI (34.8 vs 24.5 kg/m², p < 0.001), and a higher proportion of patients with American Society of Anesthesiologists (ASA) Classification of 2 or greater (77.7% vs 56.4%, p < 0.001) than group 1.

The intraoperative parameters and success rates are described in Table 2. There were no statistically significant differences in surgical success rate, defined as no residual stones >4 mm on POD1 CT scan (group 1 = 61.8% vs group 2 = 51%, p = 0.08). There were no differences in the transfusion rate (group 1 = 2.9%, group 2 = 4.2%, p = NS), total number of complications greater than Clavien grade 1 (group 1 = 13.6%, group 2 = 13.8%, p = NS), and length of hospital stay between the groups (group 1 = 64.6 hours, group 2 = 52.4 hours, p = NS).

There were no differences in overall or major surgical complications between the groups (Table 3). The major complications in group 1 (nonobese) were as follows: two cases of collecting-system perforation with prolonged nephrostomy drainage and one case each of colon perforation and splenic injury, both of which were successfully managed conservatively without additional intervention. Five patients developed septic shock, and one patient died of septic shock. The major complications in group 2 (obese) were as follows: one case of pleural effusion that was managed conservatively without additional intervention, and one case of septic shock that resulted in death.

Discussion

Obesity poses several problems in the management of stone disease, from diagnosis and imaging to anesthesia and surgery, for example, respiratory compromise would be a significant risk in general anesthesia, ventilation pressure may need to be high, and the risk of general postoperative complications (e.g., wound infections, atelectasis, and thromboembolism) would be high. ¹⁰ Extracorporeal shockwave lithotripsy has lower stone-free rates in obese patients than in nonobese patients due to table weight restrictions, difficult stone localization, and increased skin-to-stone distance. ¹¹ Flexible ureterorenoscopy and laser fragmentation represent a good treatment option for obese patients, but the large size of stones leads to prolonged operation times, secondary procedures, and anesthetic risk in this population. ¹⁰

PCNL remains the gold standard for patients with large renal stones. ^2,3 The impact of obesity on PCNL has been studied in patients undergoing PCNL in the prone position. Studies have shown that prone PCNL in normal-weight, obese, and superobese individuals had similar outcomes. ^12,13 The CROES Percutaneous Nephrolithotomy Global Study ¹⁴ reported longer operation time, inferior stone-free rate, and a higher reintervention rate in obese patients than in nonobese patients. However, this study did not standardize the PCNL technique. Standard prone positioning presents challenges for maintaining a safe airway in overweight patients, especially when rotating the patient, and may also impair venous blood flow. ¹⁵

Over the last 20 years, the supine position for PCNL has gained acceptance and popularity as an alternative to the standard historical prone position. ¹⁶ One main advantage of performing PCNL in the supine position for obese patients is that surgery can be performed without turning the patient prone–this may prevent positioning difficulties associated with the prone position and help protect the airway. Supine PCNL offers a safe alternative for patients with compromised cardiorespiratory function and morbidly obese patients. ¹⁷ Multiple studies have shown the safety and effectiveness of PCNL in the supine position. ¹⁶

There are very few comparative studies on obese patients that underwent supine and prone PCNL. A retrospective study comparing the outcomes between prone and total supine position PCNL in 42 obese patients showed similar final stone-free (83.3% vs 78.1%; p = 0.74) and complication rates (33.3% vs 31.3%; p = 0.77), but significantly shorter operative time (p = 0.0017), lower transfusion rates (p = 0.01), and shorter hospital stay (p = 0.014) in the supine position group than in the prone position group. ¹⁸ In addition, very few studies looked at the efficacy of PCNL in overweight and obese patients. Mazzucchi et al. ¹⁸ evaluated 30 obese patients who underwent supine PCNL and showed an immediate success rate, based on immediate CT scan, of 68.8%, and final stone-free rate of 78.1%. Manohar et al. ¹⁹ performed supine PCNL in high-risk and morbidly obese patients and observed a high stone-free rate of 95% and few intraoperative or postoperative complications. However, only 11% of the patients in this retrospective, nonrandomized study had a BMI of >30 kg/m².

To the best of our knowledge, this is the first study to evaluate the impact of obesity on the outcome of PCNL performed in complete supine position. We compared a large number of patients (∼400) using a standardized surgical technique and imaging regimen that included pre- and postoperative CT scans. In our study, the immediate success rate (i.e., no residual stones >4 mm on POD1) was slightly higher in the nonobese group than in the obese group, but this difference was not statistically significant. Stone size of 4 mm or smaller was used as a parameter to determine immediate success because it has been found to be a cost-effective threshold in the management of patients with residual fragments after PCNL, in prior studies. ²⁰ We used CT scan on POD1 to ensure the highest level of imaging accuracy, but if CT scans were obtained at a later time point (e.g., 3 months postoperatively), the stone-free rates could have been high due to the spontaneous elimination of small residual fragments. Moreover, the high number of complex stones in our series (∼45% GSSs of 3 and 4 for the entire cohort) is consistent with the observed stone-free rates and complication rates for our patients. ⁹

In the current study, there was no significant difference in the number of major or total complications between the groups (p = 0.6 and p = 0.8, respectively). One nonobese patient (0.2%) had extraperitoneal colon perforation with intraoperative diagnosis and was managed conservatively with separate drainage of the urinary and gastrointestinal tracts with satisfactory outcomes. We also observed one case of transsplenic puncture (0.2%) that was successfully managed by strict immobilization and maintenance of the nephrostomy tube for 7 days to tamponade the injury site. The tube was removed uneventfully in the operation room for safety. Collecting system injury was reported in up to 5.2% of the cases. ²¹ We noted two cases (0.4%) of collecting-system perforation treated with prolonged nephrostomy drainage (around 7 days). One case of death from sepsis was reported in each group. These complications are well within the previously reported incidences of complications for PCNL, especially in a population with a high percentage of complex stones.

Several weaknesses of the current study need to be acknowledged. A single-institution, retrospective study is subject to inherent bias. However, our study sample is much larger compared with any prior publication on this topic, and the strict use of CT imaging adds weight to our findings. Due to the lack of evidence in the medical literature, future studies are required to characterize the stone-related and postoperative outcomes associated with modified positioning for patients with obesity.

In conclusion, our study suggests that obesity does not affect the outcomes and postoperative complications in patients who undergo a complete supine PCNL. This technique can be used safely in patients with a high BMI. Multicentric studies with a robust casuistic analysis involving obese patients operated in the supine position are needed to elucidate the role of supine PCNL in these patients.