Ultrasound-guided percutaneous nephrolithotomy (PCNL) success rates in patients with elevated body mass index: a comparative study

Percutaneous nephrolithotomy (PCNL) is considered gold standard treatment of renal stones larger than 20 mm. Several studies have shown that ultrasound guidance during this procedure is more effective and safer than fluoroscopy. A higher body mass index (BMI) can make ultrasound-guided renal access more difficult and unsuccessful. We present a prospective analysis and comparison of ultrasound-guided PCNL in patients with normal and increased body mass index. We performed a prospective comparison of patients who underwent ultrasound-guided PCNL to remove renal stones by a single surgeon between 2020 and 2022. Patients with BMIs greater than 30 (mean 33.87—obese) were compared to those with BMIs less than 30 (mean 25.69—non-obese). Demographic, perioperative, and follow-up data were collected, analyzed, and included in this study. Total of 98 consecutive patients, with 49 patients in each group were analyzed. No statistically significant differences were observed in terms of stone volume (P = 0.085), stone density (P = 0.5590), location of renal access (P = 0.108), surgery duration (P = 0.38), blood loss (P = 0.54), or laboratory changes after surgery (P = 0.60). 87.76% of obese patients were stone free per CT scan at follow-up, compared to 73.47% of normal-weight patients (P = 0.1238). According to Clavien–Dindo classification, six patients in the non-obese group experienced grade II (10%) and grade III (2%) complications, as opposed to six patients in the obese group with grade I (2%), grade II (6%), and grade III (2%) complications. There was no significant correlation between body mass index and the success or safety of ultrasound-guided PCNL. Although more challenging, a higher BMI should not be an impediment to performing this approach. This method is safe, with no increased incidence of postoperative complications or compromise in stone-free status postoperatively and can diminish or avoid both patient’s and medical team’s exposure to ionizing radiation.


Introduction
The prevalence of obesity is on the rise worldwide, and it has been firmly linked to the occurrence of urolithiasis.This link has been documented repeatedly, with a prevalence of obesity ranging from 10 to 35% in patients with kidney stone disease [1][2][3] and its association with increased urinary excretion of stone-forming solutes such as calcium, oxalate, and sodium and with a low urine pH [4].
Percutaneous nephrolithotomy (PCNL) has been the mainstay surgical treatment for large renal calculi (> 20 mm) [5] since it was first described in the 1970s [6], as well as for large lower calyceal stones, calyceal diverticular stones, and following a failure of less invasive approaches [7].
The Clinical Research Office of the Endourologic Society collected data from 5,803 patients treated with PCNL and found it to be a safe approach in obese patients, but with longer operative times and lower stone-free rates (SFR) than in the non-obese population [8].
Gaining access to the pelvicalyceal collecting system, usually by fluoroscopy or ultrasound (US), is a vital step in performing PCNL.An accurate puncture to the desired calyx will ensure a higher SFR while reducing intraoperative and postoperative complications.Fluoroscopy-guided renal access in PCNL is the most reported imaging modality worldwide [9].Nonetheless, it may expose the patient and the surgical team to harmfully high doses of ionizing radiation over time [10].In addition, fluoroscopy does not typically allow the detection of surrounding vital organs 111 Page 2 of 7 during renal puncture, hence increasing the risk of inadvertent harm [11].Alternative methods, such as US-guided renal access, have been studied and applied to overcome these drawbacks.US guidance provides a secure method for PCNL access and has demonstrated advantages in efficacy, safety, and feasibility for upper urinary tract stone treatment [6], as well as effectively reducing or completely avoiding ionizing radiation exposure [12].However, US use can be challenging as BMI increases and thought to be limited due to concerns of poorer image quality and difficulties with probe placement [13].
The primary goal of this study was to determine whether an increased BMI affects perioperative outcomes, radiation exposure, success rate, and safety of US-guided PCNL.

Study population
Following institutional review board (IRB) approval, we conducted a prospective comparative analysis of patients who underwent US-guided PCNL by a single surgeon (IK) between 2020 and 2022.
The patients were categorized into two groups according to their BMI (mass (kg).height 2 (m) = kg/m 2 ): those with a BMI > 30 kg/m 2 (group I) and those with BMI < 30 kg/m 2 (group II).
Adult patients (≥ 18 years old) were eligible if PCNL was considered the best therapy choice.Patients with uncorrected coagulopathy, congenital renal abnormalities, pregnancy, transplanted kidneys, or current infection were all excluded.
We gathered patient demographic data: age, sex, body mass index (BMI), stone size and location, perioperative data, and follow-up clinic visits following discharge, stone-free status by CT scan following discharge was also documented.
All patients underwent the same preoperative evaluation: history and physical examination, routine laboratory investigation (blood chemistry, complete blood count, coagulation panel, urine culture, and sensitivity), and anesthesia assessment.In addition, a non-contrast low-dose CT scan of the abdomen was performed in all patients prior to surgery.Stone location was evaluated as calyceal, renal pelvic, ureteropelvic, or multiple locations.The puncture site was reported as a lower, middle, or upper calyceal, and/or multiple.

Surgical procedure
Under general anesthesia, a 7Fr open-ended ureteral catheter is inserted through flexible cystoscopy into the ipsilateral ureter in a complete supine position; female patients are positioned with the contralateral leg in stirrups to facilitate urethral access.Fluoroscopy is available and used if needed.Renal ultrasound determines the calyceal, renal, and peri-renal anatomy, stone position, and best puncture site.To gain access to the kidney, an 18G Chiba echo-tip needle is used, and a guidewire is inserted into the collecting system under ultrasound guidance.Next, skin is incised, and an 8-10 Fr facial dilator (Boston Scientific) is advanced under the US over the guidewire to advance a second safety wire; this guide wire attempts to pass down the ureter into the urinary bladder if possible.The tract is dilated with an access sheath, either a metallic (Stortz Medical) one-step dilator or balloon dilator (Boston Scientific), depending on the tract size used (for tracts larger than 16FR, balloon dilation is used), also under US guidance.In case additional accesses are needed, the tract is usually obtained before advancing the first access sheath.
Stone fragmentation is performed using an ultrasonic lithotripter (Olympus Medical) (tracts > 16 Fr) or laser energy (Lumenis) (tracts < 16 Fr).Flexible nephroscopy is usually performed at the end of the procedure to assess the residual stone fragments [14] , and a double J stent (Boston Scientific) is introduced retrogradely at the conclusion of the procedure.In circumstances when serious bleeding, perforation of the collecting system, or infection of the system is likely, a nephrostomy tube is left in place.A 14Fr Foley catheter is left in the bladder.The access sheath is removed, and the incision is closed at skin level using histoacryl glue.In patients requiring contralateral ureteroscopy or retrograde intrarenal surgery (RIRS) during the same session for contralateral stone disease, fluoroscopy is used to advance a ureteral access sheath, and the total fluoroscopic time is recorded.
After the procedure, a complete blood count and blood chemistry are drawn in POD-1, the catheter is removed, and the patient is discharged if clinically amenable.After 5-7 days, the tethered stent is routinely removed in the clinic.

Follow-up
We determined SFR at the postoperative follow-up visit to avoid false positive results of stone fragments, with an average of 60 days after surgery [15].Patients were classified as stone free (any remaining stone ≤ 4 mm) or not stone free (any remaining stone > 4 mm) based on a new low-dose non-contrast CT scan of the abdomen.All postoperative complications were recorded within 30 days after the surgery and categorized according to the Clavien-Dindo classification system.

Statistical analysis
Continuous variables were expressed as the mean ± standard deviation when customarily distributed or as a median and interquartile range for skew distribution data-multivariate analyses by Mann-Whitney test (T test).Fisher's exact test was used to compare SFR between the groups.Statistical analysis was performed using Prism by GraphPad Software version 9.0.1;P< 0.05 was considered statistically significant.

Results
A total of 98 consecutive patients who underwent USguided PCNL were included.Each group consisted of 49 patients.Detailed demographic and patient characteristics are presented in Table 1.In brief, stone diameter (P = 0.084), stone density in Hounsfield units (P = 0.059), and duration of surgery (P = 0.388) were not found to have a statistical difference between groups.The operative data are shown in Table 2.There were no statistical differences between groups in postoperative blood loss or serum creatinine levels.A tubeless procedure although not statistically significant was more frequent in both groups (93.8% vs. 96%).Complication rates are shown in Table 3.There were no differences in complication rates between the two groups.
Fluoroscopy was used in both groups.Four patients in group I required fluoroscopy to guide double J stent insertion at the conclusion of the procedure, and three patients underwent contralateral flexible ureteroscopy.In group II, fluoroscopy was used to guide double J stent insertion at the conclusion of the procedure in 17 patients; in 3 of them, it was used concurrently with flexible ureteroscopy on the contralateral kidney during the same surgical session.
There was no statistically significant difference in SFR between the two groups (P = 0.1238).The number of procedures to render patients stone free did not differ between groups, summarized in Table 4. Stone composition (when available) is shown in Fig. 1.

Discussion
Obesity prevalence in children and adults worldwide has risen significantly, reaching epidemic proportions.Over 50% of the global population is believed to be overweight or obese [16], and there is a rise in the incidence of urolithiasis as a function of body weight in both sexes [17].Accepted modalities in renal stone treatment, such as extracorporeal shock-wave lithotripsy (ESWL), retrograde intrarenal surgery (RIRS (, and PCNL, are widely available but can be challenging in obese patients.Utilization of ESWL is limited due to large skin-to-stone distance and difficulty in stone localization both by fluoroscopy and by US.While, the patient positioning required for RIRS is problematic in the obese patient and can cause respiratory restriction [18].PCNL is the gold standard for kidney stones > 2 cm, and mainly performed with fluoroscopy.Although the use of US in PCNL is gaining popularity, it can be technically challenging in obese patients due to poor image quality acquisition, making it difficult to identify critical anatomic landmarks [19,20].
The recurrence rate of kidney stones is dismally high in the obese population.Rule et al. reported recurrence rates of 11%, 20%, 31%, and 39%, at 2, 5, 10, and 15 years, respectively [21].Furthermore, Zeng et al. discovered that 43.8% of stone recurrences were in people who were overweight or obese [22].This high incidence of recurrence can lead to repeated surgeries over time, increasing patient's exposure to radiation and fluoroscopy complications.US utilization diminishes or even eliminates this exposure.
As mentioned before, the puncture site is of particular importance.Ng et al. [11] reported that US-guided PCNL puncture sites in normal-weight individuals are mainly at the lower calyx in 81.9% of cases, lowering the risk of pleural injury; in our study, a lower calyceal puncture was performed in almost 90% of patients in group II but only in 59% of patients in group I (37% middle calyceal, 4% upper calyceal).We believe this might be attributed to extensive peri-renal fat or adjacent organ enlargement, which may cause the kidney to be positioned anatomically lower than in non-obese individuals giving the advantage of having more options to freely perform a puncture in any calyx, thus improving stone clearance without increasing complications.
Due to physiological and anatomical changes caused by obesity, anesthesia-related problems can arise, including cardiopulmonary changes such as preload reduction and impaired venous blood flow due to vena cava compression as well as decreased total lung capacity and functional residual capacity [8].Placing patients in a supine position can overcome this problem.Mazzucchi et al. [23] compared supine and prone positioning for PCNL in obese patients.Surgical outcomes were similar between the two groups, except for shorter surgical time and hospital stay in the supine group.Manohar et al. [24] examined supine positioning in 62 individuals, showing an SFR of 95%, with seldom intraoperative or postoperative complications.In our series, both groups were only positioned in the supine position.There were no differences in stone-related or perioperative outcomes.Another benefit of the supine technique is the ability to perform flexible ureteroscopy on the ipsilateral or opposite kidney during the same session.Three patients in each group underwent contralateral RIRS without significantly affecting surgery duration or postoperative complications.
Research has indicated that US-PCNL is as safe as fluoroscopy-guided PCNL in obese patients, but the procedure time is longer, and the stone-free rate is lower compared to individuals of normal weight [8].Other studies have linked obesity to an increased risk of complications like bleeding and infection, as well as higher rates of pain, longer hospital stays, and higher hospital costs [25,26].Despite the aforementioned, some studies have shown that kidney stone surgery success rates in obese patients are comparable to those in non-obese patients [27,28].Through our research, we observed the efficacy and safety of PCNL in both obese and normal-weight patients.Fuller et al. [8] and Simsek et al. [29] retrospectively reported longer mean operative times in obese patients undergoing PCNL when compared to non-obese patients, we did not find a significant difference between groups (P = 0.388).The reported complication rates for PCNL range from 3 to 18%.[30].Similar rates of complications were seen for both groups in our series (12.2%), and although we reported the development of pseudoaneurysm in one patient of each group, they were successfully treated without the need for blood transfusion, which has been reported in 3% to 11.2% of cases [30].Factors influencing the risk of blood transfusion following PCNL include: operative technique, patient status, stone complexity, and number of punctures.In our case, those multivariates were similar among groups and thought to have had no impact on the outcomes.Our findings are consistent with previous studies: a higher BMI does not affect surgical outcomes such as operative time, length of hospitalization, or complication rate.
Basiri et al. compared patients undergoing US-PCNL vs. fluoroscopy-PCNL and found SFRs of 79% and 65%, respectively [31].Chi et al. presented a 7-year singlecenter experience with ultrasonography combined with fluoroscopy in PCNL with a 90.5% SFR [32].In contrast, Fuller et al. according to the CROES database showed a lower SFR in obese patients [8].Our results showed an SFR comparable with the published literature, with no statistically significant difference between groups.The number of auxiliary procedures required to render patients stone free was equal among the two groups in our study.The stone burden in group II was substantially higher, necessitating a second PCNL in two patients.
A few limitations of our study should be mentioned.Sample size is relatively small, there is relatively small number of patients who have a BMI < 25 kg/m 2 (underweight) or > 35 kg/m 2 (extreme obesity), to accurately represent the population of interest, all surgeries were performed by a single surgeon.Multi-center research involving surgeons with varying levels of experience may have a different impact on outcomes.The learning curve for successful US-guided PCNL, on the other hand, is influenced by case number.Furthermore, we did not conduct a comparison between US and fluoroscopy in similar patients; however, previous reports have found no significant differences in both approaches [32].

Conclusions
Using ultrasonography to guide access during PCNL in obese patients has no discernible effect on the procedure's complication rate or outcomes.US -PCNL is a feasible, safe, and effective treatment for obese patients with large renal stones.It can significantly reduce radiation exposure to patients and medical staff, thereby preventing long-term damage.Further studies with larger patient populations are needed.