Outcomes of ESWL for Ureteral Stones According to Intensity

In This Article

Abstract and Introduction

Abstract

Background: We evaluated the treatment outcomes of ureteral stones according to energy intensity generated by extracorporeal shock wave lithotripsy (ESWL).

Methods: We retrospectively analyzed 150 patients who underwent ESWL for treatment of ureteral stones between September 2018 and February 2020. All stones were confirmed by a computed tomography examination, and the size, location, skin-to-stone distance, and Hounsfield units (HU) of the stones were assessed. In addition, patient characteristics including body mass index and estimated glomerular filtration rate, which can affect treatment outcome, were also evaluated. The success or failure of ESWL was confirmed according to the session, and the factors affecting the treatment outcome were analyzed using a logistic regression model.

Results: Of the 150 patients, 82 (54.7%) had stones in the proximal ureter, 5 (3.3%) in the mid, and 63 (42.0%) in the distal ureter. Patients underwent ESWL an average of 1.5 times, and the success rate according to session was 65.3% for the first, 83.3% for the second, and 90.0% for the third session. A multivariate analysis revealed that stone size [odds ratio (OR) 0.81, 95% confidence interval (CI), 0.66–0.99, P=0.049] and HU (OR 0.99, 95% CI, 0.98–0.99, P=0.001) were significant factors affecting the success rate after the first ESWL session; ESWL intensity was not related to success rate. Stone size (OR 0.78, 95% CI, 0.62–0.96, P=0.022) was the only significant factor affecting the success rate in the third session.

Conclusions: Stone size and HU affected the ESWL success rate. ESWL intensity was not significantly related to the success rate, so it should be adjusted according to patient pain and the degree of stone fragmentation.

Introduction

Extracorporeal shock wave lithotripsy (ESWL) has been used widely due to its relatively high efficacy and non-invasive nature for treating urinary stones.^[1] ESWL has a satisfactory treatment effect, particularly for ureteral stones <1 cm. ESWL can replace invasive treatment using a ureteroscope, and the European Association of Urology (EAU) guidelines suggest ESWL as the first treatment option for stones <1 cm.^[2,3]

The success rate of ESWL depends on various factors, such as stone (location, density, size, and components), renal anatomy (hydronephrosis, calyceal diverticulum, ureteral obstruction and stenosis, and urinary anomaly), and patient-related factors (skin-to-stone distance, obesity, and renal function).^[4] Also, various methods to improve the success rate of ESWL have been studied. A shock wave frequency of 60–90 shock waves/min improves the stone-free rate (SFR), and tissue damage decreases with a low shock wave frequency.^[5,6] Recent studies recommend increasing the power step-by-step, as this ramping technique improves stone fragmentation and reduces renal injury during ESWL.^[7–11] Additionally, medical expulsive therapy (MET) is efficient for relieving pain and the passage of a stone;^[12] however, the types of shock waves generated by other methods are not successful.^[9]

However, no study has investigated the final energy intensity of ESWL. A higher final energy intensity may generally induces a better SFR in clinical practice. Although a higher final energy intensity may lead to better performance, it can cause other side effects, such as pain and injury to the urinary tract. If there was no difference in the SFR according to intensity, we would be able to safely perform ESWL without increasing the energy intensity. Therefore, we performed this study to evaluate the treatment outcomes of ureteral stones according to ESWL energy intensity. We present the following article in accordance with the STROBE reporting checklist (available at https://dx.doi.org/10.21037/tau-20-1397).

Table 1. Shock wave parameters-intensity setting
Intensity levels	a	b	c	1	2	3
Focal pressure (MPa)	6.7	10.5	16	21	31.5	55
Disintegrating energy E (12 mm)	2.5	3.7	7.0	11.0	20.0	29.0
Energy flux density ED (mJ/mm²)	0.03	0.07	0.11	0.15	0.28	0.44
Max. pulse repetition frequency (RPF) shot/min	180	180	180	120	120	120

Table 2. Per procedure and outcome data
Variable	Value
No. patients	150
Age (years)	52.1±14.2 [19–87]
Sex
Male	104 (69.3)
Female	46 (30.7)
BMI (kg/m²)	23.3±4.5 (17.7–36.2)
Stone size (mm)	7.2±3.4 (3.0–37.0)
SSD (mm)	95.6±15.2 (65.3–134.7)
HU	673.6±267.9 (184–1,416)
Side
Right	73 (48.7)
Left	77 (51.3)
Location
Proximal	82 (54.7)
Mid	5 (3.3)
Distal	63 (42.0)
Preoperative Cr	0.93±0.23 (0.56–1.6)
Preoperative eGFR	92.5±51.2 (45.3–129.5)
Total No. ESWL	1.5±0.9 [1–8]
ESWL intensity
a–c	6 (4.0)
1	36 (24.0)
2	85 (56.7)
3	23 (15.3)
Pain during ESWL
None	77 (51.3)
Mild	69 (46.0)
Severe	4 (2.7)
SFR after each session
1st	98 (65.3)
2nd	125 (83.3)
3rd	135 (90.0)
Complications (Clavien-Dindo)
Grade 1	3 (2.0)
Voiding difficulty	1
Hematuria	2
Alpha blocker	120 (80.0)

Values are presented as mean ± SD (range) and/or frequency (%) appropriate. BMI, body mass index; SSD, skin-to-stone distance; HU, Hounsfield units; ESWL, extracorporeal shock wave lithotripsy.

Table 3. Logistic regression analysis for different factors affecting the success rate after the first ESWL session
Variable	Univariate		Multivariable*
Variable	OR (95% CI)	P value	OR (95% CI)	P value
Age (continuous)	0.99 (0.97–1.02)	0.563	–	–
Sex (female)	0.99 (0.48–2.06)	0.984	–	–
Body mass index (continuous)	0.99 (0.87–1.12)	0.878	–	–
Stone size (continuous)	0.67 (0.55–0.81)	0.001	0.81 (0.66–0.99)	0.049
SSD (continuous)	1.01 (0.98–1.03)	0.519	–	–
HU (continuous)	0.95 (0.98–0.99)	0.001	0.99 (0.98–0.99)	0.001
Ureteral stone location
Proximal	Ref
Mid	0.86 (0.14–5.47)	0.878
Distal	1.24 (0.62–2.48)	0.544
Preoperative eGFR (continuous)	0.99 (0.97–1.01)	0.988
ESWL intensity
a–c	Ref
1	1.50 (0.23–9.61)	0.669
2	0.79 (0.14–4.54)	0.790
3	0.94 (1.40–6.28)	0.947

*, significant variables (P<0.2) according to the univariate analysis were included in the multivariable analysis. SSD, skin-to-stone distance; HU, Hounsfield units; ESWL, extracorporeal shock wave lithotripsy.

Table 4. Logistic regression analysis for different factors affecting the success rate after the third ESWL session
Variable	Univariate		Multivariable*
Variable	OR (95% CI)	P value	OR (95% CI)	P value
Age (continuous)	1.01 (0.97–1.04)	0.815	–	–
Sex (female)	1.24 (0.37–4.13)	0.724	–	–
Body mass index (continuous)	1.12 (0.93–1.35)	0.241	–	–
Stone size (continuous)	0.75 (0.62–0.91)	0.004	0.78 (0.62–0.96)	0.022
SSD (continuous)	1.03 (0.99–1.07)	0.158	1.02 (0.97–1.07)	0.402
HU (continuous)	0.99 (0.98–0.99)	0.001	0.99 (0.99–1.00)	0.106
Ureteral stone location
Proximal	Ref
Mid	0.43 (0.04–4.35)	0.477
Distal	1.03 (0.34–3.12)	0.963
Preoperative eGFR (continuous)	1.01 (0.98–1.03)	0.607
ESWL intensity
a–c	Ref
1	1.60 (0.15–17.38)	0.699
2	1.69 (0.18–16.11)	0.649
3	4.40 (0.23–82.97)	0.323

Table 5. Stone characteristics according to surgical condition
Variable	URS	No URS	P value
No. patients	15	135	–
Stone size (mm)	10.5±8.2	6.8±2.2	0.001
SSD (mm)	88.5±21.3	96.2±14.5	0.124
HU	919.7±282.8	651.9±256.6	0.001
Location			0.381
Proximal	10 (66.7)	72 (53.3)
Mid	1 (6.7)	4 (3.0)
Distal	4 (26.6)	59 (43.7)

Values are presented as mean ± SD and/or frequency (%) appropriate. URS, ureteroscopy; SSD, skin-to-stone distance; HU, Hounsfield units; ESWL, extracorporeal shock wave lithotripsy.