Since being first introduced in the 1980s extracorporeal shock wave lithotripsy (SWL) has become the treatment of choice for renal and ureteral stones up to 20 mm. It is relatively non-invasive, and has a reasonable efficacy in treating stones in all locations.
Several studies have shown that treatment of lower pole stones (LP) stones has a lower stone free rate (SFR) than other renal or ureteral stones due to the LP anatomy.
In this study, we looked at the outcomes for SWL in both lower pole (LP) and non-lower pole (NLP) stones during the same time period to evaluate if there was a difference in SFR after treatment.
MATERIALS AND METHODS
Between January 2012 and August 2013, 148 patients were treated for renal and ureteral stones using a mobile piezoelectric lithotripter (Wolf Piezolith 3000). Data were collected retrospectively on all patients who underwent SWL in this time period with stone size ranging from ≥4 mm to <20 mm. All patients had IVU or non-contrast CT (NCCT) for diagnosis of stones and plain X-Ray (if radio opaque stone) or USS for follow-up.
Pre-operatively, patients were given a combination of diclofenac 100 mg per rectum, intramuscular pethidine (100 mg/2 ml) and intramuscular metoclopromide (10 mg/2 ml). The maximum number of shocks administered within each operation was limited to 4000 (mean shock numbers were: Lower pole: 3015; non-lower pole: 3140). Post-operatively all patients were given a 7 day course of trimethoprim and oral ibuprofen as pain relief if required.
Multiple variables including patient characteristics such as age, sex and number of procedures, procedural characteristics such as number of shocks and energy of shocks, stone characteristics such as size, location and number, and treatment characteristics such as SFR, stone fragmentation rate and complications were recorded. Following treatment, stone fragmentation was defined as evidence of fragmentation on follow-up imaging and stone free rate (SFR) was defined as stone fragments ≤3 mm on X-Ray KUB or USS.
Stones were classed as lower pole (LP) if they were exclusively within the LP. Stones in all other locations within the kidney and ureter were classed as non-lower pole (NLP). Stones were also called as NLP in case of multiple stones one of which was in LP. This included stones in the upper pole, mid pole, renal pelvis, pelviureteral junction (PUJ), mid ureter or upper ureter.
Data were collated using Microsoft excel. Data analysis was performed using SPSS version 21 and P values were achieved using the χ2test.
The mean age of the patients for LP and NLP was 54 years (30-86 years) and 52 years (30-80 years), respectively [Table 1]. Male to Female ratios for LP and NLP were 60:33 and 60:48, respectively. A total of 148 patients underwent 201 procedures of which 93 (46%) were for LP stones and 108 (54%) were for NLP stones. Multiple stones were seen in 68/201 (34%). The non-lower pole stones included upper pole (n = 36), mid pole (n = 40), renal pelvis (n = 10), PUJ (n = 8), mid ureter (n = 3), upper ureter (n = 5) and a combination of upper, middle and/or lower pole (n = 6). The mean stone size was 7.4 mm (4-16) for LP stones and 8 mm (4-17) for NLP stones. The majority of stones were left sided for both LP and NLP with right: Left ratios being 41:51 for LP stones and 41:64 for NLP stones. Each side was treated and counted individually.
The mean number of procedures each patient received for LP and NLP stones was 1.6 and 1.3 respectively [Table 2]. The average number of shocks patients received for LP and NLP stones were 3015 (2000-3500) and 3140 (2000-4000), respectively. The average energies for LP and NLP were 17.3 and 16.7 kV, respectively.
Although the stone fragmentation rates for LP and NLP stones were 61% and 63% respectively the SFR after treatment completion was 25% (23/93) for LP stones and 40% (43/108) for NLP stones. This was statistically significant with a P value of P = 0.023.
There were three complications treating the LP stones, all of which were renal colic and there were six complications from the NLP stone treatment; five patients had renal colic and one patient suffered from abdominal pain. There were no other major or minor complications.
Shock wave lithotripsy (SWL) is the treatment of choice for renal and ureteral stones up to 20 mm as it is non-invasive and has a high efficacy. Our stone fragmentation rates are comparable to other studies using the same lithotripter. Our average stone free rate was 32.8%, which is slightly less than the 36% and 45% achieved by Ng et al. and Wang et al., respectively.
A study by Öbek et al. showed that there was no significant difference in efficacy of treating LP stones compared to middle and upper pole stones but there was reduced SFR for LP stones in comparison to middle and upper pole stones. This is comparable to our study that shows no statistically significant difference between stone fragmentation rates of LP vs NLP stones but does show a statistically significant difference in SFR's (P < 0.05).
This disparity in SFR's is, as several studies have shown, due to the anatomy of the lower pole. A study by Sampaio showed that the position of lower pole stones impedes gravity dependent drainage of fragments after SWL. Other studies have shown that a shorter infundibular length (<22 mm) as well as larger stone size (>10 mm) and burden can decrease stone free rates.
Despite having lower SFR's than percutaneous nephrolithotomy (PCNL) and flexible ureterorenoscopy (fURS), SWL remains the treatment of choice for some stones as hospital stay, complication rate and duration of treatment are lower in comparison. Although our study is a retrospective study, the comparison of outcomes between LP and NLP stones is based on treatment carried out by the same team using the same lithotripter in the same time period minimizing any potential for bias.
The other issue identified in this paper is the disparity in the definition of SFR. Several papers define stone free varying from no evidence of stones either by radiological imaging or ultrasonography to residual stones up to 4 mm in size. We have defined stone free as fragments ≤3 mm after a 3 month follow-up. Clarification of stone free rate and the size of clinically insignificant fragments is needed when comparing results from different studies. A consensus should be achieved on a standard definition of levels of stone free rate.
SWL is generally considered a satisfactory treatment for renal and ureteral stones with a moderately high stone fragmentation rate and low complication rate. However the SFR depends on how it is defined and is statistically significantly reduced in patients with LP stones as opposed to NLP stones.
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Source of Support: Nil
Conflict of Interest: None.