***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
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    Input: BACKGROUND AND PURPOSE The ureteral access sheath has positively impacted ureteroscopy by decreasing operative times and increasing success rates . However , as previous studies have suggested that large-caliber endoscopes may cause ureteral ischemia , concern has been raised about the impact of access sheath insertion on ureteral blood flow . We sought to determine whether the access sheath compromises ureteral blood supply and , if so , causes ischemic damage to the ureter during ureteroscopic procedures . MATERIAL S AND METHODS Using a swine animal model , ureteral blood flow was measured with a laser Doppler flowmeter . Eleven ureteral units were r and omized into four study groups : those dilated with 10F-12F , 12F-14F , and 14F-16F access sheaths  ( N = 3 per group ) and an  undilated control group  ( N = 2 ) . Blood flow measurements were obtained from the proximal ureter via laser Doppler flowmetry for 70 minutes at 5-minute intervals . Hemodynamic variability was controlled for through intraoperative heart rate and oxygen saturation monitoring , as well as a second Doppler probe that was placed on the animal 's skin , from which readings were also taken every 5 minutes . Results were correlated with histopathologic findings . RESULTS The control group demonstrated little ureteral blood flow variability over the course of 70 minutes . The study groups that were dilated with sheaths , however , all showed a decrease in ureteral blood flow after access sheath insertion , with the flow in animals dilated with 12F-14F and 14F-16F sheaths dropping below 50 % of baseline . This initial drop in blood flow was followed by a gradual increase from nadir toward baseline values over the course of the study . On average , the 14F-16F group reached nadir more quickly and took longer to restore its ureteral blood flow . All animals remained hemodynamically stable throughout the study , showing only minimal variability in heart rate , oxygen saturation , and skin blood flow over the 70-minute experiment . Histologically , there was no evidence of ischemic damage in any of the study groups at 72 hours . CONCLUSIONS In this animal model , the access sheath does cause a transient decrease in ureteral blood flow . Nonetheless , compensatory mechanisms of the ureteral wall restore blood flow to near-baseline rates and preserve urothelial integrity , suggesting that use of the ureteral access sheath remains a safe adjunct to flexible ureteroscopy . Because the chronic effects of the access sheath have yet to be eluci date d , care must be taken in selecting an appropriate-size sheath for each individual case . Preventive measures may be available to help avoid sheath-related ureteral injury in those patients identified as high risk PURPOSE The use of ureteral access sheaths during ureteroscopy is common but there are sparse data on the safety and outcomes of ureteral access sheath use in children . We compared the outcomes of ureteroscopy with vs without a ureteral access sheath in children . MATERIAL S AND METHODS We retrospectively review ed all ureteroscopy procedures for urolithiasis in patients younger than 21 years at our hospital from 1999 to 2009 . The primary outcome was intraoperative complications . Secondary outcomes were postoperative hydronephrosis , emergency room visit/hospital readmission within 90 days , stone-free status and need for re-treatment . We analyzed associations of a ureteral access sheath with outcomes . RESULTS A total of 34 boys and 62 girls with a mean age of 13 years underwent ureteroscopy .  A ureteral access sheath was used in 40 of the 96 patients ( 42 % ) . The mean stone burden was 9.6 mm . Median followup was 11 months ( range 0.2 to 110 ) . Intraoperative complication occurred in 7 cases , including perforation/extravasation in 4 , a submucosal wire in 2 and stent migration in 1 . Intraoperative complications were more common when a sheath was used ( 15 % vs 2 % , adjusted OR 8.2 , 95 % CI 1.3 - 50.9 , p = 0.02 ) . Postoperative hydronephrosis was observed in 7 of 73 cases ( 10 % ) but it was not significantly more common when a sheath was used . No ureteral stricture was identified . Sheath use was not associated with postoperative telephone calls , emergency room visits or rehospitalization . Although the stone-free rate tended to be higher in cases without a sheath ( 78 % vs 59 % , p = 0.09 ) , this association was not significant in a multivariate model ( p = 0.6 ) . CONCLUSIONS Although intraoperative complications occur more commonly during ureteroscopy with a ureteral access sheath , no increase in longer term adverse effects were observed . Future prospect i ve studies of ureteral access sheath use in children with longer followup are warranted PURPOSE Ureteral access sheaths were initially developed to facilitate difficult ureteroscopic access . However , to our knowledge no formal evaluations have been performed to assess the routine use of ureteral access sheaths . Therefore , we prospect ively analyzed intraoperative time , symptomatic outcome , major complications , stone-free rate and overall costs related to the routine use of a new ureteral access sheath during st and ard ureteroscopic procedures . MATERIAL S AND METHODS Patients undergoing 6.5Fr semirigid or 7.5Fr flexible ureteroscopy were prospect ively r and omized to unaided ureteroscopy with no access sheath or ureteroscopy via a 12 - 14Fr ureteral access sheath .  Patients who required ureteral dilatation were r and omized to the ureteral access sheath used as a dilator or a st and ard 18Fr ureteral balloon dilator . Patients were evaluated postoperatively on days 0 , 1 and 6 with a question naire to assess pain , irritative symptoms and complications . The stone-free rate and long-term complications were determined by excretory urography or computerized tomography at 3 months . RESULTS Enrolled in the study were 59 consecutive patients , who underwent a total of 62 ureteroscopic procedures . Of the 47 patients ( 76 % ) who did not require ureteral dilatation 23 ( 49 % ) underwent ureteroscopy via the ureteral access sheath and 24 ( 51 % ) underwent unaided ureteroscopy . Seven of the 15 patients ( 28 % ) who required ureteral dilatation underwent access sheath dilatation , while balloon dilatation was performed in 8 . There was no significant difference in postoperative symptoms , complication rate or stone-free status in the access sheath and nonaccess sheath groups in patients not requiring ureteral dilatation ( p < 0.05 ) . A significant increase in postoperative symptoms was noted when the balloon was used as a dilator compared to the access sheath . Operative time and costs in all patients who underwent access sheath dilatation were less than in those in whom the access sheath was not used . In the 15 patients who required dilatation 71 % of access sheath and 100 % of balloon dilatations were successful . CONCLUSIONS Routine use of a ureteral access sheath appears to facilitate semirigid and flexible ureteroscopy by decreasing operative time and costs , allowing direct visualization of ureteroscope insertion with simple ureteral re-entry and assisting renal and ureteral access with minimal associated morbidity . A ureteral access sheath should be considered for routine ureteroscopic procedures Purpose To describe the differences in the treatment and the outcomes of renal stones treated with flexible ureteroscopy ( URS ) either with or without the support of a ureteral access sheath ( UAS ) . Methods The Clinical Research Office of the Endourological Society URS Global Study involved the collection of prospect i ve data from consecutive patients treated with URS at centers around the world over a 1-year period . Baseline characteristics , stone location , treatment details , postoperative outcomes and complications were recorded . Inverse-probability-weighted regression adjustment ( IPWRA ) analyses were conducted on outcome from patients treated with or without the use of a UAS to determine the impact on stone-free rates ( SFRs ) . Results Of 2239 patients treated with flexible URS , 1494 ( 67 % ) patients were treated with the use of a UAS and 745 ( 33 % ) without a UAS . The IPWRA analyses conducted on 1827 patients with complete data and based on treatment and outcome models showed that if URS procedures were performed without the use of an UAS , the average stone-free rate would be 0.504 compared with 0.753 with a UAS . This average treatment effect of 0.248 was not significant ( P = 0.604 ) . Using IPWRA analysis on only the treated population in the estimations revealed no significant difference between using and not using a UAS ( 31 % ; ATET : 0.311 ; P = 0.523 ) . Conclusions The study showed no difference in SFR when a UAS was used or not . Whereas UAS did not increase the risk of ureteral damage or bleeding , postoperative infectious complications were reduced PURPOSE The safety of using a ureteral access sheath during retro grade intrarenal surgery remains controversial . Using a novel classification , we prospect ively evaluated the incidence and severity of ureteral access sheath driven ureteral wall injury after flexible ureteroscopy for retro grade intrarenal surgery . MATERIAL S AND METHODS Data on a total of 359 consecutive patients who underwent retro grade intrarenal surgery for kidney stone were prospect ively collected at 2 academic centers . We propose what is to our knowledge a novel endoscopic classification of iatrogenic ureteral wall injury . Ureteral injuries after retro grade intrarenal surgery were assessed visually with a digital flexible ureterorenoscope . The primary outcome measure was the incidence and nature of ureteral injuries . We sought factors predisposing to such injuries . RESULTS Ureteral wall injury was found in 167 patients ( 46.5 % ) . Severe injury involving the smooth muscle layers was observed in 48 patients ( 13.3 % ) . Males vs females ( p = 0.024 ) and older vs younger patients ( p = 0.018 ) were at higher risk for severe ureteral access sheath related ureteral injury . The most significant predictor of severe injury was absent ureteral Double-J ® stenting before retro grade intrarenal surgery ( p < 0.0001 ) . Pre-stenting vs no pre-stenting  decreased the risk of severe injury by sevenfold . Body mass index , a history of diabetes mellitus , vascular disease or abdominopelvic radiation therapy and operative time were not associated with severe ureteral injury . CONCLUSIONS Ureteral access sheath use for retro grade intrarenal surgery should involve systematic visual assessment of the entire ureter to recognize severe ureteral injury . The incidence of severe ureteral injury is largely decreased by preoperative Double-J stenting We present our experience with the use of the ureteral access sheath for the management of  small impacted lower third ureteral stones  , in comparison with more st and ard techniques . Ninety-eight consecutive patients , aged 18–73 years ( mean 48.5 ) , with small ( diameter < or = 10 mm ) impacted lower third ureteral stones ( < 5 mm in 56 , and 5–10 mm in 42 patients ) were r and omly managed with either a 12/14F coaxial ureteral dilator/sheath and a 7.5F flexible ureteroscope ( group A ; 48 patients ) , or with balloon dilatation and the 7.5F flexible ureteroscope ( group B ; 50 patients ) . In both groups , stones were grasped and extracted with a basket , and when necessary they were disintegrated with a 1.9F electrohydraulic lithotripsy ( EHL ) probe . Postoperatively , excretory urography was performed at 1 month and patients were followed-up for 1 year . The mean operative time was 45.5 min in group A , and 58.5 min in group B ( P<0.05 ) . EHL was performed in 16 ( 33.3 % ) patients of group A , and in 12 ( 24 % ) patients of group B. In group B ,  balloon dilatation  was performed in 28 ( 56 % ) patients . Ureteral perforation was revealed in 4 ( 8 % ) patients of group B. The follow-up imaging tests showed stone-free status in 46 ( 95.8 % ) patients of group A and in all ( 100 % ) patients of group B. No long-term complications were recorded . Endoscopic management of small impacted lower third ureteral stones with the ureteral access sheath is a quicker and safer procedure , in comparison with the more st and ard approach , bearing comparable efficacy 
    Output:
    
Evidence from meta- analysis indicated that the use of UAS during ureteroscopy did not manifest advantages .