Urolithiasis is common in dogs and cats. Untreated uroliths can result in persistent hematuria; stranguria; dysuria; recurrent urinary tract infections; and, potentially, urinary obstruction and postrenal azotemia. Surgery has been the time-honored method of removal of uroliths from dogs. Surgery carries the risks of perioperative hemorrhage, clot formation and obstruction of the urethra, intra-abdominal adhesions, dehiscence, and pain. During the past decade, the use of various types of lithotripsy has been considered the standard of care in human urology for the management of urolithiasis.1–3
Lithotripsy is crushing or fragmenting uroliths by use of shock waves or laser energy. Types of lithotripsy include ESWL, EHL, and laser lithotripsy. Extracorporeal SWL uses repeated shock waves generated outside the body to fragment uroliths into many small fragments, which can pass spontaneously through the urinary system. Extracorporeal shock wave lithotripsy has been described in veterinary patients for treatment of nephroliths and ureteroliths4–7; however, its use may be limited because of equipment cost and availability, urolith size, and patient size. Although EHL can be performed in large female dogs,4 EHL has been largely replaced by laser lithotripsy in human medicine because it is more effective and safer than EHL.1,8
Laser lithotripsy is an innovative technique involving the intracorporeal fragmentation of uroliths, which is achieved with a rigid or flexible cystoscope or ureteroscope. The first reports9,10 of laser lithotripsy using the Ho:YAG laser in humans were in 1995. In humans, laser lithotripsy by use of the Ho:YAG laser has become the procedure of choice for intracorporeal fragmentation of large cystic uroliths, ureteroliths, and nephroliths.1–3,11
An Ho:YAG laser is a solid-state pulsed laser that emits light at an infrared wavelength of 2,100 nm.3,12 The energy is transmitted down small-diameter flexible silica quartz optical fibers, making it well suited for endoscopic surgery. The mechanism of urolith fragmentation with the Ho:YAG laser is mainly photothermal and involves a thermal drilling process rather than a shock wave effect.13,14 The 2,100-nm wavelength of the Ho:YAG laser is absorbed in < 0.5 mm of fluid; therefore, it can be used to safely fragment uroliths within the urethra, ureter, or urinary bladder, with limited risk of urothelial damage.1–3 It combines both tissue cutting and coagulation properties as well as the ability to fragment uroliths upon contact.3
Laser lithotripsy has been reported in humans, horses, ruminants, and pigs.3,10,11,15–18 Results of a recent in vitro study19 confirm that the Ho:YAG laser can efficiently fragment canine uroliths of varied chemical composition. Likewise, implanted urethroliths in male dogs were safely fragmented by use of laser lithotripsy with an Ho:YAG laser.20 There are 2 preliminary reports21,a of laser lithotripsy of naturally occurring uroliths in dogs. The purpose of the retrospective case series reported here was to describe transurethral cystoscope–guided laser lithotripsy for the fragmentation and removal of cystic and urethral uroliths and to determine procedure duration and short-term and longterm outcomes in dogs.
Materials and Methods
Criteria for selection of cases—Dogs treated for uroliths of the lower portion of the urinary tract by use of laser lithotripsy from January 1, 2005, to June 30, 2006, at PUVTH (n = 37) and MJR-VHUP (36) were included in the study. Male dogs weighing < 5 kg (11 lb) were excluded from the study because prior cystoscopy experience revealed that male dogs < 5 kg are usually too small for retrograde passage of a 2.7-mm-diameter ureteroscope.
Laser lithotripsy procedure—Laser lithotripsy was performed with a 20-W Ho:YAG laser with a 350-microsecond pulse width.b Cystoscopy was performed with an appropriately sized rigid cystoscope (females) or flexible ureteroscopec-e (males) in anesthetized dogs. Anesthesia consisted of a narcotic premedication, induction with propofol, and maintenance with isoflurane inhalant anesthesia. Female dogs were positioned in dorsal recumbency, and male dogs were positioned in lateral recumbency. The cystoscope was passed retrograde transurethrally until the urolith could be seen. An appropriately sized, flexible silica quartz optical fiber (fiber core diameter, 200, 365, or 550 μm)f was passed through the working channel of the cystoscope and visually guided to achieve direct contact of the fiber tip with the urolith. A red aiming beam was used to guide and optimize targeting of the laser energy because the 2,100-nm wavelength of the Ho:YAG laser is not visible. The laser was activated, starting at 0.6 to 0.8 J/pulse and 6 to 10 Hz to attempt fragmentation of the urolith. The laser power settings were increased as needed to achieve fragmentation of the uroliths. Cystic uroliths were fragmented within the urinary bladder in female dogs (Figure 1) until the largest urolith fragments were small enough to be removed via basket extraction with a flexible stone basketg-i (Figure 2). In male and female dogs, urethroliths were fragmented within the urethra until the fragments could be removed via basket extraction (Figure 3). In male dogs, either cystic uroliths were fragmented within the bladder or smaller uroliths were moved into the urethra with a stone basket, released within the urethra, and fragmented within the urethral lumen to minimize urolith movement. The location of laser fragmentation of the uroliths was recorded as within the bladder, within the urethra, or both.
Basket extraction was performed by capturing one of the larger urolith fragments with the stone basket via cystoscopic guidance. If the urolith fragment was not spherical, an effort was made to orient the fragment such that the long axis of the fragment was parallel to the urethral lumen. The urolith fragment was positioned near the tip of the cystoscope, which was slowly withdrawn through the urethra while observation for smooth passage of the urolith fragment past the urethral mucosa was performed. If the fragment was too large for extraction, the mucosa bulged toward the cystoscope and tension was palpable on the shaft of the stone basket. For fragments that were too large for basket extraction, the fragment was released within the urethra and additional laser lithotripsy was performed within the urethral lumen until the fragments were small enough to be safely removed. Once all urolith fragments were small enough that the largest fragments could be safely removed by basket extraction, the remaining smaller fragments were removed by use of VUH.22,23
Information collected from the medical records regarding the dogs included age, breed, sex, neuter status, body weight, urine bacteriologic culture results, success or failure of complete removal of urolith fragments, number of lifetime urolith episodes including the current episode, and urolith composition, as determined via quantitative analysis performed by the University of Minnesota Urolith Center. Abdominal radiographs and ultrasonographic images obtained prior to urolith removal were reviewed to determine urolith location (bladder, urethra, or both), number of uroliths, and maximum diameter of the largest urolith. Successful removal of uroliths was defined as complete removal of all discrete urolith fragments on the basis of results of cystoscopy and abdominal radiography. For male dogs with radiolucent uroliths, abdominal ultrasonography or contrast cystourethography was also used to confirm complete removal of all urolith fragments. The need for cystotomy for removal of uroliths was considered a failure of cystoscope-guided laser lithotripsy. The reason for cystotomy was recorded. Complications during or after laser lithotripsy were recorded.
Variables recorded for the laser lithotripsy procedure included core diameter of the laser fiberf used, mean laser settings (Joules and Hertz) as recorded by the laser unit, location of laser fragmentation of uroliths (bladder, urethra, or both), laser time (as recorded in total minutes from the time the laser was first activated until last time activated), procedure time (including cystoscopy, laser lithotripsy, basket extraction, and VUH), number of laser lithotripsy sessions required for fragmentation of all urolith fragments into removable fragments, and number of anesthetic episodes during hospitalization. For dogs that received more than 1 laser treatment on separate days, the laser time and procedure time for the 2 procedures were added and recorded as total time for each variable for comparison with other dogs.
For statistical analysis, the maximum urolith diameter and number of uroliths were recorded as categoric data. For maximum urolith diameter, the following categories were recorded: ≤ 3 mm, 4 to 6 mm, 7 to 10 mm, 11 to 15 mm, and > 15 mm. For the number of uroliths, the following categories were recorded: 1, 2 to 5, 6 to 10, 11 to 20, and > 20.
Postprocedural radiography or ultrasonography was performed at the end of the procedure or within 24 hours. For dogs at PUVTH, meloxicam (0.2 mg/kg [0.09 mg/lb], IV or SC) was administered after anesthetic recovery, followed by orally administered meloxicam (0.1 mg/kg [0.05 mg/lb], q 24 h) for an additional 2 days. For dogs at MJR-VHUP, meloxicam (0.05 mg/kg [0.023 mg/lb], SC) was administered after anesthetic recovery, followed by orally administered meloxicam (0.05 mg/kg, q 24 h) for an additional 2 days. Dogs were monitored for voiding after the procedure and were discharged from the hospital after they were able to urinate without difficulty. At PUVTH, most dogs were discharged the day following the procedure. At MJR-VHUP, male dogs were discharged the day of the procedure and female dogs were discharged the day following the procedure. At both universities, dogs with indwelling urinary catheters were discharged the day the urinary catheter was removed.
Follow-up evaluations of dogs from PUVTH were performed at PUVTH or by the referring primary care veterinarian and included recommendations for urinalysis approximately 2 weeks after lithotripsy and urinalysis and for abdominal radiography a minimum of every 6 months to monitor for urolith recurrence for a minimum of 2 years. Owners and referring veterinarians were contacted by telephone to record follow-up information from radiography and urinalyses. For patients from MJR-VHUP, bacteriologic culture of urine was performed at 2 weeks by the referring veterinarian or at MJR-VHUP, with urinalysis and abdominal radiography performed at 1, 3, 6, 9, and 12 months and then every 3 to 6 months thereafter. Owners and referring veterinarians were contacted at each time point. All follow-up radiographs and results of the urinalysis and bacteriologic culture were evaluated by one of the authors (LGA or ACB). The duration of follow-up for each dog was recorded as the number of months from lithotripsy to the last date of abdominal radiography (or ultrasonography for radiolucent uroliths). Urolith recurrence date was listed as the number of months from lithotripsy until urolith recurrence, when applicable.
Statistical analysis—Numeric data were tested for normal distribution by use of the Shapiro-Wilk test. Measures of central tendency were compared by use of the Wilcoxon rank sum test for nonparametric data and for parametric data by use of the Student t test. Proportions of categoric data were compared by use of the χ2 test of independence. Correlation between ordinal variables was tested by use of the Spearman rank correlation coefficient. Time until urolith recurrence was calculated and graphed via Kaplan-Meier estimates for censored data. Comparison of times until urolith recurrence for censored data was performed via the log-rank test for equality. All analyses were performed with commercial software.j For all comparisons, P < 0.05 was considered significant.
Results
There were 45 male dogs and 28 female dogs. Two female dogs were sexually intact, and 26 were spayed. Eight males were sexually intact, and 37 were neutered. Median body weight of male dogs (10.1 kg [22.2 lb]; range, 5.5 to 36.4 kg [12.1 to 80.1 lb]) was greater (P = 0.04) than that of female dogs (8.9 kg [19.6 lb]; range, 4.0 to 34.8 kg [8.8 to 76.6 lb]). Mean ages of male dogs (7.6 ± 3.4 years) and female dogs (6.6 ± 3.7 years) were not significantly (P = 0.254) different. Mixed-breed dogs were most common (17/73 [23.3%]), followed by Bichon Frise (9/73 [12.3%]), Miniature Schnauzer (7/73 [9.6%]), Lhasa Apso (6/73 [8.2%]), Shih Tzu (6/73 [8.2%]), Pug (5/73 [6.9%]), Dalmatian (4/73 [5.5%]), Dachshund (2/73 [2.7%]), and Jack Russell Terrier (2/73 [2.7%]). Fifteen (15) other breeds were each represented by 1 dog.
Fifty-three (72.6%) dogs had only cystic uroliths, 10 (13.7%) had cystic uroliths and urethroliths, and 10 (13.7%) had only urethroliths. Urethroliths were more common in male (17/45 [37.8%]) dogs than in female (3/28 [10.7%]) dogs (P = 0.035). Laser lithotripsy was performed within the urinary bladder in 43 dogs, in the urethra in 16 dogs, and in the urinary bladder and the urethra in 14 dogs. More uroliths were fragmented within the urethra than the number of dogs with urethroliths because of intentional repositioning of smaller cystic uroliths into the urethra prior to fragmentation for the ease of the procedure.
Fifteen (20.5%) dogs had 1 urolith, 27 (37.0%) had 2 to 5 uroliths, 14 (19.2%) had 6 to 10 uroliths, 3 (4.1%) had 11 to 20 uroliths, and 14 (19.2%) had > 20 uroliths at admission. Maximum urolith diameter was < 3 mm for 7 (9.6%) dogs, 4 to 6 mm for 29 (39.7%) dogs, 7 to 10 mm for 31 (42.5%) dogs, 11 to 15 mm for 5 (6.8%) dogs, and > 15 mm for 1 (1.4%) dog. The maximum urolith diameter and the number of uroliths were not significantly correlated (U = −0.104; P = 0.38). Thirty-six dogs were treated for their first episode of uroliths, and 37 dogs had urolithiasis previously. The mean number of urolith episodes was 1.8 (range, 1 to 4).
Urolith composition was significantly (P < 0.001) different between sexes. Composition was calcium oxalate in 35 of 45 (77.8%) male dogs and 15 of 28 (53.6%) female dogs. Ten (35.7%) female dogs had struvite uroliths, whereas none of the male dogs had struvite uroliths. Urolith composition in the remaining dogs was urate (5 male dogs), cystine (2 male dogs), silica (1 male dog), and compound (2 male and 2 female dogs). Urolith composition was unknown for 1 female dog because the urolith was lost in shipping.
In 66 of 73 dogs, bacteriologic culture of urine, a urolith, or both was performed before or during laser lithotripsy. No growth was obtained from aerobic bacterial cultures in 45 of 66 (68.2%) dogs. Gram-positive bacteria were isolated from urine or uroliths of 13 of 66 (19.7%) dogs. Seven of 66 (10.6%) dogs had gramnegative bacteria in urine or uroliths, and 1 dog (1.5%) had both gram-negative and gram-positive bacteria. All 10 dogs with struvite uroliths had gram-positive bacteria (Staphylococcus or Proteus spp) in their urine or urolith.
Laser lithotripsy resulted in complete removal of all uroliths in all 28 female dogs and a majority of male dogs (39/45 [86.7%]; P = 0.076). Six (6/45 [13.3%]) male dogs required cystotomy for removal of uroliths because of the following reasons: 1 dog was too small to permit manipulation of the 2.7-mm flexible ureteroscope after the ureteroscope was passed retrograde up the urethra, 2 dogs had bladder perforation during attempted VUH, and 3 dogs had too many uroliths and urolith fragments remaining after initial laser lithotripsy to justify continuation of this minimally invasive approach. For the 39 successfully treated male dogs, laser lithotripsy resulted in adequate fragmentation of the uroliths in 1 session in 35 dogs and 2 sessions in 4 dogs. Laser lithotripsy resulted in adequate fragmentation of the uroliths in 1 session in all 28 female dogs. Six dogs were anesthetized 1 to 3 days after laser lithotripsy for basket extraction and VUH to remove remaining urolith fragments that had been trapped in intravesical blood clots during the initial procedure. The remaining urolith fragments were successfully removed in all 6 dogs.
Median laser time was 25 minutes (range, 2 to 328 minutes). Dogs with only urethroliths had shorter median laser time than dogs with only cystic uroliths (15.5 vs 40 minutes, respectively; P = 0.026). Similarly, median laser time was significantly (P = 0.011) shorter for dogs in which laser lithotripsy was performed in the urethra only, compared with dogs that had the uroliths fragmented within the bladder (6 vs 25 minutes, respectively). Dogs that had uroliths fragmented in both the bladder and urethra had the longest laser times (median, 78 minutes; range, 11 to 328). It should be noted that the recorded laser time was in excess of the actual time that the laser was actively being used because the laser unit recorded laser time in minutes from the time the laser was initially started until end of the last activation. This did not take into account the time between activations.
Median procedure time was also significantly (P = 0.026) shorter for dogs in which laser lithotripsy was performed in the urethra only (median, 70 minutes; range, 35 to 180 minutes), compared with dogs in which uroliths were fragmented within the bladder (105 minutes; range, 35 to 382). Dogs in which uroliths were fragmented in both the bladder and urethra also had the longest procedure time (median, 153 minutes; range, 48 to 439 minutes).
For female dogs, there was no significant difference between location of lithotripsy in the bladder or urethra for laser time (P = 0.802) or procedure time (P = 0.841; Table 1). For male dogs, lithotripsy in the urethra was significantly shorter than that in the bladder for laser time (P = 0.01) and procedure time (P = 0.025).
Variables associated with laser lithotripsy performed in various locations in the urinary tract of male and female dogs
Variable | Male dogs (n = 45) | Female dogs (n = 28) | ||||
---|---|---|---|---|---|---|
Urinary bladder (n = 20) | Urethra (n = 14) | Bladder and urethra (n = 11) | Urinary bladder (n = 23) | Urethra (n = 2) | Bladder and urethra (n = 3) | |
Laser time* (median [range]) | 27.5 (2–235) | 6 (2–30) | 90 (11–328) | 19 (3–192) | 26 (25–27) | 50 (20–91) |
Procedure time* (median [range]) | 142.5 (40–382) | 70 (35–180) | 179 (80–439) | 90 (35–330) | 103.5 (65–142) | 120 (48–135) |
Joules (median [range]) | 0.8 (0.6–1) | 0.6 (0.6–0.9) | 0.7 (0.6–1.6) | 0.65 (0.6–2) | 0.6 (0.6) | 0.6 (0.6–0.7) |
Hertz (median [range]) | 9 (6–14) | 8 (6–11) | 9 (6–12) | 8 (6–17) | 6 (6) | 8 (8–15) |
Minutes.
The number of uroliths was significantly, but weakly, correlated with laser time (U = 0.250; P = 0.035) and procedure time (U = 0.254; P = 0.03). Maximum urolith diameter was not significantly correlated with laser time (U = 0.099; P = 0.41) or procedure time (U = 0.079; P = 0.5). The university at which laser lithotripsy was performed did not significantly differ with respect to laser time (P = 0.42) or procedure time (P = 0.46).
The laser power settings (Hertz and J/pulse) used for lithotripsy were similar for uroliths fragmented in the urethra and urinary bladder for male and female dogs (Table 1). Median power settings for all dogs were 0.7 J/pulse and 8 Hz or 5.6 W. Fourteen clinicians functioned as the primary laser operator during laser lithotripsy. At PUVTH, there were 10 primary laser operators, and at the MJR-VHUP, there were 4 primary laser operators. One of the authors (LGA or ACB) was the primary laser operator or directly assisted with laser lithotripsy in 69 of the 73 dogs or had previously trained the laser operator (LGA) for the other 4 cases.
Complications related to cystoscopeguided laser lithotripsy occurred in 5 of 28 (17.9%) female dogs and 6 of 45 (13.3%) male dogs. Most complications occurred either during or within 24 hours of the laser procedure. The most common complication was the need for an indwelling urinary catheter for 1 to 5 days after the procedure in 3 female dogs and 2 male dogs because of partial to complete urethral obstruction as a result of urethral swelling. At MJR-VHUP, 5 additional female dogs had a urinary catheter placed empirically during the first 12 to 24 hours after laser lithotripsy for anticipated discomfort and urethral spasm.
Other complications included laser perforation of the bladder wall in 1 male dog, an endoscopic longitudinal urethral tear in 1 female dog, urethral obstruction 4 to 5 days after lithotripsy in 2 male dogs, bladder perforation in 2 male dogs during VUH, and hemorrhage for 24 hours following lithotripsy in 1 female dog secondary to von Willebrand disease. Meloxicam was not administered to the dog with von Willebrand disease. Laser perforation of the bladder and the longitudinal urethral tear were viewed cystoscopically, and a urinary catheter was left indwelling in those 2 dogs for 3 and 7 days, respectively. Positive-contrast cystography or urethrography was performed when the urinary catheter was removed to confirm healing of the perforation. Neither dog required surgery for repair of the perforation.
The female and male dogs that had the longest procedure times for removal of urethroliths required additional management because their case was complicated. A spayed female mixed-breed dog was evaluated for traumatic urethral perforation and urethral obstruction by a urethrolith 24 hours after cystotomy. The urethrolith was fragmented and removed via laser lithotripsy and basket extraction, and a urinary catheter was placed past the urethral perforation over a urologic guidewire and left indwelling for 72 hours to allow the urethral perforation to heal. Positive-contrast urethrography was performed when the urinary catheter was removed to confirm healing of the perforation.
A male Doberman Pinscher was evaluated for multiple urethroliths and an apparent urethral stricture. The urethroliths were fragmented and removed via laser lithotripsy, basket extraction, and VUH. The apparent urethral stricture was determined to be an extraluminal urethrolith that was completely covered by urethral mucosa (Figure 4). Several attempts were unsuccessful to dislodge the urethrolith. A mucosal incision was made directly over the urolith with the Ho:YAG laser, and the urolith was successfully expelled into the urethral lumen. The urolith was fragmented and removed by laser lithotripsy and basket extraction.
Dogs were monitored for clinical signs of urolith recurrence or other clinical signs of urinary tract disease for 12 to 24 months after laser lithotripsy. Follow-up urinalysis and bacteriologic culture of urine revealed urinary tract infections in 4 of 73 dogs. Two female dogs had UTI with gram-negative organisms 2 weeks after lithotripsy; both had indwelling urinary catheters for 1 to 5 days, which may have contributed to ascending infection. All dogs with UTI before or after lithotripsy were treated with antimicrobials on the basis of in vitro susceptibility testing. Urinalysis results were used to guide dietary and medical management to prevent urolith recurrence.
All dogs had follow-up abdominal radiography or ultrasonongraphy at least once during the 2-year follow-up period. Median duration of follow-up (on the basis of the date of last follow-up radiographs) was 12 months (range, 6 to 24 months). Fourteen of the dogs (12 males and 2 females) had recurrence of uroliths a median of 10.5 months after laser lithotripsy (range, 1 to 24 months). Male dogs had a significantly (P = 0.004) shorter time to recurrence than female dogs (Figure 5). Four dogs died or were euthanatized for causes unrelated to urinary diseases 8 to 21 months after laser lithotripsy. Medical management to prevent urolith recurrence was recommended in all dogs on the basis of urolith composition.24 Medical management included diets to minimize the risk of urolith recurrence in 53 of 73 dogs, antimicrobials selected on the basis of results of culture and susceptibility testing, and oral administration of potassium citrate in 30 dogs.
Minimally invasive removal of the recurrent uroliths was successful in all 14 dogs, including repeated cystoscope-guided laser lithotripsy in 5 dogs and cystoscopy with VUH or basket extraction in 9 dogs. There were no long-term complications attributable to laser lithotripsy in any of the 73 dogs during the follow-up period. There was no evidence of urethral stricture in any dogs, including the 14 dogs that had follow-up cystoscopy for minimally invasive urolith removal.
Discussion
In humans, the relative roles of shock wave lithotripsy, laser lithotripsy, and percutaneous nephrolithotomy have been established through prospective clinical trials.25–28 However, the relative roles of these procedures have not yet been determined in dogs and cats. In the present study, laser lithotripsy was successful for fragmentation and removal of uroliths from the bladder and urethra in 28 of 28 (100%) female dogs and 39 of 45 (87%) male dogs (92% of dogs overall). This indicated that minimally invasive urolith removal is possible for most dogs, provided the urethra is large enough to permit retrograde passage of a rigid cystoscope or flexible ureteroscope. Prolonged procedure times were required in some dogs with larger stone burdens, and the durations of laser lithotripsy and cystoscopy were related to the number of uroliths.
Dogs with urethroliths were the easiest to treat via laser lithotripsy and had significantly shorter laser and procedure times. Therefore, laser lithotripsy is an excellent option for dogs with urethroliths or a few small cystic uroliths that can be repositioned into the urethra prior to laser fragmentation. In humans, the principal use of laser lithotripsy has been for fragmentation of ureteroliths and small nephroliths.1,8,11,29 Although the Ho:YAG laser can also be used to fragment and remove large staghorn nephroliths via ureteroscopy in humans, in some situations, this approach is not practical because the large stone burden requires prolonged procedure time and repeated procedures are required in some patients.30 Likewise, management of large cystic uroliths in humans may require a combined percutaneous and transurethral cystolithotomy approach.31 Similarly, we concluded that male dogs with large numbers of uroliths in the bladder and urethra may require prolonged procedure times such that a transurethral laser lithotripsy approach is not practical. In 3 male dogs that had cystotomy for removal of uroliths in the present study, the decision was made that there were too many uroliths remaining to justify continuation of this minimally invasive approach after prolonged attempts to fragment and remove the large number of uroliths in each dog. In dogs with large numbers of uroliths, other minimally invasive options include laparoscopic-assisted cystoscopy, percutaneous cystolithotomy, or shock wave lithotripsy followed by VUH.4,31,32 Our current recommendation for male dogs that weigh < 7 kg is to only attempt transurethral cystoscope–guided laser lithotripsy in dogs with low stone burdens and to consider other minimally invasive approaches for small male dogs with large stone burdens.
Various commercial models of Ho:YAG lasers for lithotripsy are available, and the pulse duration ranges from 250 to 750 microsecond, the pulse energy ranges from 0.2 to 4.0 J/pulse, the frequency ranges from 5 to 45 Hz, and the power output ranges from 3.0 to 100 W. The power that one chooses is based on the desired application. The Ho:YAG laser used in the present study had a maximum power output of 20 W with a 350-microsecond pulse width. This laser had sufficient power for fragmentation of the uroliths in all 73 dogs. Most uroliths could be fragmented at low-power settings (median, 0.7 J/pulse and 8 Hz or 5.6 W). Therefore, low-power Ho:YAG lasers are sufficient for laser lithotripsy in dogs.
Pulsed Ho:YAG laser energy is absorbed by water inside the urolith, resulting in a photothermal effect, which causes urolith fragmentation.13,14,33 The holmium laser effect on uroliths occurs by means of a vapor bubble that is created when the pulse of laser energy traveling through water from the tip of the fiber is trapped within a bubble (so-called Moses effect).13,14,33 If the fiber tip is > 0.5 mm away from the urolith, the vapor bubble collapses, the water absorbs the energy, and no effect occurs. As the fiber tip is advanced < 0.5 mm from the calculus, the vapor bubble comes in contact with and affects the urolith. To achieve optimum results, the fiber tip should be in direct contact with the surface of the urolith during laser activation.8,12 The irrigation fluid surrounding the urolith absorbs the laser energy, provided the tip of the laser fiber is > 0.5 mm from the mucosa, which permits urolith fragmentation with minimal or no mucosal trauma.
Laser lithotripsy is performed with cystoscopic guidance, which requires some cystoscopy experience. Given that 14 individuals performed laser lithotripsy in the present study, we concluded that laser lithotripsy can be successfully performed by clinicians with prior cystoscopy experience. All 14 operators were closely supervised or previously trained on the lithotripsy technique by 1 of 2 clinicians with extensive experience in cystoscopy and lithotripsy.
A prior study20 revealed that surgically implanted urethroliths are fragmented with the Ho:YAG laser with minimal damage to the urethral mucosa; however, the urolith fragments were not removed by VUH or basket extraction in that study. Only 3 of 7 dogs voided the remaining fragments, whereas in 2 dogs, urolith fragments remained in the urinary bladder, and in 2 dogs, urolith fragments lodged in the urethra, indicating that laser lithotripsy fragmentation alone was insufficient for complete removal of urethroliths.20 In the present study, use of basket extraction and VUH was successful for removal of the urolith fragments in all female dogs and 87% of male dogs. This indicated that laser lithotripsy fragmentation of uroliths should be combined with other techniques.
Complications encountered in this study were short-lived and resolved with medical management, except for the 2 dogs with bladder perforation that occurred during VUH. Bladder perforation is an uncommon but recognized risk of VUH.23 The bladder perforations that occurred during VUH were thought to be a result of excessive focal pressure applied to the urinary bladder during transient urethral obstruction by small urolith fragments. There were also 2 laser perforations of the urinary bladder or urethra that healed within 3 to 7 days with an indwelling urinary catheter. A prior study19 that used cadaveric porcine urethra revealed that mucosal injury was minimal, provided the laser energy was at an angle of < 30° relative to the mucosal surface. Our results also confirmed that laser lithotripsy can be safely performed in most dogs with minimal risk of injury to the urinary tract. Laser perforations of the ureter are uncommon in humans treated via ureteroscopy and laser lithotripsy, which is partially attributable to the relatively straight ureter that permits the laser energy to be delivered parallel to the mucosa, thus minimizing the risk of laser injury.3,8,9,11,34 Clinical experience in humans indicates that patients at risk of excessive bleeding can be safely treated via laser lithotripsy as the sole treatment modality.35 One dog with von Willebrand disease had hemorrhage after laser lithotripsy, basket extraction, and VUH, which required placement of a urinary catheter. This dog was not known to have von Willebrand disease at the time of lithotripsy. The bleeding resolved without additional treatment within 72 hours, and the dog did not require any RBC products. The dog received 4 units of cryoprecipitate after starting treatment with desmopressin.
Most other complications were related to urethral swelling that required an indwelling urinary catheter. In humans, ureteral stents are often placed following ureteroscopy and laser lithotripsy to avoid obstruction of the distal portion of the ureter from swelling; however, ureteral stent placement is not required in uncomplicated ureteroscopy, provided the ureteral orifice is not balloondilated to facilitate access.1,36 Because of risk of urethral obstruction from swelling, urethral catheters were routinely placed in 5 female dogs toward the end of the study period at MJR-VHUP. There were no long-term complications in the present study. None of the dogs developed urethral stricture during a 12- to 24-month follow-up period.
Thirty-seven of 73 (51%) dogs had had recurrent stone formation when referred for laser lithotripsy. Additionally, 19% of the dogs had urolith recurrence during the 1- to 2-year follow-up period. Therefore, minimally invasive removal of uroliths via cystoscope-guided laser lithotripsy is an option that should be considered to minimize the number of open surgical procedures for dogs with recurrent urolithiasis.
Laser lithotripsy was successful for fragmentation and removal of uroliths from the bladder and urethra in most dogs with minimal complications. This novel treatment option can be used for minimally invasive removal of uroliths that traditionally have been removed by use of open cystotomy. Other potential urologic applications for the Ho:YAG include incision of urethral and ureteral strictures; laser ablation of polypoid lesions, superficial transitional cell carcinoma, or prostatic adenocarcinoma within the urethra or bladder; and laser ablation of ectopic ureters.3,8,37
ABBREVIATIONS
EHL | Electrohydraulic lithotripsy |
ESWL | Extracorporeal shockwave lithotripsy |
Ho:YAG | Holmium:yttrium-aluminum-garnet |
MJR-VHUP | Matthew J. Ryan Veterinary Hospital of the University of Pennsylvania |
PUVTH | Purdue University Veterinary Teaching Hospital |
UTI | Urinary tract infection |
VUH | Voiding urohydropropulsion |
Grant DC, Gevedon ML, Sroufe MA. Laser lithotripsy of naturally occurring canine uroliths (abstr). J Vet Intern Med 2007;21:600.
VersaPulse PowerSuite Holmium 20W, Lumenis, Santa Clara, Calif.
Flex-X2, Karl Storz Veterinary Endoscopy, Goleta, Calif.
DUR-8 Elite flexible ureteroscope, Gyrus ACMI Inc, Southborough, Mass.
Storz 7.5-F pediatric ureteroscope, Karl Storz Endoscopy, Goleta, Calif.
SlimLine laser fibers, Boston Scientific Corp, Natick, Mass.
NCircle Nitinol Tipless stone extractors, Cook Urological, Spencer, Ind.
Dimension Articulating stone basket, Bard Urological, Covington, Ga.
Sur-Catch NT No Tip basket, Gyrus ACMI Inc, Southborough, Mass.
STATA, version 9.2, StataCorp, College Station, Tex.
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