A laser is a device that transmits light of various freuqnecies into an extremely intense, small and nearly non-divergent beam of monochromatic radiation...
In the 1970s, it was fortuitously recognized that shock waves generated by collision with raindrops produced unusual pitting patterns on the metal surface of supersonic aircraft during high-speed flight.
Researchers theorized that the elliptical contour of one part of the plane's fuselage resulted in the convergence of shockwaves onto another focal point of the aircraft, accelerating metal fatigue. Based on these observations, scientists at Domier, a German aerospace firm, embarked on a program to develop a system for the production of shock waves that could be reproducibly focused at a solitary point with the goal of fragmenting urinary stones.
This technique was called extracorporeal shockwave lithotripsy (ESWL). On Feb. 7, 1980, at the University of Munich, ESWL was used successfully to fragment uroliths in the kidney of a human patient (Chaussy 1989). Since that time, the development of ESWL has become a smashing success in terms of revolutionizing treatment of uroliths in man. Extracorporeal shock wave lithotripsy was followed by the development of intracorporeal techniques whereby instruments used to generate high-energy wavelengths were applied directly to the surface of uroliths viewed through an endoscope. Compared to surgery, lithotripsy is highly effective, minimally invasive and associated with less risk to the patient. Notably, use of ESWL shock wave lithotripsy and intracorporeal endoscopic laser lithotripsy to treat human patients with uroliths resulted in a significant reduction in iatrogenic loss of renal function associated with surgical intervention (Holman 2002).
Widespread knowledge of the success of extracorporeal and intracorporeal techniques of lithotripsy to treat kidney and ureteral stones in humans promoted veterinarians to consider its feasibility to manage uroliths in dogs. The result was development of effective protocols for ESWL to treat kidney and ureteral stones in dogs (Adams 1999). However, the majority of naturally occurring uroliths in dogs are located in the lower urinary tract rather than the kidneys and ureters. For example, uroliths from more than 200,000 dogs have been submitted to the Minnesota Urolith Center for analysis since 1981; in 98 percent of cases, uroliths were retrieved from the urinary bladder and/or urethra.
Unfortunately, ESWL is not a reliable form of treatment for bladder stones because of their tendency to move out of the focal point of converging shock-waves. Movement of the uroliths does not allow repeated shock waves to converge on a solitary point, resulting in reduced fragmentation. Fortunately, this obstacle has been over-come by fragmenting bladder stones with the aid of cystoscopes to guide various types of newly developed lithotriptors directly to the stone's surface. Although several forms of energy (electrohydraulic, ultrasonic, ballistic) can be delivered through the cystoscope, holmium:YAG laser technology effectively fragments all types of biogenic stones in dogs (Wynn 2002).
As illustrated in the following case report, cystoscopic laser lithotripsy is an effective alternative to surgical removal of urocystoliths.
A 5-year-old female spayed Miniature Schnauzer was referred by a colleague in Arizona to the University of Minnesota Veterinary Teaching Hospital because of a two-year history of recurrent bladder stones. Urocystoliths removed one year previously were composed of 95 percent magnesium ammonium phosphate and 5 percent calcium phosphate carbonate. Since that episode, this patient was fed canned and dry formulations of an adult maintenance food designed to reduce magnesium and to promote formation of acidic urine.
Physical examination revealed an alert dog weighing 12 kilograms. Temperature (102° F), respirations and pulse rate were normal. Serum concentrations of creatinine (1.1 mg/dl), urea nitrogen (SUN = 19 mg/dl), phosphorus (3.5 mg/dl), calcium (10.7 mg/dl) and bicarbonate (20 mmol/L) were normal. Results of a hemogram revealed values within the normal reference range (hematocrit = 56 percent and WBC = 7,880/ul). Analysis of a urine sample collected by cystocentesis prior to any form of therapy revealed that specific gravity was 1.032. The urine was alkaline (pH = 8.0) and contained numerous red blood cells (>50/hpf).
White cells and epithelial cells were not detected in urine sediment. However, a few amorphous calcium phosphate crystals were observed. Aerobic culture of an aliquot of urine collected by cystocentesis did not result in in-vitro growth of bacteria.
Survey radiographs of the urinary tract revealed three radiodense urocystoliths (Figure 1). Ultrasonography of the entire urinary tract confirmed that the uroliths were localized to the urinary bladder.
Figure 1: Survey lateral abdominal radiograph of a 5-year-old spayed, female Miniature Schnauzer with three urocystoliths. Note that the outer layer of these uroliths is more radiodense than their core (i.e. compound uroliths).
The following day cystoscopic laser lithotripsy was performed while the patient was anesthetized. A 550-um quartz laser fiber was passed through the working channel of a cystoscope. Uroliths were fragmented using a Coherent Versapulse 20W holmium:YAG laser. With the quartz fiber tip in direct contact with the surface of the urolith (Figure 2), laser energy was applied until the urolith fragments were small enough (approximately less than 0.35 cm in diameter) to pass through the urethra easily. This degree of fragmentation required approximately 30 minutes of relatively constant application of laser energy (1.4 joules and 6 Hertz). Urolith fragments were removed subsequently by voiding urohydropropulsion and submitted for analysis. Double contrast cystography following voiding urohydropropulsion confirmed that all urocystoliths were removed (Figure 3).
Figure 2: Cystoscopic view of the lumen of the bladder of the female Miniature Schnauzer described in Figure 1 illustrating fragmentation of a urolith with holmium: YAG laser energy transmitted through a 520-um quartz fiber.
What is the origin of intracorporeal laser lithotripsy?
Figure 3: Lateral view of a double-contrast cystogram of the Miniature Schnauzer described in Figure 1 obtained following lithotripsy and voiding urohydropropulsion. There is no evidence of uroliths in the bladder lumen.
The term "laser" is an acronym for "light amplification by stimulated emission of radiation." A laser is a device that transmits light of various frequencies into an extremely intense, small and nearly non-divergent beam of monochromatic radiation in the visible region with all the waves in phase. Lasers are capable of mobilizing immense heat and power when focused in close range.
The term lithotripsy is derived from the Greek words "lith" meaning stone, and "tripsis" meaning to crush. A lithotriptor is a device for crushing or disintegrating uroliths.
Use of laser energy for intracorporeal lithotripsy is a relatively new concept. In 1968, investigators first reported in-vitro fragmentation of uroliths with a ruby laser. However, because fragmentation of stones was associated with generation of sufficient heat that likely would damage adjacent tissues, it could not be used to treat patients.
Likewise, use of carbon dioxide laser energy was considered to be unsuitable for clinical use because it could not be delivered through nontoxic fibers. However, in 1986 researchers using a 504 nm, pulsed dye laser successfully and safely treated human patients with ureteroliths. The holmium: YAG laser is the newest device available for clinical lithotripsy.
What is the origin of the name holmium: YAG laser lithotripsy? Holmium (Ho) is a rare earth element named after Sweden (the Greek word "holmia" means Sweden) in honor of the Swedish chemist who discovered it. A holmium YAG (Ho: YAG) laser is a laser whose active medium is a crystal of yttrium, aluminum and garnet (YAG) doped with holmium and whose beam falls in the near infrared portion of the electromagnetic spectrum (2100 nm).
How do holmium YAG lithotriptor fragment uroliths? The mechanism of stone fragmentation with the Holmium:YAG laser is mainly photothermal and involves a thermal drilling process rather than a shock-wave effect. Ho:YAG laser energy is transmitted from the crystal to the urolith via a flexible quartz fiber. To achieve optimum results, the quartz fiber tip must be guided with the aid of a cystoscope so that it is in direct contact with the surface of the urolith.
Laser lithotripsy has been reported to eliminate urinary stones in humans, horses, goats and pigs effectively (Razvi 1996, Howard 1998, Hallard 2002). In-vitro studies revealed that the holmium: YAG laser consistently shattered canine stones of all types (i.e. calcium oxalate, cystine, struvite, silica and urate) into extractable fragments (<3.5mm in diameter) in less than 30 seconds (Wynn 2002).
It is logical to question whether or not lasers capable of shattering stones would also damage tissues comprising the urinary bladder. However, damage to the bladder wall is minimal because the energy of the holmium:YAG laser is delivered in a pulsed fashion and readily absorbed by water. Therefore, continuous irrigation of the urinary bladder during lithotripsy quickly absorbs and disperses stray energy. Under these conditions, the thermal effect of the holmium laser is localized to within 1 to 2 millimeters of the quartz fiber tip. In a prospective study of 598 human patients with kidney or ureteral stones fragmented by laser lithotripsy, complications were only observed in one patient (ureteral trauma) (Sofer 2002). These results suggest that when properly used, laser lithotripsy can be used safely in dogs.
Figure 4: Urolith fragments obtained by voiding urohydropropulsion following lithotripsy. The outer layer (white arrow) of the urolith was composed of 100 percent calcium oxalate monohydrate. The interior (black arrow) of the urolith was composed of 95 percent magnesium ammonium phosphate and 5 percent calcium phosphate carbonate.
In dogs, the size of the os penis and flexure of the male urethra limits the size and deflectability of cystoscopes that can be introduced into their urinary bladders. However, small-diameter (7.5 Fr), flexible endoscopes used for evaluation of human ureters can be inserted easily in the urethra and bladder of most dogs weighing more than 15 pounds. When using flexible cystoscopes, the fiberoptic lithotriptor probe can be inserted through the working channel of the ureteroscope to deliver laser energy to shatter stones. In an experimental study, in which uroliths were lodged in the proximal end of the os penis of dogs, laser lithotripsy shattered stones without damaging the os penis (Davidson 2004).
When the dog spontaneously voided six hours following lithotripsy, there was no evidence of dysuria or gross hematuria. The dog was discharged from the hospital the next day. To minimize iatrogenic urinary tract infection, oral enrofloxacin was prescribed for five days.
Analysis of the urolith revealed that it was a compound stone. The urolith's interior was composed of 95 percent magnesium ammonium phosphate and 5 percent calcium phosphate carbonate; its outer shell was composed of calcium oxalate monohydrate. Based on the location of minerals, it is logical to assume that urinary tract infection by urease-producing bacteria precipitated formation of a core of magnesium ammonium phosphate. Attempts to acidify the urine might be one factor explaining why a layer of calcium oxalate surrounded uroliths. Likewise, Miniature Schnauzers are recognized as a breed that is predisposed to calcium oxalate urolith formation. We recommend early detection and control of urinary tract infection to prevent recurrence of magnesium ammonium phosphate, and appropriate dietary modification to minimize recurrence of calcium oxalate.
Dr. Lulich is professor of clinical sciences in the Department of Veterinary Clinical Sciences at the University of Minnesota College of Veterinary Medicine.
Dr. Osborne, a diplomate of the American College of Veterinary Internal Medicine, is professor of medicine in the Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Minnesota. DVM