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
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.
•Is laser lithotripsy effective?
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).
•Is lithotripsy safe?
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.
•Male dog considerations
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.