Laser Lithotripsy Using the Holmium: yag laser

Laser Lithotripsy Using the Holmium: YAG Laser



Larry G. Adams, DVM, Ph.D., DACVIM

Jody P. Lulich, DVM, Ph.D., DACVIM
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). The Ho:YAG laser has 350 μsec pulse duration. The laser energy is absorbed in <0.5 mm of fluid making it an ideal surgical laser for endourologic applications like laser lithotripsy.^1-3 The laser energy is delivered to the surface of the uroliths using flexible quartz fibers with multiple different diameters (200, 365, 550 microns). The nominal hazard zone if the laser is discharged in air is approximately 1 meter. As with all lasers, safety goggles are recommended to prevent ocular injury in the event of accidental firing of the laser. Because the laser energy is absorbed so efficiently in fluid, there is little or no danger to personnel provided proper laser usage protocols are observed.
Laser lithotripsy
A recent in vitro study confirmed that the Ho:YAG laser is able to fragment all types of canine uroliths.⁴ Implanted urethroliths were safely fragmented using the Ho:YAG laser in research dogs.⁵ In clinical use, we have confirmed that common types of canine and feline stones are easily fragmented using a 20 watt Ho:YAG laser. There is minimal or no trauma to adjacent mucosa unless the activated laser fiber directly contacts the urothelial lining.
The mechanism of stone fragmentation with the Ho:YAG laser is mainly photothermal, and involves a thermal drilling process rather than a shock-wave (compared with SWL that relies exclusively on shock wave effects).⁶ The pulsatile firing of the laser creates a weak acoustical shock wave that can cause stones to move within the urinary tract.
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 should be guided with the aid of a cystoscope so that it is in direct contact with the surface of the urolith. The greatest effect occurs directly in front of the fiber tip. During the pulse of laser energy, there is a microscopic air bubble on the tip of the fiber that allows the laser energy to travel further than through the fluid medium directly transmitting the laser energy into the stone. This “parting of the water” has been termed the “Moses effect”.^1,7
Because uroliths are fragmented by photothermal mechanism, the chemical composition of small fragments may be different than the original urolith ; thus stone analysis of small urolith fragments could be misleading. Laser lithotripsy can result in formation of the following additional compounds:⁶

• 
  Calcium oxalate monohydrate → calcium carbonate
  
• 
  Struvite → ammonium carbonate and magnesium carbonate
  
• 
  Cystine → cysteine and free sulfur
  
• 
  Uric acid → cyanide
  

Notably, a small amount of cyanide is liberated into the irrigation solution during laser lithotripsy of urate stones.⁸ Large urate uroliths have been successfully fragmented using the Ho:YAG laser in humans and 1 dog without any clinical evidence of cyanide toxicity. Continual irrigation of isotonic fluids in and out of the urinary tract is recommended during laser lithotripsy of urate containing stones.

Preventing problems
Certain safety precautions should be observed to prevent injury to personnel or equipment during laser lithotripsy. The laser is kept in “standby” mode whenever:1) the laser fiber tip is outside the patient’s body, 2) the tip of fiber is not visible on the video monitor, 3) the blue external clatting on the fiber is not visible, or 4) during any side conversations by veterinarians or laser safety technicians. We also use laser catheters to protect scope and the fiber when possible to provide additional protection to the cystoscope.
Holmium laser technique
Contact with tip of the laser fiber is required for fragmentation of stones or incision/cutting of tissues. Coagulation of bleeding vessels is possible by firing the laser tip 0.2 – 0.4 mm from the vessel. During laser lithotripsy, maintain constant fluid irrigation to clear visual field of stone debris. This must be done without over-distension of the urinary bladder to avoid iatrogenic trauma and bleeding. The operator should alternate filling and draining of the irrigation fluid to prevent bladder overdistention.
Although patient positioning is often the choice of the operator, we position female dogs in dorsal recumbency. A rigid cystoscope is passed retrograde into the vestibule and into the urethra to allow visualization of urocystoliths. The bladder is lavaged with sterile warm isotonic solutions (i.e. normal saline) and then refilled. During lithotripsy continuous irrigation is provided to wash debris from bladder as stones are fragmented. The laser energy is delivered via a quartz lithotripsy fiber that is passed through the working channel of the cystoscope. The fiber is placed in contact with the stone surface using the aiming beam of the fiber. A foot-operated switch will activate and deactivate the release of energy from the lithotriptor. The energy selected will vary depending on the size and location of the stones; however, initial settings to fragment most stones have been 0.6 to 0.7 Joules at 8 to 10 Hertz. The force and frequency of laser pulses can be increased to fragment a larger more stationary urolith, or decreased if excessive stone movement following deployment of higher energies inhibits laser fiber to stone contact.
Uroliths can be fragmented either in the urinary bladder or urethra provided the tip of the fiber does not come into contact with the urinary mucosa. In male dogs, it is usually more efficient to basket bladder stones and place them in the urethra prior to fragmentation. Urethroliths are more easily fragmented because their movement out of the laser field is inhibited.
Cystoscope selection for laser lithotripsy
For female dogs and cats, an appropriate size of rigid cystoscope is utilized. There are advantages to having both a 30° telescope and 0° telescope available. The 0° telescopes are useful as an operating scope because they allow direct visualization in front of the scope tip during stone fragmentation. However, the 30° telescopes are superior for diagnostic evaluation prior to and after laser lithotripsy. We generally use the following scope for various size patients.
Female cats: 1.9 mm with 9 Fr sheath

Small to medium sized female dogs: 2.7 mm with 14.5 Fr sheath

Large female dogs (> 20 kg): 4.0 mm with 19 Fr sheath
Urethral mucosal edema causes a narrowing of the urethral lumen. If mucosal edema occurs, a smaller scope may be required for re-entry into the urinary bladder in some cases. This is an even more profound problem in male dogs and may limit which patients are considered large enough for a transurethral approach.
For male dogs, we utilize a flexible ureteroscope (7.5 Fr diameter x 70 cm length) as cystoscope. Not all male dogs are acceptable candidates for laser lithotripsy. Factors affecting patient selection for the procedure include urethral diameter (minimum body weight 3-4 kg), relative stone burden and whether or not a ureteral access sheath can be placed transurethrally. There are a few special precautions in performing laser lithotripsy in male dogs. Do not pass fiber with scope tip deflected because the tip of the laser fiber can damage lining of the working channel. We attempt to use a laser catheter whenever possible to minimize the risk of injury to the working channel. Similar to female patients, one should keep the fiber clatting and laser tip visible prior to activation of the laser. You must also keep track of volume infused (<10 ml/kg) to avoid overdistention or rupture of the urinary bladder.
Goals of laser lithotripsy
The goal of laser lithotripsy is to fragment the uroliths small enough to allow removal. The largest urolith fragments are initially removed by basket retrieval to confirm that they are small enough to pass through the urethra. The remaining fragments may be removed by voiding urohydropropulsion or by flushing the bladder lumen using an Ellik evacuator. The ultimate goal of laser lithotripsy and removal of the stone fragments is to render the patient stone free. Double contrast cystography and or cystoscopy may be used to confirm complete urolith removal.
The following table lists accessories that facilitate laser lithotripsy and urolith retrieval.

References
1. Dushinski JW et al. Urologic applications of the Holmium laser, Techniques in Urol. 1997, 3: 60-64.

2. Grasso M et al. Principles and applications of laser lithotripsy: Experience with holmium laser lithotrite, J.Clin.Laser.Med.Surg. 1998, 16: 3-7.

3. Sofer M et al. Holmium:YAG laser lithotripsy for upper urinary tract calculi in 598 patients, J.Urol. 2002, 167: 31-34.

4. Wynn VM et al. In vitro effects of pulsed holmium laser energy on canine uroliths and porcine cadaveric urethra, Lasers.Surg.Med. 2003, 33: 243-246.

5. Davidson EB et al. Laser lithotripsy for treatment of canine uroliths, Vet.Surg. 2004, 33: 56-61.

6. Chan KF et al. Holmium:YAG laser lithotripsy: A dominant photothermal ablative mechanism with chemical decomposition of urinary calculi, Lasers in Surgery & Medicine 1999, 25: 22-37.

7. Wollin TA et al. The holmium laser in urology, J.Clin.Laser.Med.Surg. 1998, 16: 13-20.

8. Corbin NS et al. Laser lithotripsy and cyanide, J.Endourol. 2000, 14: 169-173.