Background and Mechanism of Action

Minimally invasive urologic procedures are becoming more common in veterinary medicine. Lithotripsy, including pulse dye, extracorporeal shock wave, and holmium:yttrium-aluminum-garnet (Ho:YAG) has been reported for use in animals. Ho:YAG lithotripsy was introduced in human medicine in the early 1990s and has been used for a variety of procedures; it is becoming the treatment modality of choice for renal and ureteral calculi. In fact, Ho:YAG was associated with shorter operation times and postoperative hospitalization periods. It was also more effective than the Lithoclast lithotripsy with regards to the immediate stone free rate.¹ Recently, Ho:YAG has been introduced to veterinary medicine, and published papers and abstracts have documented its effectiveness for treatment of cystic and urethral calculi in a variety of species.^2-4

Available medical lasers range in wavelength from 504nm (pulse dye laser) to 10,600nm (carbon dioxide laser).⁵ The Ho:YAG laser has a wavelength of approximately 2100nm, which falls in the near infrared portion of the electromagnetic spectrum. Because it is not visible to the naked eye, a red helium-neon aiming beam is used for laser targeting. Unlike the pulse dye lasers, Ho:YAG laser energy is absorbed in <0.5mm of fluid making it an ideal laser for surgical endourologic procedures.⁶ Direct contact of the laser fiber with the urolith is required for effective stone fragmentation.

The laser energy is delivered to the stone surface via a flexible quartz fiber. These fibers come in a wide variety of diameters including: 200, 365, and 550 microns. The fibers are inserted through the working channel of a cystoscope. The mechanism of stone fragmentation is primarily a photothermal process and involves a thermal drilling process rather than a shock wave. During the pulse of laser energy, a cavitation bubble is formed by vaporization of water molecules. The cavitation bubble travels further than the fluid medium, directly transmitting the laser energy into the stone; a phenomenon often referred to as the 'Moses effect'.⁷

All stones commonly seen in veterinary patients have been reported to fractionate with the Ho:YAG laser in vitro,⁸ however calcium carbonate stones, which are commonly seen in horses, pigs and goats were not evaluated in that study. It has been determined by personal experience as well as case reports that calcium carbonate will fragment, but generally requires more energy and time. Their surfaces are often more 'slippery' making contact with the laser more difficult. Because the fragmentation occurs by a photothermal mechanism, the chemical composition of the fragments may not resemble the original compound. The following compounds have been reported to form after Ho:YAG lithtotripsy.⁹

 Calcium oxylate monohydrate (COM) calcium carbonate

 Calcium hydrogen phosphate dihydrate calcium pyrophosphate

 Cystine free sulfur

 Magnesium ammonium phosphate (struvite) ammonium carbonate and

 magnesium carbonate

 Uric acid: cyanide

Because the compositions can be altered after lithotripsy, it is important to submit fragments that are large enough to not have been altered, yet small enough to pass through the urethra with ease. Although one would think the cyanide produced from the fragmentation of urate stones could cause toxicity for the patient, only a small amount is liberated¹⁰ and no signs of toxicity have been noted in humans undergoing this procedure. The author has performed lithotripsy on dogs and tortoises with urate stones without any clinical evidence of cyanide toxicity. Proper irrigation of the urinary tract with sterile saline during the procedure is recommended.

Performing the Procedure

At this time, the author performs Ho:YAG laser lithotripsy for cystic calculi in female dogs and cats and urethral calculi in male dogs. Lithotripsy is an ideal tool to have for removing urethral obstructions in male dogs. Although this procedure can be performed in male dogs with bladder stones, in the author's experience, male dogs who have a stone >1cm or contain numerous cystic calculi require too much time and this technique is not recommended. Because the laser energy is pulsatile in nature (350 microseconds), this causes the stones to 'bounce' around the bladder, which is especially difficult in male dogs. As mentioned earlier, lithotripsy has been reported to be effective for urethral calculi in pot bellied pigs, steer, and goats. It was not recommended for horses with large cystic calculi due to the time needed to fractionate these large stones that often occur in this species. The author has also used this technique, in conjunction with surgery, to aid in the removal of large (6-7cm) urate stones in two desert tortoises.

Radiographs will identify the patients that are good candidates for this procedure. If possible, a urine culture should be performed prior to the procedure, and animals should be started on appropriate antimicrobial therapy if the culture is positive. This is particularly important in the case of infected struvite stones in dogs because liberation of bacteria can occur once the lithotripsy has begun. The patient should be anesthetized and positioned in either dorsal or right lateral recumbency for female dogs and cats or right or left lateral recumbency for male dogs. A rigid cystoscope of appropriate size should be used for female cats and dogs and a flexible scope should be used for male dogs. Cystoscope size recommendations can be found in the section entitled 'Advanced diagnostics for canine and feline lower urinary tract disorders'.

Prior to inserting the laser fiber through the working channel of the scope, the fiber should be thoroughly evaluated for any defects in the blue external clatting. The aiming beam should be perfectly round to ensure the energy coming from the laser will be straight. If the beam is not straight, the fiber should be clipped and cleaved with strippers and scissors that are appropriate for the fiber. We generally use the 365micron fiber for most calculi, unless it is a large stone burden or we are working with presumed calcium carbonate stones. For the latter two, the 500micron fiber is more durable and stripping and cleaving the fiber during the procedure is required less often. Whenever the fiber is in the scope or out of the patient, we place the laser machine on standby to ensure that accidental firing cannot occur. When passing the fiber through a flexible scope, be certain the tip of the scope is not deflected to minimize trauma to the working channel of the scope. The nominal hazard zone for this laser is approximately 1meter, and protective eyewear is required for those working within that zone.

The bladder should be lavaged with sterile saline and then refilled. The laser fiber is then introduced through the scope and the aiming beam of the fiber should be in direct contact with the stone. A foot-operated switch will activate and deactivate the release of energy from the lithotripter. The energy selected will depend on the porosity of the stone, however, in our experience most canine and feline stones fragment at 0.6 Joules and 10 Hertz. The force and frequency can be increased or decreased as needed throughout the procedure. Decreasing the Joules can be helpful when the stone is moving excessively. In male dogs, one can try to basket the stones and lodge them in the urethra for fragmentation which alleviates this problem. In order to provide good visualization, continuous irrigation during the procedure to remove the stone debris that is liberated is absolutely necessary. Be certain to monitor the amount of fluid being infused so as not to over distend or rupture the bladder during the procedure.

Once the stones have been fractionated into small pieces, the largest piece should be removed via basket retrieval. I prefer the nitinol tipless baskets (Cook Urologic) which pass easily through the working channels; the tipless end is less likely to cause iatrogenic trauma to the bladder or urethra. The stone is grasped and gently removed through the urethra as sterile saline is being continuously infused to dilate the lumen. If the stone passes easily, the rest of the fragments can be removed via voiding urohydropropulsion. If the stone does not pass well, further use of the laser is required.

Complications From the Procedure

Because Ho:YAG laser energy is not strongly absorbed by water, it is very safe for urologic procedures and should cause no damage to the tissues of the lower urinary tract. However, when performing the procedure, the laser fiber must stay at least 1mm away from the bladder/urethra mucosa so as not to cause trauma to these tissues. If a small hole is made in the bladder or urethra, it will often heal quickly with a urinary catheter in place. Large tears in the bladder would require surgical intervention.

In addition to bladder or urethral trauma, introducing the cystoscope in and out of the urinary tract can lead to urethra edema and swelling. In the author's experience, the use of a urethral access sheath can help to decrease this potential problem. If one is concerned about urethral edema and secondary obstruction, a urethral catheter can be left in place for 24-36 hours. Post procedure non-steroidal anti-inflammatory drugs are often given as well.

References

1.  Jeon SS, Hyun JH, Lee KS. A comparison of holmium:YAG laser with Lithoclast lithotripsy in ureteral calculi fragmentation. Int J Urol 2005;12:544-547.

2.  Streeter RN, Washburn KE, Higbee RG, et al. Laser lithotripsy of a urethral calculus via ischial urethrotomy in a steer. J Am Vet Med Assoc 2001;219:640-643.

3.  May KA, Pleasant RS, Howard RD, et al. Failure of holmium:yttrium-aluminum-garnet laser lithotripsy in two horses with calculi in the urinary bladder. J Am Vet Med Assoc 2001;219:957-961, 939.

4.  Halland SK, House JK, George LW. Urethroscopy and laser lithotripsy for the diagnosis and treatment of obstructive urolithiasis in goats and pot-bellied pigs. J Am Vet Med Assoc 2002;220:1831-1834.

5.  Adkins WC, Dulabon DA, Chorazy ZJ, et al. Consider Ho:YAG for low-cost, effective laser lithotripsy. Clin Laser Mon 1994;12:139-141.

6.  Sofer M, Watterson JD, Wollin TA, et al. Holmium:YAG laser lithotripsy for upper urinary tract calculi in 598 patients. J Urol 2002;167:31-34.

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

8.  Davidson EB, Ritchey JW, Higbee RD, et al. Laser lithotripsy for treatment of canine uroliths. Vet Surg 2004;33:56-61.

9.  Chan KF, Vassar GJ, Pfefer TJ, et al. Holmium:YAG laser lithotripsy: A dominant photothermal ablative mechanism with chemical decomposition of urinary calculi. Lasers Surg Med 1999;25:22-37.

10. Corbin NS, Teichman JM, Nguyen T, et al. Laser lithotripsy and cyanide. J Endourol 2000;14:169-173.