Calcium Oxylate Stones

Nov 26, 2018

Calcium Oxylate Stones

Although it has always been common for us to see urolithiasis in our patients, calcium oxalate stones have definitely been increasing in frequency over the past decade. Unfortunately, these stones cannot be dissolved with medical management and recurrent formation occurs in 24-48% of patients within one year. This can require patients to have multiple cystotomies over their lifespan. So is there anything we can do to help avoid repeated cystotomy for these pets and their owners?

calcium dnaFormation of calcium oxalate uroliths is multifactorial with genetics, diet, level of water consumption, serum calcium levels, and the presence of urinary promotors being just some of the factors involved. Certain breeds such as Miniature Schnauzers, Lhasa Apsos, Yorkshire Terriers, Bichon Frises, Shih Tzus and Poodles are at increased risk. Other risk factors for the development of calcium oxalate stones include age (8-12 years), being a neutered male, or being overweight.


dog food bowlBladder stones should be removed if they are causing obstruction, inflammation, lower urinary tract signs (dysuria, pollakiuria, hematuria), or recurrent infections. Once stones are eliminated, a management protocol should be instituted to try and prevent recurrence. Firstly, any urinary tract infections need to be addressed and treated based on culture and susceptibility results. Diet manipulation is an important part of control as well. A protein-restricted, alkalinizing diet such as Royal Canin SO or Hill’s Science Diet u/d is recommended. However, both of these diets can cause weight gain and should be avoided in dogs that are prone to pancreatitis. If desired pH levels are not achieved with diet alone, the addition of urinary alkalinizing medications such as potassium citrate may be required. As a last option, a diuretic medication such as hydralazine is occasionally added to further dilute the urine. (Patients receiving hydralazine should be closely monitored for possible electrolyte deficiencies which can be caused by this drug.) Even with medical management, calcium oxalate stones may recur and recurrence within one year is seen in up to 40% of dogs. Additionally, studies have shown a 20% failure rate in removing all stones at the time of surgery and also that 9.4% of recurrent stones are suture-induced. So again, with the odds stacked so badly against us, how do we minimize the number of repeat surgeries for our patients?

refractometer deviceThe first step in minimizing the need for recurrent surgery is early and regular monitoring, because if stones are identified when small, we can often use some of the less invasive techniques outlined below for removal. A complete urinalysis should be performed periodically to monitor urine specific gravity, pH and presence of crystals. I recommend performing that first urinalysis one month after surgery and then again every three months for the first year. Remember that crystals can form when urine has been refrigerated or if it has been stored for some time; therefore presence of crystals alone doesn’t always indicate presence of stones. At each recheck a lateral abdominal radiograph or ultrasound examination of the bladder should also be performed. Ultrasound is more sensitive and will pick up smaller stones, whereas a stone typically has to measure 3mm before it can be detected radiographically. After the first year I typically find that I have a feel for how quickly or slowly each patient is likely to have recurrence and then will adjust the monitoring interval to six months if they have not formed stones already or if their rate of stone formation is relatively slow.


Non-surgical methods for Stone Removal(from least invasive to most invasive)

Catheter Retrieval

This is the simplest technique and can easily be performed in your practice. In male dogs, it may not even require any sedation. A red rubber catheter (sometimes with the end cut off) is inserted into the urinary bladder and the bladder is drained until it is fairly small. Then, with the bladder small so that the catheter is more likely to sit in the pool of stones, suction is applied using a 30cc syringe.This technique can sometimes be used to remove all stone fragments, but is most often used to obtain a sample for stone analysis. Stone fragments less than 3 mm can be retrieved (3 French is equivalent to 1 mm) with this technique.

Voiding Urohydropropulsion (VUH)
While a little more involved, my preferred technique for removing small stones is Voiding Urohydropropulsion. VUH can be performed on male and female dogs and female cats. The body weight of the patient provides a guide as to the maximum size of stone that can be retrieved with this method:

Urohydropropulsion chart

For this technique, patients are placed under general anesthesia and a urinary catheter is passed to remove urine from the bladder. Using sterile saline, the bladder is distended until it is easily palpable and turgid. The urinary catheter is removed and the patient is then held in an upright position with the spine almost 90 degrees to the table. The bladder is grasped with both hands and agitated so that the stones fall into the trigone. Manual expression is performed with steady force. Patients must be kept under deep anesthesia to facilitate expression of the bladder. This procedure is repeated until no further stones are obtained. Urine and stones can be caught in a bowl. Should you ever wish to learn this technique I am happy to have you come visit us and see how it is performed.


Cystoscopy with Intracorporeal Laser Lithotripsy and Basket Retrieval
Cystoscopy allows direct visualization of the vestibule, urethra, bladder, ureteral openings and vaginal canal. It also allows for the acquisition of biopsies. This is my preferred non-surgical technique for larger stones and we currently perform this procedure at UVS. Stones that are 5-7 mm may be grasped with a basket and removed directly from the bladder. For stones that are too big to be removed, a holmium: YAG laser can be used to fragment the stones into small enough pieces that can be removed with a combination of basket retrieval and VUH. Cystoscopy and laser lithotripsy help to ensure that all stone fragments are removed. The best success rates are seen with appropriate patient selection. Stones in female dogs and stones lodged in the urethra of male dogs are most easily removed with this procedure. In addition, a lower stone burden leads to better success. This procedure is contraindicated in very small dogs, dogs with a large stone burden and in dogs with bleeding tendencies. Complications from this procedure are typically short-lived and can include urethral swelling, hematuria, or bladder or urethral perforation (usually resolved with an indwelling urinary catheter for 2-3 days).


Extracorporeal Shock Wave Lithotripsy (ESWL)
This procedure is used primarily for stones in the ureter and kidneys and is limited to canines. Unfortunately, only a few facilities (Purdue University, AMC NY) offer this procedure due to the need for specialized equipment. A lithotripter is used to apply shock waves to the outside of the body. The shock waves have to move through a water medium (wet lithotripter) or a gel component (dry lithotripter). The shock waves break the stones into smaller fragments and/or sand so that the fragments can pass. Typically ureteral stents are placed to prevent a ureteral obstruction. Once the sand passes into the bladder (several days post-procedure), VUH is performed to remove the debris from the bladder.

Percutaneous Cystolithotomy (PCCL)
One of the concerns with surgery alone is the successful removal of all of the stones and stone debris. A technique that is beginning to gain popularity is PCCL. With PCCL, surgery is combined with cystoscopy and this increases complete stone removal rates to 96% (from about 80% with routine surgery). For this procedure, a small incision is made in the body wall and the bladder is pulled to the surface and held in place with a stay suture. A screw-tip cannula is inserted into the bladder and the scope is passed through the cannula into the bladder. A stone basket can then be used to remove the stone fragments and laser can be performed if larger stones are present.

take home message iconTake home message: Early and regular monitoring of our patients after removal of oxalate crystals allows for prompt detection of stone recurrence and provides the option of non-surgical stone removal.

No matter which technique for removal is used, once the stones are obtained, they should be submitted for analysis. This will help to guide further recommendations for management of stone recurrence. The University of Minnesota has one of the largest veterinary stone labs in the country. They offer stone analysis at no cost; however, a donation to help further research is recommended. We charge our clients a minor fee and then pass that money on as a donation. They have a very user-friendly website as well which can be found at

Veterinary Professionals