A minimally invasive option to treat this common occurrence in pets.
Urolithiasis is a common clinical problem in small animal veterinary patients. See how a minimally invasive technique helped a Bichon frise.
Urine specific gravity = 1.021; pH = 8; too numerous to count cocci bacteria/hpf; 10 to 20 WBC/hpf; 5 to 10 RBC/hpf; protein 3+
> 100,000 colonies
All results normal
Serum chemistry profile:
Blood urea nitrogen = 38 mg/dl; creatinine = 2.1 mg/dl (slightly elevated)
Figure 1: A radiograph of a female dog with bilateral large staghorn nephroliths.
Because of the positive Staphylococcus species urine culture results with staghorn stones in both renal pelves and an elevated urine pH, the cause of the azotemia is likely related to chronic pyelonephritis that is associated with struvite nephrolithiasis. Typically, if stones are made of struvite, or magnesium ammonium phosphate, they can be medically dissolved with a dissolution diet (either acidifying or neutralizing) and appropriate antibiotic therapy. This must be followed strictly for effective dissolution. For nephroliths, it can take three to nine months for appropriate dissolution. Many struvite nephroliths contain some calcium apatite or a calcium oxalate shell and, thus, will not dissolve well.
A radiographic examination and a urine culture should be performed every month to confirm no bacterial growth and the stones are not getting larger. If by three months the stones are not getting smaller, then removal may be necessary.
This patient was followed for five months with serial negative urine cultures and radiographs. The nephroliths did not change in size, and minimally invasive removal was elected with the use of staged bilateral percutaneous nephrolithotomy (PCNL) scheduled six weeks apart. With the use of intracorporeal ultrasonic lithotripsy, the stones were able to be successfully removed without complication, avoiding a nephrotomy.
The left side was treated first, and the entire stone was able to be removed through a small sheath using nephroscopy and lithotripsy. A ureteral stent was then placed prophalactically to prevent any edema or fragments from obstructing the ureter. Six weeks later, the same procedure was done on the right side. Four weeks after both kidneys were completed, the patient underwent a cystoscopy for bilateral ureteral stent retrieval.
Over the course of the following year and a half, there were no recurrent urinary tract infections, no uroliths were detected on serial radiographic or ultrasonographic examinations and the creatinine concentration improved to 1.2 mg/dl. The dog was maintained on a neutralizing stone diet.
In the past, nephroliths were treated medically, via dissolution, or removed by surgical nephrostomy, nephrectomy or pyelotomy. Management of calculi can be challenging in small animals as the incidence of stone recurrence in dogs and cats is high. Many canine and feline nephroliths remain static in size and clinically silent for years.
Some controversy still exists as to whether nonobstructive kidney stones worsen underlying kidney disease. Their removal is typically recommended if the stones are growing, if there is a partial or complete ureteropelvic junction obstruction resulting in progressive hydronephrosis or if renal parenchymal loss, worsening renal function, chronic hematuria, pain or recurrent infections are occurring.
In small animals, traditional surgical interventions are met with complications that can be severe and life-threatening. Nephrotomy has been associated with severe hemorrhage, decreased renal function, ureteral obstruction cause by nephrolith remnants and urinary leakage.
In a study in normal cats, there was a 10 to 20 percent decrease in the glomerular filtration rate (GFR) of the ipsilateral kidney after a nephrotomy. This was clinically insignificant in normal cats, but in a clinical patient that has maximally hypertrophied the remaining nephrons because of prior nephrolith-induced damage, the significance could be dramatic. So patients with an already compromised GFR from chronic stone disease may develop a clinically significant decline in renal function. Also, since more than 30 percent of adult cats will develop chronic kidney disease in their lifetime, resulting in a 67 to 75 percent decline of renal function, these cats cannot tolerate a 10 to 20 percent further decline in GFR from a nephrotomy. Hence, this procedure should ultimately be avoided whenever possible. Similar studies have not been done in clinical dogs to date—only normal dogs without exhausted hypertrophy mechanisms have been studied, which would not be considered clinically equivalent to our patient population.
In people, the treatment of choice is typically minimally invasive, consisting of extracorporeal shockwave lithotripsy (ESWL) for nephroliths smaller than 2 cm and PCNL for nephroliths larger than 2 cm. Open surgery and laparoscopy are usually considered necessary after other less invasive options have failed or have been deemed inappropriate. These and many other human studies have shown ESWL and PCNL to have a minimal effect on the GFR of clinical stone forming patients, particularly when compared with traditional surgical nephrotomy. These procedures, particularly PCNL, have been shown to be highly effective in removing all stone fragments, as endoscopic calyceal inspection is superior for visualization and fragment retrieval.
In small animals, PCNL is considered if ESWL fails, ESWL is not available, cystine stones are present (which are ESWL-resistant) or the stone is larger than 15 mm. PCNL has been performed in a handful of dogs to date, as well as in a cat. Typically, this is done by using a combination of ultrasonographic, endoscopic and fluoroscopic guidance. Patient size is less of a factor for PCNL than for ureteroscopy, as the smallest dog that had successful PCNL was only 3.1 kg.
Typically, for PCNL, the renal pelvis is accessed through the greater curvature of the kidney with ultrasound guidance using a renal access needle. Subsequently, with fluoroscopic guidance, a sheath (12- to 30-Fr) and balloon dilation catheter combination is advanced through the renal parenchyma into the renal pelvis over a guide wire and onto the offending nephrolith. A mini-PCNL approach using an 18- or 24-Fr access kit is used.
Figure 2: An endoscopic image during intracorporeal lithotripsy for removal of a large struvite nephrolith. Note the large yellow stone inside the renal pelvis. The lithotrite is fragmenting the stone into small pieces for removal.
Once the sheath is in the renal pelvis, a nephroscope is used to identify the stone or stones. If small enough, a stone-retrieval basket is used to remove the stone. If the stone or stones are larger than the sheath, then intracorporeal lithotripsy is used for stone fragmentation (ultrasonic, electrohydraulic or Hol:YAG laser) (Figure 2). This is all performed with fluoroscopic (Figure 3) and endoscopic guidance. Once the stones are small enough to fit through the sheath, they are removed, and a locking-loop nephrostomy tube (5- or 6-Fr) is left in place to allow the small hole to seal and form a nephropexy.
Figure 3: Fluoroscopic images during a PCNL procedure: A) A patient with sheath (black arrow) inside the kidney onto large nephrolith (white asterisk). A safety guide wire is down the ureter (white arrows). B) A nephroscope (white arrow) inside the renal sheath as the large nephrolith is fragmented (black arrowheads). C) Image after the nephrolith is removed. D) A ureteral stent (white arrows) in place to protect the ureter.
Surgical-assisted nephroscopic lithotripsy is performed by using a rigid endoscope during an open laparotomy, resulting in excellent magnification of the renal pelvis for stone retrieval. The same procedure as above (PCNL) can be performed via laparotomy rather than percutaneously, termed nephroscopic guided lithotripsy.
The greater curvature of the kidney is punctured with a renal puncture needle, and a pyelogram is performed. A guide wire is then advanced into the renal pelvis with fluoroscopic guidance, and a balloon dilation catheter, preloaded with a matching sheath, is advanced over the wire and onto nephrolith. The balloon is dilated, and the sheath slides right over the balloon with a smooth transition. The balloon is withdrawn, and the scope is inserted through the sheath for stone removal.
Little risk of hemorrhage exists when using a balloon-sheath combination since this creates a tamponade effect, and the renal parenchyma is spread apart with the balloon, not cut with a scalpel blade, making renal damage dramatically less then that with a nephrotomy. If this is done surgically, then a nephrostomy tube is not necessary because the hole can be closed by directly suturing the renal capsule to prevent leakage. Because a balloon is used to dilate the renal parenchyma, minimal nephron loss occurs, and the tissue is compressed via balloon dilation rather than being incised. This is now being performed routinely on canine kidneys in our practice and has been highly successful.
Unlike a nephrotomy, these minimally invasive procedures do not require transient occlusion of renal vessels and result in a much smaller hole in the renal parenchyma than a nephrotomy. Placing sutures in the capsule and apposing the small incision can close the renal access point. Care must be taken to assess all areas of the renal pelvis and renal calyces, as small calculi can remain if fluoroscopy and contrast nephrography are not used concurrently. We recommend concurrent fluoroscopy, as is recommended in human urology (Figure 3).
After the procedure, a ureteral stent is placed to protect the ureter from any small fragments that could an obstruction (Figure 4). This stent is then removed four to six weeks later cystoscopically.
For more case examples and to see how interventional radiology and interventional endoscopy can benefit your patients, visit amcny.org/interventional-radiology-endoscopy.
Interventional radiology and interventional endoscopy tip
• Raza A, Turna B, Smith G, et al. Pediatric urolithiasis: 15 years of local experience with minimally invasive endourological management of pediatric calculi. J Urol 2005;174(2):682-685.
• Donner GS, Ellison GW, Ackerman N, et al. Percutaneous nephrolithotomy in the dog: An experimental study. Vet Surg 1987;16(6): 411-417.
• Al-Shammari AM, Al-Otaibi K, Leonard MP, et al. Percutaneous nephrolithotomy in the pediatric population. J Urol 1999;162(5):1721-1724.
• Lennon GM, Thornhill JA, Grainger R, et al. Double pigtail ureteric stent versus percutaneous nephrostomy: effects on stone transit and ureteric motility. Eur Urol 1997;31(1):24-29.
Dr. Berent is the director of Interventional Endoscopy Services in the Department of Diagnostic Imaging at The Animal Medical Center in New York City. Dr. Weisse is the director of Interventional Radiology Services in the Department of Diagnostic Imaging at The Animal Medical Center in New York City.
For a complete list of articles by Drs. Berent and Weisse, visit dvm360.com/interventional