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An Urban Experience
the renal pelvis resulting in improvement of urine  ow and azotemia.
Dialysis.(2, 3) In some patients dialysis may be
required for stabilization. Dialysis is the process of removing solutes and/or water from one solution (plasma) to another solution (dialysate) by an osmotic gradient through a semi-permeable membrane. The
2 types of dialysis are peritoneal dialysis and hemodialysis. Dialysis involves the use of one type of semi-permeable membrane that allows the passage
of small molecules while preventing the transfer and
loss of large molecules. In peritoneal dialysis, the peritoneal membrane, interstitial tissue, and capillary endothelium act as the membrane. In hemodialysis, blood is circulated outside of the body through a dialyzer. The foundation of peritoneal dialysis is the dynamics
of  uid and solute exchange across a semi-permeable membrane. Large molecules such as proteins pass slowly through such a membrane. Smaller molecules (urea and glucose) and ions (sodium and potassium) move easily across the membrane down a concentration gradient until equilibrium is reached on either side of
the membrane. Water moves across the membrane from the solution of lower osmolality to that of higher osmolality until equilibrium is reached. Hemodialysis
is a renal replacement therapy that provides a bridge
of metabolic stability to patients who would otherwise die from the systemic effects of severe uremia. The principles of hemodialysis are similar to those of peritoneal dialysis except that blood is shunted outside of the body, passed through a dialyzer (“puri ed”), and returned to the body. The composition of uremic blood is normalized by exposure to a contrived solution, the dialysate, across a semi-permeable membrane in a device called a hemodialyzer. During hemodialysis, water and small molecular weight solutes and uremic toxins pass readily through the membrane pores (diffusion channels), along diffusive and hydrostatic gradients, but the movement of larger solutes (such as plasma proteins and cells), are limited by the size of the pores. Excessive body water and additional solute can be forced
through the membrane by ultra ltration produced by hydrostatic or osmotic forces imposed across the dialysis membrane. Net removal of uremic solutes is in uence
by: (a) the concentration gradient for diffusion, (b) the diffusivity of the solute, (c) permeability characteristics and surface area of the membrane, (d) blood and dialysate  ow within the dialyzer, (e) duration of dialysis, (f) distribution volume of the solutes, and (g) amount of ultra ltration (convective transfer).
Nephrostomy tubes. A nephrostomy tube may be placed for urinary diversion in order to relieve pressure on obstructed kidney. One end is curled (pig-tailed) that has a locking mechanism to maintain it in the dilated renal pelvis. Tubing exits the body wall and is connected to external urine collection bag. This requires anesthesia
and either surgery or use of interventional radiology and usually is not performed in lieu of more de nitive intervention.
Surgical management. Surgical removal is not necessary for all nephroureteroliths. Surgery is not indicated for relatively small, non-obstructive upper urinary tract uroliths. Small upper urinary uroliths may pass into the urinary bladder. It is unknown what a safe time period
to allow for ureteroliths passing without resulting in irreversible renal damage. Renal function does not recover in dogs with unilateral ureteral ligation for 40 days. If a dog or cat is ill due to ureteral obstruction, it
is not probably appropriate to wait for the ureteroliths to pass or to medically manage them for a prolonged period of time if renal function is to be preserved. If a urinary tract infection is present, removal of the upper urinary tract urolith may be necessary to control the infection; however, it is not possible to determine prior to surgery
if the uroliths are the source of infection. If bilateral upper urinary tract surgery is required, the procedures should be staged with an approximate separation of 4 weeks in order to re-evaluate renal function and to allow recovery from the  rst procedure. In general, the side with the most renal function should be operated  rst in order to preserve as much renal function as possible. Nephroliths can be removed either nephrotomy or by pyelolithotomy; however with either technique, it is not always possible to retrieve all uroliths. Nephrotomy should be avoided,
if possible, because it causes renal damage and loss of nephrons. Pyelolithotomy is preferred because renal parenchymal tissue is not incised and so loss of renal function is less likely to occur when compared with nephrotomy; however, pyelolithotomy cannot
be performed unless the renal pelvis is dilated. Often
it requires microsurgical technique. Ureterotomy is technically dif cult to perform because of the diameter of the ureter, especially in cats and stricture formation often occurs. Ureterotomy can occasionally be performed if the ureter is dilated around the ureterolith. Nephrectomy and ureterectomy are indicated for a severely hydronephrotic, non-functional kidney, but is not recommended in most cases as functional renal tissue may be present.
Comparison of medical vs surgical management.(4) In
a retrospective analysis of medically versus surgically managed patients with ureteroliths, surgically managed patients tended to do better over a longer time. Surgery, though, was associated with more complications primarily in the perioperative period. There was a recurrence rate of 40%.
Lithotripsy. Extracorporeal shock wave lithotripsy (ESWL) is a standard of care for many human patients with upper urinary tract uroliths, and has been performed successfully in dogs and cats.(5) In the 1970’s, it was recognized that when raindrops or small meteorites collided with aircraft surfaces, pits

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