| Some mineral solutes precipitate to form crystals in urine; these crystals may aggregate and grow to macroscopic size, at which time they are known as
uroliths (calculi or stones). Uroliths generally contain an organic matrix that is believed to vary minimally among uroliths and that constitutes ~2-10% of the stone’s chemical composition. The remaining 90-98% of the urolith is composed of minerals that vary depending on the type of urolith. Urolithiasis is a general term referring to stones located anywhere within the urinary tract. Uroliths can develop in the kidney, ureter, bladder, or urethra and are
referred to as nephroliths, ureteroliths, urocystoliths, and urethroliths, respectively. |
| Uroliths in all animal species are composed of ~10 different minerals. Identification of the minerals in uroliths by quantitative analysis is unreliable. The type of minerals in uroliths can be readily identified by optical crystallography, infrared spectroscopy, and/or x-ray diffraction. Minerals found in uroliths have a chemical name and often a mineral or crystal name (Table:
Urolith Names). Variation in urine characteristics over time can result in more than one
crystal type within a single urolith. In such instances, the urolith core corresponds to conditions that were present when the urolith initially formed, and the outer layers correspond to more recent conditions that favored continued growth. |
| Mechanisms involved in stone formation have not been completely defined. However, three main contributing factors are: 1) matrix—the inorganic protein core may facilitate initial urolith formation, 2) crystallization inhibitors—organic and inorganic crystallization inhibitors may be lacking or dysfunctional in animals with uroliths, and 3) precipitation crystallization factors—a complex relationship among urine solutes and other chemical factors in the urine can
lead to conditions favoring crystallization. Regardless of the underlying mechanism(s), uroliths are not produced unless sufficiently high urine concentrations of urolith-forming constituents exist, and transit time of crystals within the urinary tract is prolonged. For selected stones (eg, struvite, cystine, urate), other favorable conditions (eg, proper pH) for crystallization must also exist. These criteria can be affected by urinary tract infection, diet, intestinal absorption,
urine volume, frequency of urination, therapeutic agents, and genetics. |
| Clinical signs associated with urolithiasis are seldom caused by microscopic crystals. However, formation of macroscopic uroliths in the lower urinary tract that interfere with the flow of urine and/or irritate the mucosal surface often results in dysuria, hematuria, and stranguria. Nephroliths often are asymptomatic unless pyelonephritis exists concurrently or they pass into the ureter. Ureteral obstruction may produce signs of vomiting, lethargy, and/or flank and renal pain,
particularly if there is acute total obstruction with distention of the renal capsule. The only clinical sign associated with unilateral urethroliths may be pain, which can be difficult to detect in dogs and cats. If these initial signs of ureteral obstruction do not lead to a diagnosis, unilateral ureteral obstruction may result in hydronephrosis with loss of function of the ipsilateral kidney. Ureteroliths may also precipitate a uremic crisis in cats with previously compensated
chronic renal failure. Because clinical signs of renal dysfunction are generally not apparent until two-thirds or more of total functional renal parenchyma is lost, clinical signs may not be observed unless both ureters are obstructed, there is contralateral chronic kidney disease, or a renal infection develops. Unilateral ureteroliths may be identified serendipitously during abdominal imaging studies or surgery.
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| Abdominal palpation is helpful in detecting urocystoliths; the bladder wall may be thickened and a grating sensation may be felt when the bladder is palpated. Although palpation may reveal a single large urolith or multiple uroliths by their crepitation, it cannot dependably identify all animals with uroliths; urethral calculi may be detected by rectal palpation or located by passing a catheter. Because multiple uroliths may be present throughout the urinary tract, a complete
radiographic examination of the tract is indicated; radiodense calculi >3 mm in diameter are usually visible on radiographs. Urate, and occasionally cystine, uroliths may be radiolucent, requiring contrast radiography or ultrasonography to confirm their presence. Urinalysis, including identification of crystals on microscopic examination of fresh, warm urine and bacterial culture and sensitivity testing, is a critical part of the evaluation and may be helpful in determining the
type of urolith present. Ultrasonography and cystoscopy may also be useful.
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Urethral Obstruction: |
| Urethral obstruction is common in male dogs and cats. It may occur suddenly or may develop over days or weeks. Initially, the animal may exhibit frequent attempts to urinate and produce only a fine stream, a few drops, or nothing. Animals may also exhibit extreme pain manifested by crying out when attempting to urinate. Complete obstruction causes uremia within 36-48 hr, which leads to depression, anorexia, vomiting, diarrhea, dehydration, coma, and death within ~72 hr.
Urethral obstruction is an emergency condition, and treatment should begin immediately. |
| If the bladder is intact, it is distended, hard, and painful; care should be used when palpating the bladder to avoid iatrogenic rupture. If the bladder has ruptured, it cannot be palpated and urine can sometimes, but not always, be obtained from the abdominal cavity by paracentesis. Animals with spontaneous bladder rupture may appear temporarily improved because the pain associated with bladder distention has been relieved; however, peritonitis and absorption of uremic toxins
and potassium occur rapidly and lead to depression, abdominal distention, cardiac arrhythmias, and death. |
| Hyperkalemia and metabolic acidosis are life-threatening complications of urethral obstruction. An ECG (to record cardiac rhythm and rate) and a serum potassium are indicated. Initial emergency care involves immediate relief of obstruction by catheterization and fluid therapy with normal saline. Occasionally, an obstruction at the external urethral orifice can be dislodged by gentle massage. Sometimes, when a portion of the urethra is dilated with fluid under pressure and then
suddenly released, urethral calculi can be flushed out. The urolith nearly always can be flushed back into the bladder by using the largest catheter that can be easily passed to the calculus, occluding the distal end of the urethral lumen around the catheter, and infusing a sterile mixture of equal parts of isotonic saline solution and an aqueous lubricant. If the urethrolith cannot be flushed back into the bladder, a urethrotomy should be performed to remove the obstructing
stone(s). Depending on the clinical circumstances, the urethrotomy site may be sutured or a permanent urethrostomy created. Calculi that are flushed back into the bladder should be removed by cystotomy to prevent recurrence, although in some cases they can be dissolved. The stone should be sent for quantitative analysis, and the animal managed medically to prevent stone recurrence based on the results. |
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