Feline Calcium Oxalate Urolithiasis: Risk factors and rational treatment approaches

Abstract

Practical relevance: Uroliths occur commonly in the bladder and/or urethra of cats and can be lifethreatening if urethral obstruction occurs. Calcium oxalate accounts for 40-50% of urocystoliths and these stones are not amenable to medical dissolution; therefore, removal by surgery or minimally invasive techniques is required if uroliths must be treated. Medical protocols for prevention involve decreasing urine saturation for minerals that form uroliths.

Etiopathogenesis: Formation of uroliths is not a disease, but rather a complication of several disorders. Some disorders can be identified and corrected (such as infection-induced struvite urolith formation); others can be identified but not corrected (such as idiopathic hypercalcemia). In most cats with calcium oxalate urolith formation the underlying etiopathogenesis is not known. A common denominator of all these disorders is that they can from time to time create oversaturation of urine with one or more crystal precursors, resulting in formation of crystals.

Basic concepts: In order to develop rational and effective approaches to treatment, abnormalities that promote urolith formation must be identified, with the goal of eliminating or modifying them. It is important, therefore, to understand several basic concepts associated with urolithiasis and the factors that promote urolith formation that may be modified with medical treatment; for example, the state of urinary saturation, modifiers of crystal formation, potential for multiple crystal types, and presence of bacterial infection or urinary obstruction.

Figure 1

Lateral abdominal radiograph of an 8-year-old, castrated male domestic shorthair cat showing one calcium oxalate dihydrate urocystolith (arrow). Renal mineralization is also present (asterisk)

Figure 2

Ultrasonographic image of the urinary bladder of a 14-year-old, castrated male domestic shorthair cat with urolithiasis. Urocystoliths appear as shadowing hyperechoic structures

Figure 3

Crystalluria: struvite (double arrow) and calcium oxalate (single arrow) crystals in a urine sample collected from a 6-year-old, castrated male domestic shorthair cat

Figure 4

(a) In voiding urohydropropulsion, the urinary bladder is distended with sterile fluid. (b) The cat is held in a vertical position after the urinary bladder is distended. (c) The transurethral catheter is removed and the urinary bladder is gently agitated by grasping it through the abdominal wall. (d) The urinary bladder is gently compressed inducing micturition and voiding of the urocystoliths into the cup (arrowheads). (e) Most cats experience hematuria after voiding urohydropropulsion

Figure 5

(a) Lateral abdominal radiograph of an 8-year-old, castrated male domestic shorthair cat with a single urocystolith (white arrow). (b) A small incision (usually less than 2–3 cm) is made on the ventral midline over the apex of the urinary bladder. (c) The urinary bladder is grasped through a small incision and tacked to the body wall. A stab incision is made through the bladder wall and a rigid cystoscope is inserted. The urocystolith, being grasped with four-prong Nitinol graspers, is projected on the endoscopic monitor. (d) Cystoscopy provides magnification and better visualization of small uroliths (asterisks). (e) Urocystolith following retrieval. (f) The body is closed in routine fashion with an intradermal pattern used for skin closure

Figure 6

(a) Cystoscopic appearance of the urethral orifice (top) in a 10-year-old, spayed female domestic shorthair cat. Cystoscopy is performed in dorsal recumbency. (b) Calcium oxalate urocystolith located at the trigone. (c) Laser lithotripsy is performed using a Ho:YAG laser by passing a fiber through the operating port of the cystoscope. The green light is the aiming beam as Ho:YAG laser energy is outside of the visible spectrum. (d) Urocystolith fragments are retrieved using retrieval devices and/or voiding urohydropropulsion