Urolithiasis

Introduction

Urolithiasis is the main cause of obstruction in the lower urinary tract. Although it occurs in both males and females, urolithiasis is more common in males due to their long, narrow urethra.

Common sites of obstruction in the different species include:

cats - the terminal urethra

dogs - base of the os penis

bulls - the proximal end of the sigmoid flexure or the ischial arch

rams - vermiform appendage

birds - cloaca

The most common location for urolith formation is the bladder lumen although they may also occur in the renal pelvis, ureters or urethra.

Consequences include obstruction, ulceration, cystitis, urethritis, nephritis, hydronephrosis and rupture of the bladder or urethra.

Predisposing factors include bacterial infection, alterations in pH of the urine, heredity, sex, species and decreased water intake.

In birds, the condition arises during breeding or following egg incubation. Some birds sit very tight on their eggs and fail to void their cloacal contents regularly. Urates can precipitate in the cloaca and lead to the formation of a urolith.

Types of uroliths

Struvite most common urolith in small animals

commonly associated with infection by urease producing organisms such as Staphylococcus or Proteus

more common in females due to increased susceptibility to urinary tract infections

formation enhanced by the presence of alkaline urine

Calcium oxalate form due to supersaturation of urine with calcium

often have spine-like projections

Urate Dalmations predisposed, urolith of birds

Cystine occurs with cystinuria

Calcium carbonate occurs in horses

Silica occurs in pastured ruminants

development may be related to diet

Clinical signs

Clinical signs are due to the macroscopic uroliths in the lower urinary tract which interfere with the flow of urine or irritate the mucosal lining. This leads to signs such as: dysuria, stranguria and haematuria.

Nephroliths are often asymptomatic unless pyelonephritis develops or the stone passes into the ureter.

Ureteroliths may present as pain which is difficult to detect, or signs of vomiting, lethary, especially if there is distension of the renal capsule and developing hydronephrosis. Clinical signs may not be apparent unless both ureters are affected, there is contralateral chronic kidney disease, or a renal infection develops.

Urethral obstruction may occur suddenly or may develop over days or weeks. There will be dysuria and stranguria, and complete obstruction leads to: depression, anorexia, vomiting, dehydration, coma and death.

Birds may present weakened, straining frequently and passing only small volumes of faeces and uric acid.

Diagnosis

Abdominal palpation is helpful in finding bladder stones. Rectal palpation may help identify urethral calculi.

However, multiple uroliths can be present throughout the urinary tract, and radiographic examination is always indicated.

Radiodense calculi >3mm in diameter can usually be seen, but contrast radiography and ultrasonography may be necessary to identify radiolucent calculi.

Contrast studies include: excretory urogram for nephroliths and ureteroliths, double contrast radiography for bladder stones, retrograde urethrogram for urethroliths.

Urinalysis, including examination of urine, urine culture and sensitivity and cristal identification on microscopic exmination, is also indicated.

Urethral obstruction is diagnosed by palpating a distended, hard and painful bladder.

Once removed, stones can be sent for quantitative assessment.

Treatment

Nephroliths and ureteroliths: if only one side is affected, benign neglect can be used if the other kidney is functional. If hydronephrosis has occurred, ureteronephrectomy should be performed on the affected side.

Surgical removal of the stone may be possible, and dietary dissolution can be considered if the ureter is not obstructed.

Bladder stones can be dissolved medically, or stones removed by voiding urohydropropulsion or cystotomy.

Urethral stones should be flushed back into the bladder and removed by cystotomy, or a urethrostomy can be performed in refractory or recurrent cases.

Medical management can be attempted if there is no obstruction, and the stone is amenable to dissolution (struvite, cystine or urate). Options include: diet or drug therapy, increasing water intake, changing uring pH.

Treatment of a urethral obstruction is an emergency and the animal should be stabilised prior to sedation and manipulation, due to the hyperkalaemia which predisposes to cardiac arrhythmias.

In birds, cloacal urolithiasis is treated by breaking up the urolith with a pair of forceps and removing it piecemeal. Antibiotics should be administered for 3-5 days after due to the local trauma which can be caused by the removal.

References

Kahn, C. (2005) Merck veterinary manual Merck and co

Forbes, N. Self-assessment colour review: avian medicine Manson Publishing

Syme, H. (2009) Urogenital system: urolithiasis RVC student notes

Definitions

Urolithiasis - the presence of calculi, or uroliths, in the urinary passages.
Urolith (calculus, stone) - polycrystalline concretion of precipitated urinary solutes, urinary proteins and proteinaceous debris.