«History and Clinical Signs The sources of discomfort in acute urethral obstruction appear to be traumatic injury to the urinary tract epithelium and ...»
Urolithiasis in Small Ruminants: Surgical and Dietary Management
David C. Van Metre, DVM, DACVIM
Colorado State University
Urinary calculi, or uroliths, are concretions of solid mineral and organic compounds that cause
disease through direct trauma to the urinary tract and obstruction of urinary outflow. The urethral
process is the most common site of obstruction in sheep and goats; in those whose urethral process
has been amputated, the distal aspect of the sigmoid flexure is the usual site for blockage. In camelids, uroliths tend to become lodged at or distal to the distal aspect of the sigmoid flexure. Multiple calculi are usually present in the urinary tract of affected small ruminants.
History and Clinical Signs The sources of discomfort in acute urethral obstruction appear to be traumatic injury to the urinary tract epithelium and bladder distention following obstruction. However, the presenting complaint provided by the owner or caretaker may have little apparent relation to urinary tract disease. In a case series of 94 cases of urolithiasis admitted to the Veterinary Medical Teaching Hospital at the University of California at Davis, anorexia and bloat were the most common primary clinical complaint at admission. Owners of affected animals frequently misinterpret these clinical signs as being reflective of an acute gastrointestinal disorder. It is possible that the mild bloat that seems so common to these cases occurs as a result of activation of the sympathetic nervous system; in other words, the painful abdominal process results in rumen atony. Owners of camelids appear to commonly misinterpret stranguria as tenesmus, as several camelid urolithiasis cases in the author’s practice received previous treatment for colonic impaction prior to referral. Consequently, if the patient is an ill male or castrated male ruminant, camelid, or pig, it is critical to determine if that animal is capable of urination. Prior to veterinary examination, males or castrated males with compatible historical data should be moved to a dry, unbedded area so that urine production can be assessed. The African Pygmy goat may be predisposed to urinary tract obstruction, as this breed had significantly higher representation in urolithiasis admissions than other breeds in a 1995 study.
Early on, the animal postures repeatedly to urinate, and the tail may be seen to “pump” up and down as the animal strains to void urine. The abdominal musculature may heave with the forceful attempts to void.
These forceful voiding attempts may result in frequent passage of small volumes of urine or no urine at all.
As bladder distention progresses, the animal may tread, stretch, and kick at its abdomen. Vocalization is common in goats experiencing pain during urination Fig. 1 attempts. Blood or crystals may be adhered to the preputial hairs.
Confirmation of the diagnosis can be achieved by deep abdominal palpation in sheep and goats. Using the fingertips of both hands placed in the flanks, the examiner gently presses each hand toward the midline, as shown in Figure 1. A firm, spherical mass can be palpated in the caudal abdomen in obstructed sheep and goats that have an intact bladder. Given that camelids have a less pliant abdominal wall than sheep and goats, confirmation of a diagnosis of urolithiasis in llamas and alpacas requires transrectal or transabdominal ultrasonographic examination. Ultrasonographic examination of the perineum may be used to determine the location of the obstruction, if this information is helpful in selection of the surgical procedure.
Signs of pain usually subside upon rupture of the bladder or urethra. The empty bladder is no longer palpable. Anorexia and lethargy progress with these complicating conditions and are accompanied by progressive ascites or ventral abdominal wall edema with bladder or urethral rupture, respectively.
Abdominocentesis and abdominal fluid creatinine measurement can be used to confirm the presence of uroperitoneum.
Surgical Treatment of Urolithiasis Preoperative Evaluation Whenever possible, the author prefers to perform Fig. 2 ultrasonographic evaluation of the kidneys of animals whose clinical history is suggestive of prolonged disease, defined as signs of obstruction for greater than 24 hours duration. Detection of severe hydronephrosis (complete absence of renal cortical tissue) would warrant a grave prognosis and obviate the decision for surgery. Figure 2 shows the transabdominal ultrasonogram from a 3 year-old intact male alpaca with a history of urinary obstruction of 2.5 days duration.
Note the presence of renal pelvic dilation but the presence of visible cortical tissue; the latter finding prompted continuation with treatment with a guarded prognosis for long-term renal function. Azotemia did resolve in this animal after surgery.
The animal suffering from urolithiasis may be relatively stable if the disease is detected early in its course. More advanced signs of hypovolemic crisis may develop if the animal has developed severe prerenal azotemia and/or electrolyte imbalances, which usually accompany prolonged (24 hour) history of disease, or if the animal suffers from uroperitoneum and the associated electrolyte and acid-base imbalances.
Intravenous fluid therapy is indicated in cases of moderate or severe dehydration or if any of the above conditions exist. Physiologic (0.9%) saline solution is the fluid of choice for animals with uroperitoneum because these animals are typically hyponatremic and hypochloremic. Owing to the variability of serum potassium concentration in ruminants with obstructive urolithiasis, supplementation of potassium salts to polyionic fluids should be performed only if the animal is determined to be hypokalemic. Animals with chronic impairment of urine egress, metabolic acidosis, and/or uroperitoneum appear to be at risk for hyperkalemia. Hypocalcemia may also occur secondary to bladder rupture, and saline can be supplemented with calcium salts.
If general anesthesia is to be used, dehydration and severe electrolyte imbalances should be corrected prior to induction; normalization of blood sodium and potassium concentrations is of central importance. This guideline should be followed even if intravenous fluid therapy may result in bladder rupture. Bladder rupture can be repaired; anesthetic complications associated with urolithiasis are often difficult to correct. The author prefers to perform cystocentesis in obstructed small ruminants that require large-volume fluid therapy prior to anesthesia. While urine leakage through the cystocentesis site inevitably results in uroperitoneum, the site of cystorrhexis is made focal and is far easier to repair than spontaneous bladder rupture. Preoperatively, any urine in the abdominal cavity can be slowly drained with a trocar or large-bore indwelling catheter. Drainage of urine reduces pressure on the diaphragm, which is critical for animals restrained in lateral or dorsal recumbency, and slows the progression of azotemia and electrolyte imbalances.
The normal epithelial defenses of the lower urinary tract are damaged in urolithiasis, making the animal more prone to ascending urinary tract infection. Antibiotic therapy, therefore, is a sound choice if treatment is elected. Beta lactam antimicrobials or sulfonamides are appropriate choices. Aminoglycosides and fluoroquinolones are also concentrated in the urine, but their use is limited in food animals, owing to prolonged residues and federal laws limiting use, respectively.
Issues Related to Informed Client Consent
Owner consent for surgical treatment should include a discussion of the following potential complications: 1) Cardiac or respiratory arrest during sedation, anesthesia, or restraint / recumbency; 2) renal failure secondary to hydronephrosis, even if ultrasonographic evaluation reveals normal renal structure); 3) intraoperative and postoperative hemorrhage caused by the platelet and coagulation cascade dysfunction that can accompany uremia; and 4) formation of additional uroliths and/or movement of renal uroliths into the urethra, causing recurrent urethral obstruction postoperatively. The latter complication appears to occur in a significant number of cases. In a 1996 study, 10 of 23 cases successfully unblocked by various surgical methods developed recurrent urethral obstruction within a few months to years after discharge. Owner compliance with dietary recommendations was not determined but was subjectively assessed as inconsistent. Therefore, it is critical that the owners understand the inherently high recurrence rate associated with surgical management of urolithiasis in small ruminants. Adherence to preventive dietary measures may limit recurrence, but supportive data is currently lacking.
Urethral Process Amputation
The urethral process should be examined in all cases of suspected urolithiasis in small ruminants. Sedation and/or lumbosacral epidural anesthesia facilitate penile extrusion. Although it produces sedation and is a potent analgesic, xylazine is not recommended for use in ruminants with urolithiasis, as its diuretic effect may worsen bladder distension if urethral obstruction is complete. The author prefers diazepam (0.1 mg/kg IV, slowly) to provide sedation without analgesia.
Once sedated, the sheep or goat should be propped up on its rump. The examiner should grasp the penis through the skin at the base of the scrotum and force the penis cranially. As the glans protrudes from the prepuce, it can be grasped with a dry gauze, and the penis can be exteriorized completely. If obstructed, the urethral process can be amputated at its base, near the glans of the penis. Removal of the urethral process has no adverse effect on breeding ability or fertility.
In larger rams and bucks, resistance to penile extrusion may limit one’s ability to visualize the urethral process. Lumbosacral epidural anesthesia provides complete analgesia for penile extrusion and eliminates resistance to extrusion caused by the retractor penis muscle. One ml of 2% lidocaine per 5 kg to 10 kg of bodyweight is injected into the epidural space at the lumbosacral junction. The total dose should not exceed 15 ml in any small ruminant. Motor blockade of the hindlimbs lasting 1-3 hours will occur, and a well-bedded area should be available for larger patients to allow for safe recovery of motor function. If the sacrococcygeal space is palpable, 1-2 ml of 2% lidocaine can be injected at this site for perineal analgesia, although in the author’s experience, penile motor blockade occurs far less predictably than with lumbosacral anesthesia.
Another possible limiting factor for exteriorization of the penis a the persistent frenulum. The frenulum is the normal anatomic attachment that exists between the penis and the preputial mucosa. It normally breaks down at puberty. This typically occurs at 4-8 months of age in sheep and goats and 1.5-2.5 years of age in alpacas. If present, the frenulum will effectively prevent exteriorizing the penis.
The persistent frenulum may be most problematic in prepubertal animals with urolithiasis, animals castrated at a young age, and (rarely) the intact yearling or two year-old ram or buck. If a persistent frenulum is present, one may have to anesthetize the animal and force the penis out of the prepuce for examination.
Immediate restoration of urethral patency (a.k.a. urine voiding and emptying of the distended bladder) has been reported to occur in 37.5% to 66% of amputations. However, if urethral patency is restored with this procedure, it may be maintained only for hours to a few days, as additional calculi in the bladder or urethra often cause recurrent urethral obstruction. This procedure maintained urethral patency in only 4 of 14 cases in a California study. Thus, if it is the only procedure performed, urethral process amputation may not result in a long-term cure.
Urethral catheterization and flushing
Retrograde urethral catheterization and saline flushing (termed urohydropulsion) is often successful in relieving urethral obstruction in cats and dogs. However, this procedure does not appear to be successful in most cases in small ruminants; only 1 of 35 small ruminants treated at admission with urohydropulsion became unblocked. Retrograde catheterization of the ruminant bladder is very difficult because catheters tend to become lodged in the urethral recess (formerly termed urethral diverticulum), which lies on the dorsal aspect of the urethra at the level of the ischium.
Perineal Urethrostomy Perineal urethrostomy is a popular surgical method for correction of obstructive urolithiasis in small ruminants. This procedure can be performed with sedation and local anesthesia, epidural anesthesia, or general anesthesia. It is a surgical option for ruminants not intended to be used for breeding. Its primary limitations involve stricture of the stoma and/or recurrent obstruction with additional calculi. These complications can occur within weeks to months after surgery. In four reports, half or more of small ruminants treated with perineal urethrostomy developed these complications in less than 12 months after 4-7 surgery. Therefore, this procedure may not be optimal for ruminants kept as pets.
Urethrostomy can be performed at any site along the perineum, but the author prefers to place the urethrostomy site low in the perineum in order to minimize urine scalding of the skin and to allow room for repeated urethrostomy higher in the perineum if stricture at the original site occurs.
The skin and subcutaneous incisions are made on midline for a total length of 6-8 centimeters.
Deep to the subcutis, the paired retractor penis muscles are found on midline. These muscles are superficial to the penis and easily separable into the two component muscles. These can be transected or simply retracted aside. The penis is relatively firm, 0.5-2 cm in diameter in small ruminants, and is covered by the white tunica albuginea. It may be located quite deep in the perineum in larger sheep, and as a rule, the higher (more dorsal) in the perineum that the approach is made, the deeper is the penis. While traction is placed to exteriorize the penile shaft, sharp and blunt dissection are used to free the penis from the surrounding fascia until a 4-6 cm segment of the penis can be exteriorized without tension. Adequate exteriorization of the penis is essential to minimize tension on the urethrostomy suture line.