The most common cause of urinary obstruction in small animal surgery is urinary stones in the urethra. Other causes of obstruction are neoplasia in the urethra, and neoplasia compressing the urethra. Generally, small cystic calculi migrate to the neck of the bladder during micturition and pass into the urethra. In the male, urethral calculi most commonly lodge caudal to the os penis. In the female, calculi may lodge at any location along the length of the urethra. Urethral obstruction is more common in the male than female.
Urinary obstruction requires medical treatment to stabilize the patient to surgery and surgical treatment to release the obstruction. Dogs and cats with urinary obstruction are presented with hematuria, stranguria, and pollakiuria. Plain radiographs or radiographs with contrast material (excretory radiographs, cystourethrograms) will confirm the presence of a urinary stones or mass in the urethra.
Blood work with complete blood count, and biochemistry is required to evaluate kidney function of the patient. Urinary obstruction is associated with severe post renal azotemia and severe electrolytes imbalance. Hyperkalemia is the most common electrolyte abnormality. Hyperkalemia can induce severe bradycardia if the concentration is over 5 mEq/dl. Therefore, it is important to reduce the hyperkalemia before anesthetizing patients with urinary obstruction.
The most efficient technique to lower the potassium concentration is to diaries the patient. If the patient can still urinate on its own intravenous fluid therapy will induce a diuresis. If the patient cannot urinate, cystocentesis can be performed to empty the bladder. The concentration of potassium is monitored very closely to be able to anesthetize the patient as soon as the potassium is back to normal limits. If diuresis is not efficient, insulin/glucose and bicarbonate can be used. While the glucose moves intracellular it brings the potassium in the intracellular space. Potassium and hydrogen particles are exchanged potassium within 30 minutes. If the potassium concentration is not reduced, then calcium gluconate is used to protect the myocardium form the effect of potassium.
When the potassium concentration is within normal limits general anesthesia can be performed. After induction of general anesthesia the urinary stones lodged in the urethra can be retropulsed in the bladder. All the efforts should be made to retroflex the uroliths in the bladder. It is easier to perform a cystotomy than an urethrotomy, and there is less morbidity associated.
To perform a successful repulsion the following steps have to be performed after inducing deep general anesthesia with intubation:
A. Thoroughly mix 45 cc of sterile saline and 15 cc of Surgilube in a 60 cc syringe and attach to the largest high density polyethylene urinary catheter that will pass through the os penis (5 to 8 French).
B. Pass the catheter up to and against the calculus. If the patient is a male, place a gauze sponge around the tip of the penis and occlude the penis around the catheter by squeezing it with thumb and finger.
C. Using a back and forth action on the catheter, simultaneously inject the saline/lubricant mix under pressure. The calculi and urethra are lubricated and the viscosity of the mix encourages the calculus to dislodge and flush into the bladder. This technique is attempted regardless of how many stones are in the urethra.
D. If the above technique fails, place a finger in the rectum, palpate the urethra, and occlude its lumen, repeat step 3 above and when maximum pressure is exerted on the urethra by the saline/lubricant mix, suddenly release digital urethral occlusion allowing lodged calculi to flush into the bladder. This technique allows maximal dilatation of the urethra.
This technique is successful in most of the cases. Lidocaine can also be injected while the catheter is in contact with the stone to release a spasm of the urethra around the stones.
If the stones are successfully retroflexed in the bladder, the urinary catheter is left in place to prevent migration of the stones back into the urethra.
If the stone cannot be retroflexed in the bladder, then a urethrotomy or urethrostomy is performed.
After a midline incision in the caudal abdomen, a ventral cystotomy is performed to expose the inside of the bladder. After placing three or four stay sutures in the wall of the bladder to minimize trauma during manipulation of the bladder, the uroliths are removed either with a spoon, or suction. The bladder neck and lumen should be explored with a finger to detect remaining large uroliths. A biopsy of bladder wall should be performed then for culture and sensitivity. Bladder wall culture gives more reliable results regarding bacterial infection than urine culture. Bladder neck is flushed with warm sterile saline to remove small uroliths. The urethra is flushed with the catheter placed to retroflushed the stones in the bladder. While sterile, saline is flushed profusely in the urethra the catheter is removed. With this technique little uroliths left in the urethra are flushed in the bladder. Then a catheter is introduced in the proximal urethra and large amounts of saline are used to flush the urethra normograde. Before closing the bladder a catheter is introduced from the bladder in the urethra to confirm patency of the urethra. It is not necessary to maintain a urinary catheter postoperatively.
It is recommended to take a postoperative radiographs to make sure all the stones have been removed.
A urethrotomy (an incision over the calculi) may be performed to remove calculi that cannot be retropulsed. It is usually performed in the prescrotal or perineal region. With a urinary catheter in place to the obstruction a 5 cm midline incision is made over the uroliths. The subcutaneous tissue is dissected and the retractor penis muscle is retracted on one side. The urethral is dorsal to the retractor penile muscle. A 15 scalpel blade is use to longitudinally incise the urethra over the uroliths. The uroliths are removed and the catheter advanced. If other uroliths are present, they can either be retrieved through the urethrotomy or they can be flushed back in the bladder. Then a cystotomy is required. The urethrotomy incision can be left open or suture with an absorbable suture in a continuous pattern on the urethra. Subcutaneous tissue and skin are closed. If the incision is left open, it is going to granulate and close by second intention. Urine is leaking through the incision for several days. The corpus spongiosum will bleed when the dog urinates or becomes excited.
A urethrostomy (a permanent opening to allow calculi to pass) may be indicated in animals that are chronic recurrent calculi formers (e.g., urate calculi in Dalmatians).
Scrotal urethrostomy is the technique of choice in male dogs because the urethra has its largest diameter at the level of the scrotum. The dog is placed in dorsal recumbency and a urethral catheter is placed. After castration and scrotal ablation, the retractor penile muscle is retracted on the side to expose the ventral aspect of the urethra. The urethra is incised longitudinally over 3–4 cm. The periurethral tissue is sutured to the subcutaneous tissue with a 4-0 absorbable suture in a simple interrupted pattern. The urethral mucosa is then sutured to the skin with 4-0 nonabsorbable monofilament in a simple continuous pattern. The urethra is more superficial in the scrotal area, surrounded by less cavernous tissue. Complications of an urethrostomy are hemorrhage, stricture, and dermatitis from urine scalding. Hemorrhage happens for 6 to 7 days after surgery when the dog urinates or becomes excited. Sedation might be required for a week to 10 days after surgery to help control bleeding. Stricture mostly occurs is the dogs is self- traumatizing the surgical site. An E collar is recommended for 10 days. After a urethrostomy, dogs are at more risk of ascending UTI because the urethra is shorter.
In cats, a perineal urethrostomy is performed. The cat is positioned in ventral recumbency at the end of the table. An elliptical incision is performed around the prepuce and the scrotum. The cat is castrated. The penis is isolated and the ischiocavernosus muscles are exposed by blunt dissection and transected to their attachment to the ischium. After care, blunt dissection ventrally posterior displacement of the penis is possible. The retractor penile muscle is transected near the external anal sphincter muscle. The penile urethral is incised dorsal to the bulbourethral glands. At this point, the urethra is wide (4 mm). The pelvic urethra and 3 cm of the penile urethra are sutured to the skin with 4-0 monofilament in a simple interrupted pattern. The remaining of the penile urethra and penis are amputated. An Elizabethan collar is used to prevent self-mutilation. Complications are hemorrhage, cystitis, urethral stricture, self-mutilation, and wound dehiscence.
Figure 2: Pyelothotomy
(VIN editor: Figure was not provided at the time of publication.)
Postoperatively, patient needs to be monitored for signs of uroabdomen. Animals presenting with complete urinary obstruction and postrenal azotemia are continued on crystalloid IV therapy until serum urea nitrogen and creatinine return to normal. Postobstruction diuresis happened after the obstruction has been released. Fluid rate should be increased to maintain the hydration status of the patient.
Treatment for UTI and dietary management are required to prevent reoccurrence. After a cystotomy hematuria is possible for 2–3 days postoperatively. After a urethrotomy or a urethrostomy, hemorrhage from the urethral stoma is common in the immediate postsurgical period. It generally occurs 4–5 days postoperatively, but occasionally will last up to 2 weeks. Apply an Elizabethan collar to prevent self-mutilation.