Anatomy and Diagnostic Imaging

Kidney

Right kidney is more cranial than the left and attached to the liver by the hepatorenal ligament. Both kidneys are retroperitoneal with a hilus at their medial aspect where the renal vessels and the ureters enter and exit. In most dogs, the right renal artery is single but in about 13% of dogs the left renal artery is a paired structure. The gonadal vein enters the left renal vein. The renal vein is cranial and ventral to the artery; the ureter is the most caudal of the three. They may be seen on plain film radiographs but their visibility is enhanced following intravenous administration of iodinated contrast agent. Ultrasonography can be used to give anatomical information about the kidneys. Relative renal function can be estimated most accurately using renal scintigraphy but this is still not a very sensitive investigative tool. Excretory urography, however, is a poor determinant of relative renal function.

Ureters

Both the ureters are fibromuscular organs which travel in the retroperitoneum until they join the bladder wall within peritoneal reflections (the lateral ligaments of the bladder). The transport of urine to the bladder from the renal pelvis is an active process brought about by peristalsis. Unless they are abnormal (dilated) these can only be imaged by excretory urography.

Urinary Bladder

Two lateral (umbilical vein remnants) and one ventral ligament (urachal remnant) the trigone where the two ureters enter at an oblique angle in the dorsolateral wall forming a small slit-like opening in the mucosal epithelium. This is an extremely distensible muscular organ with sympathetic (hypogastric nn), parasympathetic (pelvic nn) and somatic (pudendal nn) innervation. Often divided, arbitrarily into the neck (trigone) and body. The urine-filled bladder is a good subject for ultrasound evaluation. Useful radiographic studies include negative contrast cystography, positive contrast cystography, and double contrast cystography.

Urethra

The urethra is a muscular tube extending from the bladder neck to the vagina in female dogs or tip of the penis in male dogs. There are skeletal muscle fibers at the proximal urethra under somatic control; the remainder of the urethral wall contains smooth muscle fibers under autonomic control. It can be imaged in the male dog by retrograde urethrography and in the female dog by retrograde vaginourethrography or voiding cystourethrography.

Congenital Abnormalities of the Urinary Tract

Ureteric Ectopia and Ureterocele

An anomaly resulting from poor differentiation of the mesonephric ducts and the ureteric bud results in one or both ureters entering the prostate, seminal vesicles, urethra or vagina. Breed and familial predisposition for Siberian huskies and Golden retrievers has been suggested. It is commonly associated with other congenital abnormalities such as hydronephrosis, short urethra, disorders of the bladder sphincter mechanism, renal hypoplasia and hydroureter. Ureterocele is a dilation of an intracystic portion of the (usually ectopic) ureter. This may be associated with a duplex urine collection system (i.e., two renal pelves and two ureters from the same kidney). The condition is usually treated in a similar manner to other cases of ectopia but may require excision of the redundant collection system.

History and Clinical Features

This is most commonly diagnosed in female dogs that have had a dribbling incontinence since birth. Affected animals commonly can void small amounts of urine normally but often dribble constantly and have staining of the perivulvar skin and hair and occasionally dermatitis on the ventrum. It has also been seen in aged female dogs with late onset incontinence, and in the cat. It is relatively infrequently diagnosed in males but may be just as common. Ectopia causes incontinence less frequently in males probably because of the length of the urethra and urethralis muscle (and therefore the sphincter mechanism). If incontinence does not occur, hydronephrosis and or pyelonephritis may be the presenting disease.

Diagnosis

A cystocentesis or catheter urine sample should be obtained for urinalysis, culture and sensitivity. Routine CBC and serum chemistry should be performed. The diagnosis of ureteric ectopia is confirmed radiographically, under general anaesthesia, by excretory urography (fluoroscopy is helpful) and/or retrograde vaginourethrography. Remember to prepare the patient adequately for this study (i.e., several enemas). Urethrocystography may also aid diagnosis of ureteric ectopia in female dogs.

Surgery

Prior to surgery any infection in the bladder should be appropriately treated. A ventral midline incision and ventral cystotomy are performed. The trigone is carefully examined for any ureteral orifices. (be gentle the urothelium becomes oedematous very easily) If found the ureter(s) should be catheterised using a soft rubber catheter. The ectopic ureters usually join the bladder in a normal position and tunnel caudally in the submucosa to open in the distal urogenital tract occasionally two openings will be present, one in the bladder and one more distal; the ureteric orifice should therefore be probed in both directions. Once ectopia is confirmed the only treatments to be considered are ureteronephrectomy or some type of ureteroneocystostomy (reimplantation). If the kidney or ureters are involved in advanced disease, but involvement is unilateral, removal is more satisfactory.

Post-Operative Complications

1.  Persistent UTI--requires careful monitoring and therapy if persists could be hydroureter & hydronephrotic kidney acting as a reservoir for infection, therefore, ureteronephrectomy will be needed.

2.  Persistent incontinence--could be due to re-canalisation of a submucosal tunnel or a poor sphincter mechanism, occasionally poor vaginal conformation causes pooling of urine at urination which then slowly leaks out. A repeat radiographic study may help determine the problem also a voiding cystogram will show any functional abnormality of the vagina.

Technique for Ureteronephrectomy

Generous ventral midline incision from just caudal to the xiphoid to beyond the umbilicus. The left kidney is found by elevating the colon thereby reflecting the remaining small intestine to the right which is held in place by the mesocolon. The right kidney is found by locating the descending duodenum and using the mesoduodenum to reflect the other abdominal structures in a similar way. The kidney is packed off with moist laparotomy towels and the perirenal fascia stripped from the capsule by blunt dissection. The renal vessels and ureter are identified and the renal artery is dissected free and ligated close to the aorta (prevents blind sac for thrombus to form) the renal vein is treated similarly. Both should be double ligated with silk or PDS. The ureter is then dissected free as close to it's attachment to the bladder as possible and divided between ligatures (prevent blind sac as focus for infection) Gentle traction on the kidney should then pull the ureter out to complete the removal. Closure is routine.

Sphincter Mechanism Incompetence (SMI)

Sphincter mechanism incompetence has also been called: estrogen-responsive/dependent incontinence, pelvic bladder incontinence short urethra incontinence. It may be analogous to urge or stress incontinence in women. This type of incontinence arises when the sphincter mechanism of the urinary bladder becomes overwhelmed and permits "overflow" of urine. The sphincter mechanism of the bladder is found at the trigone and the proximal urethra. It has active smooth muscle and skeletal muscle components and passive elastic components within the wall of the distal bladder and proximal urethra. The sphincter mechanism is under autonomic and somatic neural control and is, therefore, under conscious and unconscious control. In addition, the sphincter mechanism is influenced by changes in intraabdominal pressure as well as intravesicular pressure. Sphincter mechanism incompetence may be congenital (see earlier) but most commonly it is an acquired form of incontinence which becomes evident in older female dogs (although it has been diagnosed in male dogs). Risk factors for the development of this form of incontinence appear to be breed, age when ovariohysterectomy was performed and overall age.

History and Clinical Features

Historically, affected patients have a "postural" incontinence and or leak urine at times of rest or, occasionally, excitement. Classically, dogs with this condition will leave puddles of urine after sleeping or when lying down. The incontinence may be intermittent and inconsistent. Physical examination of the animal is non-remarkable.

Diagnosis

Diagnostic investigations are aimed at ruling other causes of incontinence out. Serum chemistry, complete blood count, urinalysis, and radiographic imaging (excretory urography, retrograde vaginourethrography), cystoscopy will help rule out other pathology that could explain incontinence. Attempts to make urethral pressure profilometry a "test" for SMI have not been successful but a urethral pressure profile is considered useful information by many investigators. Sphincter mechanism incompetence is, therefore, a diagnosis by exclusion.

Therapy

The mainstay for therapy is medical. Alpha adrenergic agents such as phenylpropanolamine can be curative for many patients. Judicious use of estrogens alone or in combination can help control animals who fail alpha-adrenergic therapy. Caution must be used in prescribing prolonged estrogen therapy because of the potential side effect of bone marrow suppression.

If medical therapy fails a surgical therapy may be indicated. Several procedures have been reported but only one procedure, colposuspension, has been scientifically proven. Research on this procedure has shown it to be helpful in 85% of cases and unhelpful for the remaining 15%. The procedure has only been used in the female dog and involves sutures passed through the vaginal stump and around the prepubic tendon on either side of the bladder neck. This procedure has the effect of pulling the bladder into a more intra-abdominal position, thereby allowing an increase in the influence of intraabdominal pressure over the urethral sphincter mechanism. In addition, it will gently entrap the proximal urethra. A variation on this operation in female dogs has been used in male dogs with some good effect. Fixation of the deferent ducts to the lateral body wall can also draw the male urinary bladder into a more intraabdominal position. This procedure has not been evaluated in as many animals as colposuspension.