Female urethra

Embryologically the female urethra is formed from an elongation between the bladder and the urogenital sinus. The urethra is positioned between the pelvic floor and the vagina and it empties into a groove on the ventral floor of the vestibule at the external urethral orifice.

The cranial two-thirds of the female urethra is covered by detrusor fibers, connective tissue, and smooth muscle which comprises the internal urethral sphincter; the caudal third of the urethra is initially encircled by striated urethralis muscle, which comprises the external urethral sphincter. In the caudal urethra, the urethralis muscle expands dorsally and encircles both the vagina and urethra. The urethral smooth muscle contains circular fibers which act as a sphincter and longitudinal fibers that contract to dilate the urethra. The epithelium changes from transitional to stratified squamous in the caudal quarter of the urethra.¹

Male urethra

The male cat has an elongated bladder neck or preprostatic urethra, a short wider prostatic section, a postprostatic or pelvic section, and the distal penile urethra that narrows significantly at the glans (Fig. 38-1). Embryologically, the prostatic urethra is equivalent to the female urethra, the pelvic portion of the urogenital sinus forms the membranous portion, and the penile urethra is derived from the phallic portion of the sinus.

The prostate and the bulbourethral glands are accessory sex glands that are situated dorsolaterally to the urethra. The prostate is located at the level of the acetabulae and the bulbourethral glands empty dorsally into the postprostatic urethra at the ischial arch; their entry marks the start of the penile urethra.

The preprostatic urethra consists of epithelium, submucosa, and three layers of smooth muscle, which are continuous with the detrusor muscle at the vesicourethral junction. At the prostate, the muscle changes from smooth to striated fibers. The postprostatic urethra has circumferentially orientated striated muscle fibers, the urethralis muscle, which corresponds to the external urethral sphincter. This contributes to resting urethral tone but also contains fast twitch fibers that are capable of responding to sudden changes in vesicular pressure and thus maintaining continence during coughing or sudden movement.² The muscle mass increases just caudally to the bulbourethral glands with the insertion of the ischiocavernosus muscles ventrally and the bulbocavernosus muscles dorsally. The penile urethra has longitudinally orientated muscle fibers ventrally but not dorsally; however, in this area the urethra is surrounded by highly vascular collagenous connective tissue and the corpus cavernosus dorsally.³ The epithelium of the male urethra is mainly transitional with stratified squamous epithelium near the urethral orifice.⁴

The male urethra is approximately 8.5–10.5 cm long³ and the dimensions of each part are shown in Figure 38-1. The urethral circumference is not affected by castration although fibrocyte density increases with castration.⁴

Blood supply

In the female, the urethral blood supply is via the vaginal arteries from the urogenital artery.

In the male, the prostatic artery (from the internal pudendal artery) supplies the prostatic and membranous urethra along with branches from the urethral artery, which extends to the cavernous tissue and distal urethra.

Innervation

Normal micturition requires a brainstem reflex and can be initiated or suppressed consciously. The striated urethralis muscle is innervated by the pudendal nerve, arising from segments S1–S3. The pelvic plexus is located bilaterally on the lateral surface of the parietal peritoneum lining the pelvic cavity and this has sympathetic input via the hypogastric plexus and parasympathetic input via the pelvic nerve. The hypogastric nerve originates from L2–L5 and stimulation of the sympathetic input inhibits detrusor contraction, stimulates the internal urethral sphincter, and inhibits parasympathetic synapses in the pelvic plexus. The pelvic nerve originates from S1–S3 and stimulates detrusor contraction.

Functional anatomy

The functional anatomy of the feline urethra has been studied experimentally by urethral pressure profile measurements.3,5,6 Urethral tone results from the interaction of the smooth muscle, striated skeletal muscle, fibroelastic and vascular tissue. The smooth muscle offers most resistance to urine leakage in the non-voiding animal. The external urethral sphincter is maintained at a low tone but activates with sudden increases in intra-abdominal or intravesicular pressure rises. The external urethral sphincter tone increases in response to fluid flow along the urethra or distension of the urethra.

In the female, the maximal urethral closure pressure is predominantly in the distal third of the urethra at the level of the urethralis muscle⁶ (Fig. 38-2A). Interestingly, unlike in the dog, there have been no significant differences found in the urethral pressure profiles of intact and ovariectomized females.⁶

In the male, the urethral pressure profiles reveal a pressure peak at the prostatic and early postprostatic urethra followed by high-pressure peaks in the bulbourethral and penile areas that usually have the maximal urethral closure pressure (Fig. 38-2B). The prostatic and postprostatic pressure peaks arise at the level of the urethralis muscle and the external urethral sphincter whilst the penile increase in urethral pressure results from the small diameter of the penile urethra.

Diagnostic imaging

A combination of imaging modalities are used to evaluate the urethra.

Feline lower urinary tract obstruction

Feline lower urinary tract disease (FLUTD) encompasses urethral plugs (20–50%), feline interstitial cystitis (20–50%), bacterial cystitis (10–20%), and urolithiasis (10–20%).8–11 Cats at most risk of FLUTD are kept indoors, male, overweight, and fed on dried diets.¹² Bacterial cystitis is more commonly seen in elderly cats with renal compromise which prevents concentration of the urine.¹³ Clinical signs of FLUTD include inappropriate urination, stranguria, hematuria, dysuria, and pollakiuria progressing to include various systemic signs (pain, bradycardia, depression, and vomiting) with complete obstruction. Males have a much higher risk of blocking. The diagnostic approach depends on the frequency and severity of episodes but should include urine analysis and culture even on first presentation. Obstructed cats require serum biochemistry profiles with electrolytes to allow stabilization of cases with hyperkalemia or marked azotemia prior to sedation or anesthesia. Risk factors for mortality include hypocalcemia and hyperkalemia.¹²

Many cases of FLUTD can be treated medically without resorting to surgery; however, recurrence rates after an episode of urethral obstruction are in the range of 30–40%, with obstruction rates being higher for urethral plugs.¹² A 2002 multiple center study in North America reported a 70% reduction in perineal urethrostomy in a 20-year period from 1980–1999.¹³ This decrease in surgical management of FLUTD may reflect an improvement in medical treatment of the condition. However, if a cat has regular recurrence of urethral obstruction despite appropriate medical management, a perineal obstruction should be considered. In certain cases (e.g., if the owner is regularly away overnight) surgery can be considered earlier.

In cats with urolithiasis it is preferable to retropulse calculi into the bladder and perform a cystotomy rather than operate on the urethra. If the calculus is firmly lodged, and a retrograde urethrogram confirms the obstruction, a temporary cystostomy tube can be placed to allow urinary diversion or repeat cystocentesis can be used. Retropulsion can be attempted again after a few days of treatment with anti-inflammatory medication and urethral relaxants. Alternatively, a perineal urethrostomy can be performed to relieve the obstruction once the cat is systemically stable.

In a study of 43 cats with urethral obstruction, 26% died or were euthanized because of their disease. Perineal urethrostomy was performed in 22% of cats and resulted in resolution of clinical signs in 50% of cases.¹⁴ Cats that had a perineal urethrostomy were half as likely to be euthanized due to their disease although this was a relatively small study. It is important to counsel owners, however, that if surgery is performed for obstructive cases of FLUTD the disease will still be present and further medical treatment may be required although re-obstruction is less likely to occur.

Other complications reported following perineal urethrostomy include urinary tract infection, stricture formation, periurethral urine extravasation, and stoma dehiscence. The incidence of postoperative urinary tract infection is reported to be from 23–50%.15–18 The range in results may be due to the fact that the infections are often asymptomatic and self-limiting, though in chronic cases struvite urolithiasis can result.¹⁹

Urethral trauma

Urethral trauma may occur after blunt or penetrating injuries but is most commonly seen in males after traumatic urethral catheterization in cases of obstructed FLUTD.20,21 With iatrogenic catheter damage the frequent sites for damage are the distal penile urethra and the ischial arch. Damage at the level of the distal penile urethra (Fig. 38-6) is due to inadequate caudal straightening of the penile urethra prior to catheterization or an inadequately sedated or anesthetized cat. Damage at the ischial arch (Fig. 38-7) is likely due to a combination of the caudal traction on the penile urethra being maintained whilst the catheter is entering the postprostatic pelvic urethra and traumatic catheters, particularly those with a stylet still in place. With an obstructed cat hydropulsion via a urethral catheter rather than the catheter itself should be used to relieve the obstruction. Clinical signs of urethral rupture are listed in Box 38-1.

Cases of penile or postprostatic urethral rupture will often have urine leakage into the perineum and the dorsum around the tail base. If a fluctuant swelling is evident in these areas in a cat where there is a possibility of urethral rupture, the fluid should be drained with aspiration or placement of a small closed suction drain to prevent skin necrosis22,23 (Fig. 38-8). If the urethral rupture is further proximal, the urine will leak into the abdomen, resulting in uroabdomen and a chemical peritonitis, which will be septic if there was a pre-existing urinary tract infection present.

The ability to urinate normally or pass a urinary catheter into the bladder does not exclude a urethral tear. Diagnosis is confirmed by imaging, including positive contrast urethrocystography. If contrast is seen both in the bladder and extravasating from the urethra a partial rupture is present. If no contrast agent is seen in the bladder the rupture can be partial or complete.²⁴ Ultrasound examination may be useful to determine disruption of the proximal urethra and bladder neck. Biochemical analysis of blood, including electrolytes, should be checked prior to sedation or anesthesia to rule out hyperkalemia, hyponatremia, hypochloremia, metabolic acidosis, or azotemia. If uroabdomen is present, abdominocentesis with biochemical analysis of the peritoneal fluid will support the diagnosis if the peritoneal creatinine, urea or potassium levels are higher than serum levels.²⁵

Treatment options for urethral rupture include urinary diversion, primary repair, or proximal urethrostomy. Small lacerations will generally heal in three to five days with urinary diversion using an indwelling urethral catheter or cystostomy tube. Larger lacerations will heal if a strip of uroepithelium remains intact but strictures may result. In cases where catheterization is difficult, normograde placement of a catheter via a small cystostomy followed by placement of a guidewire aids retrograde placement of the urethral catheter.²⁴ An indwelling catheter is required for up to three weeks depending on the size of the tear.

The extent of damage may be difficult to assess by a retrograde urethrogram and surgical exploration of the urethra is sometimes advisable to gauge the size of the deficit, particularly if there is excessive contrast leakage on a urethrogram. In cases that have suffered a road traffic accident, the urethra can be damaged ventrally, presumably by crushing against the pubis. In these cases the urethral damage can be considerable and stricture is likely with conservative management.

Most lesions of the penile urethra in cats are treated by a perineal urethrostomy.²⁰ Complete transection of the more proximal urethra is treated by urethral resection and anastomosis, which carries the risk of stricture formation, or a prepubic urethrostomy if the damage is severe. A vaginourethoplasty has recently been reported in a female cat after primary repair of a pelvic urethra rupture resulted in a stricture.²⁶ Prognosis is worse for cats with multiple traumatic injuries.²⁰ Complications which may occur after urethral rupture include strictures, incontinence, and urethrocutaneous fistulas.

Urethral tumors

Urethral tumors are rare and more commonly seen in males than females. Affected cats tend to present later in life than those affected with FLUTD and often have a lean body condition. Presenting clinical signs include dysuria, hematuria, inappetence, constipation, and weight loss. Cases reported in the literature include a transitional cell carcinoma of the preprostatic urethra and a cat with leiomyoma.27,28 Prostatic tumors (usually carcinomas) are rare in the cat but can cause extramural obstruction.²⁹ Urethral tumors generally carry a poor prognosis and due to their rarity effectiveness of treatment protocols is difficult to assess. Chemotherapeutic protocols including piroxicam, doxorubicin, and cyclophosphamide have been reported for urethral transitional cell carcinomas,³⁰ but the few cases reported have had poor survival times. A case report of surgical excision of a transitional cell carcinoma followed by radiotherapy on recurrence had a survival time of over a year.²⁷ Self-expanding metallic stents have been used in cases of urethral carcinoma but the survival times have been disappointing due to tumor occlusion of the stents.^31,32