Normal urethral anatomy
The female urethra is a muscular tube approximately 4 cm long and 6 mm wide. It derives embryologically from the urogenital sinus. It lies within the adventitia of the anterior vagina, extending from the bladder base to the external meatus, which opens into the vestibule. The wall of the urethra is made up of four layers. The innermost layer is stratified squamous epithelium, which becomes transitional near the bladder. This epithelium is surrounded by a submucosa with a rich vascular supply and multiple glands, which open into the urethra. The largest of these are the Skene’s glands, adjacent to the distal urethra. Surrounding the submucosa are two layers of smooth muscle: an inner longitudinal layer and an outer circular layer. The outermost layer of the urethra, the striated urogenital sphincter muscle, is made up of slow-twitch fibers which maintain the constant resting tone of the urethra and can contract to provide additional pressure when needed. The anterior urethral and the pubourethral (posterior urethral) ligaments form a sling that suspends the urethra behind the pubic symphysis.
Urethral diverticula are epithelium-lined sacs pouching out from the urethral lumen. They lie within the periurethral fascia and have no surrounding muscular wall. They generally range in size from 3 mm to 3 cm in diameter. It is believed that repeated infection of the periurethral glands leads to ductal obstruction and eventual rupture into the urethral lumen. This rupture tract will eventually epithelialize and become a diverticulum. A noncommunicating diverticulum can occur if the diverticular neck becomes obstructed. As these glands are present throughout the urethra, a diverticulum can occur anywhere along the length of the urethra, but is most commonly located posteriorly. Diverticula are prone to urine stasis and infection. Inflammation and chronic irritation can rarely lead to malignant degeneration.
Urethral diverticula most commonly present in women in their 20s to 40s. Their reported incidence is 0.6–6%, but the actual incidence may be higher as the diagnosis can be missed. Symptoms are often nonspecific. Some women complain of a suburethral vaginal mass, but many do not. Other common symptoms include dyspareunia, recurrent urinary tract infections, postvoid dribbling, urinary urgency and frequency, and symptoms of stress incontinence. The classic symptom triad of dysuria, dyspareunia, and dribbling is rarely seen. A urethral diverticulum should be ruled out in all women with recurrent refractory urinary tract infections.
Physical exam may reveal a suburethral cyst, but is often unremarkable. In these cases, additional studies are needed. Positive pressure urethrography (PPUG) was described in 1958 as a method of diagnosing diverticula. In this procedure, a double-balloon catheter blocks the urethra at both ends as it fills with contrast. If present, a diverticulum will be seen to fill with dye in up to 90% of cases. Transvaginal or transperineal ultrasonography is equally effective in making the diagnosis and is simpler to perform and more comfortable for the patient. Pelvic MRI has better sensitivity and better positive predictive and negative predictive value than PPUG for the diagnosis of urethral diverticula. MRI also has the advantage of delineating the extent of a diverticulum, which can aid in surgical planning. Urethroscopy may identify diverticular openings up to 90% of the time. Voiding cystourethrogram can be performed, but only has a diagnostic accuracy of 65–70%.
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