Chapter 71: Treatment with Injectable Bulking Agents

Web Chapter 71

Urinary Incontinence

Treatment with Injectable Bulking Agents

Few problems in small animal practice are as frustrating to the owner and veterinarian as urinary incontinence. Although not life threatening, urinary incontinence is unacceptable to most pet owners. Affected dogs that cannot be managed successfully often are euthanized, surrendered to a humane shelter, or kept permanently outdoors. A search of records submitted to the Veterinary Medical Database revealed that 0.5% of dogs seen at veterinary teaching hospitals from 1994 to 2003 were diagnosed with urinary incontinence (VMDB, 2005).

The causes and manifestations of urinary incontinence vary; thus accurate diagnosis and elucidation of the underlying cause are essential for therapeutic success. However, even for patients in which the disease is localized to urethral sphincter mechanism incompetence (USMI), complete continence with medical treatment is not guaranteed (see Chapter 201). Recently, use of injectable bulking agents has been increasing in treating dogs that fail medical therapy. This approach, adopted from human therapeutics, shows promise in reducing the need for medical therapy and improving response to it.


Maintenance of urinary continence relies on many factors. Normally the major component of urethral tone is made up of smooth muscle. Mucosal integrity, vasculature, and connective tissue surrounding the urethra also play important roles in preserving continence. In addition, healthy urothelium, surface tension from glandular secretions, and the pliability of the mucosa contribute significantly to coaptation of the urethral walls.

Bladder position and the exposure of the vesicourethral junction and proximal urethra to intraabdominal pressure also are important in preserving continence. When sudden increases in abdominal pressure occur, an increase in pressure occurs within the bladder and in the proximal and midurethra, termed pressure transmission (Gregory and Holt, 1994). Several studies have evaluated the relationship of urethral position and length, as well as the vesicourethral angle to incontinence (Gregory, 1994; Gregory, Cripps, and Holt, 1996). However, these measurements overlap in normal and affected dogs; many incontinent dogs have short urethras and intrapelvic bladders. Despite lack of definitive evidence for a direct cause-and-effect relationship, bladder and urethral position appear to be an important risk factor for incontinence.

The relationship of hormone status to incontinence has long been recognized. As many as 20% of neutered female dogs are expected to develop some degree of urinary incontinence during their lives, and 75% of these dogs will do so within 3 years of neutering. Decreased estrogen concentrations in women are associated with loss of urethral muscle tone, urethral vascular atrophy, and decreased glandular secretions, affecting major components of the continence mechanism. Decreased estrogen concentrations are likely a factor in the development of incontinence in dogs, but the lack of clinically recognized incontinence in anestrus-intact females (and the many ovariectomized dogs without incontinence) supports a more complex mechanism than a simple lack of trophic effect (Richter and Ling, 1985). Recent evidence suggests that increases in luteinizing hormone and follicle-stimulating hormone associated with estrogen decrease also may play a role in the development of incontinence in dogs, but the mechanism has yet to be determined (Reichler et al, 2005).

Other risk factors for incontinence are breed, body weight, and tail docking. Among the breeds at increased risk for urinary incontinence are Old English sheepdogs, Doberman pinschers, German shepherd dogs, boxers, weimaraners, rottweilers, and Irish setters. Of the breeds evaluated, Labrador retrievers appeared to have a decreased risk of incontinence, particularly among large-breed dogs (Holt and Thrusfield, 1993). Large- and giant-breed dogs and dogs weighing more than 20 kg have a significantly increased risk of developing incontinence, whereas small-breed dogs have decreased risk. Tail docking is a suspected contributor to the onset of incontinence in dogs because damage to the muscles of the pelvic floor is considered to be a contributing factor in the development of stress incontinence in women, but no studies have related tail docking directly to incontinence. However, tail docking is common; and among the breeds with increased risk of incontinence many females are docked as neonates but develop the disorder only after they are neutered, sometimes as older adults.

Although the risk factors remain unclear, the most common type of urinary incontinence recognized in female dogs is USMI. Incontinent female dogs have lower urethral closure pressures than do continent dogs, and closure pressure increases in patients successfully managed with medical therapy. As noted previously, the closure pressure of the urethra depends on multiple factors; however, α-adrenergic receptor number and stimulation have been identified as primary therapeutic targets.


History and Minimum Database

The International Continence Society defines incontinence as “the complaint of any involuntary leakage of urine.” The client complaint of “leaking urine” has several underlying reasons, including behavioral, endocrine, and infectious causes, as well as USMI. The most important point to establish with owners is the involuntary nature of the problem. Many owners perceive submissive urination, pollakiuria, or polyuria as incontinence. For this reason the owner must be questioned extensively to establish the diagnosis. Water and food intake; general health; medical treatments; and appearance, frequency, and volume of urine voiding are factors that should be discussed. For accurate diagnosis of incontinence the dog must seem unaware of the leakage at the time it occurs, although it may “clean up” afterwards or lick the vulvar area frequently. Irritation of the vulva can occur because of a constantly wet environment or secondary bacterial infection. In addition, although difficult to diagnose definitively without urodynamic studies, detrusor instability (overactive bladder) may play a role in some incontinent patients.

Many incontinent dogs, especially those older than 7 years of age, develop incontinence after becoming polyuric from an unrelated disease process or after diuretic therapy of heart failure. Development of incontinence therefore can signal another, more serious problem that has otherwise escaped the owner’s observation. The authors recommend that all incontinent dogs receive a general health screen, including complete blood count, serum electrolyte and chemistry evaluation, urinalysis (collected by cystocentesis), and systolic blood pressure measurement. In addition, because pollakiuria may be misinterpreted as incontinence by some owners and because of the increased risk of urinary tract infection in incontinent dogs, a quantitative urine culture should be performed.

Urodynamic Evaluation

The term urodynamics refers to several tests that evaluate lower urinary tract function. The rationale behind the development of this technique is the theory that to maintain continence urethral pressure must be higher than intravesical pressure during the filling and storage phases of micturition. The three most commonly performed tests are the cystometrogram (CMG), an evaluation of bladder storage and contractile function; the Valsalva leak point pressure (VLPP); and the urethral pressure profile (UPP). These tests can be used to differentiate between detrusor instability and USMI, and they can help to establish severity of disease. The predictive value of urodynamic results still is debated, and improved standardization of techniques will increase its reliability and diagnostic worth. No studies have been published evaluating the predictive value of the UPP or VLPP on response to periurethral bulking agents in dogs. Studies in women have had mixed results, with the complicating factor that the VLPP does not always increase in patients with real improvement in continence. Based on the authors’ experience, there does not appear to be a strong correlation between the UPP and the success of periurethral bulking agent therapy. However, this does not mean that urodynamics has no role in the assessment of patients for bulking agent treatment. The presence of USMI and any component of overactive bladder must be verified before placing bulking agents because the relative contribution of each of these disorders guides therapeutic decisions. Many dogs may have a significant component of overactive bladder and require anticholinergic therapy in addition to bulking agents to attain continence.


Treatment of urinary incontinence has taken several avenues. Pelvic floor muscle training and pharmacologic treatments are considered primary therapy in humans, but surgical procedures for stress incontinence in women have been described for more than 100 years and still are applied extensively. Medical and surgical treatments have been adapted for small animals, including sympathomimetic drugs, estrogens, periurethral injections, and colposuspension and artificial urethral sphincters (see Chapter 202). The use of injectable bulking agents is becoming more widespread and, with the advent of newer materials, shows significant promise in treating incontinence in dogs.

Injectable Bulking Agents

Injection of bulking agents to prevent urine leakage has been reported as early as 1938, with a description of cod liver oil injection in women. Although many materials have been investigated, the theory behind all injectable bulking agents is to narrow the diameter of the urethral lumen, thus creating outflow obstruction, increasing stretch in sphincter muscle fibers, and allowing the urethral sphincter to close more effectively. In veterinary medicine injection of bulking agents has been performed in selected centers for many years but its application after the introduction of safer and more effective bulking materials is now more widespread.

Patient Selection

Because of cost and concerns about duration of postprocedure continence, patient selection for urethral injection is important. The ideal patient for this procedure has USMI without additional active lower urinary tract disease, including urinary tract infection. Ideally, patients should be screened via the CMG and any significant degree of detrusor instability managed medically before the injection procedure. As discussed in the following paragraphs, patients with ectopic ureters may benefit from the procedure; however, the location of the ureteral stomata is important. The majority of the authors’ periurethral injection patients previously have tried and failed medical therapy. In spite of this, many of these patients have an improved response to α-agonists after injection. Based on anecdotal data, previous response to medical therapy does not appear to predict outcome of injection therapy. Currently injection therapy is recommended most frequently to those patients with USMI and failure or declining efficacy of α-agonist or estrogen therapy.

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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Chapter 71: Treatment with Injectable Bulking Agents

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