Acquired urinary incontinence (urethral sphincter mechanism incompetence, hormone-responsive incontinence) is a common disorder of older dogs characterized by intermittent involuntary urination due to urinary bladder dysfunction.
Unlike post-menopausal women with incontinence, where weakening of the pelvic floor muscles is critical to development of incontinence, changes in pelvic organ support structures may not play an important role in urinary incontinence in neutered female dogs, presumably due to the horizontal positioning of the pelvic floor.
Congenital incontinence differs from acquired incontinence in the former being present virtually from birth or soon afterwards, and has different anatomical etiologies such as spina bifida, hypospadias, renal agenesis, ectopic ureters, ureteroceles and rectovaginal fistulas.
Acquired incontinence, because of its hormone-responsive nature and higher incidence with ovariohysterectomy, is thought to involve hormone-related muscular changes within the urinary bladder and pelvic muscles, resulting in reduced urethral tone.
In some dogs however, incontinence may precede neutering and this condition should therefore be considered a multifactorial disorder. Causes of acquired incontinence may include decreased urethral tone, caudal displacement of the bladder, a shorter urethra, hormonal deficiency, obesity, altered secretion of the hypothalamic and pituitary hormones GnRH, FSH and LH due to neutering, reduced levels of glycosaminoglycans within the bladder and urethral connective tissue or changes in α1-adrenergic receptor populations within the urethral sphincter muscles.
Acquired incontinence can also be stress-induced and aggravated by post-whelping trauma to pelvic organs.
Post-neutering incontinence occurs in roughly 20% of female dogs that have been neutered post-puberty, and in pre-pubertal dogs, the incidence is less than 10%, suggesting an ovarian hormonal role in the etiopathogenesis of pelvic floor (pubococcygeus) and bladder (detrusor) muscle weakening. In large-breed female dogs, incontinence has been reported as high as 30%.
Incontinence can be exacerbated in dogs with pelvic bladder syndrome, where there is abnormal elongation of the bladder, persistent caudal displacement of the bladder and bladder neck into the pelvic canal on distention, an indistinct or blunted vesicourethral junction and a shortened urethra.
Clinically affected dogs are often middle- to old-aged neutered bitches which present with a protracted history of intermittent incontinence. Passive urinary leakage is frequently observed when the dog is asleep or recumbent.
Perivulvar dermatitis and dermal hyperpigmentation due to constant urine staining may be evident in some dogs.
Diagnosis of urethral sphincter mechanism incompetence is one of exclusion. Retrograde contrast urography, ultrasonography and excretory urography may be required to attempt to visualize and physical defects in the lower urinary tract and exclude other causes.
A differential diagnosis would include secondary incontinence associated with:
- Neurological deficits - e.g. discospondylitis, hip dysplasia, cauda equina syndrome, polymyositis
- Neoplasia - e.g. transitional cell carcinoma
- Infections - e.g. primary cystitis
- Polyuria-associated diseases - e.g. diabetes mellitus, chronic renal disease
- Post-operative incontinence - e.g. urolithiasis, vestibulovaginal stenosis, gossypiboma
- Anatomical disorders - e.g. vaginal hyperplasia
Surgical intervention is usually necessary in non-medically-responsive cases and includes:
- Urethral lengthening
- Urethral sling
- Urethral bulking - collagen injections
- Miscellaneous - urethral hydraulic occluder devices, transobturator vaginal tape
Canine acquired incontinence should be considered at worst an inconvenience rather than a reason for euthanasia.
Client annoyance with inappropriate urination in their pet is a common reason for veterinary intervention, but in unsuccessful management cases, restricting dog access to indoors or providing adequate and regularly-changed bedding may be an acceptable method of dealing with this disorder that is rarely life threatening.
This condition is also seen frequently with cognitive dysfunction in geriatric dogs, which should be addressed separately.
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