SURGERY
Article sponsored by Petcam ®
Surgical Procedures for
Urethral Sphincter Mechanism
Incompetence in Bitches
Prof G L Coetzee
Department of Companion Animal
Clinical Studies
Small Animal Surgery Section
Faculty of Veterinary Science
University of Pretoria
Acquired urethral sphincter mechanism incompetence
(USMI) is the most common storage disorder in neutered
female dogs 1 . The urethral sphincter mechanism involves
the smooth muscle of the urethra as well as the surrounding
support tissues, sub-mucosal vasculature, and urothelium 7 .
Female dogs have no true bladder neck sphincter and
urinary continence is maintained by a complex mechanism
of interacting factors. Urethral sphincter mechanism
incompetence (USMI) is the most common diagnosis made
in adult female dogs and is second only to ureteral ectopia
in juvenile dogs, especially in females 1-3 . USMI in male
dogs in uncommon, but if present it may be associated
with gross prostatic or pelvic urethral deformities such as
prostatic urethral diverticulae 4 .
Normal control of urinary continence
During filling of the urinary bladder, urethral resistance is
maintained by a complex mechanism of factors. These
include tone in the urethral smooth muscle (the internal
bladder neck sphincter), tone in the urethral striated muscle
(the external sphincter or ischio-urethralis muscle), natural
elasticity of the urethral wall tissues (not only musculature),
physical properties of the urethra (length and diameter of
the urethra) and the degree of engorgement of the sub-
urethral venous plexuses 7 . In a variety of species, including
dogs, direct sympathetic and parasympathetic interactions
occur in the urethra and bladder during storing of urine
and during voiding.
Urethral tone
Poor urethral tone is implicated in urinary incontinence
based on investigations of urethral pressure profilometry.
Urethral tone is maintained by a complex interaction of
neuromuscular, vascular, and passive elastic components;
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it is unclear which of these is deficient in female dogs with
urethral sphincter mechanism incompetence 1 .
Urethral length
Urethral length varies considerably among female
dogs of different sizes. When body size is taken into
consideration, then bitches with urethral sphincter
mechanism incompetence tend to have shorter urethras
than continent animals 1, 2 .
Bladder neck position
Radiographic finding of a “pelvic bladder” and a more
caudal positioning of the bladder neck when a bitch moves
from a standing to a relaxed recumbent position is more
pronounced in bitches with bladder sphincter mechanism
incompetence than in normal animals 2, 6 . Figure 1 & 2.
There may be a deficiency in supporting mechanisms in
the lower urinary tract of affected animals. The role of the
bladder neck position in bitches with USMI is thought to
be from changes in transference of abdominal pressure
to the urethra 6 .
SURGICAL OPTIONS
The main objective of surgical treatment is to increase
the urethral outflow resistance or the urethral length
or to relocate the intra-pelvic bladder to a more intra-
abdominal position.
Options for increasing urethral outflow resistance include
peri-urethral slings, artificial sphincters, or intra-urethral
injection of collagen bulking agents with the use of an
urethro-cyctoscopy technique 8 . The urethral length can
be increased with bladder neck reconstruction techniques.
The bladder neck can be relocated to a more intra-