Page 1249 - Clinical Small Animal Internal Medicine

127 Micturition and Associated Disorders 1187

detrusor contraction and urination. It may affect the midazolam, or other skeletal muscle relaxants, includ­
VetBooks.ir smooth muscle internal urethral sphincter, the striated ing acepromazine and methocarbemol, may be more
effective if the external urethral sphincter is involved.
external sphincter, or both. It is possible that the lesion is
cranial to the associated spinal segments and involves
minutes before voiding to decrease external urethral
the loss of inhibitory signals to the pudendal and Midazolam or diazepam is typically administered 30
hypogastric nerves, but may be a more local lesion to the sphincter pressure. Dantrolene and baclofen have been
nerves, the neuromuscular junction, or the sphincter used in the past as skeletal muscle relaxants; however,
itself. Unlike the “upper motor neuron bladder” seen in the potential for adverse effects has decreased their use
animals with thoracolumbar intervertebral disc disease in veterinary patients. In severe and refractory cases,
and other spinal cord lesions, these animals typically intermittent sterile catheterization by the owner at
have an otherwise normal neurologic examination. home may be necessary. Medical therapy of associated
The disorder affects primarily middle‐aged, large‐ and bladder atony should only be started after adequate
giant‐breed male dogs. One case series of 22 dogs relief of the functional urethral obstruction has been
reported a mean age of 4.9 years. Clinical signs are simi­ reached. Close monitoring of these patients for residual
lar to those of mechanical obstruction. The animal often urine volume and UTI is needed to assess efficacy of
postures to urinate and is able to produce a urine stream treatment and prevent complications.
that quickly becomes attenuated or stops completely. It is possible that patients with controlled DUD will
The animal may continue to posture to urinate or make experience relapse in times of stress such as hospitaliza­
several attempts without fully emptying the bladder. The tion or boarding. These animals may need periodic addi­
presence of large amounts of residual urine typically tional therapy or catheterization even if usually able to
leads to overflow incontinence and may be mistaken for urinate normally.
USMI. This leakage can occur because the hypertonicity Prognosis for recovery of normal voiding is good, but
of the involved sphincter is often dynamic and is trig­ most dogs will require life‐long therapy for DUD.
gered by the act of voiding. In chronic cases, bladder Attempts to taper medications to the lowest effective
overdistension and subsequent atony may develop. dose may be hampered by relapse of clinical signs after
Unlike animals with mechanical obstruction, these dogs months of normal voiding. Prognosis appears to be
are typically easy to catheterize and usually do not show worse in patients with bladder atony or UTI secondary
increases in urethral pressure on urodynamic evaluation to urine retention. Anecdotally, urethral stenting may
unless actively voiding. Contrast urethrography may be improve clinical signs, but this is considered a salvage
normal or reveal areas of narrowing of the urethra procedure with several potential complications, and
(urethrospasm). indicated only in the most refractory of cases.
Presumptive diagnosis of DUD is often made by
observing the dog urinate with a typical interrupted pat­ Functional Obstruction – Neurogenic
tern, documentation of a large residual urine volume,
easy passage of a urinary catheter, and ruling out of a Neurogenic functional urethral obstruction is generally
mechanical obstruction. Normal residual urine volume caused by spinal cord lesions cranial to the sacral seg­
in 48 normal dogs was reported to be 0.1–3.4 mL/kg ment. This leads to loss of inhibitory signals to the
body weight with a mean of 0.2 mL/kg. The author uses hypogastric and pudendal nerves which prevents sphinc­
<0.5 mL/kg as a general guideline. Ultrasonography is ter relaxation upon voiding. This is the classic “upper
recommended to assess the ureters and renal pelves for motor neuron bladder” in which the patient is unable to
dilation secondary to chronic obstruction and ureterore­ urinate normally and is difficult to express manually. The
nal reflux of urine. Additional diagnostics, including most commonly affected patients are those with interver­
contrast urethrography, urethroscopy or urodynamic tebral disc disease and associated paresis. These animals
evaluation, may be necessary to verify the diagnosis in typically have additional neurologic deficits, including
patients who fail to respond to medical therapy. paresis and nociceptive loss. Treatment of the underly­
Treatment of the hypertonic urethral sphincter ing lesion typically leads to partial or complete return to
generally consists of alpha‐adrenergic blockade with normal voiding function after days to weeks. Until nor­
prazosin, an alpha‐1‐specific antagonist with demon­ mal voiding resumes, the patients are managed as for
strated effects on both the internal and external ure­ DUD with alpha‐adrenergic blockade, and manual
thral sphincter. Tamsulosin, which is specific for the expression or catheterization. Monitoring for overdis­
alpha‐1A subtype found in the internal urethral sphinc­ tension and UTI is critical in these patients, as is nursing
ter, has also been successful in these dogs. Some dogs care, particularly since the overflow incontinence that
will require additional therapy if the striated muscle is can accompany this process may lead to skin breakdown
more significantly affected. Benzodiazepines, such as in these recumbent patients.