Page 758 - Small Animal Internal Medicine, 6th Edition

730 PART V Urinary Tract Disorders

CHAPTER 45
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Disorders of Micturition

ANATOMY AND PHYSIOLOGY DEFINITIONS AND TYPES OF
URINARY INCONTINENCE
Micturition depends on the coordinated actions among the
sympathetic, parasympathetic (PS), and somatic nervous Often owners will present their pet for evaluation of urinary
systems and central control centers (Fig. 45.1). The coordina- incontinence (UI); however, there are several types of UI that
tion among these systems in animals takes place in the a clinician must consider. Usually veterinarians use the term
pontine micturition center (PMC), also known as Bar- UI when referring to a dog or cat that unconsciously voids
rington’s nucleus, which is located in the dorsomedial urine. Incontinence is usually due to failure of urine storage
pontine tegmentum in the brainstem. The PMC receives during filling, although multiple etiologies may contribute to
input from other sensory stimuli to determine the onset of UI. UI is more common in spayed bitches but can be seen
micturition. in intact female dogs as well as male dogs and occasionally
The thoracolumbar sympathetic pathway provides excit- cats. Disorders associated with UI can be caused by anatomic
atory input to the bladder neck and urethra, and inhibitory problems as well as by alterations in urethral closure pres-
input to the PS ganglia. Sympathetic preganglionic fibers exit sure. Animals may also consciously void small amounts of
the lumbar spinal cord (L1-L4 in dogs and L2-L5 in cats) and urine in inappropriate locations (pollakiuria), and this is
synapse in the caudal mesenteric ganglia. Postganglionic what is commonly referred to as urge incontinence. Further-
fibers (hypogastric nerve) release norepinephrine (NE) to more, it is important to evaluate if the dog is polyuric and/
activate β-receptors in the urinary bladder and α-receptors or polydipsic (PU-PD). An animal can also present with
in the smooth muscle of the proximal urethra. This allows multiple problems, such as UI and PU or for behavioral
the bladder to relax and fill continuously, with little increase causes of abnormal voiding. Obtaining a detailed history and
in intravesical pressure (via β-receptors), and provides tone ascertaining whether the animal is aware of micturition are
to the smooth muscle near the trigone of the proximal essential to help formulate proper differential diagnoses and
urethra of female dogs, which is traditionally known as the an appropriate diagnostic plan.
internal urethral sphincter (via α-receptors). In male dogs the
sphincter mechanism is likely associated with the striated ECTOPIC URETERS
muscle in the prostatic and postprostatic urethra. Ectopic ureters (EUs) are the most common cause of UI in
The PS preganglionic motor neurons arise from the young dogs. An EU is defined as a ureteral opening in an
sacral spinal cord segments S1 to S3. Preganglionic fibers area other than the normal position in the trigone of the
travel in the pelvic nerve and synapse in the peripheral bladder (Fig. 45.2). UI is the most common clinical sign in
ganglia in the wall of the bladder. Short postganglionic dogs with EUs, and this disorder is usually diagnosed in dogs
fibers provide excitatory input to the bladder through ACh younger than 1 year of age; however, EUs should be consid-
acting on cholinergic (muscarinic) receptors in the bladder ered in any dog with UI, particularly when the history is
and also provide inhibitory input to the urethra, thus unknown. The severity of UI is variable, and some dogs may
facilitating voiding. only be incontinent when at rest or at night (i.e., nocturia).
Somatic innervation, supplied via the pudendal nerve, Breeds that appear over-represented include: Golden
also arises from the sacral spinal cord segments S1 to S3 and Retriever, Labrador Retriever, Siberian Husky, Newfound-
provides stimulation (via ACh on nicotinic receptors) to the land, and English Bulldog. EUs are uncommon in male dogs
external urethral sphincter, an area of striated muscle. Cell and, if present, affected dogs may have few or no clinical
bodies of this nerve are located in the ventrolateral nucleus signs or present with clinical signs at an older age. EUs are
of Onuf. extremely rare in cats.

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