The mechanism of continence

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CHAPTER 50 The mechanism of continence

Urinary Incontinence

Anatomy of the lower urinary tract

The pelvic floor

For the context of this chapter, the pelvic floor structures that will be described are the levator ani muscles, the endopelvic fascia and condensations of this fascia which form the ligaments. The levator ani muscles are often described as funnel-shaped structures, but in-vivo imaging has revealed that they are, in fact, more horizontal. The fibres of each side pass downwards and inwards. The two muscles, one on each side, constitute the pelvic diaphragm. Defects in the levator ani allow the urethra, vagina and rectum to pass. The levator ani is subdivided into the pubococcygeus, iliococcygeus and coccygeus. The pubococcygeus arises anteriorly from the posterior aspect of the pubic bone and from the anterior portion of the arcus tendineus (white line), which is a condensation of the obturator internus fascia. The medial fibres of the pubococcygeus merge with the fibres of the vagina and perineal body, and have been given various names such as puborectalis, pubourethralis and pubovaginalis. The iliococcygeus arises from the remainder of the arcus tendineus, partly overlapping the pubococcygeus on its perineal surface and extending to the medial surface of the ischial spine. The coccygeus or ischiococcygeus is a rudimentary muscle arising from the tip of the ischial spine, and quite often constitutes only a few muscle fibres on the sacrospinous ligament. Posteriorly, the muscles of the levator ani or pelvic diaphragm insert into the sides of the coccyx and the anococcygeal raphe, which is formed by the interdigitation of muscle fibres from either side. The most medial fibres of the pubococcygeus that pass round the rectum at the anorectal junction form the puborectalis muscle (Figure 50.1).

The endopelvic fascia invests all the structures that lie within the pelvis and is mainly composed of loose areolar tissue, although smooth muscle cells have been identified. The layer between the bladder, urethra and vagina is termed the ‘vesicovaginal fascia’, and the layer between the vagina and rectum is known as the ‘rectovaginal fascia’ or ‘rectovaginal septum’. Laterally, both these layers attach to a condensation of fascia known as the ‘arcus tendineus fasciae pelvis’. This is a condensation of the endopelvic fascia which runs from the posterior part of the pubic ramus to the ischial spine, and attaches to the stronger underlying obturator internus muscle fascia. The urethra has an intimate attachment to the lower one-third of the vagina, and therefore the supports for these structures are identical. The urethra has a further condensation of the endopelvic fascia which attaches to the symphysis pubis. These are known as the ‘pubourethral ligaments’ and lie at the midportion of the urethra. They are fairly loose attachments, and allow movement of the bladder neck during straining and also by voluntary contraction of the levator ani muscles.

The uterus, cervix and upper third of the vagina are attached to the pelvic side wall by broad condensations of endopelvic fascia with a high content of smooth muscle cells, known as the ‘cardinal and uterosacral ligaments’. These ligaments originate from the region of the greater sciatic foramen and the lateral aspects of the sacrum. In the erect position, these ligaments run almost vertically and suspend their attached structures.

The levator ani muscles, ligaments and endopelvic fascia work in synergy. If any of these structures is not intact, the mechanisms of support and continence may be affected.

Innervation of the bladder and urethra

The bladder has a rich parasympathetic nerve supply (Figure 50.2). The postganglionic cell bodies lie either in the bladder wall or pelvic plexuses, innervated by preganglionic fibres that originate from cell bodies in the grey columns of S2–4. There is little sympathetic innervation of the bladder, although greater quantities of noradrenergic terminals can be detected in the bladder neck or trigone. Noradrenergic effects can be either inhibitory or excitatory, depending on the receptor type present.

The cell bodies of the sympathetic nerves originate in the grey matter of T10–L2, and pass through the sympathetic chain via the lumbar splanchnic nerve and left and right hypogastric nerves to the pelvic plexus. It is thought that the sympathetic nervous system exerts its effects by inhibiting the parasympathetic nervous system rather than by direct action. The visceral nerve afferents pass along the sacral and thoracolumbar visceral efferents, relaying sensations of touch, pain and distension. However, transection has little or no effect on micturition. The urethra possesses similar autonomic innervation. Parasympathetic efferents cause contraction but the functional significance of this remains in doubt. There is no obvious sphincteric function, but contraction produces shortening and widening of the urethra along with detrusor contraction during micturition. Sympathetic efferents innervate the predominantly α-adrenoreceptors.

Central nervous control of continence

The connections of the lower urinary tract within the central nervous system are complex (Figure 50.3). There are many discrete areas which influence micturition, and these have been identified within the cerebral cortex, in the superior frontal and anterior cingulate gyri of the frontal lobe and the paracentral lobule; within the cerebellum, in the anterior vermis and fastigial nucleus; and in subcortical areas, including the thalamus, the basal ganglia, the limbic system, the hypothalamus and discrete areas of the mesencephalic pontine medullary reticular formation. The full function and interactions of these various areas are incompletely understood, although the effects of ablation and tumour growth in humans and stimulation studies in animals have given some insights.

The centres within the cerebral cortex are important in the perception of sensation in the lower urinary tract, and the inhibition and subsequent initiation of voiding. Lesions in the superior frontal gyrus and the adjacent anterior cingulate gyrus reduce or abolish both the conscious and unconscious inhibition of the micturition reflex. The bladder tends to empty at low functional capacity. Sometimes, the patient is aware of the sensation of urgency; sometimes, micturition may be entirely unconscious. These areas, which have been localized by functional magnetic resonance imaging (MRI), are supplied by the anterior cerebral and pericallosal arteries; spasm or occlusion of these arteries produces incontinence.

Localized lesions more posteriorly in the paracentral lobule may produce retention rather than incontinence because of the combination of impaired sensation and spasticity of the pelvic floor.

The thalamus is the principal relay centre for pathways projecting to the cerebral cortex, and ascending pathways activated by bladder and urethral receptors synapse on neurones in specific thalamic nuclei which have reciprocal connections with the cortex. Electrical stimulation of the basal ganglia in animals leads to suppression of the detrusor reflex, whereas ablation has resulted in detrusor hyper-reflexia; patients with Parkinsonism are commonly shown to have detrusor instability on cystometric examination.

Within the pontine reticular formation are two closely related areas with inhibitory and excitatory effects on the sacral micturition centre in the conus medullaris. Lesions of the cord below this level always lead to incoordinate voiding with a failure of urethral relaxation during detrusor contraction; lesions above this level may be associated with normal although involuntary micturition.

The mechanism of urinary continence

The mechanism of urinary continence is a complex dynamic process which relies on intact fascia, ligaments and muscles with their accompanying nerve and vascular supply. It relies on maximum urethral pressure being higher than maximum detrusor pressure. There are special features of physiology and anatomy that contribute to the maintenance of a low detrusor pressure, and adequate urethral closure and positioning. These features are discussed below.