Bladder, prostate and urethra

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CHAPTER 75 Bladder, prostate and urethra

URINARY BLADDER

The urinary bladder is a reservoir. Its size, shape, position and relations all vary according to its content and the state of neighbouring viscera. When the bladder is empty, it lies entirely in the lesser pelvis, but as it distends it expands anterosuperiorly into the abdominal cavity. An empty bladder is somewhat tetrahedral and has a base (fundus), neck, apex, a superior and two inferolateral surfaces.

RELATIONS

The base of the bladder is triangular and located posteroinferiorly. In females it is closely related to the anterior vaginal wall (Fig. 75.1; see Fig. 77.25B); in males it is related to the rectum although it is separated from it above by the rectovesical pouch, and below by the seminal vesicle and vas deferens on each side and Denonvillier’s fascia (Fig. 75.2A,B). The neck, which is most fixed, lies most inferiorly, 3–4 cm behind the lower part of the symphysis pubis and just above the plane of the inferior aperture of the lesser pelvis. The bladder neck is essentially the internal urethral orifice, which lies in a constant position, independent of the varying positions of the bladder and rectum. In males the neck rests on, and is in direct continuity with, the base of the prostate; in females it is related to the pelvic fascia, which surrounds the upper urethra. In both sexes the apex of the bladder faces towards the upper part of the symphysis pubis. The median umbilical ligament (urachus) ascends behind the anterior abdominal wall from the apex to the umbilicus, covered by peritoneum to form the median umbilical fold (see below).

The anterior surface of the bladder is separated from the transversalis fascia by fat in the potential retropubic space (of Retzius). This is more adherent to the bladder than to the anterior surface of the prostate, which aids reliable identification of the region of the bladder neck surgically. In males, each inferolateral surface is related anteriorly to the pubis and puboprostatic ligaments. In females the relations are similar, except that the pubovesical ligaments replace the puboprostatic ligaments. The inferolateral surfaces are not covered by peritoneum. The triangular superior surface is bounded by lateral borders from the apex to the ureteric entrances, and by a posterior border which joins them. In males the superior surface is completely covered by peritoneum, which extends slightly onto the base and continues posteriorly into the rectovesical pouch and anteriorly into the median umbilical fold: it is in contact with the sigmoid colon and the terminal coils of the ileum (Figs 75.1, 75.2 and 75.3; see page 1105). In females the superior surface is largely covered by peritoneum, which is reflected posteriorly onto the uterus at the level of the internal os (the junction of the uterine body and cervix), to form the vesicouterine pouch. The posterior part of the superior surface, devoid of peritoneum, is separated from the supravaginal cervix by fibroareolar tissue.

These relationships are important in managing bladder trauma. Extraperitoneal injuries can often be managed conservatively because urine is contained, whereas peritoneal injuries usually require surgical repair.

As the bladder fills it becomes ovoid. Anteriorly, it displaces the parietal peritoneum from the suprapubic region of the abdominal wall. Its inferolateral surfaces become anterior and rest against the abdominal wall without intervening peritoneum for a distance above the symphysis pubis which varies with the degree of distension, but is commonly 5–7 cm. The distended bladder may be punctured just above the symphysis pubis without traversing the peritoneum (suprapubic cystostomy): surgical access to the bladder through the anterior abdominal wall is usually by this route. The summit of the full bladder points up and forwards above the attachment of the median umbilical ligament, so that the peritoneum forms a supravesical recess of varying depth between the summit and the anterior abdominal wall: this recess often contains coils of small intestine. At birth, the bladder is relatively higher than in the adult because the true pelvis is shallow, and the internal urethral orifice is level with the upper symphysial border. The bladder is then abdominal rather than pelvic, and extends about two-thirds of the distance towards the umbilicus. Urine samples may therefore be obtained in children by performing suprapubic needle puncture. The bladder progressively descends with growth, and reaches the adult position shortly after puberty. Congenital abnormalities of the bladder are described on page 1312 (see also Fig. 78.11).

LIGAMENTS OF THE BLADDER

The bladder is anchored inferiorly by condensations of pelvic fascia which attach it to the pubis, lateral pelvic side-walls, and rectum.

In both sexes, stout bands of fibromuscular tissue, the pubovesical ligaments, extend from the bladder neck to the inferior aspect of the pubic bones. They are derived from the detrusor muscle, part of the detrusor apron (Fig. 75.4A,B). In the female, they constitute the superior extensions of the pubourethral ligaments. In the male, the detrusor apron is described as an extension of detrusor that extends over the anterior surface of the prostate, and condenses distally and anteriorly to form the puboprostatic ligaments.

The pubovesical ligaments lie on each side of the median plane, leaving a midline hiatus through which numerous small veins pass. A number of other so-called ligaments have been described in relation to the base of the urinary bladder, namely the lateral and posterior vesical ligaments in the male and the cardinal and ureterosacral ligaments in the female. The reflections of the peritoneum from the bladder to the side-walls of the pelvis form the lateral ligaments, and the sacrogenital folds constitute the posterior ligaments. They are all condensations of connective tissue around major neurovascular structures rather than true ‘anatomical’ ligaments, nevertheless they are described as ligaments in routine clinical use.

The apex of the bladder is connected to the umbilicus by the remains of the urachus, which forms the median umbilical ligament (Fig. 75.3). It is composed of longitudinal muscle fibres derived from the detrusor, and becomes more fibrous towards the umbilicus. It usually maintains a lumen lined with epithelium which persists into adult life but is only rarely complicated by a urachal cyst, sinus, fistula or adenocarcinoma. From the superior surface of the bladder the peritoneum is carried off in a series of folds, the ‘false’ ligaments of the bladder. Anteriorly there are three folds, the median umbilical fold over the median umbilical ligament (urachus) and two medial umbilical folds over the obliterated umbilical arteries. The inferior epigastric vessels are lateral to these folds on the anterior abdominal wall.

BLADDER INTERIOR

Vesical mucosa

Almost all of the vesical mucosa (Fig. 75.5A–C) is attached only loosely to subjacent muscle: it folds when the bladder empties, and the folds are stretched flat as it fills. Over the trigone, immediately above and behind the internal urethral orifice, it is adherent to the subjacent muscle layer and is always smooth. The anteroinferior angle of the trigone is formed by the internal urethral orifice, its posterolateral angles by the ureteric orifices. The superior trigonal boundary is a slightly curved interureteric bar, which connects the two ureteric orifices and is produced by the continuation into the vesical wall of the ureteric internal longitudinal muscle. Laterally this ridge extends beyond the ureteric openings as ureteric folds, produced by the terminal parts of the ureters which run obliquely through the bladder wall. At cystoscopy the interureteric crest appears as a pale band and is a guide to the ureteric orifices.

Ureteric orifices

The slit-like ureteric orifices are placed at the posterolateral trigonal angles (Fig. 75.5A). In empty bladders they are approximately 2.5 cm apart, and 2.5 cm from the internal urethral orifice: in distension these measurements may be doubled.

BLADDER NECK

The smooth muscle of the bladder neck is histologically, histochemically and pharmacologically distinct from the detrusor muscle proper and so the bladder neck should be considered as a separate functional unit. The arrangement of smooth muscle in this region is quite different in males and females, and therefore will be described separately.

Female

The female bladder neck (Fig. 75.5C) consists of morphologically distinct smooth muscle. The large diameter fasciculi characteristic of the detrusor are replaced in the region of the bladder neck by small diameter fasciculi which extend obliquely or longitudinally into the urethral wall. In the normal female, the bladder neck sits above the pelvic floor supported predominantly by the pubovesical ligaments (Fig. 75.4A), the endopelvic fascia of the pelvic floor and levator ani. These support the urethra at rest; with elevated intra-abdominal pressure the levators contract, increasing urethral closure pressure to maintain continence. This anatomical arrangement commonly alters after parturition and with increasing age, such that the bladder neck lies beneath the pelvic floor, particularly when the intra-abdominal pressure rises, which means that the mechanism described above fails to maintain continence and women may experience stress incontinence as a result of urethral hypermobility.

Male

In the male (Fig. 75.4B), the bladder neck is completely surrounded by a circular collar of smooth muscle, with its own distinct adrenergic innervation, which extends distally to surround the preprostatic portion of the urethra. Distinct from the smooth muscle bundles which run in continuity from the bladder neck down to the prostatic urethra, and from the smooth muscle within the prostate, these smooth muscle bundles surround the bladder neck and preprostatic urethra. The bundles which form this ‘preprostatic sphincter’ are small in size compared with the muscle bundles of the detrusor and are separated by a relatively larger connective tissue component rich in elastic fibres.

The bladder neck is sometimes called the proximal sphincter mechanism, to distinguish it from the distal urinary sphincter mechanism. However, it is not a urinary sphincter and is not responsible for continence. It is a genital sphincter that allows antegrade ejaculation of semen. Unlike the detrusor and the rest of the urethral smooth muscle (common to both sexes), the preprostatic sphincter is richly supplied with sympathetic noradrenergic nerves and is almost totally devoid of parasympathetic cholinergic nerves. Contraction of the preprostatic sphincter serves to prevent the retrograde flow of ejaculate through the proximal urethra into the bladder. It is extensively disrupted in the vast majority of men undergoing bladder neck surgery, e.g. transurethral resection of the prostate, which results in retrograde ejaculation.

Whether this preprostatic sphincter replaces, or is additional to, the bladder neck muscle pattern seen in the female is unclear, but it is probably additional.

VASCULAR SUPPLY AND LYMPHATIC DRAINAGE

Veins

The veins which drain the bladder (see Fig. 77.3A,B) form a complicated plexus on its inferolateral surfaces and pass backwards in the lateral ligaments of the bladder to end in the internal iliac veins.

Lymphatics

Lymphatics which drain the bladder (see Fig. 77.3A,B) begin in mucosal, intermuscular and serosal plexuses. There are three sets of collecting vessels, most of which end in the external iliac nodes. Vessels from the trigone emerge on the exterior of the bladder to run superolaterally. Vessels from the superior surface of the bladder converge to the posterolateral angle and pass superolaterally to the external iliac nodes (some may go to the internal or common iliac group). Vessels from the inferolateral surface of the bladder ascend to join those from the superior surface or run to the lymph nodes in the obturator fossa. Minute nodules of lymphoid tissue may occur along the vesical lymph vessels.

INNERVATION

The nerves supplying the bladder arise from the pelvic plexuses, which are a mesh of autonomic nerves and ganglia on the lateral aspects of the rectum, internal genitalia and bladder base. They consist of both sympathetic and parasympathetic components, each of which contains both efferent and afferent fibres. The innervation of the bladder has been reviewed in some detail by Mundy (1999).

Efferent fibres

Parasympathetic fibres arise from the second to the fourth sacral segments of the spinal cord and enter the pelvic plexuses on the posterolateral aspects of the rectum as the pelvic splanchnic nerves. The sympathetic fibres are derived from neuronal cell bodies in the lower three thoracic and upper two lumbar segments of the spinal cord and form the coeliac and mesenteric plexuses around the great vessels in the abdomen, from which the hypogastric plexuses descend into the pelvis as fairly discrete nerve bundles within the extraperitoneal connective tissue posterior to the ureter on each side. The anterior part of the pelvic plexus is known as the vesical plexus. Small groups of autonomic neurones occur within the plexus and throughout all regions of the bladder wall. These multipolar intramural neurones are rich in acetylcholinesterase (AChE) and occur in ganglia consisting of up to 20 neuronal cell bodies. The majority of the preganglionic nerve terminals correspond morphologically to presumptive cholinergic fibres. Noradrenergic terminals also relay on cell bodies in the pelvic plexus: it is not known whether similar nerves synapse on intramural bladder ganglia.

The urinary bladder (including the trigonal detrusor muscle) is profusely supplied with nerves which form a dense plexus among the detrusor muscle cells. The majority of these nerves contain AChE and occur in abundance throughout the muscle coat of the bladder. Axonal varicosities adjacent to detrusor muscle cells possess features which are considered to typify cholinergic nerve terminals, and contain clusters of small (50 nm diameter) agranular vesicles, occasional large (80–160 nm diameter) granulated vesicles and small mitochondria. Terminal regions approach to within 20 nm of the surface of the muscle cells and may be partially surrounded by Schwann cell cytoplasm, or more often are naked nerve endings. The human detrusor muscle possesses a sparse supply of sympathetic noradrenergic nerves which generally accompany the vascular supply and only rarely extend among the myocytes. Non-adrenergic, non-cholinergic nerves have been identified, and a number of other neurotransmitters or neuromodulators have been detected in intramural ganglia, including the peptide somatostatin. The superficial trigonal muscle is associated with more noradrenergic (sympathetic) fibres than cholinergic (parasympathetic) nerves, a difference that supports the view that the superficial trigonal muscle should be regarded as ‘ureteric’ rather than ‘vesical’ in origin. However it must be emphasized that the superficial trigonal muscle forms a very minor part of the total muscle mass of the bladder neck and proximal urethra in either sex and is probably of little significance in the physiological mechanisms which control these regions.

The smooth muscle of the bladder neck in males is predominantly orientated obliquely or circularly. It is sparsely supplied with cholinergic (parasympathetic) nerves, but possesses a rich noradrenergic (sympathetic) innervation. A similar distribution of autonomic nerves also occurs in the smooth muscle of the prostate gland, seminal vesicles and vasa deferentia. Stimulation of sympathetic nerves causes contraction of smooth muscle in the wall of the genital tract resulting in seminal emission. Concomitant sympathetic stimulation of the proximal urethral smooth muscle causes sphincteric closure of the preprostatic sphincter, thereby preventing reflux of ejaculate into the bladder. Although this genital function of the bladder neck of the male is well established, it is not known whether the smooth muscle of this region plays an active role in maintaining urinary continence. In contrast, the smooth muscle of the bladder neck of the female receives relatively few noradrenergic nerves but is richly supplied with presumptive cholinergic fibres. The sparse supply of sympathetic nerves presumably relates to the absence of a functioning ‘genital’ portion of the wall of the female urethra.

The lamina propria of the fundus and inferolateral walls of the bladder is virtually devoid of autonomic nerve fibres, apart from some noradrenergic and occasional presumptive cholinergic perivascular nerves. However, the density of nerves unrelated to blood vessels increases closer to the urethral orifice. At the bladder neck and trigone a nerve plexus of cholinesterase positive axons extends throughout the lamina propria independent of blood vessels. Some of the larger diameter axons are myelinated and others lie adjacent to the basal urothelial cells. In the absence of any obvious effector target sites, the subepithelial nerve plexuses of the bladder and the ureter are assumed to subserve a sensory function.