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4. Clinical Evaluation of the Lower Urinary Tract
Keywords
AnatomyLower urinary tract symptomsLUTSClinical assessmentUrodynamicsIntroduction
The urinary tract consists of two distinct and mutually dependent components:
Upper Tract: Comprising The Kidneys and Ureters
Both kidneys continuously produce greater than 0.5 mL of urine per kg of body weight per hour (i.e. >35 mL per hour in a 70 kg man) when functioning properly and adequately hydrated. This urine empties into the kidney ’s collecting systems which drain via the ureters.
The ureters function as low-pressure distensible conduits with intrinsic peristalsis, which transport urine from the kidneys to the bladder. The urine drains into the bladder at the vesico-ureteric junction (VUJ) at the termination of each ureter. Each junction, if correctly functioning, only allows the one-way flow of urine and contains a mechanism to prevent retrograde transmission of urine back into the ureters from the bladder. This serves to protect the upper tract from the high pressures encountered within the bladder during voiding and to prevent infection entering the upper tracts.
Lower Tract: Comprising the Urinary Bladder and Urethra
These provide a highly sophisticated system of conduits and a reservoir that converts the continuous involuntary production of urine by the kidneys into the intermittent, consciously controlled voiding of urine (micturition at a convenient time and place). A thorough evaluation and detailed understanding of the structure, function and control of the lower urinary tract is vital for the accurate interpretation of urodynamic investigations.
Low pressure storage of urine
Expulsion of urine at an appropriate time and place.
- 1.
Serosa —an outer adventitial connective tissue layer.
- 2.
Detrusor muscle —a middle smooth muscle layer, comprising a functional syncytium of interlacing muscle bundles with fibres running in all directions. Adequate functional contraction of the bladder is essential for effective bladder emptying. Despite the commonly held view it seems unlikely that the detrusor muscle is the primary target of therapy designed to treat the overactive bladder symptom complex but the effects of antimuscarinics and botulinum toxin A on the detrusor muscle are likely to represent an unwanted ‘bystander’ effect.
- 3.
Urothelium—an innermost lining composed of transitional cell epithelium providing an elastic barrier that is impervious to urine and which has a high metabolic rate and an important role in the control of bladder function. In particular there is non neuronal release of a number of putative neurotransmitters from this layer triggered by distension of the bladder and consequent stretching of the urothelium.
- 4.
Suburothelial layer—this lies immediately beneath the urothelium, is also highly active metabolically and acts in concert with the urothelium to subserve a key afferent role. It is this afferent function which is the target for all therapies directed at overactive bladder syndrome and detrusor overactivity.
The base of the bladder extends circumferentially from the ureteric orifices. This region contains the trigone which is a small triangular muscular area between the two ureteric orifices and the bladder neck. The trigone contains a complex plexus of nerves. Above the ureteric orifices is the main body of the bladder.
The bladder neck is an important functional and genital sphincter in the male, it is less well developed in women.
- 1.
To provide an effective continence mechanism, for the majority of the time (storage phase).
- 2.
To allow adequate emptying from the bladder with the minimum of resistance during micturition (voiding phase).
A further role for the urethra which seems likely but remains hypothetical is to provide afferent feedback which may have important implications in influencing bladder function. The innermost mucosal layer in both sexes is organized in longitudinal folds and during the storage phase when the urethra is ‘closed’ this appears in a stellate configuration on cross section. Such a configuration allows significant distensibility, which is necessary during urethral ‘opening’.
The submucosal layer contains a vascular plexus which may be involved in improving the seal of a ‘closed urethra’ by transmitting the tension of the urethral muscle to the mucosal folds.
(a) Anatomy of the female urethra. The bladder neck is both anatomically and functionally poorly developed. (b) Anatomy of the male urethra. There is a powerful bladder neck, distal to which is the prostatic urethra and just below that the distal sphincter mechanism with both internal and external components
The principal innervation of the lower urinary tract
The principal innervations of the lower urinary tract | |||||
|---|---|---|---|---|---|
Type | Origin | Detrusor muscle | Sphincteric muscle | Principal neurotransmitter | |
Hypogastric | Sympathetic | T10-L2 | Relaxes | Contracts sphincteric smooth muscle | Noradrenaline |
Pelvic | Parasympathetic | S2-S4 (spinal micturition Centre) | Contracts | Relaxes | Acetylcholine |
Pudendal | Somatic | S2-S4 (Onuf’s nucleus) | N/A | Contracts sphincteric striated muscle and pelvic floor | Acetylcholine |
Note the longer urethra, a prostate and two powerful sphincter mechanisms in the male compared to the single weaker intrinsic sphincter mechanism with a weaker bladder neck and also a shorter urethra in the female.
Male Sphincteric Mechanisms
- 1.
A proximal ‘bladder neck mechanism’.
- 2.
A distal urethral mechanism at the apex of the prostate.
The proximal sphincter in the male bladder neck provides a powerful mechanism in both maintaining urinary continence and also preventing retrograde ejaculation of semen during sexual activity. In patients with a damaged distal urethral sphincter (e.g. a pelvic fracture-associated urethral disruption) continence can be maintained solely by the proximal bladder neck mechanism. Ultrastructurally it consists of a powerful inner layer of muscle bundles arranged in a circular orientation.
The distal sphincteric mechanism is also extremely important, as evidenced by its ability to maintain continence even when the proximal bladder neck mechanism has been rendered totally incompetent by surgical bladder neck incision or a prostatectomy. It is confined to the 3–5 mm thickness of the wall of the membranous urethra from the level of the verumontanum down to the distal aspect of the membranous urethra. It is composed mainly of extrinsic striated muscle which is capable of the sustained contraction necessary for continence and to a lesser degree by intrinsic smooth muscle.
Prostate Gland
The prostate is made up of smooth muscle and glandular tissue, with the proportion of smooth muscle being increased in benign prostatic hyperplasia (BPH). The prostatic smooth muscle is controlled by the sympathetic nervous system, which acts by releasing noradrenaline onto α1a-adrenoceptors located on the smooth muscle cells; the resulting contraction increases the bladder outlet resistance and further aids continence in the male.
Female Sphincteric Mechanisms
Females are much more likely to suffer from urinary incontinence due to sphincteric deficiency than males, due to the much less powerful sphincteric mechanisms. The bladder neck is a far weaker structure than the male bladder neck and is often incompetent, even in nulliparous young women. The bladder neck is poorly defined with the muscle fibres having a mainly longitudinal orientation.
Urinary continence is usually reliant upon the integrity of the urethral sphincteric mechanism, which like the male distal mechanism is composed of a longitudinal intrinsic urethral smooth muscle and a larger extrinsic striated muscle component. This sphincter extends throughout the proximal two-thirds of the urethra, being most developed in the middle one-third of the urethra. Damage to the sphincter or its innervation (in particular the pudendal nerve) by obstetric trauma reduces the effectiveness of this mechanism and predisposes to stress urinary incontinence.
Pelvic Floor Muscles
In females the pelvic floor muscles also have an important role in maintaining continence. The pelvic floor is composed primarily of the levator ani muscle group, the endo-pelvic fascia and the supporting ligaments. The pelvic organs are maintained in the correct position by this pelvic floor. These tissues form a supporting ‘hammock’ beneath the urethra and during increases in intra-abdominal pressure (such as coughing, sneezing) the urethra is compressed against this hammock, thereby keeping the urethra closed and the patient continent.
Failure of this mechanism causes descent (prolapse) and also hyper-mobility of the bladder neck and is an important cause of stress urinary incontinence as is denervation of the urethral sphincter mechanism.
Function of the Lower Urinary Tract
- 1.
The storage (filling) phase.
- 2.
The voiding phase.
The phases of bladder function
Urodynamics in practice |
|---|
Storage phase |
• Bladder fills passively—Detrusor muscle is relaxed |
• Urethral sphincter mechanisms are ‘closed’—Urethral and pelvic floor muscles are contracted |
Voiding phase |
• Bladder actively expels urine under conscious voluntary control—Detrusor muscle is contracting |
• Urethral sphincter mechanisms are ‘open’—Urethral and pelvic floor muscles are relaxed |
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