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FUNCTIONAL: PATHOPHYSIOLOGY AND CLASSIFICATION OF LOWER URINARY TRACT DYSFUNCTION

Normal Lower Urinary Tract Function

Bladder filling and urine storage requires:
1. Accommodation of increasing volumes of urine at a low detrusor pressure (normal compliance) and with appropriate sensation
2. A bladder outlet that is closed at rest and remains so during increases in intra-abdominal pressure
3. Absence of involuntary bladder contractions (detrusor overactivity [DO])
Bladder emptying/voiding requires:
1. Coordinated contraction of the bladder smooth musculature of adequate magnitude and duration
2. Concomitant lowering of resistance of the smooth and striated sphincter (no functional obstruction)
3. Absence of anatomic (as opposed to functional) obstruction

Abnormal lower urinary tract function

Simple classification:

Failure to empty: because of the bladder vs. because of the outlet
Failure to store: because of the bladder vs. because of the outlet

Failure to store

Results from (or a combination of) (3):
1. Bladder overactivity (involuntary contraction and/or decreased compliance)
2. Decreased outlet resistance
3. Altered sensation

Bladder overactivity

Overactivity of the bladder during filling/storage can be expressed as phasic involuntary contractions, as low compliance, or as a combination.
If an individual has urgency urinary incontinence (UUI), it can be assumed that an involuntary contraction (DO) has occurred.
- The symptom of urgency without incontinence suggests DO, but this is often not demonstrable on urodynamic study. Conversely, urodynamically demonstrable DO may not be associated with clinically troublesome filling/storage symptoms.
Involuntary contractions are most commonly seen in association with (6):
1. Neurologic disease or injury
2. Bladder outlet obstruction
3. SUI (perhaps because of sudden entry of urine into the proximal urethra, eliciting a reflex contraction)
4. Aging (probably related to neural degeneration)
5. Iatrogenic (radiation)
6. Increased afferent input related to inflammation or irritation of the bladder or urethral wall or an increased sensitivity
7. Idiopathic

Neurologic disease or injury
- Any neurologic process interrupting the normal suprapontine inhibition of the pontine micturition center may result in neurogenic detrusor overactivity (NDO) and cause UUI.
  - CVAs, multiple sclerosis, and Parkinson disease are among the more common neurologic processes that might result in UUI.
  - Diabetes, even early in diagnosis, has been associated with NDO and UUI
Bladder outlet obstruction
- Obstruction resulting from anti-incontinence surgery in women can lead to de novo UUI secondary to induced detrusor overactivity (DO).
- In men, BOO induced by prostatic enlargement (or other obstructive process) can be associated with DO and resultant UUI.
Poor emptying from detrusor underactivity or detrusor areflexia (causing overflow incontinence) might also cause UI. This type of detrusor dysfunction is common with neurologic diseases affecting the lumbosacral cord or conus medullaris.
Systemic diseases, which can result in peripheral neuropathies such as diabetes, tabes dorsalis, and alcoholism, can similarly cause overflow incontinence.
- Whereas early in the disease process DM can lead to UUI, later in the process sensation can be altered as can detrusor contractility, resulting in impaired bladder emptying, UTIs, and UI.
Radical pelvic surgeries (i.e., radical hysterectomy, abdominoperineal resection) can also result in significant, sometime permanent, neurogenic detrusor dysfunction leading to urinary retention and overflow incontinence.
Other processes such as traumatic cervical or upper thoracic spinal cord injury can cause detrusor sphincter dyssynergia, creating impaired bladder emptying and UI, particularly when coupled with NDO.
Pelvic external beam radiation can alter bladder compliance, increase detrusor leak point pressure, and contribute to UI.

Decreased outlet resistance
- May result from any process that damages the innervation of structural elements of the smooth or striated sphincter, or both, or damages or impairs the support of the bladder outlet in women
  - A major factor required for the prevention of urinary leakage during increases in intra-abdominal pressure is the presence of at least equal pressure transmission to the proximal urethra (the mid-urethra as well in women) during such activity. Failure of this mechanism is an invariable correlate of effort-related urinary incontinence in women and men.
  - Sphincteric incontinence in men is not associated with hypermobility of the bladder neck and proximal urethra but is similar to what is termed intrinsic sphincter dysfunction in women.
- Causes of intrinsic sphincteric deficiency (ISD)
  - Iatrogenic (most common)
    - Radical prostatectomy
      - Most common surgical cause of incontinence in men
      - Generally causes leakage via a direct impairment of sphincter function.
    - In women, urethral surgery or anti-incontinence surgery can lead to urethral scarring, periurethral fibrosis, and ISD.
  - Neurologic disease
  - Traumatic or vascular injury to the lumbosacral cord
  - Any medication with either α-antagonistic properties or skeletal muscle relaxant properties
    - Can cause UI by inhibiting outlet resistance.
  - Prolonged labor, third-degree lacerations, large birth weight, multiparity, and forceps deliveries are all aspects of labor and delivery that have been associated with sphincteric dysfunction.
Altered sensation
- May occur due to increased afferent input from inflammation, irritation, other causes of hypersensitivity, and pain.

Management
- Directed towards:
  1. Inhibiting bladder contractility
  2. Decreasing sensory output
  3. Mechanically increasing bladder capacity
  4. Mechanically increasing outlet resistance, either continuously or just during increases in intra-abdominal pressure
  5. Combination of the above

Failure to empty
- Results from (or a combination of) (2):
  1. Bladder underactivity (a decrease in magnitude, coordination, or duration)
  2. Increased outlet resistance
  - Bladder underactivity
    - May result from temporary or permanent failure or impairment in one of the neuromuscular mechanisms necessary for initiating and maintaining a normal detrusor contraction.
    - Causes:
      - Neurologic diseases affecting the lumbosacral cord or conus medullaris.
      - Systemic diseases, which can result in peripheral neuropathies such as diabetes, tabes dorsalis, and alcoholism
        
        Whereas early in the disease process DM can lead to UUI, later in the process sensation can be altered as can detrusor contractility, resulting in impaired bladder emptying, UTIs, and UI.
      - Radical pelvic surgeries (i.e., radical hysterectomy, abdominoperineal resection)
  - Increased outlet resistance
    - Much more common in men than in women
    - Most often secondary to anatomic obstruction; may be secondary to a failure of relaxation or active contraction of the striated or smooth sphincter during bladder contraction
- Management
  - Directed towards:
    1. Increasing intravesical/detrusor pressure
    2. Facilitating the micturition reflex
    3. Decreasing outlet resistance
    4. Combination of the above
    5. If other means fail or are impractical, intermittent (or continuous) catheterization is an effective way to circumvent emptying failure.

Classification Systems

See Table 3 (or Table 111-5 from Campbell's 12th edition) for ICS Urodynamic Classification of Lower Urinary Tract Dysfunction
The classification system proposed by the ICS is an extension of a urodynamic classification system
Normal bladder function during filling/storage implies no significant increases in detrusor pressure (stability).
Overactive detrusor function indicates the presence of “involuntary detrusor contractions during the filling phase, which may be spontaneous or provoked.”
- If the condition is caused by:
  - Neurologic disease, the term neurogenic detrusor overactivity (previously, detrusor hyperreflexia) is used
  - Non-neurologic diseae, the term idiopathic detrusor overactivity (previously, detrusor instability) is used.
Bladder sensation can be categorized only in qualitative terms.
Bladder capacity and compliance (Δ volume/ Δ pressure) are cystometric measurements.
- Bladder capacity can refer to cystometric capacity, maximum cystometric capacity, or maximum anesthetic cystometric capacity.
Normal urethral function
- During filling/storage, uretheral closure pressure should be positive (urethral pressure minus bladder pressure) even with increases in intra-abdominal pressure, although it may be overcome by DO.
  - Incompetent urethral function during filling/storage implies urine leakage in the absence of a detrusor contraction. This leakage may be due to genuine stress incontinence, intrinsic sphincter dysfunction, a combination, or an involuntary decrease in urethral pressure in the absence of detrusor contraction.
- During voiding, the urethra opens and is continuously relaxed to allow bladder emptying at a normal pressure.
  - Abnormal urethral function during voiding may be due to either mechanical obstruction or urethral overactivity.
During the voiding/emptying phase of micturition, normal detrusor activity implies voiding by a voluntarily initiated sustained contraction that leads to complete bladder emptying within a normal time span.

An underactive detrusor defines a contraction of inadequate magnitude or duration, or both, to empty the bladder within a normal time span.
An acontractile detrusor is one that cannot be demonstrated to contract during urodynamic testing.
Areflexia is defined as acontractility secondary to an abnormality of neural control, implying the complete absence of centrally coordinated contraction.

Dysfunctional voiding describes an intermittent or fluctuating flow rate secondary to involuntary intermittent contractions of the periurethral striated muscle in neurologically normal individuals.
Detrusor sphincter dyssynergia defines a detrusor contraction concurrent with an involuntary contraction of the urethral or periurethral striated muscle, or both.
Non-relaxing urethral sphincter obstruction usually occurs in individuals with a neurologic lesion and is characterized by a nonrelaxing obstructing urethra resulting in reduced urine flow.

References

Wein AJ, Kavoussi LR, Partin AW, Peters CA (eds): CAMPBELL-WALSH UROLOGY, ed 11. Philadelphia, Elsevier, 2015, chap 70