Bladder sensation during filling cystometry
Definition
Suggested normal range (ml)
First sensation of bladder filling
Sensation when the patient first becomes aware of the bladder filling
170–200
First desire to void
Feeling that would lead the patient to pass urine at the next convenient moment, but voiding can be delayed if necessary
~250
Strong desire to void
A persistent desire to void without the fear of leakage
~400
The ICI cites a general guideline for normal sensation parameters during filling cystometry in healthy adult subjects as follows: FSF at 170–200 ml, FDV at 250 ml, and SDV at about 400 ml, with maximum cystometric capacity at around 480 ml [1]. However this is only a guideline and actual values in the “normal” population may vary considerably. In a group of 50 normal volunteers, Wyndaele and De Wachter [4] found that cystometric sensation points were significantly lower in females than males, but also found that there was a significant difference in weight in the two groups which could influence bladder capacity. In children, the expected bladder volume (EBV) may be calculated from the Koff formula ([age in years + 2] × 30 = EBV ml) in older children, and by the formula (38 + [2.5 × age in months] = EBV in ml) in infants [5]. Ultimately, bladder sensation is dependent on a subjective report from a patient who is in extraordinarily unnatural circumstances. Such factors as indwelling urethral and rectal catheters, pain from urethral catheterization, abnormal positioning on a chair (not a toilet), while being publicly observed by the study technician during a very private act can significantly alter a patients perspective and thus likely accounts for some variability and even artifact. Techniques to improve patient comfort and understanding of filling sensation points are outlined in Chap. 7. However, ultimately there are studies that show correlation of patient report with objective data. In healthy volunteers it has been found that the cystometric bladder filling sensations correspond to the filling sensations reported on frequency volume charts [6].
Compliance
Bladder compliance is the relationship between change in bladder volume and change in detrusor pressure and is measured in ml/cm H2O. Per ICS recommendation, compliance is usually calculated over the change in volume from an empty bladder to that at MCC or immediately before the start of any detrusor contraction that causes significant leakage. Both detrusor pressure measurements (at the start of filling and at MCC respectively) in the calculation of compliance are measured excluding any detrusor contraction [1, 3].
The rise in bladder pressure in a normal individual is barely perceptible in the absence of an involuntary bladder contraction. This unique property of bladder smooth muscle is likely due to a combination of active and passive phenomena. Studies of filling cystometry with physiologic filling rates in healthy subjects reveal mean compliance values from 46 to 124 ml/cm H2O. The overall range in these studies of normal, asymptomatic subjects is broad, 11–150 ml/cm H2O in one study of 17 subjects and 31–800 ml/cm H2O in another [1]. Additionally, the actual performance of filling cystometry may provoke higher pressure increases than what is seen with physiological filling of the bladder, as has been seen in studies comparing filling cystometry to ambulatory urodynamics [7]. In a study of adult patients, Weld et al. [8] proposed a critical compliance value of 12.5 ml/cm H2O, based on studies of adults with neurogenic bladder.
In defining normative values and placing such numbers in a clinical context, it is important to understand that compliance is a mathematical calculation: intravesical pressure divided by the bladder volume at bladder capacity. From prior studies of intravesical pressure, 40 cm H2O bladder storage pressure is the value at which the antegrade transport of urine from the upper tract function is compromised and such a pressure puts the kidneys at jeopardy [9]. In patients with diminished compliance and sustained high intravesical storage pressure during filling, it is clinically advantageous to maintain low bladder volumes such that intravesical pressure is consistently kept below this critical level. This is most often done by continuous bladder drainage (catheterization) or frequent voiding or catheterization intervals such that intravesical volume and thus pressure is maintained in a safe range. It should be noted that this often quoted study regarding the risk of elevated intravesical pressure is related to children with myelodysplasia and may not be relevant for non-pediatric age groups and furthermore, the risk of upper tract deterioration may occur at pressures less than 40 mm H2O.
It has been suggested that bladder compliance is a more complicated entity in pediatric practice because it is related to bladder volume which increases with age. Small pediatric bladders may be more affected by filling rates during urodynamics, and as such, slow filling rates are preferable. The Committee of the International Childrens’ Continence Society cites a rule of thumb that 10 cm H2O or less as an absolute value at expected bladder capacity is acceptable. Additionally, they recommend that as there are no reliable reference ranges for compliance in infancy and childhood, attention be directed more toward the shape of the curve than numbers, looking to see if it is linear or nonlinear, and if nonlinear, in what way it deviates from linearity [5].
Contractions
The occurrence of involuntary detrusor contractions or detrusor overactivity (DO) may be observed in normal, asymptomatic patients during urodynamics. A variety of studies have shown DO in up to 17 % of normal patients, with a mean occurrence of roughly 8 % [1]. The significance of this is unknown. It may be situational, due to sensitivity or trauma of the urethral catheter or to infusate temperature or rate. Although it can occur in normal patients, in the majority of studies, DO comprises a significant urodynamic finding that may explain a number of storage symptoms especially in those cases where it reproduces the patient’s presenting complaints.
Continence
For continent individuals there should not be a demonstrable Valsalva or abdominal leak point pressure. Patients with adequate sphincter function and without symptoms of stress incontinence will not leak under any increased physiologic abdominal pressure [10]. There is no “normal” abdominal leak point pressure. Thus, when doing an urodynamic study, the demonstration of urinary leakage during stress maneuvers such as cough or Valsalva in an individual without complaints of urinary incontinence is potentially artifactual due to the circumstances of the study. Of course, the patient who demonstrates a Valsalva leak point pressure on urodynamics but does not have the complaint of SUI should be carefully requestioned to be sure that this does not reproduce any of their symptoms. It is important to note that a lack of leakage in a symptomatic patient with complaints of urinary incontinence should be further investigated. In such individuals, particular provocative maneuvers such as shifting position, or running water during the UDS exam will potentially unmask and demonstrate incontinence. Furthermore, 15 % of women with SUI and 35 % of men with postprostatectomy SUI who are stress continent during the UDS exam will demonstrate an ALPP with the urethral catheter removed [10]. If prolapse is present, stress incontinence should be tested for with the prolapse reduced in order to exclude occult stress incontinence.
Normal Emptying
The normal voiding sequence is a coordinated neuromuscular event initiated with the activation of the micturition reflex. The first recordable event in both voluntary and involuntary voiding is the relaxation of the urethral sphincteric complex, which can be visualized on EMG as a decreased signal. This relaxation results in a decrease in urethral pressure followed closely by a rise in detrusor pressure with opening of the bladder neck and urethra and initiation of urine flow [11]. Flouroscopy during voiding in a normal patient will demonstrate bladder neck funneling and an open urethra with no narrowing or abnormal dilation.
Contractility, Clinical Obstruction, and Complete Emptying
The ICS defines normal voiding as a voluntary continuous contraction that leads to complete emptying in a normal time span in the absence of obstruction [3]. In the normal bladder, as volume increases and the detrusor muscle fibers become more progressively stretched, there is an increase in the potential bladder power and work associated with a contraction. This is most pronounced in the range from empty up to 150–250 ml bladder filling volume. At volumes higher than 400–500 ml, the detrusor may become overstretched and contractility may decrease again [12]—however, this number may be higher in patients who have exceptionally large bladder volumes [11]. Normal detrusor function is incompletely defined solely by absolute values of detrusor pressure because emptying is also related to outlet resistance. For this reason, emptying is best defined through comparison of contractile pressures to their resulting flow rates. Multiple nomograms have been developed to classify this relationship, which will be discussed in Chap. 19. The majority of these nomograms were developed in male patients. The bladder outlet obstructive index (BOOI) (Fig. 10.1), which was derived from the Abrams Griffiths and Schafer nomograms and is the basis for the ICS provisional nomogram, plots Q max against P det@Q max to categorize (male) patients as obstructed, unobstructed, or equivocal. Men are considered obstructed if BOOI is greater than 40, unobstructed if BOOI is less than 20, and equivocal if 20–40. The bladder contractility index (BCI) is derived from Schafer’s nomogram (formula in Fig. 10.1). Strong contractility is a BCI greater than 150, normal contractility a BCI of 100–150, and weak contractility a BCI of less than 100 [10]. A composite nomogram incorporates the BOOI and BCI to categorize patients into one of nine groups characterizing the spectrum of obstruction and contractility (Fig. 10.1) [10].
