The sensation of bladder filling experienced by the patient is variable but absence of normal sensation, or delayed sensation of bladder may be indicative of neurological abnormalities. Furthermore, hypersensitivity, lack of sensation during detrusor overactivity, sensation of extreme pain, or low bladder capacity overall due to sensation of fullness may be indicative of other lower urinary tract pathology.

It is important to begin the study with an empty bladder. Thus, most often patients are catheterized prior to the start of the study. This will help ensure that the infused volumes at which sensations are recorded are accurate. It is also important to ensure that the recorded infused amount accurately reflects the actual infused amount. Such calibrations should be done regularly and periodically as routine maintenance of the urodynamic equipment. Bladder coarse sensation can be delayed in patients with poorly controlled diabetes and HIV. Sensation can be absent in patients with spinal cord injuries. Hypersensitivity at low volumes may be indicative of interstitial cystitis (Painful bladder syndrome), UTI or other disorders.

Patients should be informed of the study objectives prior to beginning testing and this is especially relevant when assessing sensation. They should be prompted to inform the person performing the study of several events in the study [1]:

Filling sensations are very subjective and as such there are not a universally accepted set of normative values hence the term “coarse sensation” is utilized. Typical ranges are: first sensation ~170–200 mL, first desire to void ~250 mL, strong desire to void ~400 mL and Maximum capacity ~480 mL [17]. Reviewing a recent voiding diary may be helpful. Sensation is affected by the placement of a catheter in the bladder, which may cause irritation, and/or pain, which may be erroneously interpreted as a sensation to void. Overly warm, cold, or too rapidly infused fluid can also affect bladder sensation. When documenting the interpretation of the UDS tracing coarse sensation is usually reported as absent, reduced or increased [9].

Compliance reflects the passive viscoelastic properties of the bladder and is defined as the relationship between change in bladder volume and change in detrusor pressure [1]. Compliance is calculated by mathematically dividing the volume change of the bladder just prior to volitional micturition or the first involuntary bladder contraction by the detrusor pressure at that same point [1]. In a normally compliant bladder and in the absence of detrusor overactivity, the detrusor pressure should remain essentially unchanged during filling. Decreased bladder compliance is generally acknowledged as a risk factor for upper tract deterioration.

Despite the importance of this data point, there exists no universally accepted normative value. Compliance of less than 20 mL/cm H₂O is commonly used as the threshold below which is considered abnormal [22]. Occasionally, a prolonged involuntary bladder contraction (detrusor overactivity or DO) can be confused with true abnormal compliance. One way to differentiate between these is to stop infusing fluid and observe for a few minutes. Typically, pressures will return to baseline after a few minutes with DO whereas pressures will remain high in abnormal compliance. Video urodynamics/VCUG can be helpful as high-grade reflux and large bladder diverticulum can act as a “pop off” masking underlying abnormal compliance.

Testing of the detrusor leak point pressure (DLPP) in patients with abnormal compliance can be helpful in risk assessment of future upper tract deterioration. DLPP is defined as “lowest value of the detrusor pressure at which leakage is observed in the absence of abdominal strain or detrusor contraction” [12]. A DLPP of greater than 40 is considered deleterious to the upper tracts [13]. However, in certain individuals; a DLPP of less than 40 may also put the upper tracts at risk (Fig. 3.1).

Pelvic radiation, denervation from radical pelvic surgery, neurogenic bladder and chronically indwelling Foley catheters are common etiologies of abnormal bladder compliance. Patients, who have abnormal compliance with a chronic indwelling Foley, should be converted to a short period of CIC to allow for bladder cycling if feasible. Often, in these patients without a high suspicion of true poor compliance, normal compliance will be noted after a short period of CIC and/or bladder cycling.

When documenting the interpretation of the UDS tracing, compliance is usually reported as normal or abnormal or can be listed as a calculated value as noted previously. It is important to recognize that an artifactual decrease in the P_abd (P₂) transducer can misinterpreted as decreased compliance, but in fact this is due to artifact or repositioning of the abdominal pressure transducer during the study (Fig. 3.2).

Detrusor overactivity (DO) is defined as a urodynamic observation characterized by involuntary detrusor contractions (IDC) during the filling phase which may be spontaneous or provoked. If there is a relevant neurologic lesion it is deemed neurogenic DO. If there is no relevant neurologic lesion it is deemed idiopathic DO [1]. It is important to ensure than any suspected detrusor overactivity is in fact accurate and not artifact. True detrusor overactivity is noted as a wavelike form on the P_det tracing along with a similar wave like form on P_ves in the absence of “permission to void”. Additionally, the interpreter must ensure that there is not drop out from the rectal/abdominal catheter (P_abd) that may artificially simulate a rise in detrusor pressure.

Often, patients will report an unintended or sudden urge to urinate, which may or may not correlate with an IDC. It is key for the interpreter of the UDS tracing to be involved in the study as this helps identify artifact from true detrusor overactivity and can confirm if the DO replicates the patients presenting symptoms. Additionally, DO can be “stress induced” by strain or cough so it is important to be aware of potential precipitating events both during the study and at home.

When documenting the interpretation of the UDS tracing, detrusor contractions during the filling phase are usually reported as absent (“stable filling”), present and suppressible, present with resulting detrusor overactivity incontinence, or terminal DO (DO related incontinence resulting in emptying of the bladder) (Fig. 3.3). DO which occurs at cystometric capacity and results in bladder emptying is referred to as “terminal detrusor overactivity”. An after-contraction (Fig. 3.4) is a large amplitude rise in P_det occurring after the cessation of voiding. The clinical significance of this finding is unclear as it may represent catheter artifact or a true abnormality. While there is no defined high/low limit of rise in P_det to be considered DO, the definitive interpretation of low amplitude DO (less than 5 cm H₂O) requires a high quality UDS study [1].