Pelvic floor muscles and the striated urethral sphincter both have a critical role in bladder storage and emptying. Electromyography (EMG) is the test that best evaluates these muscles. EMG is the study of electrical potentials generated by the depolarization of muscles [15]. In the setting of UDS, EMG is recording the motor unit action potential; this is the depolarization of the striated muscle fiber that occurs when the muscle is activated by the anterior horn nerve cell. Needle electrodes or surface electrodes can record the action potential. The quality of EMG has often been cited as variable or problematic because of a poor signal source. Needle electrodes are thought to be superior, however are often avoided because of patient discomfort and logistical difficulty [16]. The more commonly used surface EMG was described in 1980 to determine relaxation of the pelvic floor as an indirect measure of the simultaneous relaxation for the external sphincter [17].

EMG testing can be performed in isolation, however this test is usually combined with other UDS tests. As an isolated test, EMG can allow the clinician to assess the voluntary contraction of pelvic floor muscles, confirming that the corticospinal tract is intact and the cortical function required to initiate the contraction of the external sphincter is working.

Passive continence does not require external sphincter activity in most cases. However, there does exist a somatic passive guarding reflex that causes sphincter activity to increase as the bladder fills. Accurate measure with needle electrodes will often show a gradual increase in EMG activity with filling until a voluntary void is initiated. When using surface electrodes, one may not always see this pattern and may rather see a consistent signal. The EMG signal, assuming appropriate recording, should transiently increase if the patient performs a stress maneuver, i.e., straining or coughing (Fig. 1.2). When a voluntary void is initiated the first UDS evidence of this is relaxation of the external sphincter and a decrease in EMG activity (Fig. 1.2). This is then followed by an increase in detrusor pressure and initiation of flow. In a neurologically normal person, failure of the external sphincter to relax will result in inhibition of a detrusor contraction.

Normal EMG studies can lead to the exclusion of some diagnoses. However, the diagnostic utility is seen best in cases where it confirms neurological or functional causes of voiding phase dysfunction [5]. When not attributed to artifact, inappropriately increased EMG activity during the voiding phase is known as detrusor-external sphincter dyssynergia (DESD), when a neurologic lesion that can explain the dyssynergia exists (typically a suprasacral spinal cord lesion) (Fig. 1.2). When there is no underlying relevant neurologic lesion, the failure of external sphincter relaxation (or increasing EMG) leads to a diagnosis of dysfunctional voiding (a learned behavior of failed external sphincter relaxation during voiding). It is difficult to accurately predict when EMG information is going to be needed to explain voiding abnormalities. Thus, because of the relatively easy methodology and low morbidity of obtaining a surface EMG, EMG is often included as a channel in multichannel pressure-flow UDS studies [18].

EMG activity can also be increased during micturition because of external factors or artifact, sometimes called “pseudo-dyssynergia.” This includes abdominal straining, movement, guarding reflex, painful urination due to the presence of a urethral catheter, and wet or dislodged electrodes [19]. The interpretation of the study therefore should include all other available information. For example, if fluoroscopy (discussed below) is obtained during voiding on studies where EMG contains artifacts, it may be used to discriminate between voiding patterns that would otherwise be differentiated by their EMG signals, i.e., dysfunctional voiding (EMG activity is expected to be increased during voiding) and primary bladder neck obstruction (PBNO) (where EMG signal is expected to be quiescent) [7, 16]. Also, a completely normal uroflow (Q_max, Q_ave, and pattern) usually will exclude significant sphincter activity during voiding. EMG and/or uroflow abnormalities seen on invasive UDS should be confirmed with non-invasive uroflow.

The cystometrogram (CMG) assesses the bladder’s response to filling. The CMG can measure filling pressure, sensation, involuntary contractions, compliance, and capacity. Sensation is the part of cystometry that is truly subjective and therefore requires an alert and attentive patient and clinician. The filling phase starts when filling commences and ends when the patient and urodynamicist decide that “permission to void” has been given. The CMG is ideally started with an empty bladder. The bladder pressure (P_ves) is monitored and fluid is infused into the bladder. This can be achieved using two separate catheters, or more commonly, a dual lumen catheter (usually 6–8 French) usually placed transurethrally (or much less commonly via a suprapubic stab incision). Guidelines exist regarding the technical specification of these catheters [7]. It is important to note that changes in bladder pressure can represent a change in detrusor pressure (P_det), for example from a bladder contraction voluntary or involuntary, or a change in abdominal pressure (P_abd), for example from movement, Valsalva, etc. Though single channel studies that monitor only P_ves can provide information about bladder function, the recommended method to measure bladder pressure includes simultaneously measuring P_abd, usually by placing a balloon catheter in the rectum or vagina. When both P_ves and P_abd are measured, the P_det can be calculated by using the equation: P_det = P_ves − P_abd.

In addition to recording pressures during filling, the CMG study also should record the volume infused into the bladder during filling. Filling rates [1], fluid temperature [7], and fluid type [8] all need to be considered. Today most cystometry is done with liquid (most commonly saline or radiographic contrast in cases where fluoroscopy will be used). The practice of gas CMG was historically described [20–22], and is rarely performed any longer as it does not allow for studying the voiding phase.

Normally detrusor pressure should remain near zero during the entire filling cycle until voluntary voiding is initiated. That means baseline pressure stays constant (and low) and there are no involuntary detrusor contractions (Fig. 1.3a). Involuntary bladder contractions can occur with filling and are seen as a rise in P_ves in the absence of a rise in P_abd. Urodynamically, this phenomenon is known as detrusor overactivity (DO). DO may be accompanied by a feeling of urgency or even loss of urine (Fig. 1.3b). Another important parameter that the CMG measures is bladder compliance, the relationship between change in bladder volume and detrusor pressure. Normally the bladder is highly compliant and stores increasing volumes of urine at low pressure. However certain conditions may cause the bladder pressure to rise in the absence of a distinct detrusor contraction. This is known as impaired compliance (Fig. 1.3c) and can pose danger to the kidneys when this pressure is transferred to the upper urinary tracts. It is difficult to define what “normal compliance” is in terms of mL/cm H₂O. In the literature mean values for normal compliance in healthy subjects range from 46 to 124 mL/cm H₂O [23–25]. Various definitions of impaired compliance have been used (i.e., between 10 and 20 mL/cm H₂O), however there is not a consistent definition based on mL/cm H₂O. Stohrer et al. have suggested that a value of less than 20 mL/cm H₂O is consistent with impaired compliance and implies a poorly accommodating bladder [26]. However, examples can be cited (i.e., small cystometric capacity) where this may not be the case. Therefore, in practical terms, absolute pressure is probably more useful than a “compliance number” or value. For example, it has been shown that storage >40 cm H₂O are associated with harmful effects on the upper tracts [27].

For patients who have incontinence, provocative maneuvers can be performed during CMG to assess urethral competence and diagnose stress urinary incontinence (SUI). Patients can be asked to Valsalva or cough during filling. The abdominal leak point pressure (ALPP) is a measure of sphincteric strength or the ability of the sphincter to resist changes in abdominal pressure [28]. ALPP is defined as the intravesical pressure at which urine leakage occurs due to increased abdominal pressure in the absence of a detrusor contraction [1]. This measure of intrinsic urethral function is applicable to patients with stress incontinence. An ALPP can only be demonstrated in a patient with SUI. Conceptually the lower the ALPP, the weaker the sphincter.

In addition to providing information of filling pressures, the CMG can assess coarse bladder sensation and capacity. The International Continence Society (ICS) defines the following measures of sensation during bladder filling [1]:

Various methods exist, but ensuring quality control and adhering to standardized practices and interpretation guidelines can achieve good inter-rater reliability [29].

Once the bladder is filled to cystometric capacity, the voiding portion of the pressure-flow study can begin. This examines the emptying phase of micturition. The same bladder and rectal (or vaginal catheter in women) catheters are used while simultaneously collecting pressure data along with uroflowmetry (Fig. 1.4). Ideally, such a study should assess a voluntary void. When there is flow of urine during an involuntary detrusor contraction patients may contract the pelvic floor to prevent leakage. Such an event should be annotated on study. In addition, some subjects may have a difficult time voiding on demand in a public setting and with invasive monitoring in place. These stressors and the artificial environment of the testing need to be accounted for when interpreting the test. For example, some patients cannot voluntarily void during an urodynamic study due to discomfort or psychogenic inhibition. Therefore the lack of a voluntary voiding bladder contraction during UDS does not always indicate that a patient has a truly a contractile bladder. Such a finding needs to be placed in the context of other parameters (i.e., non-invasive flows, history, PVR, etc.) to determine if it is, in fact, testing artifact. Remember that in order to answer a clinical question, the symptom(s) should be reproduced during the study. For example if a man has a complaint of a slow urinary stream, and his pressure-flow study reproduced the slow stream which occurs with a high pressure detrusor contraction, this is assumed to be an accurate depiction of an obstructive process. However, if a woman, who complains of urinary incontinence and has no reported difficulty with voiding and a low PVR, is unable to generate a voluntary detrusor contraction, it is less likely to have clinical significance. In these cases a poor flow rate can be confirmed or refuted with a non-invasive uroflow done in a private setting.

The voiding phase of a pressure-flow study helps assess two critical parameters related to the bladder and bladder outlet: detrusor contractility (normal vs. impaired) and outlet resistance (obstructed vs. unobstructed) [30]. Combinations of these two features will be discussed in Chap. 2 as contractility, coordination, complete emptying and clinical obstruction. In general the pressure-flow study can identify three fundamental conditions [30]:

The most widespread application of pressure-flow studies has been to determine the presence of bladder outlet obstruction, most commonly in men. Starting in the early 1960s [24] nomograms were developed to standardize the definitions of obstruction and bladder contractility [31–33]. These nomograms are well established and broadly accepted in men (because of a single highly prevalent condition, benign prostatic obstruction—BPO). However pressure-flow nomograms are less widely agreed upon for use in women (due to the lack of a single highly prevalent condition causing obstruction) and as such have not gained widespread utilization in clinical practice [34–38].

Not all pressure-flow studies fall neatly into the three fundamental conditions. An example is a man with a poorly contractile bladder from long-term outlet obstruction. His bladder may not be able to generate a sufficient pressure to have his condition classified as obstruction, even though it is a progression of a process that occurred as a result of BPO. In such cases, it is once again important to consider all aspects of the patient’s evaluation and come to a consistent clinical conclusion.

Urethral pressure profilometry (UPP) was popularized by Brown and Wickman [39] as a method to determine resistance provided by the urethra. Using a small catheter with lateral apertures through which fluid is continuously infused, simultaneous bladder and urethral pressure is measured as the catheter is slowly withdrawn along the course of the urethra. The urethral pressure transducer measures the fluid pressure required to lift the urethral wall off the catheter side holes and thus evaluates the circumferential and radial stresses induced by the presence of the catheter in the urethra and the slow urethral perfusion. Thus, urethral pressure is defined as the fluid pressure needed to just open a closed urethra [1].

Several parameters can be obtained from the UPP:

The urethral closure pressure profile (UCP) is given by the subtraction of intravesical pressure from urethral pressure.

Maximum urethral pressure (MUP) is the highest pressure measured along the UPP.

Maximum urethral closure pressure (MUCP) is the maximum difference between the urethral pressure and the intravesical pressure.

Functional profile length is the length of the urethra along which the urethral pressure exceeds intravesical pressure in women.

UPP has been mostly used as a measure of urethral resistance in women with SUI. Despite an abundant literature on urethral profilometry, its clinical relevance is controversial. Many urologists do not routinely perform urethral profilometry. In 2002, the ICS standardization sub-committee concluded that the clinical utility of urethral pressure measurement is unclear [40]. Furthermore, there are no urethral pressure measurements that (1) discriminate urethral incompetence from other disorders; (2) provide a measure of the severity of the condition; (3) provide a reliable indicator to surgical success, and return to normal after surgical intervention [40].

Videourodynamics (VUDS) consists of the simultaneous measurement of UDS parameters and imaging of the lower urinary tract. It provides the most precise evaluation of voiding function and dysfunction. VUDS are particularly useful when anatomic structure in relation to lower urinary tract function is important, for example in localizing bladder outlet obstruction (particular in women) or in assessing vesico-ureteral reflux in relation to storage and/or voiding pressures. VUDS can be performed using a variety of different methods. Most commonly fluoroscopy is employed using a C-arm that gives the most flexibility for patient positioning. However a fixed unit with fluoroscopy table that can move from 90° to 180° may also be used. It is important that the patient be able to be positioned properly to evaluate the desired function and anatomy.