High yield information and explanation of the uroDynamic Study.

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urodynamics AND urinary incontinence

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EAU guidelines urodynamics pressure flow study terminology 2025

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urodynamic study filling cystometry pressure flow tracing showing detrusor overactivity Pves Pabd Pdet

Urodynamic Study (UDS): high-yield overview

Urodynamics are tests that objectively assess storage and voiding function of the lower urinary tract by measuring urine flow, bladder pressure, abdominal pressure, and sometimes pelvic-floor/urethral activity.

Core question answered:

Is the patient’s problem due to an abnormal bladder contraction, poor bladder compliance, outlet obstruction, sphincter dysfunction, or weak detrusor contraction?

A standard invasive study includes uroflowmetry + post-void residual (PVR) + filling cystometry + pressure-flow study.
Campbell-Walsh-Wein Urology, p. 3373.

1. When is UDS indicated?

UDS is not routinely needed for straightforward lower urinary tract symptoms or uncomplicated demonstrable stress urinary incontinence. Use it when the diagnosis is uncertain or the result will change management.

Important indications

Complex, unexplained, or refractory LUTS
Urinary incontinence with unclear type, mixed symptoms, or failed treatment
Before invasive treatment when there is concern for:
- Bladder outlet obstruction (BOO)
- Detrusor underactivity (DU)
- Voiding dysfunction
Neurogenic lower urinary tract dysfunction, such as spinal cord injury, multiple sclerosis, spina bifida, Parkinsonism
Suspected poor compliance/high-pressure bladder, especially when upper-tract damage is possible
Suspected detrusor-sphincter dyssynergia (DSD)
Persistently high PVR, poor flow, recurrent retention, or recurrent urinary infections where emptying dysfunction is suspected
Failed previous incontinence surgery or suspected postoperative obstruction

Before invasive UDS, perform basic assessment: history, examination, urinalysis, bladder diary, uroflowmetry, and PVR. The EAU specifically recommends non-invasive testing first in neuro-urological patients. EAU neuro-urology guidance

2. Components of UDS

Test	What it measures	Main use
Uroflowmetry	Flow rate versus time	Screens for poor flow or abnormal voiding pattern
PVR	Urine left after voiding	Assesses incomplete emptying
Filling cystometry (CMG)	Bladder behavior during filling	Capacity, sensation, compliance, detrusor overactivity, leakage
Pressure-flow study (PFS)	Pressure and flow during voiding	Distinguishes obstruction from weak detrusor
EMG	Pelvic floor/external sphincter activity	Detects dysfunctional voiding or DSD
Video-UDS	UDS plus fluoroscopic imaging	Detects reflux, bladder morphology, site of obstruction, diverticula

3. How the invasive test is performed

A small transurethral catheter records intravesical pressure (Pves) and fills the bladder with sterile fluid.
A rectal or vaginal catheter records abdominal pressure (Pabd).
The equipment calculates:

[ \boxed{Pdet = Pves - Pabd} ]

Where:

Pves = intravesical pressure
Pabd = abdominal pressure
Pdet = true detrusor pressure

This subtraction is essential. For example, a pressure rise from coughing or straining increases both Pves and Pabd, whereas a genuine detrusor contraction raises Pdet.
Campbell-Walsh-Wein Urology, p. 3373.

During filling, ask the patient about:
- First sensation of filling
- First desire to void
- Strong desire to void
- Urgency
- Pain
- Leakage
The patient then voids with catheters in place. The relationship between Pdet and urinary flow (Q) is assessed.

4. Key parameters to know

A. Uroflowmetry

Qmax: maximum flow rate, in mL/s
Voided volume
Flow curve shape
PVR

Typical curve patterns:

Pattern	Likely implication
Bell-shaped curve	Usually normal
Low, flat plateau curve	Outlet obstruction or weak detrusor
Interrupted/staccato flow	Dysfunctional voiding or sphincter/pelvic-floor contraction
Prolonged low flow	Possible urethral stricture, BOO, or DU

Caution: Low Qmax alone cannot distinguish obstruction from weak detrusor. That requires a pressure-flow study.

B. Filling cystometry

1. Bladder sensation

Assess whether sensation is:

Normal
Increased/early, often with overactive bladder or bladder hypersensitivity
Reduced/absent, for example in diabetic neuropathy or neurologic dysfunction

2. Cystometric capacity

Volume at which the patient can no longer defer voiding or at which filling must stop.

Reduced capacity: pain, fibrosis, severe detrusor overactivity, radiation/cystitis
Large capacity: impaired sensation, chronic retention, neuropathic bladder

3. Compliance

[ \boxed{\text{Compliance} = \frac{\Delta V}{\Delta Pdet}} ]

It describes how easily the bladder accommodates increasing volume.

Low compliance means pressure rises excessively as the bladder fills.
This is important in neurogenic bladder because sustained high storage pressure can threaten the kidneys and upper urinary tract.

4. Detrusor overactivity (DO)

Definition: involuntary detrusor contractions during the filling phase, either spontaneous or provoked.

May cause urgency and urgency urinary incontinence.
Can be neurogenic or non-neurogenic.
A rise in Pves without a corresponding rise in Pabd indicates a true rise in Pdet and supports DO.

Exam point:

Overactive bladder (OAB) is a symptom syndrome: urgency, often frequency/nocturia, with or without urgency incontinence.
Detrusor overactivity is a urodynamic observation.
They are related but not synonymous. A patient may have OAB symptoms without DO being recorded during the test.
Campbell-Walsh-Wein Urology, p. 3373.

5. Urodynamic stress incontinence

Leakage with increased abdominal pressure, such as cough or Valsalva, without detrusor contraction.

This supports genuine stress urinary incontinence due to urethral sphincter incompetence.

5. Pressure-flow study: the most tested interpretation

During voiding, compare detrusor pressure with flow.

Finding	Pdet during voiding	Flow	Interpretation
Bladder outlet obstruction	High	Low	Bladder contracts strongly against a closed/narrow outlet
Detrusor underactivity	Low	Low	Bladder contraction is weak
Normal voiding	Appropriate	Good	Effective detrusor contraction with low outlet resistance
Valsalva voiding	Little/absent detrusor contraction	Variable	Patient voids mainly by abdominal straining
DSD/dysfunctional voiding	Often high and interrupted	Intermittent/low	Outlet fails to relax during detrusor contraction

Classic rule

High pressure + low flow = obstruction
Low pressure + low flow = detrusor underactivity

This distinction is the major value of pressure-flow testing. EAU male LUTS guidance

6. Important indices in men

Bladder Outlet Obstruction Index (BOOI)

[ \boxed{BOOI = Pdet@Qmax - 2(Qmax)} ]

Interpretation:

>40: obstructed
20-40: equivocal obstruction
<20: unobstructed

Bladder Contractility Index (BCI)

[ \boxed{BCI = Pdet@Qmax + 5(Qmax)} ]

Interpretation:

>150: strong contractility
100-150: normal contractility
<100: weak contractility

These indices are most validated in men with possible benign prostatic obstruction. EAU male LUTS guidance

7. Detrusor-sphincter dyssynergia

DSD is involuntary contraction of the external urethral sphincter during a detrusor contraction.

Usually occurs in a patient with a suprasacral neurologic lesion, such as spinal cord injury or multiple sclerosis.
Causes poor/intermittent flow, high voiding pressure, residual urine, vesicoureteric reflux, recurrent UTI, and possible renal damage.
Diagnosed with pressure-flow study plus EMG and often video-UDS.

High-yield distinction:

DSD: neurologic cause.
Dysfunctional voiding: similar non-relaxing sphincter/pelvic floor pattern but in a neurologically normal person.

8. Video-urodynamics

This combines filling cystometry and pressure-flow testing with fluoroscopic imaging.

Useful particularly in:

Neurogenic bladder
Suspected vesicoureteric reflux
Complex obstruction
Suspected bladder-neck dysfunction, urethral obstruction, or DSD
Congenital or postoperative lower urinary tract abnormalities

It shows both function and anatomy, including reflux and the level/cause of obstruction. EAU neuro-urology guidance

9. Common artifacts and pitfalls

Interpret UDS in the clinical context. It is not a test to interpret from numbers alone.

Cough/straining artifact: Pves and Pabd should rise together.
Catheter displacement or kinking: unreliable pressure traces.
Rectal contractions: can falsely alter Pabd.
Non-representative voiding: anxiety, privacy concerns, catheter discomfort, unusual position, or unfamiliar setting may prevent normal voiding.
Failure to demonstrate DO does not exclude OAB/urgency incontinence.
DO may be test-induced and not explain the patient’s symptoms.
Flow rate should be interpreted only if the voided volume is adequate.
Never diagnose BOO from low flow alone.

10. Adverse effects and precautions

Usually safe, but invasive UDS can cause:

Temporary dysuria
Urinary tract infection
Hematuria
Urinary retention, rarely
Autonomic dysreflexia in susceptible patients with high spinal cord lesions

Avoid or postpone invasive testing in active symptomatic UTI. In patients at risk of autonomic dysreflexia, monitor blood pressure and heart rate during the study. EAU neuro-urology guidance

One-minute exam summary

Urodynamic study assesses lower urinary tract storage and voiding. It includes uroflowmetry, PVR, filling cystometry, and pressure-flow study. During cystometry, Pdet is calculated as Pves minus Pabd. Filling phase assesses sensation, capacity, compliance, detrusor overactivity, and stress leakage. Pressure-flow study distinguishes BOO from detrusor underactivity: high detrusor pressure with low flow suggests obstruction, while low pressure with low flow suggests weak detrusor contractility. Video-UDS and EMG are particularly useful in neurogenic bladder and suspected DSD.

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