Urodynamics (UDS)

What the Study Is

Tier	Components
Standard	Uroflowmetry + PVR + cystometry + pressure-flow study
Supplementary	EMG + imaging (videourodynamics) with continuous urethral pressure

Including: UFM · PVR · cystometry · PFS · EMG · cystogram

• Pdet = Pves − Pabd
  • Pves = bladder catheter · Pabd = rectal (or vaginal/stomal) catheter — use rectal when possible; data are limited for vaginal and stomal
  • Subtraction is what separates a true Pdet rise (contraction or lost compliance) from a Pabd artefact (straining, Valsalva, movement)

Indications for UDS

1. Helpful while diagnosis remains uncertain
2. Mixed symptoms, or who failed previous treatment
3. Urinary retention or high PVR
4. Prior to any invasive procedure

EAU framing of the same idea

Recommendation	Strength
Adhere to ICS good urodynamic practice standards	Strong
Do not routinely perform UDS for uncomplicated SUI	Strong
Do not routinely perform UDS before first-line treatment of uncomplicated OAB	Strong
Perform UDS if findings may change the choice of invasive treatment, and/or if there is diagnostic uncertainty	Weak

PFS Is Indicated Before Invasive Treatment

1. Previous unsuccessful invasive treatment for LUTS
2. Patient can't void more than 150 mL
3. 80 years or older with voiding symptoms
4. Younger than 50 years with voiding symptoms
5. Qmax > 10 mL/s when considering surgery
   • with Qmax < 10 mL/s, BOO is likely and UDS is not necessarily needed — the test earns its place when flow is equivocally preserved
6. PVR > 300 mL when considering invasive therapy

The 9 Cs — Parameters to Record

Filling cystometry

1. Coarse sensation
2. Capacity
3. Compliance
4. Contraction (involuntary detrusor)
5. Continence

Voiding PFS

6. Contractility
7. Clinical obstruction
8. Complete emptying
9. Coordination

Uroflowmetry (UFM)

Parameter	Value
Voided volume (VV) — minimum volume	150 mL
Qmax — likely obstruction	< 10 mL/s
Qmax — suspect obstruction	10–15 mL/s

• Qmax < 10 mL/s for BOO: specificity 70%, PPV 70%, sensitivity 47% — rules in, doesn't rule out
• Qmax has within-subject variation — repeat the study, especially if VV < 150 mL or the pattern is abnormal
• Instruct the patient not to Valsalva void
• Voiding efficiency = VV / (VV + PVR) × 100
• Do not offer non-invasive tests as an alternative to UDS/PFS for diagnosing BOO in men

Pattern of UFM

Click the chart to open it full size.

• The pattern suggests, it never confirms
• Curve interpretation is subjective; confirm a staccato/intermittent trace on non-invasive uroflow to exclude a test-induced phenomenon

Pattern nomenclature

Pattern	Suggests
Bell	Normal
Flattened	Fixed obstruction — the plateau trace above
Interrupted / straining / wavy	Impaired contractility, obstruction, or abdominal straining
Peaked / tower	Rapid emptying, overactivity — closest to the high Qmax then tapers trace
Staccato	Dysfunctional voiding — "intermittent or fluctuating flow rate secondary to involuntary intermittent contractions of the periurethral striated muscle in neurologically normal individuals"
Saw tooth	"Shy bladder"

Cystometry (Filling)

Parameter	Value
Filling rate	(wt/4) ≈ 20–30 mL/min
Capacity	≈ 400–500 mL normal adult
Compliance	= Δ Volume / Δ Pressure — normal less than 20
Detrusor pressure	(contraction)
Continence	LPP

• Filling rate: physiologic = filling rate below the predicted maximum of body weight (kg) / 4 mL/min; above that is non-physiologic. For a 80–120 kg adult, wt/4 lands exactly on your 20–30 mL/min.

Compliance — the practical rules

• Absolute pressure beats the compliance number. "In practical terms, absolute pressure is probably more useful than a 'compliance number'" — storage pressure > 40 cm H₂O harms the upper tracts
  • Why: a 400 mL bladder at Pdet 50 and a 40 mL bladder with an early IDC both compute to ~6–8 mL/cm H₂O — only the first is dangerous. The number alone cannot tell them apart
• ICS measures two points: Pdet at start of filling (usually zero) and Pdet at cystometric capacity — both excluding detrusor contractions
• Causes of impaired compliance (4): neurologic (SCI, spina bifida — usually via increased outlet resistance/DESD or decentralisation) · long-term BOO · radiation cystitis · TB
• False positive (looks worse than it is): rapid filling → stop filling; if pressure returns to baseline, compliance is not impaired
• False negative (looks better than it is): "pop-off" mechanisms — VUR and bladder diverticula. With VUR the pressure is transferred to the refluxing kidney and is harmful; VUDS is very useful here

Detrusor overactivity (DO)

• Definition: involuntary detrusor contraction during the filling phase. Any phasic contraction during filling is DO regardless of amplitude
• Neurogenic (relevant neurological condition) vs idiopathic · spontaneous vs provoked · single vs multiple · phasic / sporadic / terminal
  • Terminal DO = a single involuntary contraction at cystometric capacity causing incontinence
  • Non-phasic Pdet change before micturition = a compliance change, not DO
• Failure to demonstrate DO does not rule it out — detection is influenced by patient position
• Be certain the contraction is truly involuntary — patients get confused and void on first desire
• Prognostic: OAB with obstruction is more likely to resolve after TURP when DO is a single terminal IDC rather than continuous/sporadic

Leak Point Pressures

Continence, LPP

• DLPP → to assess risk of upper tract deterioration, if more than 40 cm is at risk
• ALPP → to assess SUI, if less than 60 it is due to ISD, if more than 100 it is not due to ISD

	ALPP	DLPP
Measures	Sphincter response to increased Pabd	Injured bladder response to increased outlet resistance
Definition	Pves at which leakage occurs, from raised Pabd, without a detrusor contraction	Lowest Pdet at which leakage occurs, without raised Pabd or a detrusor contraction
Direction	Lower = weaker sphincter	Higher = more dangerous to upper tracts
Question answered	Is this SUI due to ISD?	Are the upper tracts at risk?

ALPP

• There is no normal ALPP — patients without SUI will not leak at any physiologic Pabd. Only demonstrable in a patient with SUI
• Measure the total Pabd required to cause leakage, not the change — includes baseline Pabd (usually 20–40 cm H₂O standing). Read from the Pves channel provided there is no involuntary contraction
• Start at 150 mL, then every 50 mL until SUI is demonstrated. If none at capacity → remove the urethral catheter and measure via the abdominal channel
• ALPP ≠ VLPP. In the same person, Valsalva LPP < cough LPP
• Current technology cannot distinguish ISD from urethral hypermobility in women. If there is no hypermobility, SUI must be ISD regardless of ALPP — an isolated ALPP has limited utility in predicting surgical success

AUA vs EAU — the role of LPP. AUA/Campbell use ALPP to characterise ISD and to inform choice of anti-incontinence surgery ("clinicians making the diagnosis of urodynamic SUI should assess urethral function" — ALPP or MUCP). EAU 14: "Do not use urethral pressure profilometry or leak point pressure to grade severity of urinary incontinence" — Strong

DLPP

• The significance: bladder pressure is climbing too high before the pop-off of urethral leakage occurs. Low outlet resistance → leak at a safe pressure; high outlet resistance (e.g. DESD) → pressure keeps climbing and reaches the kidneys — less incontinence, more danger
• Most useful in (3): UMN lesions with high storage pressures (DO + DESD) · LMN disease causing decentralisation · non-neurogenic low compliance (multiple bladder surgeries, radiation, tuberculous cystitis)

Voiding PFS

Records: Qmax, curve, PVR

Parameter	Value
Pdet	40–60 normal, female less
EMG	? DSD — look for
BOOI	= Pdet@Qmax − 2(Qmax) → more than 40 means obstructed
BCI	= Pdet@Qmax + 5(Qmax) → less than 100 means acontractile

The three fundamental patterns

Pdet	Flow	Diagnosis
Low / normal	High / normal	Normal, unobstructed voiding
High	Low / normal	Obstruction
Low	Low	Detrusor underactivity

Full bands

Index	Value	Meaning
BOOI	≥ 40	Obstructed
	20–40	Equivocal
	≤ 20	Unobstructed
BCI	> 150	Strong contractility
	100–150	Normal contractility
	< 100	Weak contractility

BOOI/BCI caveats

• Both indices break down when underactivity and obstruction coexist — e.g. long-standing obstruction
• BOOI grossly underestimates female BOO (females void at much lower pressures) → female cutoffs: Qmax ≤ 12 mL/s + Pdet@Qmax ≥ 25 cm H₂O — but with large overlap; needs radiographic or clinical corroboration
• BOO also damages storage: ~⅔ of men with symptomatic BOO have DO, resolving 50–67% of the time with treatment of the obstruction. Reduced compliance also improves after TURP
• Significantly impaired compliance remains the only absolute urodynamic indication for treating BOO — a critical BOOI level has never been defined
• If there is no obstruction on PFS, surgical outcomes are less favourable

Coordination — EMG & Sphincter

• Normal: EMG activity decreases before a voluntary bladder contraction. Failure to relax, or to stay relaxed, during micturition is abnormal
• Pseudodyssynergia: EMG sphincter "flare" during filling from voluntarily trying to inhibit an involuntary contraction (guarding reflex) — not true dyssynergia
• DESD: involuntary increase in external sphincter activity with detrusor activity/voiding
  • True DESD requires a known neurologic lesion between the pons and the sacral micturition centre (infrapontine, suprasacral — Onuf's nucleus S2–S4)
  • No neurologic lesion → it is learned behaviour = dysfunctional voiding, not DESD
  • Detrusor contracting against a closed sphincter → high pressures → can impair compliance → urodynamic risk factor for upper tract deterioration
• DISD: lack of coordination between detrusor and internal sphincter/bladder neck

Dangerous UDS Findings — Usually Require Intervention

Six findings that threaten upper and lower tract decompensation:

1. Impaired compliance
2. Detrusor–external sphincter dyssynergia (DESD)
3. Detrusor–internal sphincter dyssynergia (DISD)
4. Detrusor leak point pressure > 40 cm H₂O
5. High-pressure detrusor overactivity present throughout filling
6. Poor emptying with high storage pressures

• These are the findings that justify treating on the tracing alone, independent of symptoms — impaired compliance from radiation cystitis or TB can only be diagnosed by CMG

Videourodynamics

• Procedure of choice for confirming bladder neck dysfunction in men and women
  • Primary bladder neck obstruction can only be diagnosed on VUDS
  • Differentiates functional obstruction (PBNO vs dysfunctional voiding) — AUA/SUFU supports VUDS in young men and women without an obvious anatomic cause of obstruction
  • Especially useful for BOO in women; can confirm sphincteric dysfunction diagnosed by EMG
• Other uses (5): localise obstruction · detect incontinence not seen on exam · evaluate VUR during storage/voiding · determine whether a known anatomic abnormality (diverticulum, VUR) is contributing · neuropathic voiding dysfunction
• Refer to a VUDS centre if a complete diagnosis can't otherwise be obtained: known/suspected neurogenic LUTD · unexplained female retention · prior radical pelvic surgery · urinary diversion · pre/post renal transplant · prior pelvic radiation
• Ambulatory UDS: more physiologic, best when standard UDS is inconclusive and treatment is uncertain

Conducting the Study — Practical

• Decide on/off medication in advance: testing the drug's effect → study on a regular dosing schedule. Uncovering the cause of symptoms after empirical treatment → consider stopping first for the highest yield
• UDS is an unnatural setting:
  • Failure to record an abnormality does not rule it out
  • Not all UDS observations are clinically significant
  • Ideally the patient's symptoms are reproduced during the study
• Define the question before you start. If it isn't answered, repeat the study in the same session — many authors advocate a second filling and/or voiding cycle
• Antibiotic prophylaxis is recommended for UDS in patients with indwelling urethral/suprapubic catheters, condom catheters, or on CIC
• Air-charged vs water-filled catheters: air-charged avoid hydrostatic effects and motion artefact and need no levelling at the symphysis; but nomograms were built on fluid-filled systems and readings are systematically higher — not interchangeable