Urodynamics (UDS)

What the Study Is

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 artifact (straining, Valsalva, movement)

---

Indications for UDS

• Helpful while diagnosis remains uncertain
• Mixed symptoms, or who failed previous treatment
• Urinary retention or high PVR
• Prior to any invasive procedure

EAU framing of the same idea

PFS Is Indicated Before Invasive Treatment

• Previous unsuccessful invasive treatment for LUTS
• Patient can't void more than 150 mL
• 80 years or older with voiding symptoms
• Younger than 50 years with voiding symptoms
• 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
• 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)

• 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

• Five reference curve types (International Children's Continence Society, 2014 terminology standardization):
  • Bell-shaped
  • Tower
  • Plateau
  • Staccato
  • Intermittent

---

Cystometry (Filling)

• 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.
• After pelvic radiation: reduced bladder capacity is the most common urodynamic finding
  • Sometimes with reduced compliance and detrusor overactivity — radiation effects evolve and may progress over time

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
  • So in practice compliance = volume / end-fill Pdet → e.g. 300 mL at 6 cm H₂O = 50 mL/cm H₂O → normal
• Causes of impaired compliance (4):
  • Neurologic (SCI, spina bifida — usually via increased outlet resistance/DESD or decentralization)
  • Long-term BOO
  • Radiation cystitis
  • TB
• Wall change: poorly compliant bladders (including BOO) show ↑ type III : type I collagen and ↑ connective tissue : smooth muscle ratio
  • Once collagen has replaced the stroma, reduced compliance usually does not respond to drugs, hydraulic distension or nerve section → augmentation cystoplasty is often needed
• 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
  • This is the current ICS definition → 40 cm H₂O is not a DO cutoff; it marks a dangerous detrusor leak point or sustained storage pressure
  • Historically ("detrusor instability"): required an involuntary Pdet rise > 15 cm H₂O during filling at a volume ≤ 300 mL
• 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
  • UDS miss DO in ~50% of patients with urgency incontinence
  • Clue on a negative study: intermittent large-volume leakage + normal compliance + low PVR + no stress leak → suggests DO
• 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
  • Exam answer: ALPP measures sphincter strength — < 60 cm H₂O suggests ISD · 60–90 equivocal · > 90 little or no ISD

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
  • Sphincteric SUI testing needs volume ≥ 200 mL and Valsalva Pabd ≥ 60 cm H₂O — missing either is a main cause of a false-negative study
  • Post-prostatectomy SUI missed despite adequate volume and Valsalva
    • → the catheter can occlude the bladder neck or cause sphincter spasm: remove it at capacity and repeat Valsalva
• Grade 3 cystocele → falsely high ALPP — it kinks and obstructs the urethra.
  • Reduce it during UDS (pessary, swab or split speculum) → unmasks occult SUI and gives the true ALPP
• 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 characterize 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 decentralization
  • Non-neurogenic low compliance (multiple bladder surgeries, radiation, tuberculous cystitis)
• After sphincterotomy: DLPP is the most reliable urodynamic predictor of upper tract deterioration — > 40 cm H₂O = the sphincterotomy has failed → guides whether to repeat it
  • Urethral function tests (urethral pressure profile, Valsalva LPP, EMG) show no correlation with upper tract deterioration
  • Abnormal compliance on CMG is worrisome, but a much less established predictor

---

Voiding PFS

Records: Qmax, curve, PVR

The three fundamental patterns

Full bands

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 favorable

Orthotopic neobladder voiding

• Voids by Valsalva with external sphincter relaxation — the bowel reservoir has no coordinated detrusor contraction
  • Pattern: Pabd ↑ · Pves ↑ · Purethra ↓ — straining raises abdominal pressure, transmitted to the pouch, while urethral pressure falls
• Emptying can fail over time: flaccid bowel wall or an abdominal-wall hernia → can't raise pouch pressure enough → some need intermittent catheterization

---

Coordination — EMG & Sphincter

• Normal: EMG activity decreases before a voluntary bladder contraction. Failure to relax, or to stay relaxed, during micturition is abnormal
  • EMG lag time: normal 2–6 s with a bell-shaped voiding curve · < 2 s common in OAB · > 6 s → primary bladder neck dysfunction
• 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 center (infrapontine, suprasacral — Onuf's nucleus S2–S4)
  • No neurologic lesion → it is learned behavior = 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
• Dysfunctional voiding (the learned, non-DESD pattern above) = external sphincter spasm during voiding
  • Pelvic floor muscle dysfunction (perineal pain, constipation, pelvic muscle tenderness)
    • → most common UDS finding is a low peak flow with an intermittent pattern — outlet obstruction from the sphincter spasm
  • DO, poor compliance and large bladder capacity are not typical — unless chronic over several years
  • Neurologically intact woman with a staccato (intermittent) uroflow, incomplete emptying and recurrent UTIs
    • → increased EMG activity during voiding confirms non-neurogenic voiding dysfunction or Fowler's syndrome
  • Pelvic floor muscle therapy (teaches pelvic relaxation) is often first-line for either; several studies show sacral neuromodulation is effective

---

Dangerous UDS Findings — Usually Require Intervention

Six findings that threaten upper and lower tract decompensation:

• Impaired compliance
• Detrusor–external sphincter dyssynergia (DESD)
• Detrusor–internal sphincter dyssynergia (DISD)
• Detrusor leak point pressure > 40 cm H₂O
• High-pressure detrusor overactivity present throughout filling
• 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
  • Man with perineal discomfort, Qmax 9 mL/s and PVR 100 mL despite antibiotics and an alpha-blocker
    • → VUDS to diagnose bladder neck obstruction
    • Synergistic voiding on uroflow EMG rules out pelvic floor dysfunction
    • In presumed bladder neck obstruction, amitriptyline worsens voiding and pelvic floor physiotherapy is not beneficial
  • Young man with severe LUTS, Qmax 8 mL/s and PVR 95 mL despite tamsulosin
    • → VUDS before any surgery — low flow fits either obstruction (including PBNO) or detrusor underactivity
• Other uses (5):
  • Localize 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 center 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
• Sedation / anesthesia: DO is the only parameter reliably affected — may be absent on a sedated study
  • Compliance, detrusor leak point pressure and bladder capacity are generally unaffected
• 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 artifact and need no leveling at the symphysis
  • But nomograms were built on fluid-filled systems and readings are systematically higher — not interchangeable