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Also known as
Cystometry, Filling cystometry, Bladder pressure test, Urodynamic cystometry, CMG
Definition
A cystometrogram (CMG) is a specialized urodynamic test that measures bladder pressure in relation to volume during the filling phase of bladder function.1 This diagnostic procedure involves the insertion of a small catheter (typically 6-7Fr) through the urethra into the bladder to fill it with sterile fluid (water or saline) while simultaneously measuring intravesical pressure.2 A second catheter may be placed in the rectum or vagina to measure abdominal pressure, allowing for the calculation of true detrusor pressure by subtracting abdominal pressure from vesical pressure.3
The cystometrogram provides critical information about multiple aspects of bladder function, including:
- Bladder capacity (how much urine the bladder can hold)
- Bladder compliance (how well the bladder stretches during filling)
- Bladder sensation (when the patient feels first sensation, first desire to void, and strong desire to void)
- Detrusor muscle activity (presence of involuntary contractions)
- Leak point pressure (pressure at which urine leakage occurs)4
During the procedure, the bladder is gradually filled at a controlled rate while pressure measurements are continuously recorded. The patient communicates sensations experienced during filling, and the clinician documents the volume at which these sensations occur. Any involuntary detrusor contractions or urine leakage is also recorded. This comprehensive assessment of bladder storage function is essential for diagnosing various lower urinary tract disorders and guiding appropriate treatment decisions.5
Clinical Context
Cystometrogram (CMG) is utilized clinically in a variety of situations to diagnose and manage lower urinary tract symptoms (LUTS).1 It is particularly valuable when the diagnosis is unclear, when empirical treatments have failed, or before invasive surgical interventions are considered. The test helps in identifying specific lower urinary tract dysfunction, such as detrusor overactivity (DO), bladder outlet obstruction (BOO), detrusor underactivity (DU), stress urinary incontinence (SUI), and impaired bladder compliance.2
Relevant Medical Conditions
CMG is often indicated in the following clinical scenarios:
1. Neurogenic bladder disorders3 – Conditions where neurological impairment affects bladder function, including:
- Spinal cord injury
- Multiple sclerosis
- Parkinson’s disease
- Diabetic neuropathy
- Stroke with urinary symptoms
2. Complex or recurrent urinary incontinence4 – Particularly when:
- First-line treatments have failed
- Mixed symptoms are present
- Symptoms don’t match clinical findings
- Previous incontinence surgery has failed
3. Voiding dysfunction in men2 – Often to differentiate between:
- Bladder outlet obstruction (BOO) due to benign prostatic hyperplasia
- Detrusor underactivity (DU)
- This distinction is especially important before prostate surgery
4. Women with LUTS4 – When initial assessment is inconclusive, to differentiate:
- Stress urinary incontinence
- Urge urinary incontinence
- Mixed urinary incontinence
- Voiding dysfunction
5. Pre-surgical evaluation1 – Before interventions for:
- Stress urinary incontinence
- Pelvic organ prolapse
- Benign prostatic hyperplasia
Patient Selection Criteria
Patient selection criteria for CMG generally include individuals with:1
- Persistent LUTS despite conservative management
- Suspected neurological causes for bladder dysfunction
- Previous pelvic surgery who develop new LUTS
- Cases where findings might significantly alter the management plan
According to guidelines from the American Urological Association (AUA) and Society for Urodynamics, Female Pelvic Medicine, and Urogenital Reconstruction (SUFU), indications for urodynamic testing include:2
- Identifying lower urinary tract dysfunction
- Predicting consequences on the upper urinary tract
- Predicting management outcomes
- Assessing intervention outcomes
- Evaluating treatment failure
Procedure Details
The CMG procedure involves several steps:3
1. Preparation:
- The patient arrives with a comfortably full bladder
- A uroflowmetry test may be performed first
- Post-void residual volume is measured
- The patient is positioned (usually supine)
2. Catheterization:
- A multi-lumen urodynamic catheter (6-7Fr) is inserted through the urethra into the bladder using aseptic technique
- A second catheter is placed in the rectum or vagina to measure abdominal pressure
- Both catheters are connected to pressure transducers
3. Filling Phase:
- The bladder is filled with sterile fluid at a controlled rate (typically 10-100 ml/min)
- The patient reports sensations (first sensation, first desire to void, strong desire to void)
- The clinician records volumes at each sensation point
- Any involuntary detrusor contractions are noted
- Cough tests may be performed to assess for stress incontinence
4. Assessment:
- Bladder capacity is determined
- Compliance is calculated (change in volume/change in pressure)
- Presence of detrusor overactivity is noted
- Leak point pressures may be measured
Expected Outcomes
Expected outcomes from CMG include:4
- A detailed physiological assessment of lower urinary tract function
- A specific urodynamic diagnosis (e.g., detrusor overactivity with incontinence, bladder outlet obstruction with normal detrusor function)
- Guidance for tailoring treatment approaches
- Prediction of treatment success
- Management of patient expectations
Studies have shown that women undergoing urodynamic testing are more likely to have changes made to their management compared to those who do not undergo testing.5 Similarly, in men, urodynamics can distinguish BOO from DU, guiding appropriate interventions and improving outcomes for those with proven BOO.2 The studies also help identify patients at risk of upper urinary tract damage due to high bladder pressures, guiding more aggressive management if needed.