A cystometry test measures bladder pressure to assess how well the organ stores and releases urine. This procedure is part of urodynamic studies, which investigate the function of the lower urinary tract. By recording pressure changes as the bladder fills, the test provides objective data on the mechanical and neurological aspects of bladder control. The goal is to understand the relationship between the volume of fluid inside the bladder and the pressure exerted by the bladder wall, which is crucial for diagnosing various urinary issues.
Why Bladder Pressure Measurement is Necessary
Measuring bladder pressure is necessary to pinpoint the cause of complex or persistent lower urinary tract symptoms that cannot be determined by symptoms alone. The test helps differentiate between various types of incontinence, such as urge incontinence (involuntary bladder muscle contractions) and stress incontinence (related to abdominal pressure). It is also valuable for evaluating patients with frequent, painful, or difficult urination that has not responded to initial treatments.
The results identify neurogenic bladder dysfunction, which occurs when nerve issues, such as spinal cord injury or multiple sclerosis, affect bladder control. Monitoring pressure is important for assessing the risk of upper urinary tract damage in patients with neurological conditions. Elevated pressure during the storage phase can signal danger to the kidneys, guiding necessary treatment. The test also determines if symptoms are due to issues with the bladder muscle or an obstruction at the bladder outlet.
Preparing for the Pressure Test
Patient preparation focuses on ensuring an accurate and safe test environment. Before the procedure, a healthcare provider reviews the patient’s current medication list, especially drugs that affect the nervous system or bladder muscle function. Patients may be asked to temporarily stop taking certain medications if they could interfere with the test results. Checking for a urinary tract infection (UTI) before proceeding is essential, as an active infection can skew results and increase complication risk.
Some patients may receive an antibiotic one or two days before the test as a preventative measure. There is usually no need to adjust normal food or fluid intake, but patients are encouraged to drink extra fluids afterward to help flush the bladder. The patient should arrive with a moderately full bladder for an initial flow measurement, and specific instructions regarding bowel movements may be given, as stool in the rectum can interfere with pressure monitoring.
The Cystometry Procedure
The cystometry procedure begins by inserting a thin, flexible catheter through the urethra into the bladder to measure pressure and infuse fluid. A second pressure-sensing catheter or probe is simultaneously placed into the rectum or vagina to measure abdominal pressure. Both catheters are connected to a machine that continuously records pressure changes.
The test relies on three specific measurements: Vesical Pressure (Pves), the total pressure inside the bladder; Abdominal Pressure (Pabd), the pressure surrounding the bladder; and Detrusor Pressure (Pdet). Pdet is the true pressure generated by the bladder muscle, calculated by subtracting abdominal pressure from vesical pressure (Pdet = Pves – Pabd). This subtraction is necessary because intra-abdominal pressure changes, such as from coughing, must be excluded to determine the bladder muscle’s contribution.
During the filling phase, sterile saline or water is slowly infused into the bladder through the urethral catheter. The filling rate is controlled, typically between 50 and 100 milliliters per minute, while the pressure is continuously monitored. The patient reports specific sensations, such as the first desire to urinate and the strong desire to void, which are correlated with the corresponding volume and pressure readings.
At specific intervals, or when the bladder approaches maximum capacity, the patient is asked to cough or strain. These provocative maneuvers determine if urine leakage occurs under stress and whether it is associated with an involuntary detrusor contraction. The test concludes when the patient can no longer delay urination (maximum cystometric capacity) or when a severe leakage event occurs.
Interpreting Bladder Pressure Results
Interpreting the cystometry results involves analyzing the pressure-volume relationship recorded throughout the test. A normal bladder exhibits compliance, meaning it stores increasing volumes of urine with only a minimal rise in Detrusor Pressure (Pdet). A significant and sustained rise in Pdet during the filling phase indicates low bladder compliance, where the bladder wall is stiff and cannot accommodate volume properly.
The test also identifies detrusor overactivity, characterized by involuntary, rapid rises in Pdet that the patient cannot suppress. These contractions may or may not result in urine leakage and are a common cause of urge incontinence symptoms. Abnormal pressure during the voiding phase, such as a high Pdet combined with a low flow rate, may suggest a bladder outlet obstruction.
Normal resting Detrusor Pressure falls within a narrow range, often between -5 and +5 centimeters of water (cm H2O). A high Detrusor Leak Point Pressure (DLPP)—the pressure at which leakage occurs without a detrusor contraction—is a specific measurement used to assess the risk of damage to the upper urinary tract in patients with neurogenic bladder dysfunction. The recorded bladder capacity, involuntary contractions, and bladder wall compliance provide a comprehensive picture for diagnosis.