Urodynamics Test (UDS)
What is Urodynamics?
If you have a problem with urine leakage or blocked urine flow, one of the tools your doctor may use to evaluate the cause of your symptoms is urodynamic testing.
Urodynamics is a study that assesses how the bladder and urethra are performing their job of storing and releasing urine. Several muscles, organs, and nerves are involved in collecting, storing, and releasing urine. The kidneys form urine by filtering wastes and extra water from the bloodstream. The ureters are tubes that carry urine from the kidneys to the bladder. The bladder stores urine until you are ready to empty it. The bladder opens into the urethra, the tube that allows urine to pass outside the body. Problems in the urinary system can be caused by aging, illness, or injury.
A uroflowmeter automatically measures the amount of urine and the flow rate. You may be asked to urinate privately into a toilet that contains a collection device and scale. This equipment creates a graph that shows changes in flow rate from second to second so your doctor can see the peak flow rate and how many seconds it took to get there. Results of this test will be abnormal if the bladder muscle is weak or urine flow is obstructed.
- Measurement of Postvoid Residual
After you have finished, you may still have some urine, usually only an ounce or two, remaining in your bladder. To measure this postvoid residual, the doctor may use a catheter, a thin tube that can be gently glided into the urethra. He or she can also measure the postvoid residual with ultrasound equipment that uses harmless sound waves to create a picture of the bladder. A postvoid residual of more than 200 mL, about half a pint, is a clear sign of a problem. Even 100 mL, about half a cup, requires further evaluation. However, the amount of postvoid residual can be different each time you urinate.
A cystometrogram (CMG) measures how much your bladder can hold, how much pressure builds up inside your bladder as it stores urine, and how full it is when you feel the urge to urinate. The doctor will use a catheter to empty your bladder completely. Then a special, smaller catheter will be placed in the bladder. This catheter has a pressure-measuring device called a manometer. Another catheter may be placed in the rectum to record pressure there as well. Your bladder will be filled slowly with warm water. During this time you will be asked how your bladder feels and when you feel the need to urinate. The volume of water and the bladder pressure will be recorded. You may be asked to cough or strain during this procedure. Involuntary bladder contractions can be identified.
- Measurement of Leak Point Pressure
While your bladder is being filled for the CMG, it may suddenly contract and squeeze some water out without warning. The manometer will record the pressure at the point when the leakage occurred. This reading may provide information about the kind of bladder problem you have. You may also be asked to apply abdominal pressure to the bladder by coughing, shifting position, or trying to exhale while holding your nose and mouth. These actions help the doctor evaluate your sphincter muscles.
- Pressure Flow Study
After the CMG, you will be asked to empty your bladder. The catheter can measure the bladder pressures required to urinate and the flow rate a given pressure generates. This pressure flow study helps to identify bladder outlet obstruction that men may experience with prostate enlargement. Bladder outlet obstruction is less common in women but can occur with a fallen bladder or rarely after a surgical procedure for urinary incontinence. Most catheters can be used for both CMG and pressure flow studies.
If your doctor feels that your urinary problem is related to nerve or muscle damage, you may be given an electromyography. This test measures the muscle activity in and around the urethral sphincter by using special sensors. The sensors are placed on the skin near the urethra and rectum or they are located on the urethral or rectal catheter. Muscle activity is recorded on a machine. The patterns of the impulses will show whether the messages sent to the bladder and urethra are coordinated correctly.
- Video Urodynamics
Urodynamic tests may be performed with or without equipment to take pictures of the bladder during filling and emptying. The imaging equipment may use x rays or sound waves. If x-ray equipment is used, the bladder will be filled with a contrast medium that will show up on the x ray instead of the warm water. The pictures and videos show the size and shape of the urinary tract and help your doctor understand your problem.