(Cystometrogram, CMG)

Procedure overview

What is cystometry?

Cystometry is a diagnostic procedure used to identify problems with the filling and emptying of the urinary bladder. This test measures the amount of volume/urine in the bladder compared with the bladder pressure and the person's perception of bladder fullness. Cystometry provides information about the muscle function, mechanics, and nerve response of the bladder and urinary tract.

A normally functioning bladder sends messages to the brain via nerve pathways when it needs emptying. The spinal cord then transmits a message to the bladder to contract and begin the reflex of urinating. A person can override this reflex voluntarily by holding his/her urine.

Some medical conditions may interfere with the muscular function or nerve pathways between the bladder and the brain. These conditions may lead to urinary incontinence (loss of bladder control) or urinary obstruction. Cystometry may be indicated to determine the source of such problems. It is often done together with Uroflowmetry, which provides a measurement of urine speed and volume, and an estimation of post-void residual (how much urine remains in the bladder). 

Other related procedures that may be used to evaluate bladder or urinary problems include cystography and intravenous pyelogram (IVP). Please see these procedures for additional information.

How does the urinary system work?

The body takes nutrients from food and converts them to energy. After the body has taken the food that it needs, waste products are left behind in the bowel and in the blood.

The urinary system keeps chemicals, such as potassium and sodium, and water in balance, and removes a type of waste, called urea, from the blood. Urea is produced when foods containing protein, such as meat, poultry, and certain vegetables, are broken down in the body. Urea is carried in the bloodstream to the kidneys.

Other important functions of the kidneys include blood pressure regulation, and the production of erythropoietin, which controls red blood cell production in the bone marrow.

Urinary system parts and their functions

  • Two kidneys. A pair of purplish-brown organs located below the ribs toward the middle of the back. Their function is to remove liquid waste from the blood in the form of urine, keep a stable balance of salts and other substances in the blood, and produce erythropoietin, a hormone that aids the formation of red blood cells.

    The kidneys remove urea from the blood through tiny filtering units called nephrons. Each nephron consists of a ball formed of small blood capillaries, called a glomerulus, and a small tube called a renal tubule. Urea, together with water and other waste substances, forms the urine as it passes through the nephrons and down the renal tubules of the kidney.

  • Two ureters. Narrow tubes that carry urine from the kidneys to the bladder. Muscles in the ureter walls continually tighten and relax forcing urine downward, away from the kidneys. If urine backs up, or is allowed to stand still, a kidney infection can develop. About every 10 to 15 seconds, small amounts of urine are emptied into the bladder from the ureters.

  • Bladder. A triangle-shaped, hollow organ located in the lower abdomen. It is held in place by ligaments that are attached to other organs and the pelvic bones. The bladder's walls relax and expand to store urine, and contract and flatten to empty urine through the urethra. The typical healthy adult bladder can store up to two cups of urine for two to five hours.

  • Two sphincter muscles. Circular muscles that help keep urine from leaking by closing tightly like a rubber band around the opening of the bladder

  • Nerves in the bladder. Alert a person when it is time to urinate, or empty the bladder

  • Urethra. The tube that allows urine to pass outside the body

Reasons for the procedure

Cystometry may be recommended to evaluate problems related to the muscle function of the bladder and urethra. Conditions that may cause functional problems of the bladder and urethra may include, but are not limited to, the following:



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