The urethral abnormality that accompanies stress incontinence may be due to either of two causes. First, the urethra may be poorly supported, referred to as urethral hypermobility. The urethra should have strong support from the pelvic floor, composed of ligaments, tendons, and muscles, so that it remains closed during exercise, coughing, and straining. These structures can be injured or weakened by childbirth, pelvic surgery, obesity, frequent prolonged straining, and strenuous exercise such as weight lifting, jogging, jumping, long distance running, and high impact aerobics. The urethra then drops and opens when exposed to physical stress or straining. Loss of urethral support is frequently associated with loss of support for the other pelvic organs (prolapse), particularly the bladder. The two conditions are nevertheless independent; stress incontinence can occur without pelvic prolapse and vice-versa. In the same way, treatments to correct one of the conditions may not necessarily correct the other.
The second cause of SUI is poor urethral function, or intrinsic sphincter deficiency (ISD). SUI can occur even when the urethra is in a perfectly normal and well-supported position. At one time it was thought that this was a rare problem that occurred after nerve injuries, radiation to the pelvis, or extensive pelvic surgery. We now know that this is a common condition and may be due to aging, hormonal changes, nerve injury during childbirth, pelvic surgery, and other factors. In this situation, the walls of the urethra simply are not able to create an effective seal. It is like a faucet that needs to have a washer replaced to correct a slow drip. While there is no specific test for ISD, it is now generally believed that many women with SUI have at least some degree of ISD.