Continent women show improved urethral innervation

Yet, urethral closure pressure measurements are limited in that they only provide an indirect measure of sphincter function and are subject to artifact distortion. But neuropathic changes as seen on quantitative electromyography (EMG) have been found to more accurately reflect the neuromuscular integrity of the muscle under study by recording electrical activity of nearby motor units, reported Kimberly Kenton, MD, MS at the 2009 International Continence Society annual meeting.

"Since urethral EMG parameters of continent women had not yet been reported, our aim was to characterize those parameters in continent women without lower urinary tract symptoms, then compare them in both continent and stress incontinent women," said Dr. Kenton, associate professor at Loyola University Health Center, Maywood, IL.

Over a 2-year period, the team collected urethral EMG data with a concentric needle electrode positioned with auditory and oscilloscope guidance into the striated urethral sphincter muscle. Three to four insertion sites were selected: 12 o'clock, 3 o'clock, and 9 o'clock-all 5 mm above the external urethral meatus.

The Keypoint EMG instrument (Medtronic, Minneapolis, MN) performed multiple motor unit action potential analysis (multi-MUP) and interference pattern (IP) analysis at baseline on an empty bladder; the Viking IVp EMG (Nicolet Instrument Inc., Madison, WI) instrument processed EMG signals during filling cystometry. At each insertion site, patients were asked to "gradually squeeze as if trying to hold in urine" in order to recruit more MUP and create an IP.

IP analysis was done at resting muscle activity and as the patient increased urethral sphincter muscle activity with a voluntary squeeze. A recording of MUP activation was made with subjects at rest with an empty bladder and during filling cystometry.

Stress incontinent women had lower levels of MUP recruitment, as measured by decreasing numbers of turns per second (5.6±2.4 for continent women vs. 4.5± 2.3 for SUI [not a statistically significant difference]); lower turns per amplitude ratio (.72±.36 for continent women vs. .48±.28 for those with SUI, p=.0003); and decreased number of short segments during the IP analysis (112±78 for continent women vs. 70± 59 for those with SUI, p=.01).

"These IP findings are consistent with those found in neuromuscular disorders," Dr. Kenton said.

When a motor unit is injured, peripheral sprouting from neighboring axons results in longer duration, higher amplitude, and polyphasic MUP findings that are frequently interpreted as abnormal, but that actually represent reinnervation, Dr. Kenton noted.

"This study will allow us to understand what's causing stress incontinence and to rethink treatment options," she said. "When we think a muscle or nerve is injured, we don't know how to make the nerve or muscle work better. We compensate for the problem with surgical procedures, like slings, but do not actually fix the problem.

"Once we can show that we're seeing a neuromuscular injury, it will open the door for neurogenerative therapeutic interventions that may dramatically alter conventional treatments and improve quality of life for women," Dr. Kenton said. With one short burst of electrical stimulation, the chances of reinnervating a targeted nerve improve significantly."

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