The Mid-Urethra is Key
The mechanism of continence and SUI is explained by the Integral Theory. The Integral Theory (also called mid-urethral theory) focuses on the importance of the supporting tissues and ligaments situated around the mid-urethra. Blood circulation and nerve connections are concentrated to the mid urethra, and it has been shown that continent women stopping or voluntary inhibiting their micturition do so at the mid-urethra (1).
Several mechanisms and muscle forces interact achieving opening and closure of the urethra including both voluntary and involuntary mechanisms. If the efficacy or balance of the muscle forces is disrupted through damage to their ligamentous anchoring points, dysfunction may be a result.
Weakness in muscles and ligaments will easily push the continence system, according to the Integral Theory, into open mode when for example the intra-abdominal pressure rises upon effort, causing incontinence.
“Active closure” includes, contraction forward of the pubococcygeus muscle (PCM) which pulls the vagina tightly around the urethra, closing it off and immobilizing it while the bladder is pulled down and back like an elastic balloon, kinking off and closing off the mid urethra like a hose. A functional urethral mucosa further strengthens this seal.
As part of the micturition reflex, the PCM relaxes. This allows the Levator Plate (LP) and the longitudinal muscle of the anus (LMA) to uninhibitedly pull, opening the bladder base, creating a “funnel”, enlarging the urethral outlet. At the same time, this stretching stimulates the nerve endings, activating and reinforcing the micturition reflex. The broken lines represent the closed position of the bladder. Without the extra seal by the kinking of the mid urethra, incontinence is a fact.
The Mechanics of Stress Urinary Incontinence
Stress urinary incontinence is defined as involuntary urine leakage associated with coughing, laughing, sneezing, exercising or other movements that increase intra-abdominal pressure. The Integral Theory states that bladder dysfunctions such as SUI are mainly caused by weak suspensory ligaments as a result of altered collagen/elastin compositions (2). When the insertion points for the ligaments appears lax, the muscle forces weakens as the muscle has nothing to contract against. This results in a dysfunctional closure of the urethra, giving incontinence (3).
How Ligaments are Crucial to Continence
A ligament is a complicated contractile structure which needs to be both elastic and strong while having the ability to contract or relax according to whether the urethra is to be closed or opened. It relies on its collagen content for strength, elastin for flexibility, smooth muscle for contractility, and nerves to coordinate all these functions. Collagen fiber work like the steel rods in cement. Single collagen fibers are “glued” together to give ligaments strength. The elastin content gives them elasticity. During pregnancy and childbirth ligaments are stretched and loosened both by the force from the baby’s head and because the “glue” between the collagen rods soften in response to hormones. The rods “re-glue” soon after delivery, but often they “re-glue” in a loose and extended position. Neither the ligaments nor the muscles can now work properly. But loose ligaments may occur in women who never had children for other reasons, for example degradation of collagen and elastin after menopause (3). The link between weak ligaments and diminished muscle force can be explained as follows: If a ligament is weak, the muscle that contracts against it lengthens. It also weakens, because an elongated muscle has weaker contractile strength. The muscles can no longer close the urethral outlet or stretch the organs sufficiently to prevent activation of the micturition reflexes (2). Consequently, incontinence might be experienced.
- D. Altman et al. Urogynekologi. s.l. : Studentlitteratur, 2010. ISBN 978-91-44-05450-6.
- EAU, Liedl et. al. Update of the Integral Theory and System for Management of Pelvic Floor Dysfunction in Females. 2017
- P. Petros. The Integral Theory System. A simplified clinical approach with illustrative case histories. 2010