Pelvic Prime's Anatomy Academy Series
Stress Urinary Incontinence
The main job of the urethral sphincter is to keep urine from leaking out, with help from the levator ani muscle and the endopelvic fascia. Problems usually show up when there’s extra stress, such as coughing hard. During a cough, the diaphragm and abdominal muscles push down hard, increasing the pressure inside the abdomen. This can cause the urethra to move downwards.
When the abdominal pressure goes up, the bladder neck and nearby tissues move and stretch. The pelvic floor then has to slow down this movement. The increased pressure pushes the urethra against the supportive tissues, helping to keep it closed and prevent leaks.
With correct pelvic floor exercises, the pelvic muscles can get stronger and provide more resistance. But if the endopelvic fascia is damaged or the levator ani muscle is injured, these support tissues become more flexible and less effective, making leaks more likely during stress or increased pressure.
Garden Hose on a Trampoline Analogy
Imagine stepping on a garden hose that's sitting on a trampoline. If the trampoline is firm, the hose flattens, and the water stops. But if the trampoline is very stretchy, stepping on the hose makes the hose and trampoline move down together, so the water keeps flowing until the trampoline starts to resist.
The same thing happens in the body; if the supportive tissues are too flexible, the pressure from the abdomen can't close the urethra quickly, causing leaks.
Urethral Support System
For normal function, the levator ani muscle supports the urethra through the endopelvic fascia. With coughing, this muscle contracts along with the diaphragm and abdominal muscles to build pressure. This contraction helps tighten the layer under the urethra, reducing bladder neck movement and increasing urethral compression.
As people age, the levator ani muscle takes longer to increase it strength and the muscle's maximum strength decreases by about 35%. However, the levator ani muscle doesn’t lose much volume with age because it has more slow-twitch fibers.
If the levator ani muscle is damaged or its nerves are impaired, it takes even longer to develop force, decreasing its strength and stiffness. If the connection between the muscle and the fascia is broken, the muscle can't function properly during a cough.
Urethrovesical Pressure Dynamics
Understanding how urine stays in the bladder and how it might leak out involves looking at two main things:
Resting Pressure: This is the pressure in the urethra when everything is relaxed. This pressure needs to be higher than the pressure in the bladder to keep urine from leaking out.
Pressure Transmission: This is how well the pressure in the urethra increases when bladder pressure rises, such as during a cough. For example, if the bladder pressure is 10 cmHO and the urethral pressure is 60 cmHO, then the difference (closure pressure) is 50 cmHO, which keeps urine in the bladder. If bladder pressure goes up by 200 cmHO during a cough, the urethral pressure must also rise by 200 cmHO to prevent leakage.
Understanding these pressures helps in figuring out why incontinence happens and what can be done about it. Unfortunately, measuring these pressures accurately is difficult because the urethra moves around a lot during a cough.
Exercises for pelvic muscles can help treat Stress Urinary Incontinence. Simple tests like coughing with a full bladder can show how much urine is leaking. If a someone can contract their pelvic muscles during a cough and reduce leakage, then learning to do these exercises regularly can be an effective treatment.
However, if the pelvic muscles are damaged, exercises might not be enough. People need to be taught when and how to use these muscles to prevent leaks. Strengthening the muscles and using them correctly during activities that increase abdominal pressure, like coughing, is key.
References: Bo, K., Berghmans, B., Mørkved, S., & Van Kampen, M. (Eds.). (2023). Evidence-Based Physical Therapy for the Pelvic Floor (3rd ed.). Elsevier.