The external urinary sphincter (EUS) is a muscular ring that acts as the final gatekeeper for urinary control. This valve allows for voluntary regulation over the release of urine from the body. As a fundamental component of the continence mechanism, the EUS provides the ability to consciously prevent or initiate urine flow.
Anatomical Placement and Structure
The external urinary sphincter is situated inferior to the bladder, where the urethra passes through the pelvic floor. This muscle is composed of skeletal muscle fibers, placing the EUS under the control of the somatic nervous system. This voluntary control allows a person to consciously interrupt or delay urination.
The EUS is distinct from the internal urinary sphincter, which is made of smooth muscle and functions involuntarily. The internal sphincter is located higher up, at the bladder neck, maintaining passive continence. The external sphincter surrounds the middle or membranous portion of the urethra, providing consciously controlled closing pressure. In men, the EUS sits just below the prostate gland. In women, it is a more complex structure that surrounds the urethra and, in part, the vagina.
The Role in Maintaining Continence
The EUS functions by mechanically compressing the urethra to maintain a high-pressure zone, preventing leakage during the urine storage phase. This constant, low-level contraction is maintained by nerve signals originating from Onuf’s nucleus, a group of neurons in the sacral spinal cord. These motor neurons are continuously active during bladder filling, keeping the EUS closed and the urethra sealed.
When the bladder fills, sensory nerves signal the central nervous system, and the brain consciously decides whether to hold or void. If the decision is to hold, the brain reinforces the EUS contraction via the pudendal nerve, especially during activities that increase abdominal pressure, such as coughing or sneezing. This reflex contraction prevents stress incontinence.
When conditions are appropriate for urination, the brain’s pontine micturition center initiates the voiding reflex. This process requires precise coordination between the bladder and the EUS, known as reciprocal action. The somatic nerve activity to the EUS is inhibited, causing the skeletal muscle to relax and open the urethral channel. Simultaneously, the detrusor muscle in the bladder wall contracts, pushing urine out, while the internal sphincter also relaxes.
Primary Causes of EUS Impairment
Damage to the external urinary sphincter often results in stress urinary incontinence, characterized by leakage during physical exertion. One common cause of EUS impairment in men is iatrogenic damage from prostate surgery, such as a radical prostatectomy. Since the EUS is located immediately beneath the prostate, surgical removal can injure the surrounding muscle and nerves, compromising continence.
Neurological conditions can also disrupt the control system of the EUS. Diseases like multiple sclerosis, spinal cord injury, or stroke interfere with the signaling pathways between the brain, spinal cord, and the sphincter muscle. This disruption can lead to detrusor-sphincter dyssynergia, a loss of coordination where the EUS contracts instead of relaxing during a bladder contraction, causing difficulty emptying the bladder.
Physical trauma, such as a severe pelvic fracture, can cause direct mechanical injury to the EUS muscle or the pudendal nerve. Beyond acute injury, age-related weakening of the pelvic floor muscles, which support the EUS, diminishes its closing pressure. In women, childbirth can also cause tissue or nerve damage that contributes to the weakening of the EUS and its supporting structures.
Intervention Strategies for Dysfunction
Management of EUS dysfunction often begins with non-invasive strengthening techniques. Pelvic floor muscle training, known as Kegel exercises, directly targets the external urinary sphincter and the surrounding supportive musculature. Consistent performance of these exercises increases muscle strength and endurance, improving urethral closing pressure and reducing stress incontinence.
When EUS damage is severe, surgical interventions may be necessary. One effective option is the implantation of an artificial urinary sphincter (AUS). This device is a fluid-filled, mechanical cuff placed around the urethra that is manually controlled by the patient to open for urination and close for continence.
The injection of urethral bulking agents into the tissues surrounding the EUS is another option. These materials add volume and bulk to the area, narrowing the urethra and increasing resistance to urine flow. For conditions like detrusor-sphincter dyssynergia, where the EUS is overactive, injecting Botulinum Toxin A directly into the muscle can relax the sphincter and improve bladder emptying.