All Products
Also known as
Rhabdosphincter, External Urethral Sphincter, Urethral Sphincter (pars urethrae), M. sphincter urethrae externus (Latin), Skeletal Urethral Sphincter, Voluntary Urethral Sphincter, Striated Urethral Sphincter
Definition
The external sphincter, also known as the external urethral sphincter (EUS) or rhabdosphincter, is a ring of striated skeletal muscle that provides voluntary control over urination.1 Located within the deep perineal pouch, this sphincter surrounds the urethra and functions as a secondary sphincter mechanism to regulate the flow of urine through the urethra.2 The external sphincter is primarily composed of striated skeletal muscle fibers under voluntary control via the pudendal nerve, originating from the S2 to S4 nerve roots.3
In males, the external sphincter is positioned at the level of the membranous urethra, with muscle fibers inferior to the caudal prostate forming a circular arrangement.4 The external fibers arise from the junction of the inferior rami of the pubis and ischium, and the sphincter lies in the urogenital hiatus of the pelvic diaphragm.4
In females, the external sphincter is more complex and intricate, properly referred to as a urogenital sphincter.5 It includes three components: (1) the true annular sphincter surrounding the urethra (urethral sphincter), (2) a part passing anterior to the urethra that attaches to the ischial rami (compressor urethral muscle), and (3) a part encircling both the urethra and vagina (urethrovaginal sphincter).5 This structure is composed of an inner mucosa with a urothelial lining, a spongy mucus-producing submucosa, a layer of smooth muscle, and an external sheath of fibroelastic connective tissue.6
When contracted, the external sphincter narrows the urethra, preventing the passage of urine.1 This voluntary control mechanism is essential for maintaining urinary continence, particularly during activities that increase intra-abdominal pressure such as coughing, sneezing, or physical exertion.7 Dysfunction of the external sphincter can lead to various forms of urinary incontinence, with stress incontinence being the most common manifestation.8
Clinical Context
The external sphincter plays a crucial role in urinary continence and is clinically significant in various urological conditions.1 Malfunctioning of the external urethral sphincter can lead to disorders of the lower urinary tract, most commonly presenting as some form of urinary incontinence.2
Stress urinary incontinence, characterized by involuntary leakage of urine during activities that increase intra-abdominal pressure (such as coughing, sneezing, or physical exertion), is often associated with weakening or dysfunction of the external sphincter.3 This condition is approximately twice as prevalent in women as in men, with risk factors including aging, decreased mobility, genetic predisposition, vaginal childbirth, pelvic floor muscle damage, pelvic organ prolapse, neuropathy, diabetes, hormonal changes, obesity, and menopause.4
In women, vaginal childbirth can cause anatomical damage to the ligaments, fascial support, and pelvic floor musculature, including the external sphincter.5 The pudendal nerve innervating the external sphincter is particularly susceptible to injury during vaginal birth as it travels between the sacrospinous and sacrotuberous ligaments.6 This nerve damage can compromise the voluntary control of the external sphincter, contributing to urinary incontinence.
In men, stress incontinence is less common but may occur following prostate surgery, particularly radical prostatectomy for prostate cancer.7 Damage to the external sphincter during surgery can result in varying degrees of urinary incontinence, significantly impacting quality of life.
Treatment approaches for external sphincter dysfunction depend on the severity and underlying cause. Conservative management includes pelvic floor exercises (such as Kegel exercises), which can be effective in strengthening the external sphincter and improving urinary control.8 For more severe cases, surgical interventions may be necessary, including the implantation of an artificial urinary sphincter (AUS).9 The AUS is a device that includes a pressure balloon, urethral cuff, and pump, which work together to mimic the function of the natural sphincter, providing mechanical control of urinary flow.10 This intervention has shown high success rates, with some studies reporting over 90% improvement in continence.10
Additionally, detrusor sphincter dyssynergia, a condition characterized by lack of coordination between detrusor muscle contractions and inappropriate urethral sphincter closure, can occur due to lesions or dysfunction of the suprasacral spinal cord.11 This condition is commonly associated with spinal cord injuries, multiple sclerosis, and spina bifida, and may be treated with medications, botulinum toxin injections, intermittent self-catheterization, or sacral neuromodulation.11