Urethral Sphincters: Location, Function, and Issues

Your body has two distinct urethral sphincters, and they work as a team to keep urine in your bladder until you decide to let it go. The internal urethral sphincter is a ring of smooth muscle that operates automatically, while the external urethral sphincter is made of skeletal muscle you can squeeze on command. When either one is damaged or stops coordinating properly with the bladder, the result can range from mild leakage to complete loss of urinary control. Understanding where these structures sit, how they work, and what can go wrong with them helps make sense of a wide range of urinary problems and the treatments designed to fix them.

Where the Two Sphincters Sit

The internal urethral sphincter (IUS) wraps around the bladder neck, right where the bladder tapers into the urethra. It is made of smooth muscle, the involuntary type your body controls without conscious input. Nerve fibers containing noradrenaline concentrate at this bladder-urethra junction, keeping the sphincter tonically contracted during urine storage.¹ Autonomic nerves from the pelvic plexus predominantly regulate this upper portion of the urethra.²

The external urethral sphincter (EUS) sits farther downstream. Unlike the IUS, the EUS is skeletal muscle under voluntary control through the somatic nervous system, meaning you can consciously tighten it to hold urine or relax it to urinate.³ Its nerve supply travels from sacral spinal roots S2 through S4, reaching the muscle through both the pudendal nerves and direct sacral nerve branches.⁴ Even though one sphincter is involuntary and the other voluntary, the smooth and striated muscle layers of the sphincter complex are physically inseparable in both sexes.⁵

How the Sphincters Differ Between Men and Women

In men, the external sphincter covers the front surface of the prostate in a crescent shape above a small anatomical landmark called the verumontanum, then becomes horseshoe-shaped below it, and returns to a crescent along the proximal bulbar urethra. The levator ani muscles form a ring around the external sphincter but leave a gap at the front. In women, the external sphincter wraps around the front of the urethra in a horseshoe shape and, farther down, grows to partly envelop the distal vagina. The levator ani muscles do not support the upper female urethra the way they do in men.⁵

These differences matter practically. A woman’s shorter urethra and the lack of levator support around her upper urethra help explain why stress urinary incontinence is more common in women. In men, the prostate’s position means that prostate surgery directly threatens the sphincter mechanism, a complication covered in more detail below.

How Storage and Voiding Actually Work

During the storage phase, both sphincters stay contracted. The EUS in particular maintains increased tonic activity through what is called the spinal guarding reflex, an automatic spinal cord loop that keeps the muscle clenched as the bladder fills. When you decide to urinate, a region in the brainstem known as the pontine micturition center sends signals that suppress that guarding reflex and actively inhibit the motor neurons controlling the EUS. The sphincter relaxes, the bladder muscle contracts, and urine flows.⁶

The coordination between bladder contraction and sphincter relaxation is precise. If the timing goes wrong, such as when the sphincter contracts at the same moment the bladder does, the result is high-pressure obstruction rather than normal voiding. That particular problem, called detrusor sphincter dyssynergia, is discussed in a later section.

Sphincter Problems in Women

Stress urinary incontinence, the kind where you leak during a cough, sneeze, or jump, is the most common sphincter-related issue in women. It often involves what clinicians call intrinsic sphincter deficiency (ISD), which means the sphincter itself is too weak to keep the urethra sealed under pressure. The clinical parameters for ISD are loosely defined, and there is no universal consensus on exact diagnostic thresholds.⁷

ISD is not a single condition. Researchers have identified at least three subtypes ranging in severity. The mildest type involves a bladder neck that looks closed on imaging but leaks at very low pressures, making it easy to miss without detailed testing. A more common form shows the bladder neck sitting slightly open at rest. The most severe form involves a fixed, non-functioning urethra with almost no resistance to leakage.⁸ The practical takeaway is that “sphincter weakness” is not all-or-nothing; it exists on a spectrum, and treatment choices depend on where someone falls.

One diagnostic finding worth knowing about is the external urethral sphincter’s susceptibility to nerve damage. In women with stress incontinence, the EUS tends to show a higher percentage of denervation compared to neighboring pelvic floor muscles like the levator ani or the external anal sphincter.⁹ This suggests the EUS may be especially vulnerable to the nerve stretching that occurs during childbirth and aging.

Sphincter Problems After Prostate Surgery

For men, the most common path to sphincter trouble is prostate surgery for cancer. The external sphincter sits extremely close to the bottom edge of the prostate, and the nerves that supply it run within a few millimeters of the surgical field. Cadaver studies have shown that nerve branches reaching the external sphincter can be as close as 3 to 13 millimeters from the prostatic apex, and in half of men studied, these branches were found only on one side or existed as a single thin branch per side.¹⁰

When incontinence develops after prostatectomy, sphincter damage is the dominant cause. A urodynamic review of men who became incontinent after prostate cancer surgery found sphincter injury was the sole cause in about 59% of cases and a major contributor in another 36%.¹¹ Direct damage appears to follow a predictable pattern, occurring most often during apex preparation and the stitching that reconnects the urethra to the bladder.¹² Surgeons have increasingly adopted nerve-sparing techniques to avoid these structures, but the margins are tight.

Detrusor Sphincter Dyssynergia

In a healthy urinary tract, the bladder contracts while the sphincters relax. In detrusor sphincter dyssynergia (DSD), that coordination breaks down: the bladder pushes while the sphincter clamps shut. The result is incomplete emptying, high bladder pressures, and in serious cases, urine backing up toward the kidneys. DSD is linked to neurological conditions, particularly spinal cord injury, multiple sclerosis, and spina bifida. If it goes untreated, the chronically elevated pressures can lead to recurrent urinary tract infections and kidney damage.¹³

People with spinal cord injuries above the sacral segments are at highest risk because the brainstem can no longer coordinate the bladder-sphincter reflex through the spinal cord. Some of these patients are also at risk for autonomic dysreflexia, a dangerous spike in blood pressure triggered by bladder overdistension, which makes proper management of DSD especially urgent.

Parkinsonian syndromes also cause lower urinary tract dysfunction, though the specific pattern varies depending on where the nervous system is affected. The sphincter issues in Parkinson’s disease tend to be different from those in spinal cord injury and can include both overactive bladder and difficulty emptying.¹⁴

Children with Spina Bifida

DSD is not limited to adults. Children born with spina bifida frequently have both detrusor overactivity and sphincter dyssynergia from birth, because the neural tube defect disrupts the spinal pathways that coordinate bladder and sphincter function. Managing these children typically involves intermittent catheterization and medication, but an often-overlooked factor is bowel management. In a study of spina bifida children with both detrusor overactivity and DSD, starting a structured bowel management program led to a significant increase in bladder capacity, a drop in bladder pressure, and reduced leak-point pressure. Post-void residual volumes also fell substantially in children who could void on their own.¹⁵ The bowel and bladder share pelvic nerve pathways, so chronic constipation and rectal distension can worsen bladder dysfunction. Getting the bowels under control turns out to be an important, sometimes underappreciated, part of managing sphincter problems in this group.

What Happens to the Sphincters with Age and Menopause

Aging affects both sphincters, but the hormonal shift at menopause adds a distinct layer for women. As estrogen levels drop, the urethra shortens, its lining thins, and sphincter contractility declines. In animal models of estrogen deprivation, roughly 60% developed stress incontinence within six weeks. When estrogen was given back, the incidence of incontinence fell by about 35%.¹⁶ In women, local estrogen therapy has been shown to increase urethral closure pressure, likely by improving blood flow to the urethral vascular network, and to reduce involuntary bladder contractions during the filling phase.¹⁶

This is worth knowing because many women assume urinary leakage after menopause is just something to live with. Local estrogen, applied as a vaginal cream or ring rather than taken systemically, can improve sphincter function specifically, without the risks associated with oral hormone therapy. It is not a fix for severe sphincter deficiency, but for the milder decline that tracks with menopause, it can make a real difference.

Pelvic Floor Training and How It Changes the Sphincter

Pelvic floor muscle exercises, commonly known as Kegels, are the first-line treatment for stress urinary incontinence in both women and men after prostate surgery. They strengthen the muscles that provide external support to the urethra, helping prevent leakage during physical stress.¹⁷ But the benefits are not just about surrounding muscle strength. Imaging studies show that a structured pelvic floor training program actually increases the cross-sectional area of the urethral sphincter itself and reduces how much the bladder neck moves during coughing, both changes that directly improve continence mechanics.¹⁸

A broader review of morphological changes confirmed these findings, showing that pelvic floor training leads to a higher bladder neck position, narrower pelvic floor dimensions, and a thicker external urethral sphincter.¹⁹ The key message is that these exercises do not just tone up the general pelvic floor; they physically remodel the sphincter, making it thicker and more effective. That said, compliance is the main obstacle. The exercises need to be done consistently over weeks to months before meaningful changes appear, and many people abandon them too early.

Medications That Target the Sphincter

Drug treatment for sphincter weakness is more limited than most people expect. The best-studied medication is duloxetine, which works by boosting serotonin and norepinephrine levels at a cluster of spinal motor neurons that control the pudendal nerve. The result is increased sphincter activity during the storage phase without interfering with normal voiding.²⁰ In healthy women, a single dose of duloxetine has been shown to lower the threshold for sphincter contraction and increase the strength of sphincter contractions without altering resting urethral tone.²¹

Duloxetine is approved for stress incontinence in some countries but not everywhere; in the United States, it is primarily marketed as an antidepressant, though it is sometimes prescribed off-label for incontinence. Side effects like nausea limit its use. No other drug directly strengthens the urethral sphincter in a clinically meaningful way, which is part of why the field has invested so heavily in surgical and rehabilitative approaches.

Surgical Options When the Sphincter Fails

When conservative measures are not enough, surgery aims either to replace sphincter function or to compensate for it. The three main categories are artificial urinary sphincters, sling procedures, and urethral bulking agents.²²

The artificial urinary sphincter (AUS) is a surgically implanted device that wraps a fluid-filled cuff around the urethra, mimicking sphincter pressure. When you want to urinate, you squeeze a pump implanted in the scrotum or labia, which temporarily opens the cuff. In a nationwide long-term analysis of men who received an AUS, incontinence severity dropped substantially after implantation. About 81% of men with the device still in place at their last follow-up described their urinary condition as causing no or minor discomfort, and 99% said they would recommend the device to someone in the same situation. However, about 23% eventually needed the device removed, most often because of urethral erosion or infection. The device survival rate was roughly 76% at two years, 70% at five years, 60% at eight years, and 57% at ten years.²³ These numbers illustrate both how well the device works when it stays in and the reality that mechanical devices in the body have a finite lifespan.

Urethral bulking agents offer a less invasive alternative. A gel-like substance is injected around the urethra to add bulk and help it close more tightly. One recent study of a polyacrylamide hydrogel found treatment success rates above 89% regardless of whether patients had intrinsic sphincter deficiency, and retreatment rates were low in both groups.²⁴ Bulking agents appeal to people who want to avoid major surgery, though the effect can fade over time and repeat injections may be needed. For neurogenic bladder patients specifically, artificial sphincters, slings, bladder neck reconstruction, and bulking agents all remain in the toolkit, with the choice depending on the patient’s hand function, mobility, and the underlying neurological condition.²⁵

How Doctors Assess Sphincter Function

If you are referred for sphincter evaluation, you will likely encounter urodynamic testing, a set of procedures that measure pressures inside the bladder and urethra during filling and voiding. Urethral pressure profilometry specifically measures how much squeeze the urethra generates along its length. Lower profile measurements correlate with stress incontinence.⁹

Electromyography (EMG) of the external urethral sphincter can detect denervation and abnormal firing patterns. One useful finding is that a pulsatile pattern on the urethral pressure profile trace is a strong predictor of abnormal EMG activity. The positive predictive value of a pulsatile trace for abnormal EMG was 82%, meaning that when the pulsatile pattern shows up, there is a high chance the sphincter’s nerve supply is compromised.²⁶ For men facing prostate surgery, combining pre-operative urethral pressure measurements with EMG data can predict post-surgical incontinence with high sensitivity, potentially guiding pre-surgical counseling and planning.²⁷

Stem Cell Research and Sphincter Regeneration

The fact that no drug can rebuild a damaged sphincter has driven interest in regenerative approaches. In animal models of stress incontinence, injection of human amniotic fluid stem cells around the urethra restored the sphincter to what appeared to be normal tissue structure and function, without triggering immune rejection or tumor formation.²⁸ This is still early-stage work, and the gap between promising animal data and a proven human therapy is notoriously large in regenerative medicine. But the concept, growing new functional sphincter muscle rather than replacing it mechanically, represents a fundamentally different approach to the problem. Several groups are also exploring muscle-derived stem cells and autologous cell therapies, though none has reached routine clinical use.

What makes this line of research especially appealing is that current surgical options all have durability limits. Artificial sphincters erode or malfunction, bulking agents reabsorb, and slings can loosen. A treatment that genuinely regenerates the sphincter muscle would address the root problem rather than working around it. Whether that possibility will mature from laboratory success to clinical reality remains an open question, but it is among the more active areas of urological research.