External Urethral Sphincter

1. Overview

The external urethral sphincter, also known as the sphincter urethrae, is a ring of skeletal (voluntary) muscle that surrounds the urethra and controls the passage of urine out of the body. It is part of the continence mechanism and plays a crucial role in the voluntary control of micturition (urination). Unlike the internal urethral sphincter, which is composed of involuntary smooth muscle, the external sphincter is under conscious control.

2. Location

In Males:

Located in the deep perineal pouch, the external urethral sphincter surrounds the membranous portion of the urethra, just below the prostate and above the bulb of the penis. It lies within the urogenital diaphragm, posterior to the pubic symphysis and anterior to the rectum.

In Females:

The external urethral sphincter surrounds the mid-portion of the urethra, also within the deep perineal pouch. In females, the muscle extends more anteriorly and merges with surrounding pelvic floor musculature, including the compressor urethrae and urethrovaginal sphincter muscles.

3. Structure

Histologically and anatomically, the external urethral sphincter includes:

• Skeletal muscle fibers: Striated, voluntary muscle that wraps around the urethra.

• Innervation: Supplied by the pudendal nerve (S2–S4), allowing voluntary contraction.

• Associated structures (especially in females):

  • Compressor urethrae: Muscle that helps squeeze the urethra from the sides.

  • Urethrovaginal sphincter: Encircles both the urethra and vagina, enhancing continence.

In both sexes, the sphincter forms part of the pelvic floor musculature and integrates with connective tissue and fascia in the perineal region.

4. Function

The primary function of the external urethral sphincter is to maintain urinary continence by closing the urethra voluntarily. It acts as the final muscular control point preventing the outflow of urine and coordinates with the detrusor muscle during micturition:

• Closed at rest: To prevent involuntary leakage of urine.

• Relaxes during voiding: In response to voluntary control and neural signaling from the pontine micturition center.

5. Physiological Role(s)

The external urethral sphincter plays a vital role in both voluntary and reflexive urinary control:

• Voluntary continence: Maintains closure of the urethra even under increases in intra-abdominal pressure (e.g., coughing, laughing).

• Guarding reflex: Contracts reflexively to prevent leakage during bladder filling.

• Coordination with pelvic floor: Works synergistically with the levator ani muscles and the rest of the pelvic floor in maintaining continence.

6. Clinical Significance

Stress Urinary Incontinence (SUI)

In females, weakening or damage to the external urethral sphincter—especially following childbirth or pelvic surgery—can result in leakage during coughing, sneezing, or lifting. Treatment may include:

• Pelvic floor therapy (Kegels)

• Pessary support

• Surgical sling procedures

Urge Incontinence and Detrusor-Sphincter Dyssynergia

Neurological disorders (e.g., spinal cord injury, multiple sclerosis) can disrupt the coordination between detrusor contraction and sphincter relaxation, leading to:

• Incomplete voiding

• Urinary retention

• Recurrent infections

Post-Prostatectomy Incontinence

In men, removal of the prostate may damage or weaken the external urethral sphincter, resulting in incontinence. Management includes:

• Pelvic floor rehabilitation

• Artificial urinary sphincter implantation in severe cases

Neurological Conditions

Conditions like cauda equina syndrome, spina bifida, and diabetic neuropathy can affect pudendal nerve function, impairing voluntary control over the external sphincter.

Voluntary Control in Toilet Training

Development of external urethral sphincter control marks an important milestone in childhood toilet training, typically achieved between ages 2–4.

Electromyography (EMG) and Urodynamic Testing

The activity of the external urethral sphincter is evaluated in patients with incontinence or voiding dysfunction via:

• Perineal EMG

• Video urodynamics