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Internal urethral sphincter (female)

The internal urethral sphincter in females is a ring of smooth muscle located at the junction of the bladder and proximal urethra. Unlike the external sphincter, it is involuntary and under autonomic nervous system control. It plays a crucial role in maintaining urinary continence by providing constant tone at the bladder outlet and preventing involuntary leakage. It also functions during micturition by relaxing to allow urine to pass into the urethra.

Synonyms

  • Bladder neck sphincter

  • Involuntary urethral sphincter

  • Smooth muscle sphincter of urethra

Location and Boundaries

  • Superiorly: Continuous with detrusor muscle fibers of the bladder neck

  • Inferiorly: Proximal female urethra

  • Anteriorly: Retropubic space (space of Retzius)

  • Posteriorly: Anterior vaginal wall and urethrovaginal septum

Relations

  • Encircles the bladder neck and proximal urethra

  • Lies anterior to the vagina and posterior to the pubic symphysis

  • Closely related to the external urethral sphincter (striated voluntary muscle) situated more distally

  • Surrounded by connective tissue and vascular plexuses of the bladder outlet

Function

  • Provides involuntary control of urinary continence by maintaining tone at the bladder neck

  • Relaxes reflexively during micturition to allow urine flow

  • Contracts during bladder filling to prevent leakage

  • Contributes to urinary continence alongside the external urethral sphincter

Clinical Significance

  • Weakness or dysfunction may lead to stress urinary incontinence in women

  • Can be affected in neurogenic bladder disorders

  • Important surgical landmark in urology and urogynecology (e.g., anti-incontinence procedures, bladder neck surgeries)

  • Thickening or irregularity may be seen in bladder outlet obstruction, tumors, or post-surgical changes

MRI Appearance

T1-weighted images:

  • Sphincter appears as a thin ring of low signal intensity around the urethral lumen

  • No fluid: Lumen appears as a collapsed dark line; surrounding fat is bright

  • With fluid: Urine in the bladder/urethra appears dark, contrasting with surrounding bright fat

T2-weighted images:

  • Sphincter demonstrates low signal intensity

  • No fluid: Urethral lumen appears as a dark slit

  • With fluid: Urine shows bright signal, sphincter remains dark, well delineated

  • Fat: Intermediate-to-bright, contrasting with dark sphincter

STIR (Short Tau Inversion Recovery):

  • Sphincter remains low signal

  • Fluid: Hyperintense if present in bladder neck or urethra

  • Fat: Suppressed, appearing dark, which improves visualization of sphincter

T1 Fat-Sat Post-Contrast:

  • Normal sphincter shows mild uniform enhancement

  • No fluid: Enhancing sphincter ring is seen around collapsed lumen

  • With fluid: Enhancing ring visible around non-enhancing fluid-filled lumen

  • Pathology (inflammation, tumor) may show irregular or asymmetric enhancement

CT Appearance

Non-Contrast CT:

  • Sphincter itself not well visualized; appears as part of bladder neck soft tissue

  • No fluid: Bladder neck appears as homogeneous soft tissue

  • With fluid: Contrast between urine (low attenuation) and soft tissue makes sphincter location clearer

  • Fat: Perivesical fat appears as low attenuation outlining bladder neck

Post-Contrast CT:

  • Bladder wall and sphincter region enhance mildly and uniformly

  • Pathological changes (tumor, inflammation) show irregular or focal enhancement

  • Surrounding fat stranding may be seen in infection or inflammatory conditions

MRI image

Internal Urethral Sphincter (Female) MRI axial anatomy image-img-00000-00000

MRI image

Internal Urethral Sphincter (Female) MRI Coronal anatomy image-img-00000-00000

CT image

Internal Urethral Sphincter (Female) ct axial anatomy image-img-00000-00000