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Internal urethral sphincter (male)

The internal urethral sphincter in males is a ring of smooth muscle located at the bladder neck. It encircles the urethral orifice and functions involuntarily to regulate the passage of urine and semen. Unlike the external sphincter, which is voluntary and skeletal in nature, the internal sphincter is composed of smooth muscle fibers under autonomic control. It plays a vital role in urinary continence and retrograde ejaculation prevention.

Synonyms

  • Bladder neck sphincter

  • Internal bladder sphincter

  • Internal urethral orifice sphincter

Location and Boundaries

  • Located at the junction of the urinary bladder and proximal urethra (bladder neck)

  • Superiorly: Continuous with smooth muscle of bladder trigone

  • Inferiorly: Surrounds the beginning of the prostatic urethra

  • Anteriorly: Related to the pubic symphysis (via retropubic space)

  • Posteriorly: Related to seminal vesicles and vas deferens at the bladder base

Relations

  • Lies at the bladder neck, surrounding the internal urethral orifice

  • Superior relation: bladder trigone

  • Inferior relation: prostatic urethra

  • Posterior relation: seminal vesicles and prostate base

  • Anterior relation: pubic symphysis (separated by retropubic fat and space of Retzius)

Function

  • Provides involuntary control of urine outflow from the bladder

  • Maintains urinary continence at rest

  • Contracts during ejaculation to prevent retrograde flow of semen into bladder

  • Works in coordination with the external urethral sphincter to maintain continence under stress (e.g., coughing, lifting)

Clinical Significance

  • Damage during prostatectomy or pelvic surgery can result in urinary incontinence

  • Dysfunction can lead to retrograde ejaculation

  • Hypertrophy may contribute to bladder outlet obstruction in conditions like BPH (benign prostatic hyperplasia)

  • Target of pharmacologic agents (e.g., α-blockers) in lower urinary tract symptoms management

MRI Appearance

T1-weighted images:

  • Appears as a thin, low-signal intensity ring at the bladder neck

  • Adjacent fat around the prostate and retropubic region appears bright, providing contrast

  • Blood or proteinaceous fluid in lumen may appear brighter

T2-weighted images:

  • Internal sphincter appears as a hypointense circular band

  • Lumen fluid is bright, highlighting the dark ring

  • No fluid: bladder neck collapsed, sphincter appears as a dark ring with surrounding intermediate bladder wall signal

  • Fat signal around sphincter appears intermediate to bright

STIR (Short Tau Inversion Recovery):

  • Normal sphincter remains dark hypointense

  • Fluid within urethra or bladder neck appears bright

  • Fat is suppressed and appears dark

  • Inflammation or edema in adjacent tissue appears bright

T1 Fat-Sat Post-Contrast:

  • Normal sphincter may show minimal or no enhancement

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

  • Fat suppressed and appears dark, improving contrast of abnormal tissues

CT Appearance

Non-Contrast CT:

  • Seen as a soft tissue density ring at the bladder outlet

  • Surrounding retropubic fat visible as low density

  • Without fluid, distinction from bladder base is subtle

  • With urine, contrast between lumen and sphincter improves visualization

Post-Contrast CT:

  • Bladder lumen enhances with contrast-filled urine

  • Sphincter itself shows soft tissue density without strong enhancement

  • Pathology (tumor, inflammation, post-surgical changes) may show focal enhancement or irregular thickening

MRI image

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

MRI image

Internal Urethral Sphincter (Male) MRI SAG anatomy image-img-00000-00000