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Q11.

Pelvic Injury: Urethral Damage and Perineal Pouch Anatomy

A 36-year-old cyclist suffered pelvic injury while riding into a pothole on the road. Following this incident, he has severe pelvic pain and is unable to pass urine. His retrograde urethrogram shows contrast limited to superficial perineal pouch.

(i) What is the most likely structure and its site of involvement, which is responsible for his inability to pass urine?

(ii) What are the parts of the principal structure involved?

(iii) With the help of a labelled diagram, illustrate the difference in the contents of superficial perineal pouch in males and females.

How to Approach

The question requires a comprehensive understanding of male urogenital anatomy and the implications of trauma. The approach should involve accurately identifying the injured structure and its site based on the clinical scenario, detailing its anatomical components, and finally, illustrating the differential contents of the superficial perineal pouch in males and females. A clear, well-labelled diagram is crucial for the third part.

Model Answer

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Introduction

Pelvic injuries, often resulting from high-impact trauma like road accidents or falls, can lead to significant morbidity due to damage to underlying vital structures, including the urinary system. Urethral injuries are a critical concern in such cases, especially in males, where the urethra's longer and more complex course makes it vulnerable. The inability to pass urine (anuria or retention) coupled with extravasation of contrast on a retrograde urethrogram into the superficial perineal pouch is a classic presentation indicating a specific site of urethral disruption. Understanding the precise anatomical location and the contents of the perineal pouches is essential for accurate diagnosis and management.

(i) Most Likely Structure and Site of Involvement

Based on the clinical presentation of severe pelvic pain, inability to pass urine, and the retrograde urethrogram showing contrast limited to the superficial perineal pouch, the most likely structure involved is the male urethra, specifically the bulbous (or bulbar) part of the spongy urethra.

  • Reasoning:
    • Pelvic injury is a common cause of urethral trauma, especially in males.
    • Inability to pass urine (urinary retention) strongly suggests urethral obstruction or disruption.
    • A retrograde urethrogram is the gold standard for diagnosing urethral injuries. Extravasation of contrast indicates a breach in the urethral wall.
    • The critical diagnostic clue is the contrast being limited to the superficial perineal pouch. The bulbous urethra (a part of the spongy/penile urethra) passes through the bulb of the penis, which is a major content of the superficial perineal pouch. Rupture of the bulbous urethra, particularly if Buck's fascia (deep fascia of the penis) is also torn, leads to extravasation of urine and contrast into the superficial perineal pouch and potentially into the scrotum and anterior abdominal wall (deep to Scarpa's fascia).
    • Injuries to the membranous urethra (posterior urethra) typically result in contrast extravasation into the deep perineal pouch or retropubic space, not the superficial perineal pouch.

(ii) Parts of the Principal Structure Involved (Male Urethra)

The male urethra is a long, S-shaped tube, approximately 18-20 cm long, that extends from the urinary bladder to the exterior, serving as a passageway for both urine and semen. It is anatomically divided into two main parts: the posterior urethra and the anterior urethra. These are further subdivided:

1. Posterior Urethra (approx. 4 cm long):

  • Pre-prostatic (Intramural) Urethra: The shortest part, located at the neck of the bladder, surrounded by the internal urethral sphincter.
  • Prostatic Urethra: The widest and most dilatable part, approximately 3 cm long, passing through the prostate gland. It receives the ejaculatory ducts and prostatic ducts.
  • Membranous Urethra: The shortest (1.5-2 cm), narrowest, and least dilatable part of the urethra (excluding the external meatus). It passes through the deep perineal pouch (urogenital diaphragm) and is surrounded by the external urethral sphincter (voluntary control). This is the most vulnerable part to injury in pelvic fractures.

2. Anterior Urethra (approx. 15 cm long):

  • Bulbous (Bulbar) Urethra: The proximal part of the spongy urethra, located within the bulb of the penis in the superficial perineal pouch. It is surrounded by the corpus spongiosum and the bulbospongiosus muscle. This part is commonly injured in "straddle injuries" and is the likely site of injury in the given case.
  • Pendulous (Penile) Urethra: The longest part, extending through the corpus spongiosum within the shaft of the penis.
  • Fossa Navicularis: A slight dilation near the external urethral orifice.

(iii) Difference in Contents of Superficial Perineal Pouch in Males and Females

The superficial perineal pouch is a space in the urogenital triangle, located between the perineal membrane (superiorly) and Colles' fascia (inferiorly). While some structures are common, there are significant differences in its contents between males and females, primarily reflecting the distinct external genitalia and reproductive functions.

Content Category Males Females
Erectile Bodies
  • Bulb of the penis (surrounding the bulbous urethra)
  • Crura of the penis (proximal parts of corpora cavernosa)
  • Part of the corpus spongiosum (containing the bulbous urethra)
  • Bulbs of the vestibule (paired erectile tissues lateral to the vaginal orifice)
  • Crura of the clitoris (proximal parts of corpora cavernosa of the clitoris)
  • Body of the clitoris (anterior part formed by crura)
Muscles
  • Ischiocavernosus muscles (cover crura of penis)
  • Bulbospongiosus muscles (cover bulb of penis, enclose bulbous urethra)
  • Superficial transverse perineal muscles
  • Ischiocavernosus muscles (cover crura of clitoris)
  • Bulbospongiosus muscles (cover bulbs of vestibule, enclose vaginal orifice)
  • Superficial transverse perineal muscles
Glands
  • Ducts of bulbourethral glands (Cowper's glands), which pass through the perineal membrane to open into the bulbous urethra (the glands themselves are in the deep perineal pouch).
  • Greater vestibular glands (Bartholin's glands) (located posterolateral to the vaginal orifice, secrete lubricating mucus).
Nerves & Vessels
  • Terminal branches of the internal pudendal artery (e.g., posterior scrotal arteries, artery to the bulb of penis, urethral artery, deep and dorsal arteries of the penis).
  • Branches of the pudendal nerve (e.g., posterior scrotal nerves, dorsal nerve of the penis).
  • Terminal branches of the internal pudendal artery (e.g., posterior labial arteries, artery to the bulb of vestibule, urethral artery, deep and dorsal arteries of the clitoris).
  • Branches of the pudendal nerve (e.g., posterior labial nerves, dorsal nerve of the clitoris).

Labelled Diagram: Superficial Perineal Pouch in Males and Females

[Please imagine a detailed, hand-drawn diagram here, as image generation is not supported. The description below outlines the key features for such a diagram.]

Male Superficial Perineal Pouch Diagram Description:

  • Show the pubic arch anteriorly, ischial tuberosities laterally, and the perineal membrane as the roof.
  • Illustrate the central bulb of the penis, containing the bulbous urethra.
  • Show the crura of the penis extending from the ischial rami.
  • Depict the bulbospongiosus muscle covering the bulb of the penis and the ischiocavernosus muscles covering the crura.
  • Include the superficial transverse perineal muscles extending laterally.
  • Mark the course of the internal pudendal artery branches and pudendal nerve branches supplying these structures.
  • Indicate the opening of the ducts of the bulbourethral glands into the bulbous urethra.

Female Superficial Perineal Pouch Diagram Description:

  • Show the pubic arch anteriorly, ischial tuberosities laterally, and the perineal membrane as the roof.
  • Illustrate the clitoris anteriorly, with its crura extending from the ischial rami.
  • Depict the paired bulbs of the vestibule lateral to the vaginal orifice.
  • Show the ischiocavernosus muscles covering the crura of the clitoris and the bulbospongiosus muscles covering the bulbs of the vestibule and encircling the vaginal orifice.
  • Include the superficial transverse perineal muscles.
  • Clearly label the greater vestibular glands (Bartholin's glands) located posterolateral to the vaginal orifice.
  • Mark the course of the internal pudendal artery branches and pudendal nerve branches.

Conclusion

The detailed clinical scenario and radiological findings strongly point to a traumatic rupture of the bulbous urethra within the superficial perineal pouch, a common consequence of pelvic trauma. A thorough understanding of the male urethral anatomy, particularly its segments and their relations to the perineal pouches, is crucial for timely and accurate diagnosis. Furthermore, appreciating the distinct contents of the superficial perineal pouch in males and females highlights the anatomical basis for gender-specific presentations of perineal injuries and infections, underscoring the importance of precise anatomical knowledge in clinical practice.

Answer Length

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Additional Resources

Key Definitions

Retrograde Urethrogram (RUG)
A radiological procedure used to visualize the male urethra by injecting contrast medium retrogradely (against the normal flow of urine) into the urethral meatus. It is the gold standard for diagnosing urethral injuries by identifying extravasation of contrast or urethral obstruction.
Superficial Perineal Pouch
An anatomical compartment in the urogenital triangle of the perineum, bounded superiorly by the perineal membrane and inferiorly by Colles' fascia. It contains erectile tissues, muscles, vessels, and nerves related to the external genitalia, with significant differences between males and females.

Key Statistics

Urethral injuries are reported in approximately 4-15% of patients with pelvic fractures. Anterior urethral injuries, particularly to the bulbous urethra, are more common from straddle injuries, while posterior urethral injuries are more frequently associated with severe pelvic fractures.

Source: American Urological Association (AUA) Guidelines / European Association of Urology (EAU) Guidelines

Male urethral length typically ranges from 18 to 20 cm, with the spongy (anterior) urethra being the longest segment (around 15 cm), compared to the female urethra which is significantly shorter, about 3-4 cm.

Source: Anatomy textbooks (e.g., Gray's Anatomy)

Examples

Straddle Injury

A common mechanism of injury for the bulbous urethra. It occurs when a person falls forcefully onto an object (like a bicycle crossbar, fence, or beam) that impacts the perineum, compressing the urethra against the pubic symphysis. This blunt trauma can rupture the bulbous urethra, leading to extravasation of urine into the superficial perineal pouch.

Colles' Fascia and Urine Extravasation

In cases of a ruptured bulbous urethra (as implied in the question), if Colles' fascia (the deep membranous layer of superficial perineal fascia) is intact, the extravasated urine and blood are contained within the superficial perineal pouch. However, if Colles' fascia is also torn or if the rupture extends anteriorly, the fluid can spread into the scrotum, penis, and lower anterior abdominal wall (deep to Scarpa's fascia), but not into the thighs or anal triangle due to fascial attachments.

Frequently Asked Questions

▶Why is the membranous urethra considered the most vulnerable part in pelvic fractures?

The membranous urethra is fixed within the urogenital diaphragm (deep perineal pouch) and is relatively immobile. In severe pelvic fractures, particularly those involving pubic rami displacement, shearing forces can tear the membranous urethra away from the prostate, leading to significant injury.

▶What is the significance of "inability to pass urine" in this scenario?

Inability to pass urine (urinary retention) after a pelvic injury with suspected urethral trauma is a critical red flag. It indicates either complete urethral transection or significant compression/edema causing obstruction. This symptom necessitates immediate investigation (like a retrograde urethrogram) before attempting any catheterization, as blind catheterization can convert a partial tear into a complete one.

Topics Covered

AnatomyPathologyUrologyPelvic AnatomyUrinary SystemTrauma
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