urogenital diaphragm

**Terminology/Anatomy** - Classic description: a triangular sheet stretched between the two ischiopubic rami, with a superior fascia and an inferior fascia (the perineal membrane). Between them lies the deep perineal space. - Current Terminologia Anatomica: the "urogenital diaphragm" is no longer used. The deep perineal pouch is bounded inferiorly by the perineal membrane, and the pouch contains the muscles and neurovascular structures. Boards (especially older question banks and texts) may still use the old term. - Location: urogenital triangle of the perineum, inferior to the pelvic diaphragm (levator ani + coccygeus) and anterior to the anal triangle.

**Contents (deep perineal space/pouch)** - Males: membranous urethra, bulbourethral (Cowper) glands (their ducts pierce the perineal membrane to enter the spongy urethra), deep transverse perineal muscle, external urethral sphincter, dorsal nerve and artery of the penis (branches of the pudendal nerve and internal pudendal artery). - Females: urethra and vagina pass through it. It contains the external urethral sphincter (including the urethrovaginal sphincter and compressor urethrae), the deep transverse perineal muscle, and branches of the internal pudendal vessels and pudendal nerve. There are no bulbourethral glands (the greater vestibular/Bartholin glands lie in the superficial pouch).

**Innervation** - Pudendal nerve (S2–S4, "S2, 3, 4 keeps the poop off the floor"), via its perineal branch. The external urethral sphincter is skeletal muscle under voluntary control.

**Function** - Supports pelvic viscera, fixes the perineal body, and provides voluntary urethral closure (external urethral sphincter) for urinary continence.

**Clinical significance (high-yield)** - **Straddle injury / urethral rupture in males**: the membranous urethra is the part that passes through the deep perineal pouch. - Membranous urethra rupture (e.g., pelvic fracture, often with a high-riding prostate): urine extravasates into the deep perineal pouch and can extend superiorly into the retropubic/prevesical space. - Spongy (bulbous) urethra rupture (straddle injury, below the perineal membrane): urine extravasates into the superficial perineal pouch and can spread into the scrotum, around the penis (under Buck fascia if intact, or under Colles fascia if Buck is torn), and up the anterior abdominal wall deep to Scarpa fascia ("butterfly" pattern in the perineum). It does not spread into the thigh (Colles fascia attaches to the fascia lata) or the anal triangle (it fuses with the posterior edge of the perineal membrane). - Therefore, the perineal membrane is a key landmark for distinguishing membranous from spongy urethral injury. - **Stress urinary incontinence/postprostatectomy incontinence**: injury or weakness of the external urethral sphincter or the pudendal nerve (e.g., childbirth trauma, radical prostatectomy). - **Pudendal nerve block** (landmark: ischial spine) provides anesthesia of the perineum for childbirth.

**Memory pearls** - Deep pouch = skeletal muscle sphincter and membranous urethra (male). Superficial pouch = ischiocavernosus, bulbospongiosus, superficial transverse perineal muscles, and erectile tissue (bulb, crura), plus the Bartholin glands in females. - Perineal membrane: the inferior boundary of the deep pouch, and the structure the penile bulb and crura attach to.