Pelvis and Perineum

Pelvic Anatomy and Structural Organization

• Bony pelvis composition: Formed by two hip bones (ilium, ischium, pubis) joined anteriorly at the pubic symphysis and posteriorly to the sacrum via the sacroiliac joints; the pelvic inlet, pelvic cavity, and pelvic outlet define three anatomical planes critical for obstetric assessment
• Pelvic floor (levator ani complex): Consists of three muscle groups—pubococcygeus, iliococcygeus, and ischiococcygeus (coccygeus)—that form a funnel-shaped diaphragm responsible for supporting pelvic organs, maintaining continence through tonic contraction, and increasing intra-abdominal pressure during Valsalva
• Perineum subdivision: Divided into anterior urogenital triangle and posterior anal triangle by the transverse perineal line connecting bilateral ischial tuberosities; this anatomical distinction is critical for understanding perineal pathology and surgical approaches

Vascular and Innervation Architecture

• Arterial supply: Internal iliac arteries branch into visceral branches (umbilical, superior vesical, middle rectal) and parietal branches (obturator, internal pudendal, inferior gluteal); the internal pudendal artery is the primary blood supply to perineal structures and follows a characteristic course exiting the pelvis through the greater sciatic foramen, looping around the ischial spine, and re-entering via the lesser sciatic foramen to enter the pudendal canal
• Venous drainage: Mirrors arterial supply with internal iliac veins draining to the common iliac system; note the portal-systemic anastomoses at the rectum (superior rectal vein to portal system; middle and inferior rectal veins to systemic circulation) clinically relevant in portal hypertension
• Innervation patterns: Somatic innervation via sacral plexus (S1-S4) provides motor control and sensation; autonomic innervation (sympathetic from thoracolumbar cord; parasympathetic from sacral parasympathetic nuclei) controls pelvic organ function; pudendal nerve (S2-S4) is critical for perineal sensation and external sphincter control

Sex-Based Anatomical Variations

• Female pelvis: Wider pelvic outlet, larger pelvic inlet, wider subpubic angle (>80°), straighter ischial spines, and shorter sacrum facilitate childbirth; round ligament of uterus traverses the inguinal canal providing landmark for anatomy
• Male pelvis: Narrower pelvic outlet, smaller pelvic inlet, acute subpubic angle (<70°), prominent ischial spines, and longer curved sacrum; prostate gland and seminal vesicles are important perineal structures

Pelvic Floor Dysfunction

• Stress urinary incontinence (SUI): Involuntary urine leakage with increased abdominal pressure (coughing, sneezing, exercise) due to inadequate urethral support and weakness of pelvic floor muscles; more common in multiparous women and those with obesity
• Pelvic organ prolapse: Descent of pelvic organs (bladder = cystocele, small bowel = enterocele, rectum = rectocele, uterus/vaginal apex = uterine prolapse) presenting with vaginal bulging sensation, pressure, incomplete evacuation, or urinary symptoms
• Fecal incontinence and defecatory dysfunction: May result from external anal sphincter damage (obstetric trauma, prior surgery), pudendal nerve injury (chronic straining, childbirth), or pelvic floor paradox (inability to relax pelvic floor during defecation)

Perineal Pathology

• Perineal trauma: Obstetric lacerations (first through fourth-degree), perineural abscess formation, or necrotizing fasciitis presenting with pain, purulent drainage, systemic toxicity, and rapid progression requiring urgent surgical intervention
• Anorectal pathology: Internal hemorrhoids (bright red bleeding, painless unless thrombosed), external hemorrhoids (palpable, often painful), anal fissures (severe pain with defecation, visible tear in anal verge), and perirectal abscess (pain, fever, fluctuance in perianal region)
• Pudendal nerve entrapment: Compression of pudendal nerve in pudendal canal (Alcock's canal) at ischial spine, presenting with perineal pain, dyspareunia, or erectile dysfunction; often missed diagnosis

Reproductive Pathology

• Testicular descent abnormalities: Cryptorchidism (undescended testis) presenting as palpable inguinal mass or absent testis; risk factors include prematurity and family history
• Female reproductive dysfunction: Dyspareunia (painful intercourse), vaginismus (involuntary pelvic floor muscle spasm), or vulvodynia (chronic vulvar pain); anatomical variants including Müllerian duct anomalies may present with primary amenorrhea or infertility

Physical Examination Techniques

• Pelvic floor assessment: Digital vaginal/rectal examination to assess muscle tone, contractility, and relaxation; use of pelvic floor muscle strength grading (0-5 scale) or Oxford scale; observation of perineal descent during straining to assess dynamic function
• Bidigital examination for perineal structures: Assessment of anal sphincter tone, detection of masses, fissures, or abscess formation; evaluation of pudendal nerve entrapment by palpation at ischial spine with pain reproduction
• Visual inspection: Anoscopy for hemorrhoids, fissures, and inflammatory conditions; speculum examination for vaginal pathology; assessment of perineal skin integrity and signs of trauma

Imaging and Advanced Diagnostic Tools

• Defecography/dynamic MRI: Gold standard for assessing pelvic floor dysfunction, prolapse, and defecatory disorders; shows anatomical relationships and dynamic movement during straining and defecation
• Ultrasound (transperineal): Non-invasive assessment of pelvic floor muscle anatomy, levator ani integrity, and prolapse; useful for postpartum evaluation
• Pudendal nerve imaging: MRI with protocol specific to pudendal canal may show nerve compression at ischial spine; pudendal nerve terminal motor latency (PNTML) testing assesses nerve conduction but has limited clinical utility
• Anorectal manometry: Measures anal sphincter pressures and reflexes; assesses both resting (internal sphincter) and squeeze (external sphincter) pressures to evaluate continence mechanisms

Specialized Testing

• Electromyography (EMG): Documents external anal sphincter or urethral sphincter denervation from pudendal nerve injury
• Post-void residual (PVR) measurement: Via ultrasound or catheterization to assess voiding efficiency in patients with pelvic floor dysfunction
• Pelvic floor physical therapy assessment: Comprehensive evaluation by specialized physical therapist for muscle dysfunction, posture, and behavioral patterns

Conservative Management (First-Line for Most Conditions)

• Pelvic floor physical therapy: Targeted muscle training with biofeedback, progressive strengthening exercises (Kegel exercises with 8-12 repetitions, 3 sets daily), and education on proper relaxation; most effective for stress incontinence and mild-to-moderate prolapse with success rates 60-80%
• Behavioral modifications: Bowel/bladder training, dietary modifications (increased fiber for constipation; reduced irritants for diarrhea), timed voiding, and fluid management
• Pessary use:

Pudendal nerve (S2–S4) — the single most tested structure

• Block landmark: the ischial spine, palpated transvaginally, because the nerve loops around it between the greater and lesser sciatic foramina before entering the pudendal (Alcock's) canal. ACOG describes pudendal block for second-stage/perineal analgesia only — the classic distractor is expecting it to abolish labor pain from uterine contractions, which is visceral and travels with T10–L1 sympathetics.
• Branches to know: inferior rectal (external anal sphincter, perianal skin), perineal, and dorsal nerve of penis/clitoris. Bilateral block is required because perineal innervation overlaps at the midline.
• Common distractor: the levator ani is supplied mainly by direct sacral branches (nerve to levator ani, S3–S5), not solely by the pudendal nerve; sphincter urethrae and external anal sphincter are pudendal.

Pectinate (dentate) line divides everything

• Above: visceral innervation → painless bleeding internal hemorrhoids, superior rectal vein → portal system, drainage to internal iliac nodes, adenocarcinoma.
• Below: inferior rectal nerve → exquisitely painful external hemorrhoids and fissures (classically posterior midline), inferior rectal vein → systemic (IVC), drainage to superficial inguinal nodes, squamous cell carcinoma.

Perineal pouches and urethral injury

• Bulbar (spongy) urethral rupture — straddle injury: urine confined to the superficial perineal pouch by Colles fascia → butterfly scrotal/penile/lower abdominal wall extravasation.
• Membranous urethral rupture — pelvic fracture: extravasation deep/extraperitoneal, with high-riding prostate. Best next step per AUA Urotrauma guideline is retrograde urethrography before catheterization.

Surgical anatomy traps

• Ureter passes under the uterine artery at the cervix — "*water under the bridge*" — the classic ligation injury during hysterectomy.
• Obturator nerve (L2–L4): injured in pelvic lymphadenectomy or compressed by obturator hernia (Howship–Romberg sign) → thigh adduction weakness plus medial thigh sensory loss.
• Perineal body integrity determines pelvic floor support; ACOG favors restrictive rather than routine episiotomy because midline incisions extend into the anal sphincter.
• Erection is parasympathetic (S2–S4), ejaculation sympathetic — "point and shoot"; pudendal somatic fibers drive bulbospongiosus expulsion.