Perineum and Anal Triangle Anatomy

Perineum

• The perineum is a diamond-shaped region inferior to the pelvic floor.
• It is bounded by the pelvic outlet, including the pubic symphysis, ischiopubic rami, ischial tuberosities, sacrotuberous ligaments, and coccyx.
• The ceiling is the pelvic diaphragm (levator ani and coccygeus), and the lateral wall is the pelvic cavity below the levator ani.
• It has two triangles:
  • A urogenital triangle, which is oriented in the horizontal plane.
  • An anal triangle tilted upwards, facing more posteriorly.
• The urogenital hiatus is a U-shaped defect within the perineum.

Anal Triangle

• Faces posteroinferiorly
• Lateral border is medial margins of the sacrotuberous ligaments
• Anterior border is a horizontal line between the two ischial tuberosities
• Posterior border is the coccyx
• The ceiling is the pelvic diaphragm
• The floor is skin
• The major muscle in the anal triangle is the external anal sphincter.

Ischioanal Fossae

• Wedged-shaped spaces located between the skin and the pelvic diaphragm
• The cavern's lateral wall is made by the ischium, obturator internus, and sacrotuberous ligament
  • Internal pudendal artery and vein, pudendal nerve, and nerve to obturator internus pass through the pudendal canal
• The medial wall is the external anal sphincter and levator ani
• Anterior recesses project into the urogenital triangle
• The fossae are filled with fat to support the canal and allow distention during defecation
• The area contains branches of the internal pudendal vessels and nerve-inferior rectal vessels and nerves

Urogenital Triangle

• The anterior half of the perineum contains roots of the external genitalia and openings of the urogenital system.
• Laterally, it is bounded by the ischiopubic rami.
• Posteriorly, an imaginary line exists between the ischial tuberosities.
• Anteriorly, it is bordered by the inferior margin of the pubic symphysis.
• The roof is made up of the levator ani.
• A strong fibromuscular support platform (perineal membrane), deep and superficial perineal pouches are included

Perineal Membrane

• A thick fibrous sheet that fills the urogenital triangle
• The deep perineal pouch lies superior to it, and the superficial perineal pouch lies inferior to it.
• Supports external genitalia and pelvic viscera.

Perineal Body

• An ill-defined fibromuscular structure (node) into which muscles of the pelvic floor and perineum attach
• Stretches between the anal and urogenital triangle
• Gynaecologists call this 'the perineum'.
• Can be damaged during childbirth or trauma, resulting in incontinence and prolapse.

Deep Perineal Pouch

• It contributes to the pelvic floor and supports elements of the urogenital system in the pelvic cavity.
• The Urethra and vagina (in females) pass through the pouch.
• Female Components:
  • Muscles: Deep transverse perineal muscle, external urethral sphincter, sphincter urethrovaginalis, compressor urethrae
  • Structures: Part of the urethra, opening for the vagina
  • Nerves: Dorsal nerve of the clitoris and internal pudendal nerve and vessels
• Male Components:
  • Muscles: Deep transverse perineal muscle, external urethral sphincter
  • Structures: bulbourethral glands, membranous urethra
  • Nerves: Dorsal nerve of the penis, internal pudendal nerve and vessels

Superficial Perineal Pouch

• Contains erectile tissue and skeletal muscles associated with erectile structures.
• Key anatomical areas:
  • Body of the clitoris/penis
  • Greater vestibular glands (Bartholin's glands)
  • Glans clitoris/penis
  • Bulb of the vestibule
  • Corpus spongiosum/cavernosa
  • Spongy urethra
• Ischiocavernosus muscle forces blood into the erect penis and clitoris
• The bulbospongiosus muscle helps compress erectile tissue and forces blood to distal regions
  • In males, it facilitates emptying the bulbous part of the penile urethra following urination and is responsible for the pulsatile emission of semen
  • In females, it tightens the vagina during orgasm
• The superficial transverse perineal muscle helps stabilize the perineal body

Fascia

• The superficial fascia of the urogenital triangle is continuous with that of the anterior abdominal wall.
• Fluids or infectious materials in the superficial pouch can track out of the perineum.
• Superficial perineal fascia consists of:
  • Superficial fatty, which provides a fatty layer replaced by the dartos muscle in males.
  • Deep membranous (Colle's) layer continuous with abdomen's membranous layer (Scarpa's) and the dartos.
• Deep investing perineal fascia (Gallaudet) surrounds the penis, continuing into the deep fascia (Buck's).

Extravasation of Urine

• Rupture of the male urethra can lead to urine extravasation into perineal and pelvic spaces
• This is contained within the perineal, pelvic, and lower abdominal wall fascial planes

Superficial Features - Female

• Vulva consists of the clitoris and vestibular apparatus.
• Glands opening into the vestibule:
  • Para-urethral glands
  • Greater vestibular glands
• Key structures:
  • Mons pubis
  • Prepuce of clitoris
  • Glans and frenulum of clitoris
  • Labia majora and minora
  • Urethral and vaginal opening
  • Remnant of the hymen

Superficial Features - Male

• Consists of the scrotum and penis, which are attached along the longitudinal midline raphe.
• The scrotum is homologous to the labia majora in females. The prepuce (foreskin) covers the glands and is removed during circumcision.
• Key structures:
  • Glans penis
  • Corona of glans
  • Neck of glans
  • External urethral orifice

Phimosis

• An uncircumcised prepuce covers all or most of the glans penis.
• The prepuce are tight and cannot be retracted easily
• Untreated conditions may result Fibrous adhesions and Infection
• Infection can significantly complicate the condition by producing an inflammatory exudate and oedema.

Paraphimosis

• Retained retraction of a tight foreskin behind the coronary sulcus
• Constriction leads to marked edematous swelling of the distal prepuce and glans

Pudendal Nerve

• Originates in the pudendal nerve
• Dorsal nerve of the clitoris and penis
• Perineal nerves
• Inferior rectal branches

Pudendal Nerve Block

• Used to relieve pain during childbirth, targeting the ischial spine as an anatomical landmark.
• Used to abolish pain from the anterior part of the perineum, an ilioinguinal nerve block is performed

Internal Pudendal Vessels

• Consists of the Internal pudendal artery and vein with the inferior rectal vessels, dorsal artery/vein and perineal artery

Episiotomy

• A surgical incision of the perineum and inferoposterior vaginal wall.
• Used during vaginal surgery and labor to enlarge the vaginal orifice, which will decrease excessive tearing of the perineum and perineal muscles
• Posterolateral incisions give the best protection against sphincter damage

Lymphatics

• Lymphatics from the perineum drain initially to the internal iliac nodes.
• Superficial tissue/skin of the scrotum/skin of labia drains initially to the superficial inguinal nodes.
• The Testis drains to the lumbar nodes.
• Glans penis and glans clitoris drains to deep inguinal nodes.