7.4 Perineal Anatomy & Classification of Obstetric Lacerations

Quick Summary: Accurate repair begins with accurate anatomy and accurate classification. This section covers the perineal body and the muscular compartments that anchor to it, the anal sphincter complex, and the standardized grading of first- through fourth-degree obstetric lacerations including the 3a, 3b, and 3c subdivisions, plus the periurethral, sulcus, and cervical tears that are easy to miss. Episiotomy and repair technique follow in the next section.

Pelvic Floor & Perineal Anatomy

The female pelvic floor is a multi-layered muscular and fascial diaphragm that supports the pelvic viscera and provides voluntary and involuntary sphincter control for the urethra, vagina, and rectum.

The Perineal Body (Central Tendon of the Perineum)

The perineal body is a dense, pyramid-shaped fibromuscular node situated in the midline between the posterior vaginal wall and the anterior anal canal. It serves as the central anchoring point for the pelvic floor musculature:

• Superficial Muscular Attachments: Bulbospongiosus (bulbocavernosus) muscles anteriorly, superficial transverse perineal muscles bilaterally, and the external anal sphincter posteriorly.
• Deep Muscular & Fascial Attachments: Deep transverse perineal muscles, perineal membrane, and pubococcygeus fibers of the levator ani.
• Clinical Significance: Integrity of the perineal body is essential to prevent long-term pelvic organ prolapse, rectocele formation, and sexual dysfunction.

Muscular Compartments of the Pelvic Floor

1. Superficial Perineal Space (Pouch):
   • Bulbospongiosus (Bulbocavernosus): Surrounds the vaginal orifice, covers the vestibular bulbs, and inserts into the perineal body.
   • Ischiocavernosus: Overlies the crus of the clitoris, originating from the ischial tuberosities.
   • Superficial Transverse Perineal: Extends transversely from the ischial tuberosities to fuse into the perineal body.
2. Urogenital Diaphragm (Deep Perineal Pouch):
   • Contains the deep transverse perineal muscle, the sphincter urethrae, and the compressor urethrae.
3. Pelvic Diaphragm (Levator Ani Complex):
   • Composed of the puborectalis, pubococcygeus, and iliococcygeus muscles, along with the coccygeus muscle. The puborectalis forms a muscular U-shaped sling around the anorectal junction, pulling it forward to create the anorectal angle (80° to 90°), which is vital for gross fecal continence.

The Anal Sphincter Complex

• External Anal Sphincter (EAS): A circular ring of striated, voluntary muscle approximately 1.5 to 2.0 cm in length and 8 to 10 mm in thickness. It surrounds the lower two-thirds of the anal canal and is innervated by the inferior rectal branch of the pudendal nerve (S2–S4). It provides voluntary squeeze pressure for acute continence.
• Internal Anal Sphincter (IAS): An involuntary, smooth muscle cylinder that represents the thickened distal continuation of the circular smooth muscle layer of the rectum. It measures 2 to 3 mm in thickness and appears clinically as a pale-pink, glistening, fibrous white band. It is innervated by autonomic sympathetic and parasympathetic fibers and is responsible for 70% to 80% of resting anal canal closure pressure and resting continence of gas and liquid stool.

Classification of Obstetric Perineal Lacerations

The standardized international classification system (Sultan / ACOG / RCOG) categorizes obstetric tears according to anatomical depth and involvement of the anal sphincter complex:

Other Intrapartum Lacerations

• Periurethral Lacerations: Superficial tears along the labia minora or urethral meatus. Often bleed briskly from vestibular erectile tissue. If repair is required, place an indwelling urinary catheter first to identify the urethral lumen and prevent iatrogenic suture occlusion or urethral stricture.
• Sulcus Tears (Vaginal Sidewall Tears): Longitudinal tears along the lateral vaginal sulci extending toward the ischial spines. Require careful exposure with right-angle or Breisky retractors to secure retracted bleeding vessels at the apex.
• Cervical Lacerations: Typically occur at the 3 o'clock and 9 o'clock positions. Suspected when brisk, bright red arterial bleeding persists despite a firmly contracted uterine fundus. Visualized by placing sponge forceps (ring forceps) sequentially around the entire circumference of the cervix.