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Summary Table

Category	Details
Risk Factors	Nulliparity, macrosomia (birth weight >4,000 g), operative vaginal delivery (forceps > vacuum), midline episiotomy, prolonged second stage of labor, occiput posterior position, shoulder dystocia, Asian ethnicity, short perineal body
Etiology	Mechanical stretching and tearing of the perineum during vaginal delivery when the tissue cannot accommodate the presenting fetal part
Patient Presentation	Postpartum woman with visible perineal laceration, perineal pain, bleeding from the laceration site; in severe cases, fecal incontinence or passage of flatus through the vagina
Classic Physical Exam	Visible tear of the perineal body extending posteriorly; must perform rectal examination to assess anal sphincter integrity and rectal mucosal involvement
Key Diagnostic Results	Clinical diagnosis by direct visual inspection and digital rectal examination; no imaging or lab test required for classification
Management	Layered surgical repair; first and second degree tears repaired by the delivering provider; third and fourth degree tears require repair in the operating room by an experienced surgeon with adequate anesthesia and lighting
Keywords	"Laceration extending to the anal sphincter," "fecal incontinence after delivery," "fourth degree tear involving rectal mucosa," "midline episiotomy complication"

1. Pathophysiology

The perineum is the fibromuscular tissue between the vaginal introitus and the anus. During the second stage of labor, as the fetal head crowns and delivers, the perineum undergoes progressive stretching. When the mechanical force exceeds the tensile capacity of the tissue, a tear occurs. The direction and depth of the tear determine the degree and clinical consequences.

The perineal body is the central tendinous point where several structures converge: the bulbospongiosus muscle, the superficial and deep transverse perineal muscles, and the external anal sphincter. This anatomical convergence explains why deeper tears have progressively more severe functional consequences. A tear that disrupts only skin and subcutaneous tissue (first degree) is clinically minor, but once the tear reaches the external anal sphincter complex, the patient is at risk for long-term anal incontinence.

The external anal sphincter (EAS) provides roughly 30% of resting anal tone and is primarily responsible for voluntary continence (squeezing to prevent stool passage). The internal anal sphincter (IAS), a smooth muscle extension of the circular rectal muscularis, provides approximately 70% of resting anal tone and maintains involuntary continence. A fourth degree tear disrupts both sphincters and the rectal mucosa, creating a direct communication between the vaginal canal and the rectal lumen. If not properly repaired, this leads to a rectovaginal fistula and permanent fecal incontinence.

The reason midline episiotomy increases the risk of third and fourth degree extension is purely mechanical. A midline cut directed straight toward the anus shortens the distance between the vaginal opening and the anal sphincter, creating a path of least resistance for tear propagation. This is in contrast to a mediolateral episiotomy, which is cut at a 45 to 60 degree angle away from the midline and directs any extension laterally, away from the sphincter complex. This distinction is one of the most tested concepts in obstetric perineal injury.

Operative vaginal delivery, particularly forceps-assisted delivery, increases the risk because forceps blades add diameter to the presenting part and apply direct pressure to the perineal tissue. Forceps carry a higher risk of sphincter injury compared to vacuum extraction, a distinction that commonly appears in exam questions.

2. Classification and Clinical Manifestation

Degree	Structures Involved	Clinical Features	Functional Impact
First Degree	Perineal skin and vaginal mucosa only	Superficial laceration, mild bleeding, minimal pain	None; heals well, some may not require suturing
Second Degree	Perineal skin, vaginal mucosa, and perineal body muscles (bulbospongiosus, transverse perineal) but NOT the anal sphincter	Deeper laceration with moderate bleeding and pain; visible muscle fibers in the wound	Perineal pain during recovery; no fecal continence issues
Third Degree	All of the above plus the anal sphincter complex	Visible disruption of the sphincter ring; must be assessed by rectal exam	Risk of flatal and fecal incontinence
Fourth Degree	All of the above plus the rectal mucosa	Complete tear through the sphincter into the rectal lumen; rectal mucosa visible in the wound	High risk of fecal incontinence and rectovaginal fistula if not properly repaired

Third degree tears are further subclassified:

Subtype	Description
3a	Less than 50% of the external anal sphincter (EAS) thickness torn
3b	More than 50% of the EAS thickness torn
3c	EAS completely torn plus disruption of the internal anal sphincter (IAS)

This subclassification matters because 3c and fourth degree tears carry the highest risk of long-term anal incontinence and require the most meticulous repair. On the exam, a vignette describing a complete sphincter tear with involvement of a smooth muscle ring deep to the EAS is pointing you toward a 3c injury, and the IAS must be repaired as a separate layer.

3. Diagnostic Workup

Test	Purpose	Key Findings
Visual inspection of the perineum (Best Initial Step)	Identify the presence and extent of the laceration	Visible tear; note the depth and whether muscle fibers or sphincter tissue is exposed
Digital rectal examination (Required for accurate classification)	Assess integrity of the anal sphincter and rectal mucosa	Loss of sphincter tone on squeeze, palpable gap in the sphincter ring, or a finger in the rectum visible through the wound
"Pill-rolling" test of the sphincter	Palpate the EAS between thumb (in vagina) and index finger (in rectum) to feel for intact muscle fibers	Intact sphincter has a firm, doughnut-like ring; disrupted sphincter has a palpable defect or gap
Endoanal ultrasound (postoperative or if delayed presentation)	Evaluate sphincter integrity if occult injury is suspected or after failed primary repair	Hypoechoic defect in the sphincter ring on cross-sectional imaging
Anal manometry (long-term follow-up)	Quantify resting and squeeze pressures in patients with persistent incontinence after repair	Reduced resting pressure (IAS injury) or reduced squeeze pressure (EAS injury)

The diagnosis of perineal rupture is entirely clinical. No laboratory test or imaging study is needed at the time of delivery. The critical step that students must remember is that every postpartum perineal laceration requires a rectal examination before classification and repair. Without inserting a finger into the rectum, a third or fourth degree tear can be misclassified as a second degree tear, leading to inadequate repair and devastating long-term consequences.

Visual inspection alone is insufficient because the external anal sphincter can retract laterally when torn, making the wound appear shallower than it truly is. The rectal exam allows the examiner to feel for a gap in the sphincter ring and to check whether the rectal mucosa is intact. If the examining finger in the rectum is visible through the perineal wound, the tear is fourth degree by definition.

Endoanal ultrasound and anal manometry are not used in the acute setting. They are reserved for patients who present weeks to months later with fecal incontinence or suspected occult sphincter injury. Endoanal ultrasound is considered the most accurate test for identifying anatomic sphincter defects in the postpartum period and is the study of choice when evaluating a patient with persistent incontinence despite prior repair.

4. Management and Treatment

Degree	Repair Setting	Technique	Key Details
First Degree	Bedside / delivery room	Simple interrupted or running sutures through skin and mucosa; some small first degree tears may heal without suturing	Absorbable suture (polyglactin 910 / Vicryl 3-0 or 4-0)
Second Degree	Bedside / delivery room	Layered closure: vaginal mucosa (running), perineal muscles (interrupted), skin (subcuticular running)	Absorbable suture; ensure dead space is closed to prevent hematoma
Third Degree	Operating room with regional or general anesthesia	Identify and reapproximate the ends of the torn EAS; IAS repaired separately if involved (3c)	EAS repaired with end-to-end or overlap technique using PDS (polydioxanone) or Vicryl 2-0/3-0 sutures
Fourth Degree	Operating room	Rectal mucosa repaired first (interrupted absorbable sutures, knots tied into the rectal lumen), then IAS, then EAS, then perineal body and skin	Must repair from deepest layer outward; rectal mucosal closure is the first step
Postoperative Care	All degrees	Stool softeners, pain management, sitz baths, pelvic floor physiotherapy referral for third/fourth degree	Lactulose or docusate sodium for 2 weeks minimum; antibiotics (broad-spectrum, e.g., cefoxitin or amoxicillin-clavulanate single dose perioperatively) for third/fourth degree

Acute repair principles. The overarching rule for third and fourth degree tears is that repair must be performed in the operating room under adequate anesthesia (regional or general) with good lighting and proper instruments. Attempting sphincter repair at bedside under local anesthesia leads to poor visualization, incomplete repair, and higher failure rates.

For fourth degree tears, the sequence of repair is critical and commonly tested: start with the rectal mucosa using interrupted absorbable sutures (Vicryl 3-0 or 4-0) with knots tied on the luminal (rectal) side to minimize suture exposure to the perineal wound. Next, repair the internal anal sphincter as a separate layer using interrupted sutures. Then repair the external anal sphincter using either an end-to-end approximation or an overlap technique. Current evidence suggests the overlap technique for the EAS may provide slightly better long-term continence outcomes for complete EAS tears, though both methods are acceptable. Finally, close the perineal muscles and skin in layers as with a standard second degree repair.

Postoperative management includes stool softeners (lactulose 15 to 30 mL twice daily or docusate sodium 100 mg twice daily) to prevent hard stools from disrupting the repair. The patient should avoid straining. A single dose of prophylactic broad-spectrum antibiotics at the time of repair is recommended for third and fourth degree tears to reduce wound infection and dehiscence rates.

Long-term management includes pelvic floor rehabilitation. Patients who undergo third or fourth degree repair should be counseled that subsequent deliveries carry a recurrence risk of 5 to 7%. The mode of delivery for the next pregnancy is determined by symptom status: if the patient has ongoing anal incontinence symptoms, elective cesarean delivery is recommended for subsequent pregnancies. If the patient is continent and asymptomatic after repair, vaginal delivery can be attempted.

5. Differential Diagnosis and Distractors

Differential Diagnosis	Why It Looks Similar	Key Discriminator
Cervical laceration	Postpartum bleeding from a genital tract laceration	Bleeding source is at the cervix (visualized on speculum exam), not the perineum; perineum is intact; occurs more commonly with precipitous delivery or cervical manipulation
Vaginal sidewall laceration (sulcus tear)	Postpartum bleeding, perineal pain, may coexist with perineal tear	Bleeding originates from the lateral vaginal walls, not the posterior perineal body; often associated with operative vaginal delivery; requires separate inspection with retractors
Vulvar/periurethral laceration	Anterior perineal pain and bleeding after delivery	Tear is anterior (near the urethra and clitoris), not posterior toward the anus; no sphincter involvement; watch for urinary retention or hematuria
Postpartum hemorrhage from uterine atony	Significant bleeding after delivery	Uterus is boggy and above the umbilicus on palpation; bleeding is from the uterine cavity, not from a visible laceration; the perineum and genital tract may be intact
Episiotomy dehiscence	Perineal wound breakdown with pain and possible infection	History of episiotomy that was initially repaired; wound breaks down days after delivery with signs of infection (erythema, purulent discharge, fever); not an acute tear
Rectovaginal fistula	Passage of stool or gas through the vagina	Presents weeks to months after delivery (not immediately); often a consequence of an unrecognized or poorly repaired fourth degree tear; confirmed by exam, dye test, or imaging
Bartholin gland abscess	Perineal pain and swelling postpartum	Swelling is lateral and inferior to the vaginal introitus at the 5 or 7 o'clock position; fluctuant mass; unrelated to the perineal body or sphincter

The most important diagnostic distinction on the exam is between a second degree and a third degree tear. Both involve visible muscle injury, and both cause pain and bleeding. The discriminator is whether the anal sphincter is involved, which can only be determined by rectal examination. A vignette describing a postpartum laceration with "loss of anal tone" or "fecal incontinence" is telling you the sphincter is torn, and the answer is third or fourth degree laceration.

Another distractor scenario involves a patient with postpartum bleeding where the uterus is firm and well-contracted. When uterine atony is excluded, the next step is to systematically inspect the genital tract (cervix, vaginal walls, perineum) for lacerations as the source of bleeding.

6. Traps and High-Yield Pearls

The most common trap in perineal rupture questions is failing to perform a rectal examination. A vignette will describe what appears to be a straightforward second degree laceration, and the question asks for the next best step before repair. The answer is digital rectal examination to rule out sphincter and rectal mucosal involvement. Students who skip this step and proceed directly to bedside repair risk missing a third or fourth degree tear, which is the exact error the question is designed to catch.

A second frequently tested concept is the comparison between midline and mediolateral episiotomy. Midline episiotomy is easier to repair, heals with less pain, and has better cosmetic outcomes, but it carries a significantly higher risk of extension into the anal sphincter (third and fourth degree tears). Mediolateral episiotomy is more painful, harder to repair, and associated with more dyspareunia, but it protects the sphincter. When a question asks which episiotomy type reduces the risk of sphincter injury, the answer is always mediolateral.

The third pearl involves the repair sequence for fourth degree tears. Students often answer "repair the external anal sphincter" as the first operative step, but the correct sequence starts from the deepest layer: rectal mucosa first, then IAS, then EAS, then perineal body. Questions may present the steps in random order and ask you to arrange them correctly.

Finally, remember the forceps versus vacuum distinction. Both are operative vaginal delivery methods that increase the risk of perineal tears, but forceps carry a higher risk of third and fourth degree lacerations compared to vacuum. If a question asks which delivery method is associated with a higher risk of anal sphincter injury, the answer is forceps. The core competency being tested across perineal rupture questions is your ability to classify the injury accurately through proper examination, sequence the repair correctly from deep to superficial, and understand the long-term functional implications of sphincter disruption.

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