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Monochorionic Twins - Complete MS Final Exam Guide

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PART 1: DEFINITION & BASIC CONCEPTS

What Are Monochorionic Twins?

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PART 2: PLACENTAL VASCULAR ANASTOMOSES (The Root of All Problems)

ANASTOMOSIS TYPES:
┌─────────────────────────────────────────────────────────┐
│  AA (Artery-Artery) - Surface, BIDIRECTIONAL            │
│  → PROTECTIVE against TTTS (present in 84% non-TTTS,   │
│    only 24% in TTTS placentas)                          │
│                                                          │
│  VV (Vein-Vein) - Surface, BIDIRECTIONAL                │
│                                                          │
│  AV (Artery-Vein) - DEEP, UNIDIRECTIONAL                │
│  → One twin's artery supplies a shared cotyledon        │
│  → The other twin's vein drains it                      │
│  → NET FLOW causes TTTS if unbalanced                   │
└─────────────────────────────────────────────────────────┘

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PART 3: COMPLICATIONS OF MONOCHORIONIC TWINS

Overview Flowchart

MONOCHORIONIC TWINS
        │
        ├──── TTTS (Twin-Twin Transfusion Syndrome) ─── 10-15%
        │
        ├──── TAPS (Twin Anemia-Polycythemia Sequence)  < 1% spontaneous
        │                                               13% post-laser
        │
        ├──── TRAP (Twin Reversed Arterial Perfusion    1-2.5%
        │          / Acardiac Twinning)
        │
        ├──── Selective FGR (Fetal Growth Restriction) ─ Unequal sharing
        │
        ├──── Monoamniotic complications ─── Cord entanglement, IUFD
        │
        └──── Conjoined twins (rare)

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PART 4: TTTS - TWIN-TWIN TRANSFUSION SYNDROME

Definition

• Incidence: 10-15% of monochorionic pregnancies
• Untreated mortality: 80-90%

Pathophysiology (Simple Explanation)

DONOR twin                          RECIPIENT twin
─────────────────                   ─────────────────
Loses blood via AV anastomoses ──►  Gains blood
Oliguria → OLIGOHYDRAMNIOS          Polyuria → POLYHYDRAMNIOS
"Stuck twin" against uterine wall   Dilated bladder
Hypovolemia → ↓ renal perfusion     Fluid overload → cardiac failure
Anemia/growth restriction           Polycythemia → hyperviscosity
Absent/reversed UA flow             Tricuspid regurgitation, hydrops

Diagnosis of TTTS

• Monoamniotic twins (no dividing membrane, cord entanglement)
• Isolated discordant growth
• Isolated polyhydramnios or oligohydramnios
• Severe intertwin hemoglobin difference at birth

• Discordant nuchal translucency >20% in 1st trimester - detects 52% of TTTS cases (PPV only 36%)
• All MCDA twins should be scanned every 2 weeks from 16 weeks
• Monitor: fluid volumes, bladder filling, growth, free-floating intertwin membrane, Doppler

Quintero Staging (Must Know for Exams!)

QUINTERO STAGING OF TTTS
┌────────┬────────────────────────────────────────────────────────────┐
│ Stage  │ Findings                                                   │
├────────┼────────────────────────────────────────────────────────────┤
│   I    │ Donor: oligohydramnios (DVP ≤2cm)                         │
│        │ Recipient: polyhydramnios (DVP ≥8cm)                       │
│        │ Donor bladder VISIBLE; Dopplers NORMAL                     │
├────────┼────────────────────────────────────────────────────────────┤
│   II   │ Donor bladder NOT visible (oliguria)                       │
│        │ Dopplers still NORMAL                                      │
├────────┼────────────────────────────────────────────────────────────┤
│   III  │ ABNORMAL Dopplers in one or both twins:                    │
│        │ • Absent/reversed EDF in umbilical artery (donor)         │
│        │ • Reversed a-wave in ductus venosus (recipient)           │
│        │ • Pulsatile flow in umbilical vein (recipient)            │
├────────┼────────────────────────────────────────────────────────────┤
│   IV   │ HYDROPS (usually in recipient)                             │
├────────┼────────────────────────────────────────────────────────────┤
│   V    │ IUFD of one or both twins                                  │
└────────┴────────────────────────────────────────────────────────────┘

NOTE: Progression is NOT always sequential - can jump Stage I → V!

Management of TTTS

TTTS DIAGNOSIS CONFIRMED
         │
         ├─── Stage I ──► Controversial; close monitoring OR laser
         │                (many centers offer laser even at Stage I)
         │
         ├─── Stage II-IV ─────────────────────────────────────────┐
         │                                                          │
         │         < 26 weeks' gestation?                          │
         │              YES ──► FETOSCOPIC LASER                   │
         │                      PHOTOCOAGULATION                   │
         │                      (Treatment of choice)              │
         │                                                          │
         │         ≥ 26-28 weeks ──► Laser OR Amnioreduction        │
         │                           OR delivery (depending on GA)  │
         │                                                          │
         └─── Stage V (IUFD) ──► Expectant / neuroprotection      │
                                  for surviving twin                │

• Post-op single IUFD: ~33% (donor and recipient equally)
• Post-op double IUFD: ~4%
• TAPS (post-laser): ~13% of double survivors
• Persistent TTTS: ~14%
• pPROM, abruption, chorioamnionitis

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PART 5: TAPS - TWIN ANEMIA-POLYCYTHEMIA SEQUENCE

Definition & Diagnosis

• Spontaneously in <1% of MC twins (small, slow, residual AV anastomoses)
• Post-laser in ~13% of double survivors

• MCA-PSV > 1.5 MoM in the anemic twin
• MCA-PSV < 0.8 MoM in the polycythemic twin
• Post-laser: usually recipient becomes anemic, donor becomes polycythemic (reverse transfusion through missed small anastomoses)

• Cordocentesis + intrauterine transfusion for anemic twin
• If anemia recurs: repeat laser or cord coagulation

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PART 6: TRAP SEQUENCE (Twin Reversed Arterial Perfusion / Acardiac Twinning)

Definition

• Incidence: 1-2.5% of MC twins
• Pump twin survival: 14-90% (variable reports)

Pathophysiology

TRAP SEQUENCE MECHANISM:
                    AA anastomosis
Pump Twin ──────────────────────────► Acardiac Twin
(normal, with heart)   REVERSED       (no heart, receives
                       FLOW           deoxygenated blood
                                      → deficient head,
                                      heart, upper limbs)
         ◄──────────────────────────
                    VV anastomosis
         Deoxygenated blood returns
         → Pump twin: ↓ venous O₂ sat
         → Brain sparing (↓ MCA resistance)
         → Risk of cardiac failure, hydrops

TWO REQUIREMENTS:
1. AA anastomosis (allows flow reversal)
2. Discordant cardiac development or cardiac IUFD of one twin

Poor Prognostic Factors for TRAP:

• High acardiac/pump twin weight ratio
• Rapid increase in acardiac mass
• Small differences in UA Doppler values between twins

Management of TRAP

TRAP SEQUENCE DIAGNOSED
        │
        ├─── Spontaneous cessation of flow to acardiac twin?
        │         YES ──► High mortality! (85% pump twin death/brain damage
        │                 even with "spontaneous resolution")
        │
        ├─── < 16 weeks ──► Monitor closely (membrane not formed yet;
        │                    procedural risk high)
        │
        └─── ≥ 16 weeks, no spontaneous arrest ──► PROPHYLACTIC INTERVENTION
                  │
                  ├─── Fetoscopic laser coagulation of acardiac cord
                  │    (80% survival rate with laser - Hecher series)
                  │
                  └─── Intrafetal ablation (RFA, laser, bipolar)
                       Umbilical cord occlusion

Intervention > conservative management (meta-analysis evidence)

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PART 7: SELECTIVE FETAL GROWTH RESTRICTION (sFGR) IN MC TWINS

Definition

• Weight discordance >20-25% between MC twins, NOT due to TTTS
• Due to unequal placental sharing, often with velamentous cord insertion

Placental Findings

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PART 8: MONOAMNIOTIC (MCMA) TWINNING

Definition

Key Features:

• Division at day 8-12 post-fertilization
• Main danger: cord entanglement leading to IUFD
• TTTS can occur even in MCMA twins - diagnosed by polyhydramnios of common cavity + discordant bladder sizes (but twins move freely; cords typically entangled)

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PART 9: QUICK REFERENCE SUMMARY TABLE

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PART 10: SURVEILLANCE PROTOCOL FOR ALL MC TWINS

MC TWIN PREGNANCY DIAGNOSED
         │
         └──► Ultrasound EVERY 2 WEEKS from ~16 weeks
                   │
                   Assess each visit:
                   ├─ Amniotic fluid volumes (DVP both sacs)
                   ├─ Bladder filling in both twins
                   ├─ Fetal growth (biometry)
                   ├─ Free-floating intertwin membrane
                   ├─ Doppler studies (UA, DV, MCA-PSV)
                   └─ Fetal cardiac function (Stage III+)
              │
              Counsel patient: warn of symptoms of TTTS
              (rapid abdominal distension, preterm contractions)
              → seek IMMEDIATE medical advice if these occur

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PART 11: EXAM HIGH-YIELD POINTS

1. TTTS diagnosis requires BOTH oligo (DVP ≤2 cm) in donor AND poly (DVP ≥8 cm) in recipient - not just growth discordance
2. AA anastomoses PROTECT against TTTS (bidirectional, equalize flow) - present in 84% of non-TTTS but only 24% of TTTS placentas
3. Quintero Stage II = donor bladder NOT visible (critical distinction from Stage I)
4. Laser photocoagulation is superior to amnioreduction - surviving twins after laser have lower rates of anemia and neurologic damage
5. Solomon technique (laser along entire equator) reduces TAPS and recurrent TTTS post-procedure
6. TRAP requires two conditions: AA anastomosis + discordant cardiac development
7. TAPS = MCA-PSV >1.5 MoM (anemic twin) + <0.8 MoM (polycythemic twin) - no fluid discordance
8. Post-laser surveillance: weekly MCA-PSV for 1 month then every 2 weeks (watch for TAPS)
9. Single IUFD post-laser occurs in ~33% of laser cases; surviving twin after laser has LOWER neurologic risk than after amnioreduction (because anastomoses are obliterated)
10. TTTS can occur in monoamniotic twins - diagnosed by poly of common cavity + bladder discordance (no stuck twin, cords entangled)

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Anatomy of the Anterior Abdominal Wall + Prevention of Incisional Hernia

MS Final Exam - 10 Mark Answer

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PART 1: LAYERS OF THE ANTERIOR ABDOMINAL WALL

LAYERS (superficial → deep) - 9 LAYERS TOTAL
═══════════════════════════════════════════════════════
1. SKIN
2. SUBCUTANEOUS TISSUE
   ├─ Superficial (fatty) layer = Camper's fascia
   └─ Deep (membranous) layer = Scarpa's fascia
      (only BELOW umbilicus; fuses with fascia lata of thigh)
3. EXTERNAL OBLIQUE (muscle + aponeurosis)
4. INTERNAL OBLIQUE (muscle + aponeurosis)
5. TRANSVERSUS ABDOMINIS (muscle + aponeurosis)
6. TRANSVERSALIS FASCIA
7. EXTRAPERITONEAL (preperitoneal) FAT
8. PARIETAL PERITONEUM
═══════════════════════════════════════════════════════
Medially also contains:
• RECTUS ABDOMINIS (within its sheath)
• PYRAMIDALIS (small, at lower end)

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PART 2: CROSS-SECTION DIAGRAM (ABOVE ARCUATE LINE)

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PART 3: THE MUSCLES - INDIVIDUAL DETAILS

3A. LATERAL FLAT MUSCLES (from outside in)

3B. RECTUS ABDOMINIS (Medial)

• Origin: Pubic symphysis and pubic crest
• Insertion: Xiphoid process + 5th, 6th, 7th costal cartilages
• Features: 3 tendinous intersections (fused to anterior sheath only, NOT posterior) - this is why epigastric vessels run freely behind it
• Enclosed in: Rectus sheath (formed by aponeuroses of the three lateral muscles)

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PART 4: RECTUS SHEATH - THE MOST IMPORTANT EXAM TOPIC

The Arcuate Line (Line of Douglas)

THE RECTUS SHEATH - COMPOSITION

ABOVE ARCUATE LINE:
┌──────────────────────────────────────────────────────────┐
│ ANTERIOR SHEATH = EO aponeurosis                         │
│                 + Anterior lamella of IO aponeurosis     │
│                                                          │
│ POSTERIOR SHEATH = Posterior lamella of IO aponeurosis   │
│                  + TA aponeurosis                        │
└──────────────────────────────────────────────────────────┘

BELOW ARCUATE LINE:
┌──────────────────────────────────────────────────────────┐
│ ANTERIOR SHEATH = EO + IO + TA aponeuroses (ALL three)   │
│                  pass in FRONT of rectus muscle          │
│                                                          │
│ POSTERIOR SHEATH = ABSENT                                │
│                 Transversalis fascia covers              │
│                 rectus posteriorly                       │
└──────────────────────────────────────────────────────────┘

EO = External Oblique, IO = Internal Oblique, TA = Transversus Abdominis

Why does this matter surgically?

• Below the arcuate line, there is NO posterior rectus sheath - only the thin transversalis fascia. This is the weakest area for hernia formation.
• Pfannenstiel, lower midline, and lower paramedian incisions all pass through this weak zone.

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PART 5: INTERNAL SURFACE ANATOMY & PERITONEAL FOLDS

PERITONEAL FOLDS (seen laparoscopically)
├── Median umbilical fold (1, unpaired midline)
│   └── Contains: Obliterated URACHUS
│
├── Medial umbilical folds (2, bilateral)
│   └── Contains: Obliterated umbilical arteries
│
└── Lateral umbilical folds (2, bilateral)
    └── Contains: Inferior epigastric vessels (IMPORTANT!)

• Supravesical fossa - between median and medial folds → supravesical hernia
• Medial inguinal fossa (Hesselbach's triangle) - between medial and lateral folds → DIRECT inguinal hernia
• Lateral inguinal fossa - lateral to lateral fold (deep ring) → INDIRECT inguinal hernia

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PART 6: BLOOD SUPPLY, NERVE SUPPLY & LYMPHATICS

Blood Supply

DEEP (MAIN) SUPPLY:
├── Superior epigastric artery
│   └── Branch of internal thoracic artery
│   └── Enters rectus sheath below costal margin
│   └── Runs on POSTERIOR surface of anterior sheath
│
├── Inferior epigastric artery
│   └── Branch of external iliac artery
│   └── Enters rectus sheath below arcuate line
│
└── Both ANASTOMOSE at the umbilicus
    (collateral: upper ↔ lower limb vasculature)

SUPERFICIAL SUPPLY:
└── Superficial epigastric, superficial circumflex iliac,
    superficial external pudendal (branches of femoral artery)

Nerve Supply

NERVE SUPPLY
├── T7–T11: Intercostal nerves (thoracoabdominal)
├── T12: Subcostal nerve
├── L1: Iliohypogastric nerve (superior branch)
└── L1: Ilioinguinal nerve (inferior branch)

All nerves run between internal oblique and
transversus abdominis, then pierce anterior rectus
sheath to supply skin.

Lymphatics

• Above umbilicus → Superficial axillary nodes
• Below umbilicus → Superficial inguinal nodes

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PART 7: CLINICALLY IMPORTANT LANDMARKS

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PART 8: INCISIONAL HERNIA - DEFINITION & AETIOLOGY

Risk Factors (Aetiology Flowchart)

RISK FACTORS FOR INCISIONAL HERNIA
             │
    ┌────────┼───────────┐
    ▼        ▼           ▼
PATIENT    WOUND      SURGICAL
FACTORS    FACTORS    FACTORS
    │          │           │
obesity    infection  wrong suture
smoking    tension    poor technique
malnutrition poor      inadequate
DM         quality    closure
steroids   tissue     short SL:WL
immunosuppn            ratio
chronic cough
cancer
collagen disorders

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PART 9: PREVENTION OF INCISIONAL HERNIA

9A. PREOPERATIVE OPTIMIZATION

BEFORE SURGERY:
┌─────────────────────────────────────────┐
│  • STOP SMOKING (essential)             │
│  • Weight loss for obese patients       │
│  • Correct malnutrition                 │
│  • Control diabetes                     │
│  • Treat chronic cough (COPD, asthma)   │
│  • Stop/reduce steroids if possible     │
│  • Optimise immune status               │
└─────────────────────────────────────────┘

9B. INTRAOPERATIVE - CORRECT CLOSURE TECHNIQUE (MOST IMPORTANT)

CORRECT WOUND CLOSURE (European Hernia Society Guidelines)

SMALL-STITCH / SMALL-BITE TECHNIQUE:
┌──────────────────────────────────────────────────────────┐
│                                                          │
│   Bite distance:  5 mm APART                            │
│   Bite depth:     5-8 mm FROM WOUND EDGE                │
│   Include:        FASCIA ONLY (no muscle!)               │
│   Avoid:          Excessive tension                      │
│                                                          │
│   Suture material: 2/0 SLOWLY RESORBABLE                │
│   (NOT heavy / non-absorbable - cuts through tissue)    │
│                                                          │
│   Suture Length : Wound Length RATIO = 4:1 (minimum)    │
│   (< 4:1 means bites too far apart or too tight)        │
│                                                          │
│   Continuous OR interrupted - both acceptable            │
│   (if continuous: don't pull too tight, don't go too    │
│    near fascial edge)                                    │
│                                                          │
│   DRAINS → through SEPARATE stab incisions              │
│   (NEVER through the wound - prevents fascial           │
│    apposition and causes hernia)                        │
└──────────────────────────────────────────────────────────┘

9C. PROPHYLACTIC MESH (High-Risk Patients)

INDICATIONS FOR PROPHYLACTIC MESH:
│
├── Morbid obesity / bariatric surgery patients
│   → Substantially reduces incisional hernia risk
│
├── High-risk midline laparotomy
│   (re-operation, stoma formation, AAA repair)
│
└── Prevention of PARASTOMAL hernia
    (occurs in up to 50% of stoma patients)
    → Prophylactic mesh placement at time of stoma

Complete Prevention Summary Flowchart

PREVENTION OF INCISIONAL HERNIA - COMPLETE FLOWCHART

PRE-OP                INTRA-OP               POST-OP
────────              ────────               ───────
Optimize patient  →   Correct technique  →   Avoid:
• Stop smoking        • Small bites           • Wound infection
• Lose weight         • 5mm apart             • Prolonged ileus
• Nutrition           • 5-8mm depth           • Early return
• DM control          • Fascia only           • to activity
• Treat cough         • 4:1 SL:WL ratio
                      • 2/0 slow absorbable
                      • Drain via stab incision
                      +
                      Prophylactic mesh
                      in HIGH-RISK patients
                      (obese, re-op, stoma,
                      AAA, malignancy)

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PART 10: SUMMARY TABLE - HIGH YIELD FOR EXAM

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