---

RETROSIGMOID APPROACH — A COMPREHENSIVE 50-MARK ANSWER

---

1. INTRODUCTION & HISTORICAL BACKGROUND

• First described by Sir Charles Ballance (1894) for acoustic neuroma surgery
• Cushing (1917) popularized bilateral suboccipital decompression
• Dandy (1925) introduced unilateral suboccipital approach with improved facial nerve identification
• Rand & Kurze (1965) introduced microsurgical technique
• Yasargil (1970s) refined microsurgical retrosigmoid technique
• Modern era: combination with endoscopy (key-hole retrosigmoid), intraoperative neurophysiological monitoring

---

2. SURGICAL ANATOMY OF THE CEREBELLOPONTINE ANGLE (CPA)

• CN V (trigeminal) — superiorly
• CN VII & VIII bundle — entering IAC at porus acusticus
• CN IX, X, XI — inferiorly at jugular foramen
• AICA (anterior inferior cerebellar artery) — loops into IAC
• PICA (posteroinferior cerebellar artery)
• SCA (superior cerebellar artery)

• Length: ~8–10 mm
• Divided by Bill's bar (vertical crest) and transverse crest
• Contains: facial nerve (anterosuperior), superior vestibular nerve (posterosuperior), cochlear nerve (anteroinferior), inferior vestibular nerve (posteroinferior)

---

3. INDICATIONS

Primary Indications:

1. Acoustic neuroma (Vestibular Schwannoma)
   • Best for tumors ≤2 cm CPA component with serviceable hearing
   • Tumors with limited IAC involvement (not reaching fundus)
   • CPA meningiomas (hearing preservation possible)
2. Microvascular decompression (MVD)
   • Trigeminal neuralgia (Jannetta procedure)
   • Hemifacial spasm
   • Glossopharyngeal neuralgia
3. Non-acoustic CPA neoplasms — epidermoid cysts, facial schwannomas, meningiomas
4. Facial nerve decompression (CPA–IAC segment)
5. CPA arachnoid cysts
6. Vascular lesions — AVMs, aneurysms of AICA/PICA

Advantages Over Other Approaches:

---

4. CONTRAINDICATIONS

• Tumor extending to fundus of IAC (relative contraindication for hearing preservation)
• Non-serviceable hearing (translabyrinthine preferred for large tumors)
• High jugular bulb obscuring access
• High-riding sigmoid sinus limiting posterior fossa dural exposure
• Patient unfit for posterior fossa surgery

---

5. PREOPERATIVE EVALUATION

Clinical Assessment:

• Pure tone audiometry (PTA) + Speech discrimination score (SDS)
• Serviceable hearing (Gardner-Robertson Class I–II): PTA ≤50 dB, SDS ≥50%
• Vestibular function tests: caloric tests, VNG
• Complete cranial nerve examination (V, VII, VIII, IX, X)
• Facial nerve grading (House-Brackmann scale — baseline)

Imaging:

• MRI brain with gadolinium — gold standard
  • Size, location, IAC extension to fundus
  • Relationship to brainstem
  • Fluid-attenuating sequences for hearing preservation planning
• CT temporal bone — mastoid pneumatization, jugular bulb height, sigmoid sinus position
• CT angiography / MRA — vascular anatomy, AICA loop

Audiological:

• BAER (Brainstem Auditory Evoked Response) — baseline for intraoperative monitoring

---

6. PATIENT POSITIONING

Park-Bench (Lateral/Semilateral) Position — Most Common:

• Patient lateral decubitus, affected side up
• Head fixed in Mayfield 3-point cranial fixation frame
• Head tilted 15° toward floor, rotated 15° toward ceiling (brings retrosigmoid area upward)
• Shoulder roll under opposite shoulder
• All pressure points padded
• Neutral cervical position to avoid jugular venous obstruction

Supine with Head Rotation:

• Simpler, used for osteoclastic craniectomy
• Head turned 90° to contralateral side

Sitting Position (Historical / Rarely Used):

• Advantages: gravity-assisted CSF drainage, reduced bleeding
• Risks: venous air embolism (major concern), paradoxical air embolism through PFO
• Requires: Doppler monitoring, G-suit, central venous catheter
• Largely abandoned except at specific centers

Prone Position:

• Rarely used; for midline lesions

---

7. SURGICAL TECHNIQUE — STEP-BY-STEP

FLOWCHART: RETROSIGMOID APPROACH — SURGICAL STEPS

PATIENT PREPARATION & POSITIONING
           │
           ▼
SKIN INCISION (Post-auricular / Lazy-S)
           │
           ▼
SOFT TISSUE DISSECTION
(Reflect scalp flap anteriorly)
(Subperiosteal reflection of neck muscles)
           │
           ▼
BONE WORK — RETROSIGMOID CRANIOTOMY
(4×3 cm bone flap posterior to sigmoid sinus,
inferior to transverse sinus)
           │
     ┌─────┴──────┐
     │            │
  CRANIECTOMY  BONE FLAP
(osteoclastic) (osteoplastic)
     │            │
     └─────┬──────┘
           ▼
SIGMOID SINUS SKELETONIZATION
& MASTOIDECTOMY (if needed)
           │
           ▼
DURAL INCISION (curvilinear/C-shaped)
(Dural traction sutures → retract sigmoid sinus anteriorly)
           │
           ▼
CSF DRAINAGE FROM CPA CISTERN
(Gradual — prevents cerebellar herniation)
           │
           ▼
CEREBELLAR RETRACTION
(Slow, gentle — over 5-10 minutes)
           │
           ▼
CPA EXPOSURE
(Identify flocculus, choroid plexus,
CN VII-VIII bundle)
           │
           ▼
TUMOR IDENTIFICATION & DEBULKING
(If large: internal decompression first)
           │
           ▼
FACIAL NERVE IDENTIFICATION
(Medially at brainstem first, then laterally)
(Nerve monitor used continuously)
           │
           ▼
REMOVAL OF POSTERIOR LIP OF IAC
(Diamond bur drilling — limited by
posterior semicircular canal)
           │
           ▼
COMPLETE TUMOR RESECTION
(Dissect tumor off facial nerve
toward porus acusticus)
           │
           ▼
HEMOSTASIS + DURAL CLOSURE
(Fat graft — obliterate IAC + mastoid)
(Dura closed watertight)
           │
           ▼
BONE FLAP REPLACEMENT / CRANIOPLASTY
           │
           ▼
LAYERED WOUND CLOSURE + PRESSURE DRESSING

---

Step-by-Step Details:

A. INCISION

• Lazy-S (or reverse C-shaped) post-auricular incision, 2–3 cm behind the post-auricular crease
• Extends from above the pinna to the mastoid tip
• Skin flap reflected anteriorly, exposing the mastoid cortex and occipital bone
• Subperiosteal dissection reflects neck muscles (sternocleidomastoid, splenius capitis, semispinalis) inferiorly

B. CRANIOTOMY

• Burr hole placed at asterion (landmark for junction of transverse and sigmoid sinuses)
• Additional burr holes placed; bone flap elevated (~3×4 cm) using craniotome
• Flap preserved in saline / antibiotic solution for replacement at closure

• Bone removed piecemeal with Kerrison rongeurs / drill
• Posterior boundary: 1–2 cm posterior to sigmoid sinus
• Superior boundary: transverse sinus
• Size: approximately 3×3 cm
• Sigmoid sinus identified anteriorly — key landmark
• Transverse sinus identified superiorly

• Mastoidectomy → skeletonize sigmoid sinus
• Allows anterior retraction of sigmoid sinus → increases CPA exposure
• Identifies posterior semicircular canal → guides drilling of posterior IAC lip
• Fig. 179.30 — Cummings (p. 3448): Mastoidectomy in conjunction with retrosigmoid exposure permits anterior retraction of sigmoid sinus

C. DURAL OPENING

• C-shaped or curvilinear incision in posterior fossa dura, posterior to sigmoid sinus
• Incise carefully — avoid injury to vessels adherent to dura
• Dural flap tented with traction sutures → sinus retracted anteriorly
• Increases lateral working angle substantially

D. CSF DRAINAGE — CRITICAL STEP

• Slow, controlled CSF drainage from CPA cistern
• Inadequate drainage → cerebellum herniates through dural incision → cerebellar contusion/necrosis/hemorrhage
• CSF also drained from cisterna magna / foramen magnum region
• Lumbar drain can be placed preoperatively for large tumors

E. CEREBELLAR RETRACTION

• Cottonoid strips (with rubber glove backing) placed over cerebellar surface
• Slow, gentle retraction over 5–10 minutes
• Once adequate CSF drained, no external retractor needed — cerebellum falls away
• Exposes: flocculus, choroid plexus, CN VII–VIII bundle
• Key view: posterior fossa face of temporal bone + porus acusticus

F. TUMOR SURGERY — ACOUSTIC NEUROMA

1. Identify CN VII and VIII at porus acusticus (medial)
2. Debulk tumor internally (ultrasonic aspirator / bipolar + scissors) for large tumors
3. Identify facial nerve at brainstem — critical before dissection
4. Remove posterior lip of IAC with diamond burs (under irrigation)
   • Lateral limit = posterior semicircular canal
   • Previously exposed mastoid dura guides surgeon to canal location
5. Dissect tumor from facial nerve — from CPA toward porus (medial → lateral)
6. Cochlear nerve preservation for hearing conservation — avoid blood supply to IAC
7. Fundus visualization: mirrors + endoscope (30°/70° rod-lens) if needed
   • Cannot use Bill's bar (translabyrinthine advantage) — relies entirely on nerve monitor for lateral IAC

G. CLOSURE

• IAC packed with abdominal fat graft — prevents CSF leak
• Mastoid air cells waxed / obliterated with fat
• Dura closed in watertight fashion (4-0 braided nylon)
• Bone flap replaced and fixed with microplates/titanium or cranioplasty material (reduces post-op headache)
• Muscles, fascia, subcutaneous tissue, skin — layered closure
• Mastoid compressive dressing applied

---

8. INTRAOPERATIVE MONITORING

---

9. INTRAOPERATIVE IMAGES

---

---

---

---

---

10. SELECTION OF SURGICAL APPROACH — DECISION FLOWCHART

                  ACOUSTIC NEUROMA / CPA LESION
                              │
                ┌─────────────┴─────────────┐
                │                           │
      HEARING SERVICEABLE?            HEARING NOT SERVICEABLE
       (PTA ≤50 dB, SDS ≥50%)              │
                │                           ▼
          YES   │                  TRANSLABYRINTHINE APPROACH
                │
        ┌───────┴────────┐
        │                │
  TUMOR SIZE           TUMOR SIZE
  ≤1.5 cm (intra-    >2 cm, CPA component
  canalicular/small)
        │                │
        ▼                ▼
  MIDDLE FOSSA      RETROSIGMOID (SUBOCCIPITAL)
  APPROACH          APPROACH
  (fundus visible,  (panoramic CPA exposure)
  young patient)

        RETROSIGMOID APPROACH — Further decision:
                              │
              ┌───────────────┴──────────────┐
              │                              │
    TUMOR NOT AT FUNDUS             TUMOR AT FUNDUS
              │                              │
              ▼                              ▼
    HEARING PRESERVATION          HEARING PRESERVATION
    ATTEMPTED                     UNLIKELY / TRANSLABYRINTHINE
    (30–65% success rate)         PREFERRED

---

11. POSTOPERATIVE MANAGEMENT

• Immediate: ICU/HDU monitoring for 24 hours
• Neurological observation: hourly GCS, cranial nerve assessment
• Wound: mastoid compressive dressing for 24–48 hours
• CSF leak prevention: head-of-bed elevation 30°, avoid Valsalva
• Medications: steroids (dexamethasone), antiemetics, analgesics
• Hearing: ABR tested on day 1 post-op
• Facial nerve: House-Brackmann grading documented
• Early mobilization: Day 1–2 post-op
• Vestibular rehabilitation: started early for balance compensation

---

12. COMPLICATIONS

Intraoperative:

Postoperative:

1. Intradural bone dust from drilling the posterior IAC lip
2. Scar and muscle adhesion to dura

---

13. RESULTS & OUTCOMES

Hearing Preservation:

• Success rate: 30–65% depending on selection criteria (Cummings, p. 3449)
• Best results: tumor <1.5 cm, limited IAC involvement, not reaching fundus
• Cochlear implantation is an option if hearing conservation fails post-operatively

Facial Nerve Preservation:

• Comparable rates to translabyrinthine approach
• Some centers report slightly better rates with translabyrinthine (due to direct Bill's bar identification)
• Modern series (with monitoring): >95% anatomical preservation for tumors <2 cm

Tumor Control:

• Gross total resection: >95% for acoustic neuromas <2 cm
• Recurrence: ~2–3% at 5 years

---

14. COMPARISON: THREE STANDARD CPA APPROACHES

          ┌─────────────────────────────────────────────────┐
          │         CPA TUMOR SURGERY — APPROACH COMPARISON │
          └─────────────────────────────────────────────────┘

FEATURE          │ RETROSIGMOID    │ TRANSLABYRINTHINE│ MIDDLE FOSSA
─────────────────┼─────────────────┼──────────────────┼─────────────
Hearing          │ Preserved       │ Always lost      │ Best preserved
Preservation     │ (30–65%)        │                  │
─────────────────┼─────────────────┼──────────────────┼─────────────
IAC fundus       │ ❌ Indirect     │ ✅ Direct        │ ✅ Direct
visualization    │                 │                  │
─────────────────┼─────────────────┼──────────────────┼─────────────
CPA exposure     │ Widest/Panoramic│ Wide             │ Limited (<1cm)
─────────────────┼─────────────────┼──────────────────┼─────────────
Tumour size      │ Any size        │ Any size         │ Intracanalicular
─────────────────┼─────────────────┼──────────────────┼─────────────
Cerebellar       │ Required        │ Minimal          │ Not needed
retraction       │                 │                  │
─────────────────┼─────────────────┼──────────────────┼─────────────
Facial nerve     │ Brainstem first │ Brainstem + fundus│ Fundus first
identification   │                 │                  │
─────────────────┼─────────────────┼──────────────────┼─────────────
Post-op headache │ 10%             │ Rare             │ Rare
─────────────────┼─────────────────┼──────────────────┼─────────────
Best for         │ CPA tumors with │ Large tumors /   │ Small intra-
                 │ serviceable     │ non-serviceable  │ canalicular
                 │ hearing         │ hearing          │ in young pts

---

15. RECENT ADVANCES

1. Endoscope-Assisted Retrosigmoid Approach

• Keyhole retrosigmoid craniotomy + rigid endoscope (30°/70°)
• Visualizes fundus of IAC without drilling posterior semicircular canal
• Reduces the principal limitation of this approach (indirect fundus visualization)
• Used for: MVD, small intracanalicular tumors, cholesteatoma of CPA
• Fully endoscopic retrosigmoid approach reported for acoustic neuromas

2. Robotic-Assisted Surgery

• Da Vinci system adaptations for CPA — early experimental phase
• Theoretical advantage: tremor-free dissection in narrow corridor

3. Intraoperative MRI

• Real-time assessment of tumor residual during resection
• Alters surgical decisions in ~20–30% of cases
• Increases gross total resection rates

4. Intraoperative CT with Navigation

• Neuronavigation / image guidance integrated with retrosigmoid approach
• Precise IAC drilling without damaging posterior SCC
• Reduces risk of CSF fistula

5. Enhanced Neuromonitoring

• Continuous transcranial motor evoked potentials (MEP)
• Antidromic facial nerve action potentials (Colletti & Fiorino — Cummings, p. 4313)
• Intraoperative ABR with automated real-time analysis

6. Hearing Preservation Modifications

• Arriaga & Gorum: posterior semicircular canal resection with retrosigmoid approach for enhanced lateral IAC exposure while preserving hearing (reported 2/3 success rate)
• Cochlear implantation post-operatively when hearing conservation fails (combined with retrosigmoid dissection)

7. Modified Retrosigmoid Approaches

• Retrosigmoid-transjugular approach: access to jugular foramen tumors
• Retrosigmoid-transtentorial approach: access to upper CPA + Meckel's cave
• Far-lateral retrosigmoid: access to anterior foramen magnum + lower CN pathology

8. Stereotactic Radiosurgery vs. Surgery

• Gamma Knife / CyberKnife: alternative for tumors <3 cm
• Current consensus: microsurgical retrosigmoid for young patients, good hearing, growing tumors; radiosurgery for elderly, small stable tumors
• Wait-and-scan: for small incidentally found tumors

9. Minimally Invasive / Keyhole Techniques

• Small (2.5 cm) retrosigmoid craniotomy with endoscope
• Equivalent tumor control with less morbidity in selected patients
• Shorter hospital stay, reduced blood loss

---

16. IMPORTANT POINTS FOR EXAM (RGUHS)

1. Dandy's approach = original suboccipital = modern retrosigmoid
2. Key landmark: Sigmoid sinus (anterior) + Transverse sinus (superior)
3. Asterion = junction of lambdoid, occipitomastoid, parietomastoid sutures = burr hole landmark
4. Critical step: CSF drainage before cerebellar retraction
5. Hearing preservation rate: 30–65%
6. Post-op headache: 10% — reduced by bone flap replacement
7. Limitation: Cannot directly visualize IAC fundus (Bill's bar inaccessible)
8. MVD (Jannetta's procedure for trigeminal neuralgia): exclusively retrosigmoid approach
9. Bone flap vs. craniectomy: Osteoplastic bone flap reduces post-op headache incidence
10. Fat graft: mandatory to obliterate IAC and mastoid air cells — prevents CSF leak

---

REFERENCES

1. Cummings Otolaryngology Head and Neck Surgery (7th Ed.) — Chapter 173 (Facial Nerve Surgery) & Chapter 179 (Acoustic Neuroma / CPA Surgery), pp. 3314, 3448–3452
2. K.J. Lee's Essential Otolaryngology (11th Ed.) — Skull Base Surgery chapter
3. Scott-Brown's Otorhinolaryngology Head and Neck Surgery (8th Ed.) — CPA approaches
4. Dhingra's Diseases of Ear, Nose and Throat — Surgical approaches to temporal bone
5. Hazarika's Ear Nose Throat — Retrosigmoid (suboccipital) approach
6. Zakir Hussain's ENT — Posterior fossa approaches
7. Stell and Maran's Head and Neck Surgery — Skull base surgery
8. Colletti V, Fiorino F — Antidromic facial nerve monitoring in retrosigmoid approach for acoustic neuroma (Cummings, p. 4313)
9. Arriaga MA, Gorum M — Posterior semicircular canal resection for enhanced retrosigmoid approach with hearing preservation (Cummings, p. 3452)
10. Jannetta PJ — Microvascular decompression via retrosigmoid approach

---