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Surgical Anatomy of the Inner Ear - Viva Questions & Answers for ENT Postgraduates

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SECTION 1: OVERVIEW AND BONY LABYRINTH

• Cochlea - the organ of hearing
• Vestibular apparatus - the organ of balance (semicircular canals, utricle, saccule)

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1. Cochlea (anterior) - for hearing
2. Vestibule (middle) - connects cochlea and semicircular canals
3. Three semicircular canals (posterior) - for angular acceleration

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• Cochlear duct (scala media)
• Saccule (spherical recess of vestibule)
• Utricle (elliptical recess of vestibule)
• Three semicircular ducts (within the bony canals)
• Endolymphatic duct and sac

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SECTION 2: COCHLEA

• Snail-shaped osseous structure, coiled 2.5 turns (2 and 2/3 turns by some descriptions) around a central bony axis called the modiolus
• Modiolus contains: spiral ganglion neurons, cochlear artery, and cochlear vein
• Base of cochlea faces the internal auditory canal (IAC) posteriorly
• Basal turn of cochlea bulges into the medial wall of the middle ear as the promontory

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1. Scala vestibuli (superior) - perilymph; connects to the oval window (stapes footplate)
2. Scala media / cochlear duct (middle) - endolymph; triangular cross-section
3. Scala tympani (inferior) - perilymph; ends at the round window

• Roof - Reissner's membrane (separates from scala vestibuli)
• Floor - Basilar membrane (separates from scala tympani) + organ of Corti
• Lateral wall - Stria vascularis (on the spiral ligament)

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• Oval window: Located in the medial wall of the middle ear; occupied by the stapes footplate attached via the annular ligament. Transmits vibrations to the scala vestibuli (perilymph). Surgical risk: perilymph fistula, labyrinthitis.
• Round window: Located inferior and posterior to the oval window, covered by the round window membrane (secondary tympanic membrane - two epithelial layers around connective tissue). Acts as a pressure-release valve; connects scala tympani to the middle ear. Surgical importance: site of intratympanic drug delivery and cochleostomy in cochlear implantation.

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• Base (narrow, stiff) - responds to high-frequency sounds (20,000 Hz)
• Apex (wide, floppy) - responds to low-frequency sounds (20 Hz)

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SECTION 3: ORGAN OF CORTI

• Inner hair cells (IHC): ~3,500; single row; flask-shaped; primary afferent transducers (receive 95% of cochlear nerve afferents)
• Outer hair cells (OHC): ~12,000-15,000; three rows; cylindrical; electromotile (cochlear amplifier via prestin protein); receive olivocochlear efferent innervation

• Inner and outer pillar cells - frame the tunnel of Corti
• Deiters cells - support each OHC
• Hensen cells, Claudius cells (laterally)
• Inner border cells, inner phalangeal cells (medially)

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• Generates the endocochlear potential (+85 mV)
• Secretes endolymph (maintains high K+ concentration)
• Is the most metabolically active epithelium in the cochlea, requiring rich blood supply

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SECTION 4: VESTIBULAR SYSTEM

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• Located in the ampulla of each semicircular canal
• Contains sensory hair cells (Type I and Type II) embedded in the cupula - a gelatinous membrane spanning the full lumen of the ampulla
• Detects angular (rotational) acceleration of the head
• The cupula has the same density as endolymph; inertia of endolymph deflects the cupula during head rotation
• Deflection toward the utricle (ampullopetal) = excitation (in horizontal canal); away from utricle (ampullofugal) = inhibition

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• Located in the elliptical recess of the vestibule
• Macula oriented in the horizontal plane (roughly same as horizontal semicircular canal)
• Detects linear acceleration in the horizontal plane and head tilt
• Striola (dividing line) forms a C-shape with the open side pointing medially

• Located in the spherical recess of the vestibule
• Macula oriented in a vertical plane
• Detects vertical linear acceleration and gravitational forces
• Striola hooks superiorly in its anterior portion

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SECTION 5: INTERNAL AUDITORY CANAL (IAC)

• Lies within the petrous bone, running laterally from the porus acousticus (opening at posterior cranial fossa) to the fundus (lateral end)
• Length: ~1 cm; diameter: ~4-5 mm
• Divided at the fundus by two bony crests:
  • Transverse (falciform) crest - horizontal divider; separates superior and inferior compartments
  • Bill's bar (vertical crest) - vertical divider in the superior compartment

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• Most vestibular schwannomas arise at the Obersteiner-Redlich zone (transition between Schwann cell myelin and central oligodendrocyte myelin) within the IAC
• Surgical approaches: translabyrinthine (no hearing preservation), retrosigmoid/suboccipital, middle cranial fossa (for intracanalicular tumors with hearing preservation)
• At the fundus, the cochlear nerve is inferior and anterior; during hearing-preservation surgery, avoid lateral-to-medial traction on the nerve as its fibers are delicate
• At the porus, nerves rotate 90 degrees: the cochlear nerve is inferior to the vestibular trunk

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SECTION 6: BLOOD SUPPLY

• Labyrinthine artery (internal auditory artery) - branch of the anterior inferior cerebellar artery (AICA) (85%) or directly from the basilar artery (15%)
• The labyrinthine artery divides into:
  • Cochlear artery (common cochlear artery → proper cochlear artery + cochleoverstibular artery)
  • Anterior vestibular artery (supplies utricle, ampullae of anterior and lateral SCCs)

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SECTION 7: APPLIED SURGICAL ANATOMY

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• Bony channel connecting scala tympani (perilymph) with the subarachnoid space
• Located at the basal turn of the cochlea, opens near the jugular foramen
• Clinically: CSF gusher during cochlear implant surgery can occur if the cochlear aqueduct is patent or if there is an enlarged vestibular aqueduct with labyrinthine-CSF communication
• Vestibular aqueduct: separate channel containing the endolymphatic duct; enlarged vestibular aqueduct (EVA) is a common cause of childhood SNHL - diagnosed when the midpoint width >1.5 mm or opercular width >2 mm on CT

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• The endolymphatic duct runs within the vestibular aqueduct (bony channel in the petrous bone)
• Connects the membranous labyrinth (utricle + saccule via utriculoendolymphatic valve) to the endolymphatic sac
• The endolymphatic sac lies in a dural fold on the posterior surface of the petrous bone, just posterior to the posterior semicircular canal
• Functions: endolymph homeostasis, immune defense, pressure regulation
• Meniere's disease: endolymphatic hydrops (distension of the membranous labyrinth) - likely due to impaired reabsorption of endolymph; surgical treatment = endolymphatic sac decompression/shunt

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• Type I: Flask-shaped; enclosed in a calyx nerve terminal; fast response
• Type II: Cylindrical; bouton-type synaptic endings; slower response

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SECTION 8: HIGH-YIELD QUICK FACTS FOR VIVA

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Additional Viva Questions & Answers - Surgical Anatomy of the Inner Ear

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SECTION 9: FACIAL NERVE IN RELATION TO THE INNER EAR

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• The lateral SCC is the universal mastoid landmark because it is the most accessible and reliably identified structure during mastoidectomy
• The facial nerve runs inferior and medial to the ampullated (anterior) end of the lateral SCC at the second genu
• Drilling posterior and inferior to the lateral SCC leads to the facial recess (posterior tympanotomy)
• Identifying the lateral SCC allows safe identification of the facial nerve before opening the facial recess
• In cholesteatoma surgery, the canal wall can erode the overlying bone, exposing the nerve

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• A small cleft of fibrocartilage in the anterior wall of the oval window, located between the cochlea and the oval window
• It is the most common site of origin of otosclerotic foci - the initial osteoclastic resorption begins here and spreads to involve the anterior stapes footplate
• As disease progresses it spreads across the annular ligament causing stapedial fixation
• If it extends medially into the cochlear endosteum, sensorineural hearing loss results from hyalinization of the spiral ligament
• Surgical implication: During stapedotomy, the anterior footplate area is opened last; if obliterative otosclerosis fills the oval window, special "biscuit footplate" techniques are required

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SECTION 10: SURGICAL APPROACHES TO THE INNER EAR

1. Extended cortical mastoidectomy
2. Bony labyrinthectomy (removing lateral, posterior, and superior SCCs + vestibule)
3. Skeletonization of jugular bulb and vertical facial nerve
4. Skeletonization of the IAM (internal auditory meatus)
5. Identification of facial nerve at lateral IAC (using Bill's bar as guide)
6. Opening posterior fossa dura
7. Tumour removal
8. Closure with abdominal fat obliteration

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• Medially: Facial nerve (posterior wall of fallopian canal)
• Laterally: Chorda tympani
• Superiorly: Fossa incudis (short process of incus)
• Inferiorly: Annular ligament / tympanic ring

1. Round window membrane (RWM) cochleostomy - directly through the RWM; now preferred for hearing preservation; electrode directed anteriorly and inferiorly toward modiolus
2. Extended RW cochleostomy - slightly anteroinferior to RWM
3. Bony cochleostomy - anterior and inferior to RWM (older technique); removes bone over the scala tympani at the basal turn

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• Scarpa's ganglion (vestibular ganglion) contains the bipolar cell bodies of the vestibular nerve
• Located within the IAC, in the midportion of the canal
• Divided into superior and inferior divisions:
  • Superior vestibular ganglion - innervates the utricle, ampullae of anterior and lateral SCCs, and superior saccule
  • Inferior vestibular ganglion - innervates the posterior SCC ampulla and inferior saccule
• Analogous structure for cochlea: spiral ganglion (in the modiolus)
• During vestibular neurectomy for Meniere's disease, the vestibular nerve is sectioned in the IAC, with care to spare the facial and cochlear nerves

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SECTION 11: OTOSCLEROSIS SURGICAL ANATOMY

1. Elevation of tympanomeatal flap - at 6-12 o'clock position; reveals the posterior middle ear
2. Visualization of ossicles - incudostapedial joint, stapes, oval window
3. Testing stapes mobility - confirms fixation (Gellerstedt test / palpation)
4. Identification of oval window - the stapes footplate sits within this oval aperture in the medial wall; the facial nerve passes above it in the tympanic segment; the round window is inferoposterior
5. Division of incudostapedial joint - with joint knife
6. Stapedius tendon section - posterosuperiorly at the pyramidal eminence
7. Crural fracture - posterior crus then anterior crus fractured
8. Stapedotomy fenestra - small hole (0.5-0.8 mm) made in the center of the footplate by microdrill, CO2 laser, or KTP laser
9. Prosthesis placement - wire-piston (typically 4-4.5 mm long, 0.4-0.6 mm diameter) placed through fenestra, crimped around long process of incus

• Facial nerve above (can be dehiscent over oval window in up to 5% of cases)
• Saccule lies immediately below the footplate - direct trauma or perilymph suction risks SNHL
• Chorda tympani anteromedially

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• Do NOT attempt to retrieve the footplate - attempting to recover it causes further perilymph loss and trauma to the saccule
• Place the prosthesis over the floating footplate (it will transmit vibrations)
• Some surgeons advocate leaving it in situ - it may reattach or be well-tolerated

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SECTION 12: BPPV - APPLIED ANATOMY

• Otoconia (calcium carbonate crystals) from the utricular macula become dislodged (due to trauma, degeneration, or Meniere's disease)
• They fall into the posterior semicircular canal (most dependent position when lying down) - "canalithiasis"
• When the head moves, the displaced otoconia move in the canal, creating abnormal endolymph flow and deflecting the cupula - causing the nystagmus and vertigo of BPPV
• Posterior canal BPPV is most common (due to its anatomical position being most inferior when supine)

• Uses gravity to move the debris through the posterior canal → through the common crus → into the utricle, where it disperses
• Each position change moves the debris one step further along the canal
• CRM is effective in ~80% of posterior canal BPPV

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SECTION 13: VASCULAR ANATOMY - EXTENDED

• Cochlear aqueduct (perilymphatic duct): a bony channel connecting the scala tympani at the basal turn of the cochlea to the subarachnoid space near the jugular foramen
• It allows equilibration between perilymph and CSF
• In neonates and children, it is wide and patent; it narrows and may close with age

• CSF gusher during cochlear implantation or stapedotomy: occurs when the cochlear aqueduct is unusually patent or when there is an abnormal communication (e.g., dilated IAC, X-linked deafness with stapes gusher = POU3F4 gene mutation, Michel aplasia)
• Management of gusher: pack the cochleostomy with muscle/fascia, tilt the table, avoid CSF loss
• On imaging, a widened cochlear aqueduct or absent modiolus (IP-III deformity) predicts a gusher risk

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• Crista of the anterior (superior) SCC
• Crista of the lateral (horizontal) SCC
• Macula of the utricle
• Part of the saccule (superior)

• Crista of the posterior SCC (via singular nerve)
• Macula of the saccule (inferior portion)
• Also carries olivocochlear efferent fibers (via vestibulo-cochlear anastomosis of Oort) to the cochlea

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SECTION 14: EMBRYOLOGY AND CONGENITAL INNER EAR

• Inner ear develops from the otic placode (surface ectoderm, week 3-4) → invaginates to form the otic vesicle (otocyst) by week 4
• The otocyst differentiates into:
  • Dorsal part → endolymphatic duct + sac, utricle, semicircular canals
  • Ventral part → saccule + cochlear duct (cochlea achieves 2.5 turns by week 25)
• The otic capsule (bony labyrinth) forms by endochondral ossification from the cartilaginous otic capsule - it is the only bone in the body that does NOT remodel after birth (no Haversian system in adults) - this is why otosclerosis (disordered remodeling) is pathological
• The membranous labyrinth is complete by week 25 gestation

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SECTION 15: HIGH-YIELD APPLIED FACTS

• Perilymph aspiration during stapedotomy or cochleostomy
• Direct trauma to the saccule (immediately below the footplate)
• Laser thermal injury to perilymph and hair cells
• Floating footplate with saccular damage
• Vasospasm or occlusion of the labyrinthine artery during manipulation
• Infection / labyrinthitis post-operatively

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• The promontory is the rounded bony elevation on the medial wall of the middle ear formed by the basal turn of the cochlea
• On its surface runs the tympanic plexus (Jacobson's nerve, CN IX branch + sympathetic fibers) providing sensation to the middle ear and contributing to the lesser petrosal nerve (parotid secretomotor)
• Surgical relevance:
  • Cochleostomy for cochlear implantation is made through or just anteroinferior to the round window niche, which is located just inferoposterior to the promontory
  • Intratympanic injections (steroids, gentamicin) are administered through the tympanic membrane, with the drug pooling at the round window niche on the promontory surface
  • Promontory stimulation test - used to predict cochlear implant benefit

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What Was Already Covered - Full Index

From the Previous Comprehensive Response:

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Additional Points Not Yet Covered - Quick Additions

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• Macula cribrosa superior - for superior vestibular nerve (utricle + anterior/lateral SCC)
• Macula cribrosa media - for saccular nerve (inferior vestibular nerve)
• Macula cribrosa inferior - for singular nerve (posterior ampullary nerve)

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• Located on the posterior surface of the petrous bone, within a dural fold between two layers of posterior fossa dura
• Lies posteroinferior to the posterior SCC
• Has a proximal (intraosseous) portion and a distal (extraosseous/rugose) portion
• The rugose portion (distal sac) is the immunocompetent region - contains the only immunologically active cells in the labyrinth (KJL)
• Functions: endolymph reabsorption, immune surveillance, pressure regulation
• The endolymphatic duct connects it back to the utricle and saccule via the vestibular aqueduct
• Surgical target: endolymphatic sac decompression (Shambaugh/Portmann procedure) for Meniere's disease - the posterior fossa dura is exposed posterior to the labyrinth and the sac is decompressed or shunted

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• Late sequela of suppurative labyrinthitis (post-meningitis, especially Streptococcus pneumoniae), autoimmune disease, or trauma
• New bone forms within the scala tympani (most common), then scala vestibuli
• The basal turn of the scala tympani is affected first
• CT shows increased density in the cochlea
• MRI: loss of normal fluid (T2) signal in cochlea
• Surgical importance: cochlear implantation becomes extremely difficult - the electrode cannot be fully inserted; requires a double-array electrode, drill-out procedure, or combined approach

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• Abnormal, profuse CSF flow from the cochleostomy during CI surgery
• Occurs when there is an abnormally wide communication between the IAC and cochlea (absent or deficient modiolus/cribriform plate)
• Associated conditions: IP-III (Michel), IP-I (Mondini with common cavity), DFN-3 (X-linked deafness, POU3F4 mutation)
• On CT: absent or markedly deficient modiolus, widened fundus of IAC
• Management: pack cochleostomy with muscle/fascia, tilt head up (Trendelenburg reverse), use lumbar drain if persistent

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• Vertigo and/or oscillopsia induced by loud sounds or pressure changes (Valsalva, nose-blowing)
• Basis: a pathological third window (most classically superior canal dehiscence) transmits sound energy directly to the labyrinthine fluid, deflecting the SCC cupula
• Named after Pietro Tullio who demonstrated it experimentally in pigeons
• Also seen in: EVA, perilymph fistula, Meniere's disease (rarely)
• Hennebert's sign = vertigo on pneumatic otoscopy (positive fistula test without a fistula - seen in Meniere's)

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THE FACIAL NERVE (CN VII) - Comprehensive Postgraduate Reference

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1. FIBER COMPOSITION

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2. NUCLEAR ORIGINS IN THE BRAINSTEM

• Motor nucleus (pons) → SVE fibers → muscles of facial expression etc.
  • The facial nerve fibers loop posteriorly around the abducens (VI) nucleus, forming the internal genu - creating a bulge in the floor of the 4th ventricle (facial colliculus)
  • Key fact: A lesion here will affect both CN VI and CN VII simultaneously (Möbius syndrome = agenesis of both nuclei) (SB block 3)
  • Upper face receives bilateral cortical innervation (via direct + indirect pyramidal fibers) → upper motor neuron lesion spares forehead; lower motor neuron lesion affects the whole hemiface (KJL)
• Superior salivary nucleus (pons) → GVE (parasympathetic preganglionic)
• Nucleus solitarius (medulla) → receives SSA taste fibers (via nervus intermedius)

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3. COURSE OF THE FACIAL NERVE - SIX SEGMENTS

Segment 1: Intracranial (Cisternal)

• Length: 17-24 mm
• Exits pontomedullary junction ~1.5 mm anterior to CN VIII
• CN VII diameter: ~1.8 mm (smaller than CN VIII at ~3 mm)
• Travels rostrally and laterally through the cerebellopontine cistern for 15-17 mm
• Key relations: AICA loops near or between CN VII and VIII; veins of the middle cerebellar peduncle
• Enters the porus acousticus (internal auditory meatus)

Segment 2: Meatal (Intracanalicular)

• Length: 8-10 mm
• Occupies the anterosuperior quadrant of the IAC throughout its length
• Separated from cochlear nerve (anteroinferior) by the transverse (falciform) crest
• Separated from superior vestibular nerve (posterosuperior) by Bill's bar (vertical crest)
• No fibrous sheath or endoneurium in the IAC - only surrounded by pia and arachnoid; the dural investment ends at the fundus
• The nervus intermedius lies between CN VII and CN VIII in the CPA cistern

Segment 3: Labyrinthine

• Length: 4 mm (shortest intratemporal segment)
• Diameter: 0.61-0.68 mm (narrowest point - the meatal foramen/fallopian canal entrance)
• Runs from the meatal foramen (fundus of IAC) to the geniculate ganglion
• Travels anteriorly, superiorly, and laterally - forms a 120-degree angle with the IAC
• The basal turn of the cochlea lies anteroinferior to this segment
• At its lateral end: the geniculate ganglion - nerve makes an acute posterior turn of ~75 degrees (first genu)
• No fibrous epineurial sheath here (acquired at and distal to the geniculate ganglion)
• Geniculate ganglion separated from middle fossa by only a thin layer of bone - dehiscent in ~25% of ears (CUM) or ~35-55% (SB) - thus a common site of nerve injury in middle fossa fractures
• Most vulnerable to ischemia - poorest intrinsic vascular network
• Surgical importance: This is the primary decompression target in Bell's palsy surgery (middle cranial fossa approach)

Segment 4: Tympanic (Horizontal)

• Length: 11 mm (KJL) or 13 mm (SHA)
• Runs from geniculate ganglion (first genu) posteriorly to the second genu
• Course: skims over the cochleariform process → forms the superior wall of the oval window niche (fossa ovalis) → runs to the pyramidal eminence
• Relations: lateral SCC above, stapes/oval window below, occupies the medial wall of the anterior attic
• Dehiscent over the oval window in approximately 35-55% of cases (SB) / 66% of all dehiscences are at this site (SHA)
• Bilateral in ~75% of cases
• The subarachnoid space around the nerve usually ends at the junction of the labyrinthine and tympanic segments - rarely extends onto the tympanic segment (if it does, may fistulize into middle ear → CSF otorrhea)

Segment 5: Mastoid (Vertical)

• Length: 13 mm (KJL) or 20 mm (SHA) - longest intratemporal segment
• Runs from the second genu (at the lateral SCC, pyramidal eminence level) to the stylomastoid foramen
• The second genu is located just anteroinferior to the lateral SCC and anterior to a line through the short process of the incus
• A superior extrapolation of the vertical segment would approximately bisect the prominence of the lateral SCC - the key surgical landmark
• Space between mastoid FN and chorda tympani = facial recess (posterior tympanotomy space)
• Greater variability in the mastoid segment's path than the tympanic segment
• Exits at the stylomastoid foramen - surrounded by the aponeurosis of the posterior belly of digastric (important: preserving this during rerouting helps maintain blood supply)

Segment 6: Extratemporal

• Enters parotid gland; divides into two main divisions at the pes anserinus (goose's foot)
• Five terminal branches:

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4. BRANCHES GIVEN OFF DURING THE INTRATEMPORAL COURSE

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5. SURGICAL LANDMARKS FOR THE FACIAL NERVE

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6. BLOOD SUPPLY

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7. HOUSE-BRACKMANN (HB) GRADING SYSTEM

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8. BELL'S PALSY - SUMMARY

• Most common cause of acute unilateral facial palsy (LMN type)
• Pathology: viral (HSV-1 reactivation) edema and compression at the labyrinthine segment (narrow bony canal, no epineurium, watershed vascular zone)
• Prognosis: 80-90% complete recovery; 95-100% recover if paralysis remains incomplete
• Poor prognostic factors: complete palsy, age >60, diabetes, hypertension, hyperacusis, decreased tearing, severe pain, >90% ENoG degeneration

• Steroids: prednisolone 1 mg/kg/day - most benefit if started within 48 hours; significantly improves recovery rates (meta-analysis: 17% better than placebo)
• Antivirals + steroids: combination therapy - moderate evidence for reducing synkinesis and excessive tearing in severe Bell's palsy (Cochrane review)
• Surgical decompression (middle cranial fossa approach): indicated if >90% degeneration on ENoG AND no voluntary motor unit potentials within 2 weeks of onset - recovery to HB Grade I/II = 92% vs 42% with steroids alone (Gantz et al., CUM)

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9. NEONATAL/PEDIATRIC DIFFERENCES

• At birth: no mastoid process (forms by age 12), incomplete tympanic ring with foramen of Huschke
• The stylomastoid foramen in the neonate is very superficial; the facial nerve exits more anteriorly (parotid is smaller and more anterior)
• The second genu is more acute and courses more laterally in neonates
• The lower division of the facial nerve in young children runs superficially over the angle of the mandible - at risk from skin incisions
• Dehiscence of the fallopian canal in the tympanic segment: 35-55% of the population
• ASOM in neonates/children can cause facial palsy through dehiscent tympanic segment
• Malformations of the 1st/2nd branchial arches (Treacher Collins, Goldenhar) almost always involve an abnormal facial nerve

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10. FACIAL NERVE ANOMALIES

• Anteriorly displaced tympanic segment running anterior and inferior to the oval window - creates extreme surgical hazard during stapes surgery
• Bipartite or tripartite mastoid segment (Proctor and Nager)
• Posterior hump of the mastoid segment (bulges posterolaterally)
• Persistent stapedial artery: runs through the tympanic cavity from the promontory to middle cranial fossa (becoming middle meningeal artery); the foramen spinosum is absent on that side on CT; dehiscent tympanic FN segment is associated
• Dehiscent nerve prolapsing through the oval window - presents as a middle ear mass (~75% bilateral when it occurs)

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11. SURGICAL APPROACHES TO THE FACIAL NERVE

• The most important element is a tension-free anastomosis
• Use interposition graft (great auricular nerve or sural nerve) when tension-free end-to-end is not possible
• For tumors: observe if HB Grade I or II; decompress if HB Grade II/III; resect + graft if HB Grade IV or worse

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12. EMBRYOLOGY OF THE FACIAL NERVE

• 3rd week: Facioacoustic crest visible on dorsolateral hindbrain near otic placode
• 4th week: Facial nerve becomes distinct as otic placode forms otocyst (membranous labyrinth)
• 5th week: Geniculate ganglion formed; facial nerve divides into main trunk (to 2nd branchial arch = muscles of facial expression) and chorda tympani (pretrematic branch = curves into 1st branchial arch)
• 7th week: Geniculate ganglion well-defined; gives rise to sensory roots forming nervus intermedius
• The chorda tympani and the main trunk are equal in size at this stage (5th week)
• As the pons expands, the abducens nucleus ascends and facial fibers loop around it forming the internal genu → explains why CN VI and CN VII palsies coexist in pontine lesions and Möbius syndrome
• The GSPN acts as an anchor during caudal migration, producing the horizontal segment and first/second genu of the vertical nerve

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13. KEY NUMERICAL FACTS - QUICK VIVA REFERENCE

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