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

SECTION 1: OVERVIEW AND BONY LABYRINTH

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

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

• 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

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

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)

• 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.

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

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)

• 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

SECTION 4: VESTIBULAR SYSTEM

• 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

• 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

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

• 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

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)

SECTION 7: APPLIED SURGICAL ANATOMY

• 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

• 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

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

SECTION 8: HIGH-YIELD QUICK FACTS FOR VIVA

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

SECTION 9: FACIAL NERVE IN RELATION TO THE INNER EAR

• 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

• 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

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

• 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

• 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

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

• 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

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

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

• 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

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

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

• 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