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Cochlear duct

The cochlear duct (scala media) is the central membranous channel of the cochlea, situated between the scala vestibuli and scala tympani within the bony cochlear spiral. It is filled with endolymph, a potassium-rich fluid essential for transducing mechanical sound vibrations into neural signals. The cochlear duct contains the Organ of Corti, the primary sensory apparatus for hearing.

Formed from the membranous labyrinth, the duct follows the spiral shape of the cochlea for approximately 2.5 turns around the modiolus. Its architecture—bounded by Reissner’s membrane, the basilar membrane, and the stria vascularis—creates the electrical and mechanical gradients crucial for auditory transduction.

It communicates indirectly with the vestibular system and plays a central role in maintaining ionic homeostasis, sound frequency discrimination, and auditory neural transmission.

Synonyms

  • Scala media

  • Membranous cochlear duct

  • Cochlear canal

Location and Structure

  • Occupies the middle compartment of the cochlea, lying between scala vestibuli (superiorly) and scala tympani (inferiorly).

  • Filled with endolymph, unlike the surrounding perilymph-filled compartments.

  • Boundaries:

    • Superior: Reissner’s membrane

    • Inferior: Basilar membrane

    • Lateral: Stria vascularis and spiral ligament

  • Houses the Organ of Corti, containing inner and outer hair cells and tectorial membrane.

  • Extends from the vestibule to the helicotrema at apex where it ends blindly.

Relations

  • Medially: Modiolus, cochlear nerve fibers

  • Laterally: Spiral ligament and stria vascularis

  • Superiorly: Scala vestibuli

  • Inferiorly: Scala tympani

  • Anteriorly: Round window niche region

  • Posteriorly: Vestibular system (utricle and saccule region)

Attachments

  • Reissner’s membrane attaches between the osseous spiral lamina and the lateral wall.

  • Basilar membrane attaches from the osseous spiral lamina to spiral ligament.

  • Stria vascularis attached to lateral cochlear wall produces endolymph.

  • Hair cells anchored via supporting cells to basilar membrane.

Function

  • Auditory transduction: Houses the Organ of Corti, converting mechanical sound waves into neural impulses.

  • Frequency discrimination: Basilar membrane tonotopy encodes specific sound frequencies along its length.

  • Endolymph regulation: Maintains unique ionic composition critical for hair cell depolarization.

  • Mechanical filtering: Provides structural boundaries controlling vibration patterns during sound processing.

Clinical Significance

  • Endolymphatic hydrops: Enlargement of cochlear duct seen in Ménière’s disease.

  • Congenital malformations: Hypoplastic or absent cochlear duct in inner ear developmental anomalies.

  • Sudden sensorineural hearing loss: Can relate to labyrinthine inflammation or vascular compromise.

  • Labyrinthitis: Fluid signal abnormalities in membranous labyrinth.

  • Cochlear implant planning: Cochlear duct morphology crucial for electrode insertion pathways.

MRI Appearance

T2-weighted images (including 3D T2 sequences such as T2 DRIVE / 3D FIESTA / 3D CISS / SPACE):

  • Endolymph fluid: Appears bright hyperintense, well-delineating the cochlear duct.

  • Scala vestibuli & tympani: Also bright but separated from cochlear duct by membranes (not directly visualized).

  • Modiolus: Dark central structure.

  • Organ of Corti region: Too small for direct resolution but may appear as a very thin low-signal line.

  • Pathology:

    • Endolymphatic hydrops: Cochlear duct appears enlarged, encroaching on scala vestibuli/tympani.

    • Labyrinthitis: Hyperintense T2 signal within duct and adjacent membranous labyrinth.

    • Obliterative disease: Loss of internal fluid signal, replaced by low-signal fibrosis or ossification.

T1-weighted images (Pre-Contrast):

  • Endolymph: Low signal (dark).

  • Cochlear bone: Very low signal cortex.

  • Modiolus: Intermediate signal.

  • Pathology:

    • Hemorrhage or proteinaceous fluid: Increased T1 brightness in cochlear duct.

    • Fibrosis or early ossification: Focal low-signal filling defects.

T1 Fat-Saturated Post-Contrast:

  • Normal cochlear duct: No enhancement of endolymph or perilymph.

  • Enhancement seen in:

    • Labyrinthitis: Diffuse enhancement of membranous labyrinth walls.

    • Neoplastic infiltration: Abnormal enhancement along cochlear turns (e.g., leukemia, lymphoma).

    • Post-operative changes: Enhancement after cochlear implant or labyrinthine surgery.

    • Hemorrhagic or inflammatory processes: Patchy or linear enhancement of cochlear wall.

CT Appearance

Non-Contrast Temporal Bone CT:

  • Cochlear duct itself: Not directly visualized (soft tissue density within bony cochlea).

  • Cochlea: Appears as a well-formed 2.5-turn spiral osseous capsule.

  • Modiolus & osseous spiral lamina: High-density bony structures clearly outlined.

  • Pathology detected on CT:

    • Otosclerosis: Lucent focus near oval window or cochlear endosteum.

    • Labyrinthitis ossificans: Partial or complete bony obliteration of cochlear duct.

    • Congenital anomalies: Incomplete partition defects (Mondini, Michel aplasia).

    • Cochlear fractures: Disruption of osseous spiral contour.

    • Cochlear implant planning: Assessment of lumen patency, modiolar integrity, and anatomy.

MRI images

Cochlear duct