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Scala media

The scala media, also known as the cochlear duct, is the endolymph-filled middle chamber of the cochlea, positioned between the scala vestibuli and scala tympani. It is a key component of the membranous labyrinth and contains the organ of Corti, the sensory epithelium responsible for converting mechanical sound vibrations into neural signals.

The scala media runs the entire spiral length of the cochlea and is bounded by Reissner’s membrane superiorly and the basilar membrane inferiorly. Its ionic composition (high potassium endolymph) is essential for generating the endocochlear potential, enabling mechanoelectrical transduction.

Synonyms

  • Cochlear duct

  • Middle cochlear chamber

  • Endolymphatic cochlear space

Location and Structure

  • Position: Sandwiched between the scala vestibuli (superior) and scala tympani (inferior).

  • Shape: Triangular cross-section in axial/oblique imaging planes.

  • Boundaries:

    • Reissner’s membrane: Separates scala media from scala vestibuli

    • Basilar membrane: Supports the organ of Corti and separates scala media from scala tympani

    • Stria vascularis: Lateral wall responsible for endolymph production

  • Contents:

    • Endolymph

    • Organ of Corti

    • Tectorial membrane

    • Spiral limbus and spiral ligament

Relations

  • Superiorly: Scala vestibuli

  • Inferiorly: Scala tympani

  • Laterally: Stria vascularis and spiral ligament

  • Medially: Rosenthal’s canal (spiral ganglion cells) and modiolus

  • Apically: Helicotrema region where scalae converge (scala media stops just before this point)

Function

  • Hearing: Houses the organ of Corti for detection of sound vibrations

  • Electrochemical role: Maintains high potassium concentration for transduction current

  • Mechanical tuning: Basilar membrane properties vary along length to encode frequency

  • Signal initiation: Converts mechanical displacement of hair cells into auditory nerve signals

Clinical Significance

  • Endolymphatic hydrops (Ménière’s disease): Enlargement of scala media

  • Sensorineural hearing loss: Damage to organ of Corti, stria vascularis, or membranes

  • Cochlear inflammation: Viral or autoimmune labyrinthitis affects the scala media

  • Congenital anomalies: Cochlear malformations may distort or obliterate the scala media

  • Imaging importance: Visualized in high-resolution inner ear MRI and CT for pathology and surgical planning

T2-weighted Images (High-resolution / 3D T2 such as SPACE / CISS)

  • Fluid-filled scala media:

    • Hyperintense (bright) due to endolymph exhibiting long T2 relaxation

    • Slightly less bright than cerebrospinal fluid depending on sequence

  • Differentiation from other scalae:

    • Scala vestibuli and tympani also bright, but boundaries defined by thin low-signal membranes (Reissner/basilar membranes)

  • Pathology:

    • Endolymphatic hydrops:

      • Scala media appears expanded, pushing Reissner’s membrane toward scala vestibuli

    • Fibrosis or ossification: Low T2 signal replacing fluid space

    • Labyrinthitis: Mild T2 blurring of fluid signal or debris

3D T2 (CISS / FIESTA / DRIVE)

  • Extremely high spatial resolution allows visualization of all three cochlear scalae

  • Scala media:

    • Appears as a distinct triangular bright chamber

    • Reissner’s membrane may be visible as a thin hypointense line

  • Pathology:

    • Hydrops produces visible bulging of scala media

    • Subtle partition anomalies (e.g., incomplete partition type II) clearly delineated

T1-weighted Images (Pre-contrast)

  • Normal scala media:

    • Very low signal (dark)

  • Context:

    • Surrounding bone: very low signal

    • Modiolus: intermediate-to-low signal

    • Spiral ganglion region: slightly higher signal

T1 Fat-Saturated Post-Contrast

  • Normal scala media:

    • No enhancement (endolymph does not enhance)

CT Temporal Bone Appearance

Non-Contrast CT

  • Scala media itself:

    • Not directly visible (soft tissue fluid density similar to other labyrinthine fluids)

  • Identifiable via bony boundaries:

    • Cochlear turns clearly seen

    • Modiolus and osseous spiral lamina well visualized

    • Partitioning membranes not visible

MRI images

Scala media MRI axial image