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Scala tympani

The scala tympani is one of the two perilymph-filled chambers of the cochlea (along with the scala vestibuli). It extends from the round window niche at the basal turn of the cochlea and spirals upward toward the helicotrema, where it communicates with the scala vestibuli. Separated from the scala media by the basilar membrane, the scala tympani plays a crucial role in sound transmission, receiving vibratory energy from the stapes footplate—via perilymph movement—and directing it toward the helicotrema for dissipation at the round window.

It contains perilymph fluid, which is similar in composition to cerebrospinal fluid, and houses the cochlear nerve fibers along the modiolus. The scala tympani is a key landmark in cochlear implant surgery, where electrode arrays are placed within it due to its size, accessibility, and surgical safety.

Synonyms

  • Inferior cochlear chamber

  • Tympanic scala

  • Perilymphatic lower duct

Location and Structure

  • Position: Inferior chamber of the cochlea, beneath the basilar membrane.

  • Origin: Begins at the round window (fenestra cochleae).

  • Course: Spirals around the modiolus through all cochlear turns.

  • Termination: Meets the scala vestibuli at the helicotrema at the cochlear apex.

  • Contents: Filled with perilymph, transmits pressure waves through the cochlea.

  • Boundaries:

    • Superior: Basilar membrane

    • Medial: Modiolus

    • Lateral: Bony cochlear wall

Relations

  • Medially: Modiolus containing cochlear nerve fibers

  • Laterally: Osseous spiral lamina and lateral cochlear wall

  • Superiorly: Scala media (cochlear duct) separated by basilar membrane

  • Inferiorly: Round window and tympanic cavity

  • At the apex: Communicates with scala vestibuli via the helicotrema

Function

  • Sound transmission: Propagates perilymphatic pressure waves generated by stapes movement

  • Energy dissipation: Waves culminate at the round window to relieve pressure

  • Cochlear implant pathway: Preferred chamber for electrode insertion

  • Mechanical–neural transduction support: Supports vibrations of basilar membrane that stimulate hair cells in scala media

Clinical Significance

  • Cochlear implantation: Electrode array is positioned in the scala tympani for optimal stimulation

  • Perilymphatic fistula: Leakage into middle ear affects hearing

  • Labyrinthitis or inflammation: Can alter perilymph signal on MRI

  • Fractures of temporal bone: May disrupt scala tympani anatomy or cause perilymphatic leakage

  • Congenital malformations: Abnormal cochlear partitioning may alter scala tympani size or shape

MRI Appearance

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

  • Perilymph: Very bright hyperintense, sharply outlining scala tympani from adjacent structures

  • Basilar membrane: Appears as a thin dark line separating scala tympani from scala media

  • Modiolus: Low-to-intermediate signal

  • Usefulness:

    • Excellent for visualizing cochlear fluid spaces

    • Identifies fibrosis, ossification, or obstruction in post-meningitis cases

    • Detects congenital anomalies of cochlear turns

T1-weighted images (pre-contrast):

  • Perilymph: Low signal relative to surrounding bone and modiolus

  • Modiolus: Intermediate signal

  • Basilar membrane and bony walls: Low signal

  • Abnormalities:

    • Hemorrhage or proteinaceous fluid increases T1 signal

    • Fibrosis or ossification may appear absent as low-signal filling defects

T1 Fat-Saturated Post-Contrast

  • Normal scala tympani: No enhancement (perilymph does not enhance)

  • Abnormal enhancement patterns:

    • Labyrinthitis: Diffuse or patchy enhancement along cochlear walls

CT Temporal Bone Appearance

Non-Contrast Temporal Bone CT:

  • Scala tympani: Appears as a low-density lumen within dense otic capsule bone

  • Cochlear turns: Clearly defined by surrounding hyperdense bone

  • Round window niche: Visible as opening into basal turn

  • Pathology visible:

    • Otosclerosis: Lucent foci around otic capsule (“ring sign”)

    • Cochlear ossification: Partial or complete bony obliteration of scala tympani

    • Fractures: May traverse cochlea, disrupting chamber continuity

    • Anomalies: Incomplete partition defects, enlarged cochlear aqueduct

MRI images

Scala tympani MRI axial image