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Interscalar septum

The interscalar septum is the thin bony partition separating the individual cochlear turns (scalae) of the cochlea within the petrous temporal bone. It lies between adjacent cochlear turns and contributes to the spiraling bony architecture that houses the scala vestibuli, scala tympani, and cochlear duct.

This septum is an essential structural element that supports the integrity of the cochlear modiolus and helps maintain the distinct separation of fluid-filled compartments required for normal hearing transduction. Its integrity is crucial, as defects or thinning may result from congenital malformations, infection, otosclerosis, trauma, or inner-ear dysplasia.

Synonyms

  • Interscalar cochlear septum

  • Cochlear turn septum

  • Spiral septum (informal)

Location and Structure

  • Located within the bony cochlea of the petrous temporal bone.

  • Forms the bony partitions between successive turns of the cochlea.

  • Contributes to the separation of the scala vestibuli and scala tympani between turns.

  • Continuous with the modiolus, the central bony core of the cochlea.

  • Appears as thin, curved osseous plates between the basal, middle, and apical turns.

Relations

  • Medially: Modiolus (central cochlear axis)

  • Laterally: Cochlear outer wall (otic capsule)

  • Superiorly & Inferiorly: Adjacent cochlear turns and scala partitions

  • Anteriorly: Basal turn and round window niche area

  • Posteriorly: Junction toward apical cochlear turns and vestibule

Function

  • Divides cochlear turns into discrete bony compartments

  • Supports cochlear architecture and spiral structure

  • Helps maintain separation of perilymphatic spaces (scala vestibuli and tympani)

  • Plays a role in maintaining inner-ear hydrodynamics and frequency mapping

  • Provides physical stability for cochlear partitions and membranes

Clinical Significance

  • Inner-ear malformations: Absence or hypoplasia of the interscalar septum is a hallmark of cochlear dysplasia (e.g., Mondini deformity).

  • Infection: Labyrinthitis ossificans may cause septal thickening or obliteration.

  • Otosclerosis: May involve the otic capsule and distort septal contours.

  • Trauma: Temporal bone fractures may disrupt cochlear septal structures.

  • Implantation relevance: Important in cochlear implant surgical planning, ensuring correct electrode insertion path and identifying malformed turns.

MRI Appearance

T1-weighted images (pre-contrast):

  • Bone: Very low signal (black), representing dense otic capsule

  • Interscalar septum: Appears as thin, sharply marginated dark lines separating cochlear turns

  • Perilymph and endolymph: Intermediate signal

  • Pathology:

    • Fibrosis or ossification may cause septal thickening (low signal)

    • Fluid loss or inflammation alters adjacent cochlear signal intensity

T1-weighted Fat-Saturated Post-Contrast:

  • Normal septum: No enhancement (bone has no vascularity)

  • Surrounding soft tissues: May show contrast around cochlear modiolus or vestibule

  • Abnormal enhancement:

    • Seen in labyrinthitis, neoplastic infiltration, or postoperative inflammation

    • Pericochlear enhancement may indicate otosclerosis (active fenestral phase)

3D T2-weighted sequences (e.g., 3D T2 SPACE, FIESTA, CISS):

  • Fluid (endolymph, perilymph): Very bright hyperintense

  • Interscalar septum: Seen as thin hypointense (dark) bony plates sharply dividing cochlear turns

  • Modiolus: Hypointense central pillar

  • Utility:

    • Best sequence for visualizing septal defects, cochlear dysplasia, or incomplete partition anomalies

    • Identifies post-infectious ossification, cochlear fibrosis, or congenital absence

CT Temporal Bone Appearance

Non-Contrast CT (High-Resolution Temporal Bone CT):

  • Interscalar septum: Appears as thin, dense osseous partitions between basal, middle, and apical cochlear turns

  • Otic capsule: Dense, uniform bone

  • Modiolus: Seen as central bony column

  • Pathologic features:

    • Dysplasia (e.g., Mondini): Partial or absent septa; fewer than 2.5 cochlear turns

    • Labyrinthitis ossificans: Irregular ossification filling scalae and thickening the septa

    • Otosclerosis: Lucent pericochlear bone (“double ring” sign)

    • Temporal bone fracture: Disruption or discontinuity of septal margins

    • Cochlear implant planning: Clear visualization of septa and turns, crucial for evaluating insertion trajectory

MRI image

Interscalar septum mri axial image

MRI image

Interscalar septum mri axial