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Saccule

The saccule is one of the two primary otolithic organs of the vestibular system, located within the vestibule of the inner ear. It resides in the medial aspect of the vestibule, lying adjacent to the cochlea and inferior to the utricle. The saccule contains a specialized sensory epithelium—the macula sacculi—responsible for detecting vertical linear acceleration and gravity-dependent head position.

Its structure includes a thin, membranous, fluid-filled sac lined by vestibular sensory hair cells embedded in the otolithic membrane, which contains calcium carbonate crystals (otoconia). These crystals shift with head movement, stimulating the underlying hair cells and transmitting signals via the vestibular nerve to the brainstem.

Synonyms

  • Sacculus

  • Saccular chamber

  • Inferior otolith organ

Location and Structure

  • Position: Medial wall of the vestibule, adjacent to the basal turn of the cochlea

  • Shape: Small, ovoid, thin-walled membranous sac

  • Macula: Vertical, oval sensory area detecting linear acceleration

  • Fluid content: Filled with endolymph and surrounded by perilymph within vestibule

  • Key connections:

    • Communicates with the cochlear duct via the ductus reuniens

    • Connects to the endolymphatic duct indirectly through utriculoendolymphatic pathways

Relations

  • Anteriorly: Basal turn of the cochlea

  • Posteriorly: Utricle and vestibule

  • Laterally: Bony vestibular wall

  • Medially: Vestibular aqueduct (posterior-superior direction)

  • Inferiorly: Proximity to spherical recess of the bony vestibule

  • Superiorly: Utricular chamber

Attachments

  • Anchored by fine connective tissue filaments to the bony vestibule

  • Sensory macula firmly attached to membranous wall

  • Connected to ductus reuniens and endolymphatic duct system

Function

  • Detects vertical linear acceleration, such as jumping or elevator movement

  • Sense of gravitational orientation and upright head positioning

  • Works with the utricle for balance, posture, and spatial awareness

  • Contributes to motion reflexes including vestibulocollic and vestibulospinal responses

Clinical Significance

  • Benign paroxysmal positional vertigo (BPPV): Otoconia detachment from saccule may lead to positional vertigo

  • Menière’s disease: Saccular hydrops can cause vertigo, tinnitus, and hearing loss

  • Vestibular neuritis / labyrinthitis: Inflammation may alter saccular function

  • Superior canal dehiscence: Pressure changes may secondarily affect otolithic organs

  • Imaging importance: Evaluation of endolymphatic hydrops and membranous labyrinth integrity

MRI Appearance

T1-weighted images (pre-contrast):

  • Saccule itself: Very low to low signal (dark) due to endolymph

  • Surrounding bone: uniformly dark cortical margin

T1 Fat-Saturated Post-Contrast:

  • Normal saccule: No intrinsic enhancement (endolymph is non-vascular)

T2-weighted 3D sequences (3D T2 SPACE / CISS / FIESTA):

  • Endolymph-filled saccule: Bright, hyperintense fluid signal

  • Bony labyrinth outline: sharply defined, dark signal cortex

  • Best sequence for structural evaluation and fluid–soft tissue contrast

  • CT Temporal Bone Appearance

Non-Contrast CT:

  • Saccule itself is not directly visualized (soft-tissue attenuation similar to other labyrinthine fluids)

  • Bony labyrinth architecture: sharply defined, high-attenuation thin osseous boundaries

MRI image

Saccule MRI axial image