Anatomy of the inner ear

Overview of the inner ear

The inner ear is the most medial portion of the peripheral auditory and vestibular system. It is embedded within the petrous portion of the temporal bone and consists of a complex series of fluid-filled spaces and sensory organs. The inner ear is divided into the osseous labyrinth and the membranous labyrinth. The osseous labyrinth is a network of cavities within the temporal bone, while the membranous labyrinth is a continuous set of soft tissue ducts and sacs suspended within the osseous spaces.

Anatomy of the inner ear: bony labyrinth

The bony labyrinth has three main regions: the cochlea, the vestibule, and the semicircular canals. These structures contain perilymph, a fluid similar in composition to extracellular fluid, which surrounds the membranous labyrinth.

Cochlea: A spiral-shaped chamber that coils approximately two and a half turns around a central bony core called the modiolus. It is responsible for hearing.
Vestibule: The central cavity of the bony labyrinth that houses the saccule and utricle, two otolith organs important for linear acceleration and static head position.
Semicircular canals: Three bony canals oriented in orthogonal planes: the lateral, posterior, and superior canals. They detect rotational head movements.

Anatomy of the inner ear: membranous labyrinth

The membranous labyrinth lies within the bony labyrinth and contains endolymph, a fluid rich in potassium. It consists of the cochlear duct, the utricle, the saccule, and the semicircular ducts. The sensory epithelium of hearing is the organ of Corti, located within the cochlear duct. The sensory epithelium of the vestibular system is located in the maculae of the utricle and saccule and in the cristae of the semicircular ducts.

The cochlear nerve and vestibular nerve arise from the sensory cells and travel through the internal auditory canal to the brainstem. The facial nerve also courses through this region, making a thorough understanding of the anatomy of the inner ear essential when evaluating temporal bone pathology.

CT inner ear

Computed tomography of the temporal bone is the preferred imaging modality for evaluating the bony structures of the inner ear. A CT inner ear study uses high-resolution thin-section acquisition to delineate the fine osseous details of the cochlea, vestibule, and semicircular canals. It is particularly useful for assessing congenital malformations, otosclerosis, labyrinthitis ossificans, fractures, and erosive lesions of the otic capsule.

On a CT inner ear scan, the normal cochlea appears as a well-defined spiral lucency with a clear modiolus. The vestibule appears as an oval-shaped cavity between the cochlea and the semicircular canals. The semicircular canals are visible as thin rings of bone surrounding the membranous ducts. The internal auditory canal is also evaluated for widening or narrowing. CT inner ear imaging provides excellent spatial resolution for bony anatomy but gives limited information about the soft tissue contents of the labyrinth.

MRI inner ear

Magnetic resonance imaging provides superior contrast resolution for the soft tissues and fluid spaces of the inner ear. An MRI inner ear protocol typically includes high-resolution T2-weighted sequences such as 3D CISS or FIESTA, which produce strong signal from the perilymph and endolymph. These sequences allow detailed evaluation of the membranous labyrinth, the cochlear nerve, the vestibular nerve, and the internal auditory canal.

MRI inner ear imaging is the study of choice for suspected labyrinthitis, intralabyrinthine schwannoma, endolymphatic hydrops, and inflammatory or neoplastic conditions involving the membranous labyrinth. It is also used to evaluate the nerve bundles within the internal auditory canal and to differentiate between cochlear and retrocochlear pathology. Gadolinium-enhanced T1-weighted images may demonstrate abnormal enhancement in cases of inflammation or tumor.

Imaging correlation

In clinical practice, CT inner ear and MRI inner ear are complementary. CT best demonstrates the osseous anatomy of the otic capsule and the labyrinthine windows. MRI best demonstrates the fluid-filled compartments and neural structures. A combined approach is often necessary for complete assessment of the anatomy of the inner ear, particularly in preoperative evaluation for cochlear implantation or in the workup of patients with congenital hearing loss.

Accurate interpretation of these studies requires a solid understanding of the normal segmental anatomy, including the basal, middle, and apical turns of the cochlea, the oval and round windows, the vestibular aqueduct, and the cochlear aqueduct. Advances in imaging continue to improve visualization of the delicate structures that comprise the anatomy of the inner ear.

Clinical relevance

Structural abnormalities of the inner ear may be identified using CT inner ear or MRI inner ear in patients presenting with sensorineural hearing loss, vertigo, tinnitus, or balance dysfunction. Disorders such as enlarged vestibular aqueduct syndrome, Mondini malformation, and semicircular canal dehiscence are readily recognized with high-resolution imaging. Precise knowledge of the anatomy of the inner ear guides both diagnosis and surgical planning.