Clinical Anatomy of the Inner Ear
The inner ear (labyrinth) lies in the petrous part of the temporal bone—the densest bone in the body—between the middle ear laterally, the internal auditory canal medially- Anterior to the mastoid air cells , and the posterior cranial fossa posteriorly. It consists of the bony labyrinth filled with perilymph, and the membranous labyrinth suspended inside it, filled with endolymph and responsible for both hearing (cochlear duct) and balance (utricle, saccule, and semicircular ducts). Anterior to the mastoid air cells.
1. The Bony Labyrinth is a system of hollow, communicating cavities carved into the dense petrous temporal bone. It is lined with periosteum and filled with perilymph.
- Posterior: Vestibule
- Superior & Posterolateral: Semicircular canals
- Anteromedial: Cochlea
A. The Vestibule (The Central Chamber)
- Location: Central part, connected to the middle ear via the Oval Window
- Contents: the membranous utricle and saccule.
- Key openings:
- Oval window (stapes footplate) → Lateral wall
- Round window → Inferior to oval window
- Five openings for semicircular canals
- Clinical Significance:
- Site for BPPV due to displacement of otoconia (from utricle).
- Oval window fixation → conductive HL (otosclerosis).
- Round window rupture → perilymph fistula → sudden vertigo.
B. The Cochlea (The Hearing Organ)
- Shape: A spiral tube making about 2.5 turns around a central bony pillar called the Modiolus.
- Parts (from base to apex):
- Basal turn (high frequencies) —> forms Promontory
- Middle turn
- Apical turn (low frequencies)
- Structure:
- Scala Vestibuli (SV): Receives vibrations from oval window.
- Scala media (cochlear duct)
- Scala Tympani (ST): Ends at the Round Window, which bulges into the middle ear to dissipate pressure waves.
- Separated by:
- Reissner’s membrane
- Basilar membrane
- Clinical Significance: The cochlea's proximity to the Internal Auditory Canal (IAC) makes it vulnerable to tumors of CN VIII (Acoustic Neuroma/Vestibular Schwannoma).
Clinical importance:
- Sensorineural hearing loss
- Cochlear implants placed here
C. The Semicircular Canals (The Balance Organs)
- Number & Orientation: Three canals (Superior/Anterior, Posterior, and Lateral/Horizontal), each oriented at right angles to the others.
- Function: Detect angular (rotational) acceleration of the head.
- Ampulla: swelling at one end containing crista ampullaris (angular acceleration receptor).
- Clinical Significance:
- Lateral canal erosion in cholesteatoma → fistula test positive (eye nystagmus on pressure). It forms a prominent bulge on the epitympanum. It is a key surgical landmark for finding the adjacent Facial Nerve and is often involved or exposed in chronic middle ear disease, which can cause vestibular fistula (causing vertigo upon applying pressure).