Anatomical Distinction Between External and Internal Ear Canal
The term "internal ear canal" is a misnomer in standard anatomical terminology—what is commonly referred to as the "internal auditory canal" (IAC) is actually a bony canal within the temporal bone that houses cranial nerves VII and VIII, while the "external auditory canal" (EAC) is the passage from the auricle to the tympanic membrane. These are completely separate anatomical structures with distinct functions and locations 1, 2.
External Auditory Canal (EAC)
Structure and Location:
- The EAC is an osseous-cartilaginous structure extending from the auricle (pinna) to the tympanic membrane (eardrum) 2.
- It consists of an outer cartilaginous portion (lateral one-third) and an inner bony portion (medial two-thirds) 3, 4.
- The canal is approximately 2.5 cm in length in adults and serves as a physical conduit between the outside world and the tympanic membrane 5, 2.
Function:
- The primary purpose is to funnel sound waves toward the tympanic membrane for transmission to the middle ear 5, 4.
- It contains specialized cerumen (earwax) glands and migrating epithelium that provide self-cleaning mechanisms 5.
- The canal acts as both a conduit for sound and a protective cul-de-sac 5.
Clinical Relevance:
- The EAC is the site of acute otitis externa (swimmer's ear), characterized by inflammation and infection of the ear canal 1, 6.
- Tenderness of the tragus or pinna when manipulated is a hallmark sign of EAC pathology 1.
- Exostoses (bony growths) can narrow the EAC, trapping cerumen and debris 7.
- Congenital anomalies include aural atresia (complete absence) or stenosis (narrowing) 8.
Internal Auditory Canal (IAC)
Structure and Location:
- The IAC is a bony canal within the petrous portion of the temporal bone, completely separate from the EAC 1.
- It extends from the posterior cranial fossa to the inner ear structures (cochlea and vestibular apparatus) 1.
- This canal houses cranial nerve VII (facial nerve) and cranial nerve VIII (vestibulocochlear nerve) along with associated blood vessels 1.
Function:
- The IAC serves as a passageway for neural structures connecting the brainstem to the inner ear sensory organs 1.
- It transmits electrical signals for hearing (cochlear nerve) and balance (vestibular nerve) 1.
- Unlike the EAC, it has no role in sound conduction—it only carries neural impulses after sound has been converted to electrical signals 1.
Clinical Relevance:
- MRI of the IAC is used to evaluate sensorineural hearing loss, not conductive hearing loss from EAC pathology 1.
- Pathology in the IAC typically affects nerve function (hearing loss, vertigo, facial paralysis) rather than causing ear canal symptoms 1.
Key Distinguishing Features
| Feature | External Auditory Canal | Internal Auditory Canal |
|---|---|---|
| Location | From auricle to tympanic membrane [2] | Within temporal bone, from posterior fossa to inner ear [1] |
| Composition | Cartilage (lateral) and bone (medial) [3] | Entirely bony [1] |
| Contents | Air, cerumen, epithelium [5] | Cranial nerves VII and VIII [1] |
| Function | Sound conduction [4] | Neural transmission [1] |
| Accessible to exam | Yes, via otoscopy [4] | No, requires imaging [1] |
| Common pathology | Otitis externa, exostoses [1,7] | Acoustic neuromas, nerve dysfunction [1] |
Common Clinical Pitfall
A critical error is confusing EAC pathology (which causes conductive hearing loss and responds to topical therapy) with IAC pathology (which causes sensorineural hearing loss and requires neurologic evaluation). 1 When a patient presents with ear pain and discharge, the pathology is in the external auditory canal, not the internal auditory canal 1. CT temporal bone evaluates the EAC and middle ear structures, while MRI of the IAC evaluates the neural structures within the internal auditory canal 1.