External auditory canal

The external auditory canal, also known as the external acoustic meatus, is a passageway that conducts sound waves from the external environment to the tympanic membrane (eardrum). It forms a part of the external ear and plays a key role in the conduction of sound.

Structure

The external auditory canal is approximately 2.5 to 3 cm in length in adults and has an S-shaped curve. It is divided into two distinct parts:

• Cartilaginous part (lateral one-third): Flexible and lined with skin containing hair follicles, sebaceous glands, and ceruminous glands that produce earwax (cerumen)
• Osseous part (medial two-thirds): Formed by the temporal bone and narrower in diameter, this segment is firmly attached to the periosteum and lacks glands

The canal is lined by thin skin that is continuous with the auricle externally and the tympanic membrane internally.

Location

The external auditory canal is located within the temporal bone of the skull. It begins at the external opening (external acoustic pore) on the lateral side of the skull and extends medially to end at the tympanic membrane. It lies anterior to the mastoid process and inferior to the squamous part of the temporal bone.

Function

The main function of the external auditory canal is to transmit sound waves from the external environment to the tympanic membrane. It also serves as a protective conduit, preventing foreign objects from reaching the delicate structures of the middle and inner ear. Additional functions include:

• Amplifying sound frequencies between 2,000–5,000 Hz
• Maintaining a warm, moist environment optimal for tympanic membrane function
• Producing cerumen, which traps dust and has antimicrobial properties

Blood Supply

The arterial supply to the external auditory canal is derived from:

• Deep auricular branch of the maxillary artery (main supply to canal wall)
• Posterior auricular artery
• Superficial temporal artery

Nerve Supply

The innervation of the external auditory canal is shared among several nerves:

• Auriculotemporal nerve (branch of CN V3): Anterior wall and roof
• Arnold’s nerve (auricular branch of CN X): Posterior and inferior wall
• Facial nerve (CN VII): Minor sensory contribution
• Glossopharyngeal nerve (CN IX): May contribute to innervation of the medial part

Clinical Significance

Otitis Externa

Also known as "swimmer's ear," this is an infection of the external auditory canal, often caused by bacteria or fungi. It leads to inflammation, pain, discharge, and narrowing of the canal.

Impacted Cerumen

Excessive buildup of earwax can block the canal and result in hearing loss, discomfort, or tinnitus. Manual removal or cerumenolytics may be required.

Foreign Bodies

Common in children, objects lodged in the canal may cause pain, infection, or damage to the tympanic membrane if not removed properly.

Arnold’s Reflex

Stimulation of the auricular branch of the vagus nerve in the canal can induce coughing. In rare cases, it may also trigger a vasovagal response.

Trauma

Injuries to the canal, including abrasions or lacerations, can result from aggressive cleaning with cotton swabs or insertion of objects. Fractures of the temporal bone may also involve the canal.

Development

The external auditory canal develops from the first pharyngeal cleft during embryogenesis. The cartilaginous portion forms from the first branchial arch mesenchyme, while the bony portion is a derivative of the temporal bone ossification.

Imaging

High-resolution CT of the temporal bone can delineate the bony canal and assess for congenital anomalies, trauma, or tumors. Otoscopic examination remains the primary clinical tool for visualizing the canal and tympanic membrane.