External acoustic meatus

Meatus acusticus externus

Also known as: ear canal, external auditory canal, EAC, cerumen, earwax

The S-shaped canal (≈2.5 cm) from the concha to the tympanic membrane: its lateral third is cartilaginous and wax-producing, its medial two-thirds bony.

Overview

Length≈2.5 cm (≈24 mm)

Cartilaginous : bony1/3 : 2/3

Diameter≈7–9 mm

Resonant frequency≈3 kHz

The meatus runs medially from the concha to the tympanic membrane. Its lateral one-third (≈8 mm) has a framework of elastic cartilage continuous with the auricle and is lined by thick skin with hairs, sebaceous glands and ceruminous glands (modified apocrine sweat glands); together their secretions form cerumen. Its medial two-thirds (≈16 mm) are formed by the tympanic part of the temporal bone and lined by very thin skin, tightly bound to periosteum, without glands or hairs — hence pain from even minor trauma or swelling here.

The canal is S-shaped: the lateral part runs upward, forward and medially, the bony part slightly backward and downward. The narrowest point, the isthmus, lies just medial to the bony–cartilaginous junction; foreign bodies impact here. Because the tympanic membrane is oblique, the floor and anterior wall are longer than the roof and posterior wall.

To straighten the canal for otoscopy, the auricle is pulled upwards and backwards in adults and downwards and backwards in infants (whose canal is mainly cartilaginous and the bony part not yet developed).

Location

From the concha of the auricle to the tympanic membrane; anterior relation is the temporomandibular joint and parotid gland; posterior, the mastoid process; superior, the middle cranial fossa.

Anatomy

Innervation

Auriculotemporal nerve (V3) — anterior and superior walls; auricular branch of the vagus (Arnold nerve) — posterior and inferior walls (stimulation can cause coughing — Arnold's ear-cough reflex — or vasovagal syncope); twigs of the facial nerve.

Arterial supply

Posterior auricular, deep auricular (maxillary) and superficial temporal arteries.

Histology

Stratified squamous keratinised epithelium; ceruminous glands (coiled apocrine glands with myoepithelium) and sebaceous glands in the cartilaginous part.

Development

First pharyngeal cleft; the meatal plug of ectodermal cells canalises in ≈week 26–28 — failure causes congenital aural atresia (conductive hearing loss).

Function

Conduct sound to the tympanic membrane

Resonate at ≈3 kHz to amplify speech frequencies

Protect the eardrum (cerumen, hairs, curvature, maintenance of temperature and humidity)

Secretions

Cerumen: mixture of ceruminous and sebaceous secretions and desquamated keratinocytes; wet (dominant) or dry (recessive; common in East Asians) type determined by the ABCC11 gene.

Clinical significance

Impacted cerumen and foreign bodies

Wax impaction is the commonest cause of conductive hearing loss in adults; cotton buds push wax deeper. Treated with softening drops, irrigation or microsuction. Insects should be drowned with oil before removal; button batteries are an emergency.

Exostoses and Ramsay Hunt syndrome

Exostoses ("surfer's ear") are bony growths from cold-water exposure. Ramsay Hunt syndrome (herpes zoster of the geniculate ganglion): vesicles in the canal and concha with ipsilateral facial palsy, otalgia and sometimes hearing loss and vertigo.