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III. FUNCTIONS AND PATHOPHYSIOLOGY OF THE EXTERNAL EAR

Objectives:

The external ear is the entry way to the auditory receptors of the inner ear. Thus, it plays an important role in normal hearing. Because it is a structure that is frequently examined in the clinic, understanding of its functions is important.

At the end of this section you should be able to:

1. Describe the two functions of the external ear, including the effects of the pinnae and ear canals on incoming sound waves.

2. State the type of hearing loss involving the external ear and the ways in which these come about.

THE EXTERNAL EAR

Objective 1: The external ear, which includes the pinna and ear canal, carries out two physiological functions: acoustic and non-acoustic.

The auditory function allows efficient sound transmission from the environment to the tympanic membrane. The nonacoustic functions of the ear canal include protection of the tympanic membrane and the maintenance of a clear passage for sound.

Non-acoustic functions of the external ear.

The protective function of the ear canal is related, in part, to its anatomical structure. The depth of the canal and its tortuous shape and rigid walls provides protections of the tympanic membrane (and the middle ear beyond) from direct injury.

The canal has a self-cleaning function, which keeps the sound pathway clear of debris. The technical problem of self cleansing faced by the ear canal is that it is a blind alley lined with skin. The usual surface contact that removes desquamated keratinocytes from skin on other parts of the body does not operate here. The solution is a mechanism of epithelial migration; the surface of the skin moves laterally from the tympanic membrane toward the ear canal opening.

The external ear canal is also protected by the arrangement of hairs and the production of wax in the outer (cartilaginous) part. These not only help to prevent objects from entering the ear canal, but they aid in the desquamation and skin migration out of the canal.

Acoustic Functions of the external ear

Although Charles Darwin had concluded in 1907 that the human pinna is vestigial and of no functional significance, several earlier investigations had (correctly) determined that this structure does play an important role in hearing. Specifically, it alters the amplitude of the incoming sound wave and, in doing so, provides a mechanism for amplifying differentially sounds within the range of frequencies that make up human speech. It also plays a role in the 'spatial' hearing of sounds. The curve that relates the amplitude of the sound in free space to that at the ear drum is referred to as a transfer function. Transfer functions are shown in FigureIII-1.

For humans, there is an amplification from the free-field to the eardrum of from 5 to 20 dB over the frequency range from about 1.5 to 7 kHz. While some of this increase is due to reflection of sound from the torso, together with diffraction of sound by the head, most of the increase arises from two mechanisms: a) resonance of the concha (the scooped-out area of the extended ear leading to the ear canal opening) around 5 kHz; and b) resonance of the external canal (closed tube, like an organ pipe) around 2.5 kHz.

At higher frequencies, above about 6 kHz, the shape of the transfer function shifts systematically as the location of a sound sources is changed, both vertically and horizontally, especially for frequencies above about 6 kHz. The external ear acts as a directional amplifier of sound. Thus, the shifts in the structure of the transfer function with changes in sound direction provide important cues for sound localization. Far from being vestigial, the complex structures of the pinna and external ear canal are now recognized as a significant components in the mechanisms that underlie the capacity of a listener to recognize and localize sounds in space.

Objective 2: Pathophysiology of the external ear

Diseases of, trauma to, and maldevelopment of the pinna and external ear canal may include skin, cartilage or bone. Because of the relationships of the external ear to the middle ear, mastoid region, scalp, skull, neck, parotid gland, and temporo-mandibular joint regions, a number of therapeutic approaches are often called for. Secondary involvement of the cochlea and vestibular systems of the inner ear is also occasionally found.

From the standpoint of hearing and hearing disorders, abnormalities of the pinna and external ear canal that result from any condition usually can only cause a blockade to sound conductance. They create a conductive hearing loss. Naturally, hearing loss may be only one of the considerations for intervention in an external ear disorder. There are a number of conditions that may present themselves, some of which are unrelated to hearing disorders. The presence of foreign bodies in the ear canal and infection of the skin of the external auditory canal are conditions most commonly associated with conductive hearing loss in the external ear.

Foreign bodies

Almost anything can become lodged in the external auditory canal. Even naturally occurring cerumen may become the impacted material which results in a noticeable conductive hearing loss. Probing the ear canal with an instrument (e.g., a Q-tip) is dangerous for it can force the impacted material further into the canal and can perforate the tympanic membrane resulting in damage to the middle ear structures

Congenital malformations

Congenital malformations of the pinna and external ear canal are related to developmental defects of the first and second branchial arches and the branchial groove which joins the first pharyngeal pouch to form the external ear canal. Malformation of the external ear canal results in an atresia, which is a conductive blockade of connective tissue or bone. Maldevelopment of the first pharyngeal pouch, leads to abnormalities in Eustachian tube, middle ear, and mastoid differentiation. These malformation may occur singly or in combination.

Recall that the ectodermal otocyst, which eventually differentiates into the inner ear cochleovestibular system, develops independently from external and middle ear primordial tissue, which is of branchial arch origin. Because of this separate development of the inner ear from that of the external and middle ears, congenital combined lesions of the external, middle, and inner ears are not common, but they do occur. When this happens, there results a sensorineural hearing loss as well as a conductive hearing loss; this combined sensorineural and conductive hearing loss is referred to as a mixed hearing loss.

Other conditions of the external ear

Other conditions that may result in a hearing loss include trauma and neoplasia (benign or malignant). Inflammatory conditions of the external ear (external otitis) may result in canal occlusion, and can have an effect on the non-acoustic protective and cleansing action of the ear canal.

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