Function of the Normal Ear

The ear is divided into three parts: an external ear, a middle ear, and an inner ear. Each part performs an important function in the process of hearing. The external ear consists of an auricle and ear canal. These structures gather sound and direct it toward the eardrum. The middle ear chamber lies between the external and inner ear. This chamber is connected to the back of the throat by the Eustachian tube which serves as a pressure-equalizing valve. The middle ear consists of the eardrum and three small ear bones (ossicles): malleus (hammer, incus (anvil and stapes (stirrup). These structures transmit sound vibrations to the inner ear. In so doing, they act as a transformer, converting sound vibration in the external ear canal into fluid waves in the inner ear.

The inner ear chamber contains the microscopic hearing nerve endings bathed in fluid. Fluid waves stimulate the delicate nerve endings which in turn transmit sound energy to the brain where it is interpreted.

TYPES OF HEARING IMPAIRMENT

The outer and middle ear conduct and transform sound; the inner ear receives it. When there is some difficulty in the outer or middle ear, a conductive hearing impairment occurs. When there is trouble in the inner ear, a sensorineural or hair cell impairment is the result. Difficulty in both the middle and inner ear results in a mixed hearing impairment.

Hearing loss may be divided further due to the cause of the hearing handicap.

1. Congenital hearing loss
   a. Genetic – an actual defect in the child’s genes which results in an abnormal development of the ear.
   b. Non-genetic – hearing loss is due to some problem which occurred during the fetal development or the immediate birth period.
2. Acquired hearing loss – This is a hearing impairment which occurs sometime after birth and is not transmitted to future children.

Conductive Impairment

A conductive type of hearing impairment occurs when sound is not conducted efficiently through the ear canal, eardrum, or tiny bones of the middle ear. Conductive losses reduce the loudness of sound that is heard.

A conductive impairment may occur from blockage of the outer ear canal, from a perforation (hole) in the eardrum, from middle ear infection or fluid due to blockage of the Eustachian tube, or from a congenital defect or disease of any of the three middle ear bones. This type of impairment is usually correctable through surgery.

The child with a conductive hearing loss will never go deaf. He will always be able to hear either through ear surgery or by use of a properly fitted hearing aid.

Sensorineural Impairment

A sensorineural hearing loss is used to describe hearing impairments which result from disturbances of defects in the inner ear and transmission of electrical signals from the hair cells. These impairment may be congenital (i.e. present at birth), hereditary, developmental, or a combination of these. In addition, these impairments may result from infections, injuries, ototoxic drug therapy, or lack of oxygen.

Congenital Factors

German measles contracted by the other during the first three months of pregnancy may interfere with inner ear development in the fetus. Occasionally, other viral diseases are at fault. The viruses of measles and mumps may cause a sensorineural hearing loss after birth, but this happens infrequently. Immunizations are now available for both of these diseases.

In addition, there are other inner ear developmental and inherited abnormalities which may be present at birth. These problems occur as a result of the inner ear not being developed properly. Mondini dysplasia or a Michel abnormality is such factors.

Problems at Birth

A very difficult and complicated labor or premature birth may also result in an inner ear hearing impairment on occasion. This may be due to lack of oxygen. There are many syndromes which can result in a hearing impairment at birth. One can have a hearing loss at birth without any hereditary relationship.

Jaundice occurring at or shortly after birth is capable of damaging the inner ear. This is most often due to Rh incompatibility between the mother’s and the child’s blood. Fortunately, this is not a common occurrence.

Hereditary Impairment

The development and function of the ear is dependent upon hundreds or even thousands of genes interacting with each other and with the intra- and extra-uterine environment. A major cause of late-onset hearing loss is genetic. There are several patterns of inheritance. In autosomal dominant disorders, one parent expresses the trait, which he transmits to 50 percent of his children. In autosomal recessive inheritance, the parents of the children are clinically normal, but carry the recessive gene to 25 percent of their children. X-link inheritance traits are transmitted from a carrier mother to 50 percent of her sons.

Most cases of hereditary childhood deafness are sensorineural rather than conductive in nature. Most examples of hereditary hearing loss are recessive. Recessive deafness characteristically is associate with retention of hearing of low frequency sounds since most of these cases are associated with abnormalities primarily affecting the first turn of the cochlea (i.e. the Scheibe inner ear abnormality). In dominant inherited deafness, the audiogram generally is flat. However, there are other dominant types of deafness in which there is a low frequency or mid-frequency sensorineural hearing loss. In X-link recessive deafness, some retention of hearing is usually seen in all frequencies.

Hereditary sensorineural hearing loss may be present at birth, or may develop later in life. This may be due to inner ear malformations or to other associated syndromes which have an associated inner ear hearing loss. One may see a genetic sensorineural hearing loss with or without associated abnormalities.

Infections

The most common type of acquired sensorineural loss is meningitis. Frequently this may effect both ears, but can involve one ear. Other types of infections would include viral diseases, such as mumps, rubella, and otitis media.

Hearing Impairment In One Ear

A hearing impairment that is confined to one ear deprives a person of the ability to distinguish the direction of sound. He will also have difficulty hearing from the involved side in a noisy background. These are minor problems in a young child. When this hearing impairment in one ear is conductive, surgery will usually be able to restore the hearing, giving a better balance of hearing. This is usually done in a child who is in his teens. When the impairment is sensorineural, it is often possible when the child grows order to restore some of this balance of hearing with a special hearing aid (i.e. CROSS hearing aid).

Cochlear Implant

The cochlear implant is an electronic device that is implanted into the inner ear of a profoundly deaf child. This device is only utilized in the child who cannot benefit from a hearing aid. It is a device which is used to bypass the diseased or nonfunctional hair cells and converts the sounds we heat to electrical impulses which directly stimulate the cochlear nerve. The implant consists of an external portion comprised of a microphone, sound processor, and external coil and an internal portion that must be surgically implanted. The surgical procedure involves the placement of an internal coil beneath the skin behind the ear and a stimulating electrode which is inserted into the cochlea or inner ear.

The cochlear implant can increase the awareness of sound in children, improve the ability to lip read and enhance communication skills. It does not restore sounds like normal hearing people hear. Following surgery, rehabilitation is necessary, as the child must learn to associate the sound signals with normal sounds.

To determine suitability for this device in a profoundly deaf child, a careful examination is required. The evaluation is performed to determine whether or not the child can receive adequate information from a powerful hearing aid, or whether or not the procedure can be performed and give the expected improvement.

Currently, there are single channel and multiple channel devices. This is related to the number of stimulating electrodes within the cochlear.