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How do we hear?
The ear consists of three parts that each have a role in hearing. Sound waves enter the ear canal of the external ear to cause the tympanic membrane (eardrum) to vibrate. Within the middle ear there are three small bones, the malleus (hammer), incus (anvil) and the stapes (stirrup). These bones continue to transmit the vibrating signal from the eardrum to the cochlea (inner ear), which is a fluid filled chamber. The movement of the middle ear hearing bones causes the inner fluids to move in waves. These waves stimulate the hair cells of the inner ear. The movement of the hair cells then stimulates the auditory nerve which carries the signals to the brain allowing us to hear. Hearing loss is considered to be either conductive or sensorineural. Conductive hearing loss occurs when there is a problem in external or middle ear. This type of hearing loss is often correctable with either surgery or a hearing aid. Sensorineural hearing loss occurs when there is a problem affecting the cochlea or auditory nerve. Hearing aids are most often used to correct this type of hearing loss. When hearing is so poor that hearing aids are of little or no benefit, a cochlear implant may be considered.
Surgery for Conductive Hearing
Correction of conductive hearing loss may be divided into the following procedures:
Canalplasty, tympanoplasty, with or without ossicular reconstruction, and stapedectomy.
Obstruction of the external ear canal may be either congenital or acquired. In these situations, creation of a new ear canal will result in improved hearing.
Perforation of the eardrum will reduce the ability to adequately vibrate the middle ear hearing bones, causing hearing loss. Perforations may occur from either infection or trauma. Repair of the perforation is performed either through the ear canal or by an incision behind the eardrum. The approach is determined by the size and location of the perforation, as well as the possible presence of infection within the mastoid. The graft material typically consists of soft tissue, which lines either muscle or cartilage.
The middle ear hearing bones may be partially destroyed or absent. This may be either congenital or a result of chronic infection. Ossicular reconstruction is usually performed through the ear canal. The repair typically utilizes a prosthesis made of titanium. The operation may require placement of a prosthesis replacing all three bones or one replacing the incus and malleus. This is an outpatient procedure with a recovery of a few days.
Otosclerosis is a metabolic condition in which there is a hardening of the base of the stapes. This cause decreased vibration of sound into the inner ear, resulting in hearing loss. Otosclerosis is an inherited condition, which while a dominant trait, it may not display itself in every generation. Otosclerosis involves both ears in about two thirds of patients. Surgical correction of this condition is performed through the ear canal under either general or local anesthesia. The arch of the stapes is removed and a piston is placed from the incus through an opening in the base of the stapes. This is usually an outpatient procedure with a recovery of a few days.
Surgery for Sensorineural Hearing Loss
Currently, most forms of hearing loss are corrected with hearing aid amplification. When hearing is so poor that hearing aids provide minimal or no benefit, a cochlear implant may be considered. A cochlear implant consists of two parts, an external speech processor and the internal device, consisting of a receiver/stimulator and electrode array. The external speech processor receives sound and converts it to an electrical signal. This is then passed through the skin behind the ear to a receiver/stimulator. This receiver /stimulator is secured to the bone under the skin. The signal then passes on to the electrode array which has been placed within the cochlea. The nerve endings are then directly stimulated allowing the individual to hear. Before considering the procedure, the patient undergoes a detailed evaluation. This includes X-ray studies, such as an MRI and CT scan. Most importantly however, a detailed evaluation is performed by the implant audiologist. Once the individual is determined to be a candidate, the surgery is scheduled. The procedure is performed under general anesthesia through an incision behind the ear. The mastoid is opened and the electrode array is placed through an opening in the cochlea. The receiver/stimulator is secured to the bone behind the ear. Patients usually spend one night in the hospital. The recovery is few days. There is however, swelling behind the ear, which may take a while to subside. As a result of this, the speech processor is not hooked up to the receiver/stimulator until four weeks after surgery. The patient then undergoes intensive rehabilitation under the care of the implant audiologist in order to learn how to listen to sound through the implant.
Bone Anchored Hearing Aid
When an individual has a profound hearing loss in one ear, with normal or near normal hearing on the other side, a conventional hearing aid will provide no benefit. An alternative is placement of a bone anchored hearing aid (BAHA). With the loss of the ability to discriminate sound in the affected ear, it is not possible to restore hearing on that side. The idea of the BAHA is to take sound from the affected side and vibrate it across the skull to the better hearing ear. The procedure may be performed under general or local anesthesia as an outpatient. A titanium implant is placed in the bone behind the ear. Three to four months later an external device similar to a hearing aid is hooked up to the titanium implant. While the device does not allow for improved sound localization, there is a significant improvement in sound awareness.
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