Cochlear Implant: Clinical Status, 1985

Abstract

Until recently, a patient who was profoundly deaf and who was unable to benefit from wearing a conventional hearing aid could not receive any type of auditory stimulation. With the development of the cochlear implant, which is an electronic prosthetic device used to provide auditory sensation to deaf patients, the lives of profoundly deaf persons can now be considerably improved. The first single gold, hard-wired electrode was implanted in the cochlea by Dr. William House in early 1961. In the United States, electrical auditory stimulation was initially reported by Simmons1Simmons FB Electrical stimulation of the auditory nerve in man.Arch Otolaryngol. 1966; 84: 2-54Crossref PubMed Scopus (234) Google Scholar in 1966 and by House and Urban2House WF Urban J Long term results of electrode implantation and electronic stimulation of the cochlea in man.Ann Otolaryngol. 1973; 82: 504-510Google Scholar in 1973. Since then, the feasibility of an electrical implantable prosthesis has become a reality that can be seriously considered in a profoundly deaf patient. To date, cochlear or extracochlear implantation of an electronic device has been performed on a limited basis throughout the world. In approximately 400 patients, either a cochlear or an extracochlear electronic prosthesis has been implanted. The basic function of the implanted prosthesis is to bypass the nonfunctioning cochlea. The normal cochlea transduces mechanical energy into coded electrochemical signals, which are transmitted through the cochlear nerve to the brainstem and auditory cortex. The cochlear implant transduces sound to electrical energy, which is transmitted through the electrodes that are microsurgically implanted into or near the cochlear part of the inner ear. Basically, the device consists of two parts, an internal implanted portion and an external portion. Currently, single electrodes are most commonly used, although bipolar electrodes have been developed, and research information is being obtained from placement of multiple electrodes. Most of these electrodes have been inserted through the round window membrane (which is the membrane that seals the scala tympani portion of the cochlea from the middle ear) into the scala tympani, and only a few of the devices have been implanted against the round window. The external portion of the device consists of a microphone (the same as the microphone on a hearing aid), a transducer that converts the sound waves into coded electrical energy, and a transmitter. The most common method of transmission, through magnetic induction coupling, is quite reliable but poses several problems. It is very inefficient and therefore necessitates use of fairly large batteries. Consequently, miniaturization has not been possible. In addition, the external transmitting coil must be precisely aligned over the internal receiving coil, and this alignment is somewhat difficult to maintain in daily life. Moreover, magnetic induction coupling can support only single-channel transmission. A newly developed method of transmission is with radio frequency. This method is much more efficient than magnetic induction coupling, and it allows multiple-channel stimulation. Furthermore, with this new method, the internal and external coils do not have to be accurately aligned. One currently used device, the Vienna-3M extra-cochlear implant prosthesis, was codeveloped by the Hochmairs in Vienna, Austria, and the 3M Company in St. Paul, Minnesota. A person in whom a profound sensorineural hearing loss has developed bilaterally after development of normal speech is a potential candidate for surgical implantation of a cochlear prosthesis. In contrast, those who have a profound congenital sensorineural hearing loss or who have development of a profound sensorineural hearing loss before development of normal speech currently may not be candidates for a cochlear implant. A profoundly deaf patient who is unable to benefit from a hearing aid can certainly be considered for implantation of an extracochlear device. Etiologic factors that should prompt consideration of cochlear implantation are profound sensorineural hearing loss caused by ototoxicity, otosclerosis, meningitis, syphilis, Meniere's disease, trauma, and idiopathic sensorineural hearing loss. Patients with profound sensorineural hearing loss who may not be candidates for implantation are those who have transverse temporal bone fractures or those who have bilateral acoustic neuromas (Recklinghausen's disease) and destruction of the peripheral nerve supply to the cochlea as a result of the tumors or surgical removal of the tumors. Additionally, some congenital defects in which development of the cochlea is incomplete, such as the Mondini malformation, may preclude implantation. The operative procedure for implantation of a cochlear device involves methods that are familiar to an otologic surgeon who is accustomed to treating acute and chronic ear diseases. The surgeon uses a modified mastoidectomy incision with a temporal extension that is considerably higher than the usual mastoid incision. The rationale for this approach is that the internal coil portion of the implant is placed in the squamous part of the temporal bone, and the incision line is placed posterior to the internal coil rather than directly over it. A complete mastoidectomy is then performed, and the facial recess is opened in order to gain access to the round window membrane. In most instances, this procedure will allow adequate exposure of the round window membrane and will not result in injury to the facial nerve. The active platinum electrode may be inserted in the round window niche, against the round window membrane, and held in place with temporal fascia, or it may be inserted through the round window membrane. The ground electrode is placed in the epitympanum between the head of the malleus and the dural plate of the middle cranial fossa or under the temporal muscle. The internal transducer coil is placed in a depression created in the squamous portion of the petrous bone. This procedure is done with the patient under general anesthesia, and 2 to 3 days of hospitalization are necessary after the operative procedure. After a satisfactory healing period of 3 to 4 weeks, the external transducer portion can be fitted, and the patient is ready for auditory electrical stimulation. During a lengthy rehabilitation period, the patient and the family receive extensive instructions, including videotape guidelines, to assist in the use of the cochlear implant prosthesis. The rehabilitation program is directed toward recognition and use of the new auditory clues available as well as improvement in communication ability. Each patient is given instructions in speechreading (lipreading) and speech production, and a home practice program is designed to continue the training process with the implant. Learning the effective and comfortable use of the signals provided by the implant involves considerable effort and time, perhaps as long as 6 months or a year before the situation is entirely mastered. The principal advantage of the cochlear implant at this stage of development is that it enables the user to hear environmental sounds at a level that approximates normality. For example, the patient can recognize the sounds of doorbells, telephones, car horns, trains, garbage disposals, running water, approaching footsteps, opening and closing of doors, and sirens on emergency vehicles; thus, the person who is profoundly deaf has much more security while driving a car. Most patients with a cochlear implant can distinguish a male from a female voice, but they describe speech as sounding very muffled or distorted, like a radio that is not tuned accurately to a station. Because lipreading is generally improved in patients with a cochlear implant, their understanding of conversations is enhanced. A tremendous psychologic gain is imparted to patients with no measurable hearing, as a cochlear implant prosthesis helps to increase self-esteem and a sense of security by enabling these patients to distinguish various environmental sounds and to recognize some noises that might signify danger. I believe that the cochlear implant is one of the most exciting developments in otology in the past several years. The currently used device is continually being modified and improved, and the indications for implantation most likely will be gradually expanded during the next several years.