ACCESS: Multiple Avenues for Deaf People

3. Cochlear Implants

38 Cochlear implants are quickly changing the landscape of hearing loss. Children with a severe-to-profound hearing loss who do not benefit from conventional hearing aids are now able to receive a cochlear implant. Children who receive an implant quickly after the onset of deafness and who use the implant consistently have the potential to develop spoken language skills that approach those of their normally hearing peers. This chapter is intended to provide a broad overview of cochlear implants and issues related to their use in children. We begin with a description of the anatomy and physiology of the human auditory system and how the electrode array of a device is implanted into the cochlea to restore, in a limited way, the function of the auditory system. In the chapter’s second section, we discuss the team approach that is used at the Alfred I. duPont Hospital for Children in Wilmington, DE, to evaluate candidacy for cochlear implantation, to educate parents on the risks, benefits, and realistic expectations for cochlear implantation, and describe the intensive (re)habilitation that is required for children who do receive cochlear implants. In the chapter’s third and final section, we review recent research on the factors that are believed to promote success in developing spoken language for pediatric cochlear implant recipients. AU D I T O R Y AN AT O M Y A N D PH Y S I O L O G Y The human ear can detect sound along a wide range of frequencies, from 20 to 20,000 Hz (Campbell 1998). The range of loudness that the Cochlear Implants ROBERT C. O’REILLY, AMANDA J. MANGIARDI, AND H. TIMOTHY BUNNELL 3 Ch_03_Pgs_38-72.PMD 8/6/2008, 1:21 PM 38 auditory system can handle (dynamic range) is enormous, with pressures from the quietest sound detectable to the loudest sound tolerable of 100 to 120 dB (Sanes and Rubel 1988). The ear performs this task through an intricate mechanism in the middle ear that enables sound pressure in air to be transferred to liquid in an efficient fashion (“impedance matching,” see later discussion). The resulting sound-pressure waves in the cochlear fluids are converted into an electrical signal that is relayed to the cochlear nerve. The signal then passes along the neural arc of the central auditory pathways and is perceived in the auditory cortex of the temporal lobe as sound. The ear has three general divisions: the outer ear, consisting of the pinna and cartilaginous and bony external auditory canal; the middle ear, which includes the tympanic membrane and ossicular chain (malleus, incus, and stapes); and the inner ear, consisting of the cochlea and vestibule and the cochlear and vestibular nerves coursing through the internal auditory canal to the brain stem (Fig. 1a). The pinna is made of skin-covered elastic cartilage folded into a complex, somewhat cone-shaped, structure that tapers to form the external auditory canal. The medial one-third of the external auditory canal is made up of the bone of the tympanic ring, which extends to form the circular groove (sulcus), into which the tympanic membrane is inserted. The membrane itself is an approximately dime-sized condensation of fibrous tissue covered with squamous epithelium (skin cells) on its lateral side and mucosa on its medial side. COCHLEAR IMPLANTS 39 Figure 1a. Anatomical structures of the outer, middle, and inner ear. Courtesy of Susan M. O’Reilly. Ch_03_Pgs_38-72.PMD 8/6/2008, 1:21 PM 39 Figure 1b. Detail of anatomical spaces (scala) of the cochlea and its relationship to the organ of corti. Courtesy of Susan M. O’Reilly. Figure 1c. Organ of corti showing hair cells, position of basilar membrane, and connections to the cochlear nerve. Courtesy of Susan M. O’Reilly. Ch_03_Pgs_38-72.PMD 8/6/2008, 1:21 PM 40 The middle ear houses the ossicular chain. The most lateral bone (malleus) is firmly attached to the tympanic membrane. The globular body of the malleus attaches to the body of the anvil-shaped incus. The long process of the incus attaches with a synovial joint to the top (capitulum ) of the stirrup-shaped stapes bone. The footplate of the stapes is rectangular and fills one of the windows into the cochlea (the “oval window”). The footplate is sealed at its periphery into the oval window by a gasket of fibrous tissue called the annular ligament. This ligament allows...