**3. Types of cochlear implants**

A 60-year-old history protects cochlear implantation technology, which has experienced multiple changes in devices and speech processing strategies. It was about 200 years ago when Alessandro Volta described the early auditory percepts induced by applying a large voltage between his own ears in 1790 [3–6]. Further investigations by Weaver and Bray were focused and resulted in this concept that it might be possible to generate electrical signals mimicking

In 1957, an electrode with receiver coil was successfully implanted for a patient with resected cochlear nerve due to cholesteatoma, which was able to stimulate the apparatus for months, and shockingly, the patient had sound awareness and simple word recognition [8–10]. Following Djourno and Eyries, House started his work in the early 1960s who implanted simple wires, wires with ball electrodes, and even simple arrays into the scala tympani, which finally led into production of implantable device in 1972; this was a beginning point for clini-

At the beginning, there was a resistance from scientific community especially neurophysiologists and otologists against CI; however, the national institute of health (NIH) approved the use of electrical stimulation of auditory nerves as a rehabilitation method in 1977, while

Multichannel CIs were produced in greater numbers due to the food and drug administration (FDA) approval because of their ability of open-set word recognition and better frequency spectrum percepts [3, 6, 14]. Another remarkable progress was occurred in 1991, while continuous interleaved sampling (CIS) strategy was introduced, which developed improved openset word recognition in comparison with previous analogue methods, so that all currently

Sensory hair cells within the cochlea have the responsibility for transforming sound vibration to neural signal in healthy individuals; then, the signal continues its way to auditory cortex through cochlear nerve. Cochlear implants take the place of these cells using electrodes which stimulate the nerve fiber electrically. **Figure 1** illustrates a cochlear implant device. Common cochlear implants have two parts: external component as a hearing aid and internal compo-

The external part is consisted of three parts: a microphone for gathering sounds, a speech processor analyzing and encoding sound into a digital code, and a magnetic headpiece which

The internal part has a receiver stimulator which receives and decodes the data and conducts decoded signal to the electrode array. In the next step, there is a flexible silicone carrier, which has variable number of electrodes. The remaining cochlear nerve fibers are stimulated by the

transmits coded signals by a transcutaneous radiofrequency link to the internal part.

electrode array, which is surgically implanted in scala tympani of the cochlea.

evaluating the outcome in patients with single channel implants [3, 4, 12, 13].

auditory input stimulus [7].

138 An Excursus into Hearing Loss

available strategies are based on CIS [15].

**2. Structure and mechanism of action**

nent which is surgically inserted in mastoid [16].

cal trials [3, 11].

#### **3.1. Totally implantable cochlear implants**

Currently available implants have an external part and need patients to wear it consisted of an external microphone, processor, and transmitting coil for empowering the electrode, which needs a dry and stable environment. Thus, development of totally implantable cochlear implants that make the whole system available underneath the skin is a new area of research. There are several challenges and requirements in the way of this progress including a tiny and sensitive microphone with ability to filter the endogenous noises, as well as a rechargeable battery with appropriate long life. There is a report of three patients with totally implantable cochlear implants [17].

#### **3.2. Unilateral or bilateral cochlear implantation**

Unilateral cochlear implantation was the only option offered at the beginning. Later, it was questioned if the patients would take more benefits from bilateral cochlear implants. Surprisingly, it was revealed that patients with bilateral cochlear implantation show better speech perception and improvement in "hearing in noise." Also, these patients showed a significantly better sound localization in comparison with their single-side implantation condition [18, 19].

Previous studies have concluded that there is no significant difference for audiologic outcomes between unilateral and bilateral cochlear implantation regarding surgical timing, as both ears can be implanted simultaneously or sequentially. Adult studies have shown that the second ear matches the first ear performance at 6 months [20]. The story has a difference when it comes to children, as it has been concluded that patients with simultaneous bilateral cochlear implantation have improved speech recognition and language when compared to children who were implanted sequentially [21].

Current contraindications for cochlear implantation are two absolute and relative categories. Absence of cochlear development, deafness due to lesions of the central auditory pathway, and massive cochlear ossification that prevents electrode insertion are among absolute contraindications. Relative contraindications include aplasia of the acoustic nerve and medical conditions or developmental delays that would severely limit participation in aural rehabilitation.

Cochlear Implants: An Excursus into the Technologies and Clinical Applications

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Cochlear implantation procedure is performed under general anesthesia associated with facial nerve monitoring. Surgeon needs to expose the mastoid, so a postauricular incision is made and soft tissue is dissected; latter, the surgeon makes a subperiosteal pocket for placement of implant magnet. A cortical mastoidectomy is performed associated with finding landmarks of temporal bone, such as incus, tegmen tympani, lateral semicircular canal, and sigmoid sinus. Then, the surgeon opens the facial recess, which is surrounded by chorda tympani, facial nerve, and incus buttress as its boundaries to identify the round window niche

There are different methods for accessing scala tympani after finding the round window; in cochleostomy, the surgeon drills a separate hole and the anterior limit of the round window in extended cochleostomy. The implant is inserted into the cochlea, once the cochlea is opened. For making sure of the proper function of implant, an integrity test is performed by an audiologist at the end of the procedure. X-ray radiography is used to ensure proper location of cochlear implant by some surgeons. At the end, the patient is discharged the same day, and

Cochlear implantation is generally a safe performed surgical procedure throughout the world with globally estimated complication rate of 16% [18]. Requiring additional surgery or cochlear explantation is categorized as major, and complications needing conservative medical management are classified as minor complications. Now, complication rates are decreasing due to improved experience, using smaller incisions and improvements in designing devices, and

Infection is one of the most important major complications of cochlear implantation. Skin infection and acute otitis media are the most common type of implant-related infections ranging from 1 to 12% in the literature. Otitis media and soft tissue infection increase the risk of cochlear implant removal if leading to receiver stimulator infection. Also, it has been reported that cochlear implantation increases the risk of bacterial meningitis as 30-fold greater than general population; however, dawn of vaccination has made these cases sporadic [28]. Facial nerve palsy is another major complication of cochlear implantation, which is estimated to occur in 0.7% of cases due to heat induced by drill, cochleostomy, or reactivation of herpes virus as a result of surgery stress [29]. Finally, device failure is another major complication of cochlear

are generally calculated to be 11.8% for minor and 3.2% for major complications [27].

cochlear implant is usually activated 2–4 weeks postoperatively.

**5. Surgery**

through the recess.

**6. Complications**

The cost-effectiveness of bilateral cochlear implantation has remained controversial despite evident advantages of binaural stimulation. A Canadian study has reported that cochlear implantation is cost-effective in adults compared to no implantation; however, sequential bilateral cochlear implantation has a slight superiority in comparison with unilateral implantation [22]. Other studies have approved cost effectiveness of bilateral simultaneous pediatric implantation and unilateral adult cochlear implantation, although they have not approved cost-effectiveness of bilateral sequential pediatric implantation and bilateral (sequential or simultaneous) adult implantation [23].
