INTRODUCTION
Cochlear implant (CI) is a widely accepted treatment for bilateral profound sensorineural hearing loss nowadays. The benefits of CI in the field of hearing and speech development as well as in the acquisition of language are presently unquestionable. The USA Food and Drug Administration (FDA) approved the use of modern, multichannel CI in adults in 1985. In 1990, FDA approved the use of CI for children down to the age of 2 years and later above the age of 12 months in 2002.1 Many publications recommend and support the use of CI as early as one year of age.2,3
HISTORY OF COCHLEAR IMPLANT
Andre Djourno and Charles Eyries were the first persons that implanted auditory prosthesis in human's ear in 1957. The receiver was able to differentiate between lower and higher frequency stimuli but could not understand speech. Their work was continued and highly studied by William House in 1962.4 More than 1000 patients were implanted between 1972 and the mid-1980s. However, the FDA only approved the House 3M single-channel CI in 1984. Later, National Institutes of Health (NIH) of USA funded extensive CI researches at several institutions to improve the quality of CI. Works of CI were funded too by a variety of sources at University of California, San Francisco, University of Melbourne, Australia and Technical University of Vienna, Austria. Later, they became the foundation of cochlear implants manufactured worldwide, namely Advanced Bionics, Cochlear Corporation and MED- EL Corporation, respectively.5
IMPLANT DEVICE
A cochlear implant is not like a hearing aid that merely amplifies the sound. It stimulates the auditory nerve directly, bypassing the cochlear hair cells. It does not restore normal hearing, but rather it allows the perception of sound sensation. It consists of external and internal parts (Fig. 1). The external part includes a microphone, speech processor and transmitter meanwhile the internal part includes a receiver and electrode array.2
The sound is picked up by the microphone from the environment, and then digitalized by the speech processor as electrical impulses. Later, these signals are sent via the transmitter to the internal portion of the device, which is surgically implanted. The receiver, which is under the scalp, picks up the signals and delivers them to the electrode array that is inserted into the cochlea. The electrodes along the array directly stimulate the auditory nerve.
INDICATIONS OF IMPLANTATION
The decision to implant CI is a team decision. The CI team should include otologist, audiologist, speech-language rehabilitation therapist, psychologist, social worker and educational specialist. The decision of implant is based on the hearing status, medical fitness for surgery, available educational options and support systems. The surgical placement of the implant alone does not result in improved hearing and in fact, is only the beginning of a long and involved learning process. Success depends on motivation and a long-term commitment to therapy and education.
- Audiological considerations (Based on FDA US1)
- Adult postlingual candidate: Bilateral sensorineural hearing loss with hearing thresholds above 70 dB for mean frequencies of 500, 1000, 2000, and 4000 Hz, free field audiometry with hearing aids above 55 dB in the same frequencies and with less than 40% speech discrimination using open lists of words at a stimulation intensity of 65 dB HL (Fig. 2).3
- Children prelingual or postlingual candidate: Bilateral sensorineural hearing loss with hearing thresholds exceeding 90 dB HL for mean frequencies of 500, 1000, 2000 and 4000 Hz, with less than 40% speech discrimination using open lists of words at a stimulation intensity of 65 dB HL (Fig. 2).
- Failure of trial of hearing aids for 3–6 months in both children and adults.
- Medical considerations
- Medical fitness for anesthesia and surgery.
- Congenital malformations manifesting with bilateral agenesis of cochlea based on CT Scan is not suitable for CI.
- Lack of functionality of the auditory pathway or presence of diseases causing central type hearing loss based on MRI scan is not suitable for CI.
- Psychological considerations
- Patients should not have any severe psychiatric illnesses or learning difficulties due to abnormalities of central nervous system.
- Patients or guardians should understand and agree for long-term rehabilitation program after the implantation.
LATEST UPDATES OF COCHLEAR IMPLANT
Nowadays, bilateral cochlear implantation is advised especially for children if the financial support is not an issue. Various studies have shown clear benefits among patients who underwent bilateral implantation.6–84
These benefits are:
- Being able to locate sounds
- Obtaining a summation effect
- Avoiding the “head shadow” effect
- Improving speech discrimination in noisy environments.
With expanding inclusion criteria for cochlear implantation, the number of prelingually deafened persons who are implanted as adults increases. Straatman LV et al concluded that the quality of life and speech recognition in prelingually deafened adults were significantly improved as a result of CI. The range of age of implantation was between 21 to 47 years in prelingual adults. Scores on the Nijmegan Cochlear Implant Questionnaire, the Total Glasgow Benefit Inventory Score and the Health Utility Index 3 were all significantly improved with CI in his study.9 Klop WM et al and Van Dijkhuizen JN et al also published similar results. 10,11
The peer-reviewed literature provides strong evidence that CI in older adults is safe. It improves speech understanding, communications, social participations and mental health in older patients who continue to experience poor word understanding despite appropriately fitted hearing aids.12 However, there is a significant gap in knowledge regarding how CI rehabilitation interacts with changing psychosocial and functional status with aging.13
Cochlear implant in single-sided deafness (SSD) appears as another choice of treatment in hearing rehabilitation. CI offers the potential to restore binaural sound processing and sound localization, and suppress tinnitus in patients with SSD. Tokita J et al have reviewed recent studies and case reports on CI in SSD that showed promising results in term of improvement of sound localization, speech discrimination and tinnitus suppression in SSD patients.14
Cochlear implant in patients with neurofibromatosis type 2 and patients with vestibular schwannoma in the only hearing ear is a reasonable hearing rehabilitation option, provided early surgical intervention or radiotherapy with preservation of the cochlear nerve.15,16,17 Although auditory brainstem implant (ABI) has been the standard surgical treatment for these patients, most of these patients can achieve good speech discrimination and telephone use after cochlear implantation. Promontory stimulation or MRI up to 1.5 Teslas for evaluation of the integrity of the cochlear nerve is recommended.
SURGICAL STEPS OF MINIMALLY INVASIVE WITH HEARING PRESERVATION COCHLEAR IMPLANT SURGERY TECHNIQUE (FIGS. 3 TO 32)
Minimally invasive with hearing preservation cochlear implant surgery has become the mainstay of surgical technique in experienced implant centres.5
Fig. 3: A 3 cm postauricular incision marking, 1–2 cm from the sulcus behind the pinna is made without posterior extension.
Fig. 4: Clean and drape the site of operation, infiltration of 2–5 milliliters of local anesthetics (Marcaine 0.5%, containing bupivacaine hydrochloride 5 mg with adrenaline 5 mcg).
The advantages include reduced blood loss and tissue trauma, improved cosmetics, less hair removal, reduced surgical time and most importantly, atraumatic electrode insertion via round window approach with hearing preservation.6
Fig. 6: A double-layer skin flap is performed. A double-layer skin flap may reduce the chance of infection because the incisions are at different locations and layers and it allows better healing too. The first layer, consisting of the skin and subcutaneous tissue, is raised and retracted. After that, the second layer, consisting of the muscle and periosteum, is incised separately and then retracted in another location. The subperiosteal pocket is created for receiver stimulator.
Fig. 8: A cortical mastoidectomy is performed without saucerization of the cavity with a cutting burr size 7–8 initially, then size 5–6 until the lateral semicircular canal is identified, while maintaining good irrigation.
Fig. 9: Place the processor template behind the ear and position the implant template on the skull in order to check its proper position. Position the implant template in such a way that the processor template is in front and they are not overlapping each other.
Fig. 10: The bed of the stimulator of the implant is drilled approximately 1–2 mm into a flat bony surface to increase stability.
Fig. 11: The adequacy of the depth of the receiver bed is checked with receiver template. The handle of the receiver template will be dislodged once the receiver template is able to stay at the bed.
Fig. 12: A smooth, deep and wide enough electrode channel is drilled in the bone leading to the mastoid for the placement of the electrode. Because the electrode exits on the lateral side of the implant, the electrode lead comes out superiorly for the left ear and inferiorly for the right ear. This may be different in different types of implant.
Fig. 14: The fossa incudis is located and the short process of the incus is identified to ensure proper orientation of the posterior tympanotomy. The posterior tympanotomy is a triangular opening made between the mastoid air cells and the middle ear space, which is referred to as the facial recess. The posterior tympanotomy is bordered by the vertical portion of the facial nerve as posterior limit, the annulus and chorda tympanic as anterior limit and the superior limit is the posterior bony buttress at the level of the fossa incudis.
Fig. 15: Using the facial monitor to check the exact location of the vertical segment of facial nerve.
Fig. 16: Posterior tympanotomy is done by drilling with a 3 mm diamond burr, with high magnification and copious irrigation.
Fig. 17: The following should be visible after posterior tympanotomy: the long process of the incus, the incudostapedial joint, the pyramid eminence, the stapedius tendon, the promontory and the round window niche.
Two approaches to scala tympanic for electrode insertion, either via (A) Round window approach or (B) cochleostomy.
Fig. 18: A portion of the anterior inferior bony margin of RW and the superior overhang of the RW niche are drilled away to expose the RW membrane.
Fig. 19: A clear view of the RW membrane is crucial for round window approach in electrode insertion.
Fig. 20: 1 mL of 4 mg dexamethasone is instilled into middle ear space before opening of the RW membrane to reduce any inflammation process secondary to manipulation and avoid further damage to residual hair cells.
Fig. 21: A micro-hook is used to incise the RW membrane before inserting the electrode array into the cochlea.
Fig. 22: Surgical claw or the angled micro forceps is used to maneuver the electrode array. The electrode lead is held very carefully at the proximal thicker part, just above the marker ring. The tip of the electrode array is guided towards the cochlea opening. After the tip is gently maneuvered further into the cochlea, gripping of the electrode array between the contacts is done.
Fig. 23: After the electrode array is fully inserted, the marker ring will seal the cochlea opening.
Fig. 24: A small piece of temporalis fascia is placed around the electrode array at the entrance to the cochlea to secure the electrode array and to seal the opening.
Fig. 25: Cochleostomy is made inferior and slightly anterior to the round window niche. The cochlea is drilled to expose intact endosteum. The bony lip of the cochleostomy site is smoothed with a diamond burr.
Fig. 27: 1 mL of 4 mg dexamethasone is instilled into middle ear space before opening of the endosteum to reduce any inflammation process secondary to manipulation and avoid further damage to residual hair cells.
Fig. 28: A micro-hook is used to incise the endosteum before inserting the electrode array into the cochlea.
Fig. 30: A small piece of temporalis fascia is placed around the electrode array at the entrance to the cochlea to secure the electrode array and to seal the opening.
Fig. 31: The electrode lead is looped in the mastoid cavity below the surface of the bone, using the cortical overhang to hold it in place. A bone wax is used to immobilize the electrode at the electrode channel. Intraoperative measurements are performed and to check that the electrode is in the scala tympani. Surgical wound is closed in layers and covered with mastoid dressing.
Tips and Pearls
REFERENCES
- Dettman S, Pinder D, Briggs RJS, et al. Communication development in children who receive the cochlear implant younger than 12 months: risks versus benefits. Ear Hear. 2007;28:11–18.
- Holt RF, Svirsky MA. An exploratory look at peadiatrics cochlear implantation: is earliest always best? Ear Hear. 2008;29:492–511.
- Doyle JB, Doyle JH, Turnbull FM, et al. Electrical stimulation in eighth nerve deafness. A preliminary report. Bulletin of the Los Angeles Neurological Society. 1963;28:148–50.
- Eisen MD. The history of cochlear implants. In J. K. Niparko (Ed), cochlear implants: Principles and practices (2nd Ed). Philadelphia: Lippincott Williams & Wilkins.
- Schleich P, Nopp P, D’Haese P. Head shadow, squelch, and summation effects in bilateral users of the Med-EL Combi 40/40+ cochlear implant. Ear Hear. 2004;25:197–204.
- Ramos-Macias A, Deive-Maggiolo L, Artiles-Cabrera O, et al. Bilateral cochlear implants in children: acquisition of binaural hearing. Acta Otorrinolaringol Esp. 2013;64:31–6.
- Litovsky RY, Goupell MJ, Godar S, et al. Studies on bilateral cochlear implants at the university of Wisconsin's binaural hearing and speech laboratory. J Am Acad Audiol. 2012;23:476–94.
- Straatman LV, Huinck WJ, Langereis MC, et al. Cochlear implantation in late-implanted prelingually deafened adults: changes in quality of life. Otol Neurotol. 2014;35:253–9.
- Klop WM, Briaire JJ, Stiggelbout AM, et al. Cochlear implant outcomes and quality of life in adults with prelingual deafness. Laryngoscope. 2007;117: 1982–7.
- Van Dijkhuizen JN, Beers M, Boermans PP, et al. Speech intelligibility as a predictor of cochlear implant outcome in prelingually deafened adults. Ear Hear. 2011;32:445–58.
- Yeagle JD, Ceh KM, Francis HW. Geriatric cochlear implantation. Oper Tech Otolaryngol Head Neck Surg. 2010;4:266–271.
- Clark JH, Yeagle J, Arbaje Al, et al. Cochlear implant rehabilitation in older adults: literature review and proposal of a conceptual framework. J Am Geriatr Soc. 2012;60:1936–45.
- Tokita J, Dunn C, Hansen MR. Cochlear implantation and single-sided deafness. Curr Opin Otolaryngol Head Neck Surg. 2014;22:353–8.
- Mukherjee P, Ramsden JD, Donnelly N, et al. Cochlear implants to treat deafness caused by vestibular schwannomas. Otol Neurotol. 2013;34:1291–8.
- Celis-Aguilar E, Lassaletta L, Gavilan J. Cochlear implantation in patients with neurofibromatosis type 2 and patients with vestibular schwannoma in the only hearing ear. Int J Otolaryngol. 2012;2012:157497.
- Trotter MI, Briggs RJ. Cochlear implantation in neurofibromatosis type 2 after radiation therapy. Otol Neurotol. 2010;31:216–9.