Sensorineural hearing loss

Introduction

The Implex TICA LZ 3001 is a totally implantable electronic hearing device for the surgical treatment of selected patients who have moderate to severe high-frequency sensorineural hearing loss (SNHL). [1] TICA has obtained CE approval for routine use in Europe. The author has been implanting TICA since 1998. [1,2] The reader is referred to a recent review of implantable hearing aids published by Huttenbrink. [3]

Materials and methods

The TICA LZ 3001 is manufactured by Implex AG Hearing Technology in Munich (www.implex.de; Implex has a U.S. office in Raleigh, N.C.). The device consists of three main parts: the actuator, the membrane sensor (microphone), and the processor module (figure 1). The entire system is designed to be implanted into the mastoid cavity and tympanum. [4]

The actuator. The actuator, which weighs 0.4 g, is made of titanium (medical grade 2, ASTM F67) and was designed to be implanted into the mastoid cavity and tympanum. [4] Transfer of mechanical oscillations to an ossicle in the middle ear is effected by a titanium coupling rod or clip. The transducer is highly tuned, with a resonance frequency in the range of 7 to 10kHz, depending on the dynamic mass load, Below this resonance and down to the low frequencies, the frequency response is flat, with a small ripple of less than [+ or -]1 dB. Vibration amplitude at the low and middle frequencies is about 60 nm, with a transducer voltage of 1 V, corresponding to an equivalent sound pressure level (SPL) of 100 dB SPL up to 1 kHz. At the higher frequencies, up to 10 kHz, the output level increases to beyond 130 dB SPL. Nonlinear distortions at maximum volume (1 V) are small (THD [less than]0.1%) throughout the entire transfer range.

Because of its short on-time (50 [micro]s and short off-time ([sim]2 ms), the dynamic properties of the transducer allow for good transmission of audio signals and fast changes in the time domain--that is, of plosives in speech signals. The electric power consumption at full volume and broadband signals is in the region of 1 [micro]W. The hermetically sealed sensor is implanted in the posterior wall of the auditory canal.

The sensor. The membrane sensor, which also weighs 0.4 g, has a membrane diameter of 4.5 mm. Despite this miniaturization, the sound pressure transfer function is flat and the bandwidth amounts to approximately 10kHz. Because of its high tuning and high no-load resonance frequency, the microphone is rather insensitive to post-surgical changes to the loading mass on the microphone membrane produced by the covering skin of the auditory canal. Transfer factor at 1,000 Hz is approximately 1.5 mV/Pa.

Because of its small mass, the sensor is highly insensitive to environmental mechanical disturbances. It is made of titanium and is hermetically sealed. Full metal encapsulation and additional internal electronic components protect the sensor against environmental electromagnetic influences. A module suited for subcutaneous implantation behind the ear contains the signal-processing electronics and an integrated battery that can be recharged transcutaneously with a portable charger. [5] The electronics' basic audiologic features are provided by a flexible, digitally programmable, three-channel AGC system with a peak clipping function. Total bandwidth is approximately 10 kHz.

Battery recharging time is about 2 hours and the implant operating time is 50 hours. However, most patients report that they recharge the battery for 30 to 45 minutes on a daily basis. When the battery's lifetime expires, detachable sensor and transducer connectors allow for the replacement of the processor module while the sensor and actuator are left in situ. A small wireless remote-control device allows the patient to turn the device on and off, adjust the volume, and select from among four different hearing programs.

The processor module. The processor module incorporates an energy source and a digitally programmable three-channel audioprocessor. [5] Similar to a CI receiver, the processor module is implanted subcutaneously behind the auricle.

The three TICA parts are not permanently coupled. Both the sensor and actuator contain wires with plugs that are connected to the processor unit by the surgeon. In case of a loss of energy, the processor module can be easily exchanged while the actuator and sensor both remain implanted.

Results and discussion

The heart of the implant is a disk-shaped heteromorph piezoelectric actuator that is coupled to the incus. [4,6,7] Unlike the loudspeaker of a conventional hearing aid, the actuator does not produce sound but micromechanical vibrations. These allow the ossicular chain to be driven in the physiologic amplitude range. The actuator heteromorph consists of two different disks: a piezoelectric disk and a titanium disk. The piezoelectric disk drives the titanium disk, resulting in the desired vibrations. These are attached to the incus with a coupling axis that has a coupling element at its tip (figure 2). Titanium clips are available for the long incus process. For the incus body, the titanium tip of the coupling rod is inserted into a laser-made bony indentation and fixed with ionomeric cement gel. [6]

In vitro studies with laser Doppler velocimetry revealed a broadband and flat frequency response from 50 Hz to 10 kHz, producing power equivalent to 100dB at 1 kHz and up to 130 dB at 7 kHz (figure 3). Thus, the TICA LZ 3001 is specifically designed for high-frequency SNHL. Output impedance is remarkably high. Output impedance plays a crucial role in overcoming the ear's input impedance and possible postoperative scar formation.

Animal experiments revealed auditory brainstem responses that were identical to those for sound stimulation. [8] Energy consumption was so low that a totally implantable device could be conceived. A totally implantable device requires an implantable sound sensor (microphone). Because of the outer ear's unique acoustic properties that contribute to noise suppression and auditory orientation, a sensor's best location is thought to be the eardrum, ossicles, or outer ear canal. Thus, a membranous sensor was developed to be implanted subcutaneously in the dorsal wall of the external auditory canal. [9] In its interior, it possesses a microphone that is stimulated by the sensor's subcutaneous titanium membrane. Similar to the actuator, the sensor's frequency response could be developed to be flat from 50 Hz to 8 kHz. For implantation, the sensor possesses an integrated silicon retention collar that holds it in place after surgery.

Eight weeks after implantation, the TICA is fitted to the patient's individual SNHL by the audiologist. This is accomplished by means of wireless digital programming through the intact skin. Various sittings over a period of weeks to months are usually required until the audiologic adjustment is optimized.

Patient selection criteria are indicated in the table and in figure 4. All patients who underwent implantation had failed to be successfully rehabilitated with conventional hearing aids. Most patients who have been successfully implanted describe their hearing as clear or natural. Moreover, patients report directional hearing (figure 5) and a median daily duration of use of 16 hours.

A prospective intra-individually controlled study with an enrollment of 20 consecutive patients who had an irreversible, chronic, bilateral, moderate to severe SNHL allowed for a 6-month followup of 19 per-protocol patients. All patients had failed to benefit from conventional hearing aids. Schuller's x-ray revealed the correct postoperative anatomic location of the implants (figure 6). [10] Discrimination of phonetically balanced monosyllables was improved significantly in 18 of the 19 patients and reached 100% in 14 patients (preoperative: 4 patients). One hundred percent of the standardized sentences were understood by 15 of the patients. In the presence of noise, the sentence recognition threshold (score of 50%) was from -2 to 1 dB signal-to-noise ratio. Functional gain reached 55 dB at 3 kHz. The articulation index was improved by 105%. Auditory localization was correct by 89.5%. Analog scaling revealed a natural sound impression of 100% (median: +17.5%,-0%; preoperative: 70%) and only 5% distortion (pr eoperative: 30%). Using the standardized Gothenburg profile subjective evaluation of hearing, patients' orientation, social behavior, and self-confidence were increased to 80 to 88% of the maximum score; there were 17 responders and three nonresponders. Only mild adverse events were observed, most of which resolved. Thus, the treatment of SNHL with the totally implantable hearing system appears to be an efficient and safe method for treating patients who cannot benefit from hearing aids.

From the Department of Otolaryngology, the University of Tubingen, Germany.

References

(1.) Zenner HP, Leysieffer H. Totally implantable hearing device for sensorineural hearing loss. Lancet 1998;352:1751.

(2.) Zenner HP, Maassen MM, Plinkert PK, et al. [First implantation of a totally implantable electronic hearing aid in patients with inner ear hearing loss]. HNO 1998;46:844-52.

(3.) Huttenbrink KB. Current status and critical reflections on implantable hearing aids. Am J Otol 1999;20:409-15.

(4.) Leysieffer H, Baumann JW, Muller G, Zenner HP. [An implantable piezoelectric hearing aid transducer for inner ear deafness. II: Clinical implant]. HNO 1997;45:801-15.

(5.) Leysieffer H, Baumann JW, Mayer R, et al. [A totally implantable hearing aid for inner ear deafness: TICA LZ 3001]. HNO 1998;46:853-63.

(6.) Lehner RL, Maassen MM, Plester D, Zenner HP. [Ossicular coupling of an implantable hearing aid transducer using an Er:YAG laser]. HNO 1997;45:867-71.

(7.) Lehner R, Maassen MM, Leysieffer H. [Cold deformation elements for attaching an implantable hearing aid transducer to ear ossicles or perilymph]. HNO l998;46:27-37.

(8.) Plinkert PK, Baumann JW, Lenarz T, et al. [In vivo studies of piezoelectric implantable hearing aid transducer in the cat]. HNO 1997;45:828-39.

(9.) Leysieffer H, Muller G, Zenner HP. [An implantable microphone for electronic hearing aids]. HNO 1997;45:816-27.

(10.) Maassen MM, Lehner R, Dammann F, et al. [Value of Schuller conventional roentgen diagnosis and computerized tomography of the temporal bone in preoperative diagnosis of the Tubingen implantable cochlear amplifier]. HNO 1998;46:220-7.

Indications for the TICA LZ 3001 implantable hearing device

Three criteria should be met

1. Problems in applying conventional hearing aids

Medical and psychosocial problems

* Intolerance of the occlusion of the auditory canal because of the earmold

* Repeated inflammation of the auditory canal

* Impairment of manual motor skills (joints, tremor, paralysis), which renders the daily use of tiny hearing aids and/or the use of tiny elements impossible

* Defamation or discrimination because of stigmatization

Auditory problems

* Unbearable feedback noise

* Distortion

* Limited speech comprehension

* Reduced ability to communicate in background noise

* Steeply sloped hearing loss with severe difficulty in the high-pitch ranges (figure 4)

Certain professions

* Musicians

* Athletes, sports instructors

* Language professionals (translators, teachers)

* Workers who use earphones or stethoscopes (call centers, telephone information services, health professionals)

* Workers for whom the telephone is indispensable and phone conversations are not possible with a conventional hearing aid

2. Moderate to severe high-frequency sensorineural hearing loss. The maximum low-frequency loss should be no more than 30 dB hearing loss at 0.5 kHz. In addition, the hearing loss sloping between 0.5 and 2 kHz should be approximately 30dB or more (30/30 rule). The difference between both ears should be no more than 20 dB.

3. Schuller's x-ray should display pneumatization grades III/IV.

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