Scala Vestibuli and Scala Tympani Double Array Cochlear Implant

[0001] This application claims priority from U.S. Provisional Patent Application 62/112,209, filed February 5, 2015, which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

[0002] The present invention relates to implantable electrodes for medical devices and specifically to a double branch cochlear implant electrode.

BACKGROUND ART

[0003] A normal ear transmits sounds as shown in Figure 1 through the outer ear 101 to the tympanic membrane (eardrum) 102, which vibrates the ossicles of the middle ear 103 (malleus, incus, and stapes). The stapes footplate is positioned in the oval window 106 that forms an interface to the fluid filled inner ear (the cochlea) 104. Movement of the stapes generates a pressure wave in the cochlea 104 that stimulates the sensory cells of the auditory system (hair cells). The cochlea 104 is a long narrow duct wound spirally around its central axis (called the modiolus) for approximately two and a half turns. The cochlea 104 includes an upper channel known as the scala vestibuli, a middle channel known as the scala media and a lower channel known as the scala tympani. The hair cells connect to the spiral ganglion cells of the cochlear nerve 105 that reside in the modiolus. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104

functions as a transducer to generate electric pulses which are transmitted to the cochlear nerve 105, and ultimately to the brain.

[0004] Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104. To improve impaired hearing, auditory prostheses have been developed. For example, when the impairment is related to operation of the middle ear 103, a conventional hearing aid or middle ear implant may be used to provide acoustic-mechanical stimulation to the auditory system in the form of amplified sound. Or when the impairment is associated with the cochlea 104, a cochlear implant with an implanted stimulation electrode can electrically stimulate auditory nerve tissue with small currents delivered by multiple electrode contacts distributed along the electrode.

[0005] Figure 1 also shows some components of a typical cochlear implant system, including an external microphone that provides an audio signal input to an external signal processor 111 where various signal processing schemes can be implemented. The processed signal is then converted into a digital data format, such as a sequence of data frames, for transmission into the implant 108. Besides receiving the processed audio information, the implant 108 also performs additional signal processing such as error correction, pulse formation, etc., and produces a stimulation pattern (based on the extracted audio information) that is sent through an electrode lead 109 to an implanted electrode array 110.

[0006] Typically, the electrode array 110 includes multiple electrode contacts 112 on its surface that provide selective stimulation of the cochlea 104. Depending on context, the electrode contacts 112 are also referred to as electrode channels. In cochlear implants today, a relatively small number of electrode channels are each associated with relatively broad frequency bands, with each electrode contact 112 addressing a group of neurons with an electric stimulation pulse having a charge that is derived from the instantaneous amplitude of the signal envelope within that frequency band.

[0007] Figure 2 shows various functional blocks in a signal processing arrangement for producing electrode stimulation signals to electrode contacts in an implanted cochlear implant array according to a typical hearing implant system. A pseudo code example of such an arrangement can be set forth as:

Input Signal Preprocessing:

BandPassFilter ( input_sound, band_pass_signals )

Envelope Extraction:

BandPassEnvelope (band_pass_signals, band_pass_envelopes) Stimulation Timing Generation:

TimingGenerate (band_pass_signals, stim_timing) Pulse Generation:

PulseGenerate (band_pass_envelopes , stim_timing, out_pulses) The details of such an arrangement are set forth in the following discussion.

[0008] In the arrangement shown in Figure 2, the initial input sound signal is produced by one or more sensing microphones, which may be omnidirectional and/or directional. Preprocessor Filter Bank 201 pre-processes this input sound signal with a bank of multiple parallel band pass filters (e.g. Infinite Impulse Response (IIR) or Finite Impulse Response (FIR)), each of which is associated with a specific band of audio frequencies, for example, using a filter bank with 12 digital Butterworth band pass filters of 6th order, Infinite Impulse Response (IIR) type, so that the acoustic audio signal is filtered into some K band pass signals, U _\ to UK where each signal corresponds to the band of frequencies for one of the band pass filters. Each output of sufficiently narrow CIS band pass filters for a voiced speech input signal may roughly be regarded as a sinusoid at the center frequency of the band pass filter which is modulated by the envelope signal. This is also due to the quality factor (Q « 3) of the filters. In case of a voiced speech segment, this envelope is approximately periodic, and the repetition rate is equal to the pitch frequency.

Alternatively and without limitation, the Preprocessor Filter Bank 201 may be

implemented based on use of a fast Fourier transform (FFT) or a short-time Fourier transform (STFT). Based on the tonotopic organization of the cochlea, each electrode contact in the scala tympani typically is associated with a specific band pass filter of the Preprocessor Filter Bank 201. The Preprocessor Filter Bank 201 also may perform other initial signal processing functions such as and without limitation automatic gain control (AGC) and/or noise reduction and/or wind noise reduction and/or beamforming and other well-known signal enhancement functions. An example of pseudocode for an infinite impulse response (IIR) filter bank based on a direct form II transposed structure is given by Fontaine et al., Brian Hears: Online Auditory Processing Using Vectorization Over Channels, Frontiers in Neuroinformatics, 2011 ; incorporated herein by reference in its entirety.

[0009] The band pass signals U _\ to C/κ (which can also be thought of as electrode channels) are output to an Envelope Detector 202 and Fine Structure Detector 203. The Envelope Detector 202 extracts characteristic envelope signals outputs Y ₁₍ ... , Y _K that represent the channel-specific band pass envelopes. The envelope extraction can be represented by Y _k = LP( \ U _k \), where |. denotes the absolute value and LP(.) is a low-pass filter; for example, using 12 rectifiers and 12 digital Butterworth low pass filters of 2nd order, IIR-type. Alternatively, the Envelope Detector 202 may extract the Hilbert envelope, if the band pass signals U ₁₍ ... , U _K are generated by orthogonal filters.

[0010] The Fine Structure Detector 203 functions to obtain smooth and robust estimates of the instantaneous frequencies in the signal channels, processing selected temporal fine structure features of the band pass signals U ₁₍ ... , U _K to generate stimulation timing signals X ₁₍ ... , X _K . The band pass signals Uj, ... , U _k can be assumed to be real valued signals, so in the specific case of an analytic orthogonal filter bank, the Fine Structure Detector 203 considers only the real valued part of U _k . The Fine Structure Detector 203 is formed of K independent, equally-structured parallel sub-modules.

[0011] The extracted band-pass signal envelopes Y ₁₍ ... , Y _K from the Envelope Detector 202, and the stimulation timing signals X ₁₍ ... , X _K from the Fine Structure Detector 203 are input signals to a Pulse Generator 204 that produces the electrode stimulation signals Z for the electrode contacts in the implanted electrode array 205. The Pulse Generator 204 applies a patient-specific mapping function— for example, using instantaneous nonlinear compression of the envelope signal (map law)— That is adapted to the needs of the individual cochlear implant user during fitting of the implant in order to achieve natural loudness growth. The Pulse Generator 204 may apply logarithmic function with a form- factor C as a loudness mapping function, which typically is identical across all the band pass analysis channels. In different systems, different specific loudness mapping functions other than a logarithmic function may be used, with just one identical function is applied to all channels or one individual function for each channel to produce the electrode stimulation signals. The electrode stimulation signals typically are a set of symmetrical biphasic current pulses.

[0012] It is well-known in the field that electric stimulation at different locations within the cochlea produce different frequency percepts. The underlying mechanism in normal acoustic hearing is referred to as the tonotopic principle. In cochlear implant users, the tonotopic organization of the cochlea has been extensively investigated; for example, see Vermeire et al., Neural tonotopy in cochlear implants: An evaluation in unilateral cochlear implant patients with unilateral deajhess and tinnitus, Hear Res, 245(1-2), 2008 Sep 12 p. 98-106; and Schatzer et al., Electric-acoustic pitch comparisons in single-sided- deaf cochlear implant users: Frequency-place functions and rate pitch, Hear Res, 309, 2014 Mar, p. 26-35 (both of which are incorporated herein by reference in their entireties).

[0013] In some stimulation signal coding strategies, stimulation pulses are applied at a constant rate across all electrode channels, whereas in other coding strategies, stimulation pulses are applied at a channel-specific rate. Various specific signal processing schemes can be implemented to produce the electrical stimulation signals. Signal processing approaches that are well-known in the field of cochlear implants include continuous interleaved sampling (CIS), channel specific sampling sequences (CSSS) (as described in U. S. Patent No. 6,348,070, incorporated herein by reference), spectral peak (SPEAK), and compressed analog (CA) processing.

[0014] In the CIS strategy, the signal processor only uses the band pass signal envelopes for further processing, i.e., they contain the entire stimulation information. For each electrode channel, the signal envelope is represented as a sequence of biphasic pulses at a constant repetition rate. A characteristic feature of CIS is that the stimulation rate is equal for all electrode channels and there is no relation to the center frequencies of the individual channels. It is intended that the pulse repetition rate is not a temporal cue for the patient (i.e., it should be sufficiently high so that the patient does not perceive tones with a frequency equal to the pulse repetition rate). The pulse repetition rate is usually chosen at greater than twice the bandwidth of the envelope signals (based on the Nyquist theorem).

[0015] In a CIS system, the stimulation pulses are applied in a strictly non-overlapping sequence. Thus, as a typical CIS -feature, only one electrode channel is active at a time and the overall stimulation rate is comparatively high. For example, assuming an overall stimulation rate of 18 kpps and a 12 channel filter bank, the stimulation rate per channel is 1.5 kpps. Such a stimulation rate per channel usually is sufficient for adequate temporal representation of the envelope signal. The maximum overall stimulation rate is limited by the minimum phase duration per pulse. The phase duration cannot be arbitrarily short because, the shorter the pulses, the higher the current amplitudes have to be to elicit action potentials in neurons, and current amplitudes are limited for various practical reasons. For an overall stimulation rate of 18 kpps, the phase duration is 27 μβ, which is near the lower limit.

[0016] The Fine Structure Processing (FSP) strategy by Med-El uses CIS in higher frequency channels, and uses fine structure information present in the band pass signals in the lower frequency, more apical electrode channels. In the FSP electrode channels, the zero crossings of the band pass filtered time signals are tracked, and at each negative to positive zero crossing, a Channel Specific Sampling Sequence (CSSS) is started. Typically CSSS sequences are applied on up to 3 of the most apical electrode channels, covering the frequency range up to 200 or 330 Hz. The FSP arrangement is described further in Hochmair I, Nopp P, Jolly C, Schmidt M, SchoBer H, Garnham C, Anderson I, MED-EL Cochlear Implants: State of the Art and a Glimpse into the Future, Trends in

Amplification, vol. 10, 201 -219, 2006, which is incorporated herein by reference. The FS4 coding strategy differs from FSP in that up to 4 apical channels can have their fine structure information used. In FS4-p, stimulation pulse sequences can be delivered in parallel on any 2 of the 4 FSP electrode channels. With the FSP and FS4 coding strategies, the fine structure information is the instantaneous frequency information of a given electrode channel, which may provide users with an improved hearing sensation, better speech understanding and enhanced perceptual audio quality. See, e.g., U. S. Patent 7,561 ,709; Lorens et al. "Fine structure processing improves speech perception as well as objective and subjective benefits in pediatric MED-EL COMBI 40+ users." International journal of pediatric otorhinolaryngology 74.12 (2010): 1372-1378; and Vermeire et al., "Better speech recognition in noise with the fine structure processing coding strategy. " ORL 72.6 (2010): 305-311 ; all of which are incorporated herein by reference in their entireties.

[0017] Many cochlear implant coding strategies use what is referred to as an n-of-m approach where only some number n electrode channels with the greatest amplitude are stimulated in a given sampling time frame. If, for a given time frame, the amplitude of a specific electrode channel remains higher than the amplitudes of other channels, then that channel will be selected for the whole time frame. Subsequently, the number of electrode channels that are available for coding information is reduced by one, which results in a clustering of stimulation pulses. Thus, fewer electrode channels are available for coding important temporal and spectral properties of the sound signal such as speech onset.

[0018] Sometimes (especially in children), the inner fluid spaces within the cochlea (scala tympani, scala vestibuli, etc.) can become ossified with bony tissue. An abnormally ossified cochlea does not accept the insertion of an entire conventional electrode array. In such situations, two openings can be drilled in the outer surface of the cochlea through which a double electrode array can be inserted, with one array inserted into the basal turn of the cochlea and the other array inserted into the higher first turn of the cochlea.

[0019] In conventional cochlear implants, the electrode array is inserted into the scala tympani, But it also has been suggested that the scala vestibuli could also be used. One problem with that approach is the extremely delicate nature of Reissner's membrane that separates the perilymph fluid in the scala vestibuli from the endolymph fluid in the scala media. Insertion of an electrode array into the scala vestibuli would have to avoid rupturing Reissner's membrane and the resulting mixing of the fluids that would cause a loss of residual natural hearing.

[0020] U. S. Patent 6074422 describes a dual branch cochlear implant electrode with a scala tympani branch and a scala vestibuli branch. This arrangement ruptures Reissner's membrane and destroys any residual natural hearing. And no attempt is made to control the geometry and locations of the electrode contacts on the two different array branches relative to each other. The electrode contacts on each branch are operated independently of each other and locally stimulate adjacent neural tissue.

SUMMARY OF THE INVENTION

[0021] Embodiments of the present invention are directed to an implantable cochlear implant electrode with two parallel branches. A scala tympani branch is configured for insertion into a cochlear scala tympani, and a scala vestibuli branch is configured for insertion into a cochlear scala vestibuli. Electrode contacts are distributed on the outer surface of each branch. There is at least one branch pair of contacts in which one electrode contact on one of the branches cooperates with a counterpart electrode contact on the other branch to develop electrical stimulation signals in the adjacent tissue. The electrode contacts are spaced along the longitudinal axis of each branch so that after surgical insertion into a patient cochlea, at least one branch pair of electrode contacts will be separated by a distance within a defined range of a minimum separation distance.

[0022] In specific embodiments, all the electrode contacts on at least one of the branches may be configured to operate as branch pairs with counterpart electrode contacts on the other branch. The electrode contacts may be uniformly or non-uniformly spaced along the longitudinal axis of each branch. The spatial separation distance may be referenced to an apical-most edge, a basal-most edge, or a center point of each electrode contact in the at least one branch pair of electrode contacts.

[0023] The scala vestibuli branch may be shorter than the scala tympani branch. The electrode branches may have a combined volume less than or equal to the volume of a conventional scala tympani-type cochlear implant electrode. And the electrode contacts may be spaced along the longitudinal axis of each branch so that after surgical insertion into a patient cochlea, multiple branch pairs of electrode contacts will each be spatially separated by distances within a defined range of a minimum separation distance.

[0024] Embodiments of the present invention also include a cochlear implant system having an implantable electrode arrangement according to any of the preceding.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 shows the anatomy of the human ear with a cochlear implant system.

[0026] Figure 2 shows various functional blocks in a signal processing arrangement for producing electrode stimulation signals to electrode contacts in an implanted cochlear implant array according to a typical hearing implant system.

[0027] Figure 3 shows an example of double branch cochlear implant electrode according to an embodiment of the present invention.

[0028] Figure 4A-B show cross-sectional orthogonal and longitudinal views respectively of a double branch cochlear implant electrode according to an embodiment of the present invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0029] Embodiments of the present invention are directed to an implantable cochlear implant electrode with two parallel branches: a scala tympani branch configured for insertion into the scala tympani, and a scala vestibuli branch configured for insertion into the scala vestibuli. By using a sufficiently high number of electrode contacts and by organizing the electrode contacts into branch pairs, the neural tissues located between the paired electrode contacts may be more effectively stimulated.

[0030] Figure 3 shows an example of double branch cochlear implant electrode 300 according to one specific embodiment of the present invention. A scala tympani branch

301 is configured for insertion into a cochlear scala tympani, and a scala vestibuli branch

302 is configured for insertion into a cochlear scala vestibuli. Multiple electrode contacts

303 are distributed on the outer surface of both branches 301 and 302 along their longitudinal axes. In specific embodiments, each branch 301 and 302 may have a different total number of electrode contacts 303, or there may be the same number of electrode contacts 303 on both branches 301 and 302. At least one subset of the electrode contacts 303 is configured to operate as a branch pair of contacts so that in a given branch pair, an electrode contact 303 on one of the branches 301 or 302 cooperates with a counterpart electrode contact 303 on the other branch 302 or 301 so as to develop the electrical stimulation signals in the adjacent tissue in between (the organ of Corti in the cochlear duct).

[0031] Figures 4A-B show cross-sectional orthogonal and longitudinal views respectively of an implanted double branch cochlear implant electrode arrangement where the modiolar axis of the cochlea is on the left side, and the outer radial wall of the cochlea is on the right side. A given branch pair electrode contact 303 on the outer surface of the scala vestibuli branch 301 in the scala vestibuli 401 cooperates with a counterpart electrode contact 303 on the scala tympani branch 302 in the scala tympani 402 to operate as a branch pair of contacts so that electrical stimulation signals to the branch pair result in the development of an electric field between them that stimulates the neural tissue 403 between them in the organ of Corti in the cochlear duct 405. To maximize the effectiveness of that stimulation, the distance between cooperating electrode contacts 303 in branch pair should be as small as possible.

[0032] As shown in the orthogonal cross-section of Fig. 3 A, the scala vestibuli branch 301 located in the scala vestibuli 401 is separated from the scala tympani branch 302 located in the scala tympani 402 by a distance having two dimensional components: a radial displacement component d _x and a vertical displacement component d _y . To avoid trauma to the Reissner's membrane 404, it is better for the scala vestibular branch 301 to be positioned as close as possible to the inner modiolar wall (as far left as possible in Fig. 3 A). Therefore, it is desirable that the scala tympani branch 302 also be positioned as close as possible to the inner modiolar wall in order to minimize the radial displacement component d _x of the separation distance. Conveniently, as a result of the cochlear anatomy, that also will result in minimizing the vertical displacement component d _y of the separation distance.

[0033] And as shown by Fig. 3B, the electrode contacts 303 on a branch pair of contacts also are spaced along the longitudinal axis of each branch 301 and 302 by a longitudinal displacement component d _z . In electrodes according to embodiments of the present invention, the longitudinal spacing of the electrode contacts 303 on the two branches 301 and 302 is controlled to minimize the longitudinal displacement component d _z so that after surgical insertion into a patient cochlea, at least one branch pair of electrode contacts 303 will be separated by a distance within a defined range of a minimum separation distance. That minimum separation distance may specifically by with reference to the longitudinal displacement component d _z , or more ideally, it may be a spatial separation distance within a defined range of a minimum separation distance based on the sum of all three displacement components, d _x , d _y and d _z .

[0034] The term "minimum separation distance" is understood to be defined to within reasonable boundaries. Such a criterion may considered to be fulfilled when the actual separation distance is within a within a defined range— e. g., 1 %, 2%, 5%, 10%, 20%, etc.— of some theoretical minimum distance. Moreover, the electrode contacts are not simple points in space, but actually are extended areas and (if curved) three-dimensional objects. Thus the separation distance is determined relative to specific defined reference points; for example, the centers of the electrode contact areas or the most apical/basal point of the electrode contacts.

[0035] In the simplest case, there may be just a single branch pair— one electrode contact on the scala tympani branch and the another electrode contact on the scala vestibuli branch— that fulfils the minimum separation distance criterion. The electrode contacts of that branch pair are located on their respective array branches so that after surgical insertion of the electrode branches into the scala tympani and scala vestibuli, there will be the desired minimum separation distance between them.

[0036] In a more advanced case, there may be multiple branch pairs of electrode contacts separated by minimum spatial distances. In such embodiments, the exact separation distance criterion can be, for example, that the sum of the individual distances of the different contact pairs is some minimum value, or that the minimum of the square of the sum of the individual distances is some minimum, or the minimum of the sum of the squares of the individual distances is a minimum. In some embodiments with multiple branch pairs, the branch pairs may be organized into one or more subgroups according to specific criteria.

[0037] Configuring the electrode contacts into branch pairs and/or into one or more branch pair groupings may take into account other factors besides separation distance. For example, the size of the electrode contacts may be required to be greater than some defined minimum size in order to ensure a minimum current density (or a maximum electrode impedance). This may lead to a boundary condition that the distance between electrode contacts together on the same branch may not be smaller than a defined threshold distance. Similarly the required (local or overall) flexibility of the electrode branch must be guaranteed and there may be a similar boundary condition.

[0038] In specific cases all the electrode contacts of one branch may have a counterpart electrode contact on the other branch such that all the electrode contacts on at least one branch are part of a branch pair. Alternatively, the individual branches may have different numbers of electrode contacts and just a subset of the electrode contacts of each branch may be organized into branch pairs. In one specific case, one branch (e.g., the scala vestibuli branch) may have just one electrode contact while the other branch has multiple contacts (e.g. , the scala tympani branch). It is also possible that one or more of the electrode contacts may actually be positioned slightly outside the cochlea (e.g., at the round window or oval window electrode opening) and still be part of a branch pair if the minimum spatial separation distance criteria is met. Since the curvature of the scala spaces varies from base to apex, it may be advantageous to have non-uniformly spaced electrode contacts on one or both electrode branches. And in general, the spatial distances between the electrode contacts on the scala vestibuli branch may vary from those in the scala tympani branch.

[0039] To manufacture such a dual branch cochlear implant electrode for a given specific patient, the size of the patient's cochlea initially can be determined, for example, by pre-operative MRI or X ray imaging and/or as described in U. S. Patent Publication 20140228909. From this overall cochlear size measurement, the actual sizes also can be determined for the scala vestibuli and scala tympani chambers. Alternatively, the cochlear size and dimensions may be assumed independently of a specific patient. Using this determined/assumed cochlear size information, the location and spacing of the electrode contacts on both electrode branches can be controlled according to a shortest distance principal calculation.

[0040] The electrode contacts may be uniformly or non-uniformly spaced along the longitudinal axis of each branch. The scala vestibuli branch may be shorter than the scala tympani branch. The electrode branches may have a combined volume less than or equal to the volume of a conventional scala tympani-type cochlear implant electrode. And the electrode contacts may be spaced along the longitudinal axis of each branch so that after surgical insertion into a patient cochlea, multiple branch pairs of electrode contacts will each be spatially separated by distances within a defined range of a minimum separation distance.

[0041] The scala tympani branch may be surgically inserted via the round window, the round window niche, or cochleostomy The scala vestibuli branch may be surgically inserted via the oval window or cochleostomy.. For preservation of residual hearing, insertion through the oval window will not be possible, so a cochleostomy may be preferred for insertion of the scala vestibuli branch. Furthermore, the overall length of the scala vestibuli branch needs to be less than that of the scala tympani branch both because of the inherent anatomy of the inner ear, and also to preserve residual hearing. Similarly, the total overall volume of both electrode branches should not exceed the volume of existing older style electrode arrays. And the scala vestibuli branch needs to be much softer than the scala tympani branch to minimize trauma to Reissner's membrane.

[0042] Stimulation signals developed by the branch pairs of electrode contacts may provide increased sensitivity and speech perception. More specifically, when two paired electrode contacts are both located in the scala tympani, the selectivity of targeting the neighbouring neural structures is significantly lower than is case when one electrode contact is located in the scala tympani and the other cooperating electrode contact is located in the scala vestibuli. This is probably because the neighbouring neural structures in the latter case are located directly between both paired electrode contacts.

[0043] Although various exemplary embodiments of the invention have been disclosed, it should be apparent to those skilled in the art that various changes and modifications can be made which will achieve some of the advantages of the invention without departing from the true scope of the invention.