WO2010034080A1

WO2010034080A1 - Medical implant with integrity testing

Info

Publication number: WO2010034080A1
Application number: PCT/AU2009/001287
Authority: WO
Inventors: John Chambers
Original assignee: Cochlear Limited
Priority date: 2008-09-26
Filing date: 2009-09-28
Publication date: 2010-04-01

Abstract

Disclosed is a system and method of testing a medical implant implanted in a user. The system applies an electrical current to the user as a test signal, and measures the resulting signal as a measurement signal. The measurement signal may be used to determine a status of the medical implant. The test signal may be applied to the user via the medical implant itself and measured by a pair of externally- applied electrodes connected to an external part of the implant system, or may be applied by the pair of externally-applied electrodes by the external part of the implant system and measured by the medical implant. In one form, the medical implant system is a cochlear implant system.

Description

MEDICAL IMPLANT WITH INTEGRITY TESTING

TECHNICAL FIELD

The present invention relates to medical implants and to testing of the implants.

PRIORITY

This application claims priority from Australian Provisional Patent Application No. 2008905066 entitled "Medical Implant with Integrity Testing", filed on 26 September 2008.

The entire content of this application is hereby incorporated by reference.

BACKGROUND

A variety of medical implants apply electrical energy to tissue of a patient to stimulate that tissue. Examples of such implants include pace makers, auditory brain stem implants (ABI), devices using Functional Electrical Stimulation (FES) techniques, Spinal Cord Stimulators and cochlear implants.

A cochlear implant allows for electrical stimulating signals to be applied directly to the auditory nerve fibres of a patient, allowing the brain to perceive a hearing sensation approximating the natural hearing sensation. These stimulating signals are applied by an array of electrodes implanted into the patient's cochlea.

The electrode array is connected to a stimulator unit which generates the electrical signals for delivery to the electrode array. The stimulator unit in turn is operationally connected to a signal processing unit which also contains a microphone for receiving audio signals from the environment, and for processing these signals to generate control signals for the stimulator.

The implant delivers controlled electrical stimulation to the neural sensors of the cochlea via twenty or more individual wires and electrodes. This multiplicity of individual electrodes and associated channels of electrical stimulation conveys a perception of sound since the neurosensory cells that line the narrow canals of the cochlea are distributed tonotopically such that stimulation applied to one location is perceived as a different frequency or pitch to that applied elsewhere. Consequently these prostheses make use of sophisticated electronic circuitry to distribute the various spectral elements of incoming sound into separate channels of electrical stimulation that must be customised for the comfort and safety of each individual user.

Figure 1 shows an arrangement of a typical prior art cochlear implant system. Figure IA shows the implanted part or medical implant 100, in one form comprising stimulator 110, containing electronics circuitry for data communications and processing, stimulation delivery and neural response measurement and tissue interface array of electrodes 111. Receiver 112 receives control data from the external part 120 (see Figure IB), via electromagnetic waves through the scalp of the user, and provides these to the internal part 110. External part 120 contains microphones, batteries, electronic circuitry, controls and connectors. Also shown in Figure IB is corresponding transmitter 121 which transmits control signals from external part 120 to the receiver 112.

Figure 1C shows the external part 120, with optional accessories such as a cabled pair of earphones 130 that allows a carer or guardian to monitor the quality of microphone signals. Also shown is another optional audio accessory 140 for use with the external part 120 such as an alternate microphone or audio program source such as an MP3 player.

Whilst the tiny cable and electrode systems of these prostheses are designed to be robust and reliable, their small size and flexibility never-the-less render them vulnerable to surgical mishandling during implantation and trauma or misuse over their 50 or more year operating life span. Whilst diagnostic imaging and post implantation testing prior to wound closure can confirm electrical operation and correct placement of electrode systems, device performance degradation or failure from mishandling, trauma or surgical migration may not become evident for several months or years.

In addition to conversational feedback from the recipient, operational integrity of cochlea implants and electrode systems invivo is currently verified from the presence of electrical signal artefacts associated with the delivery of electrical stimulation pulses from the implant as shown in prior art Figure 2. Shown in Figure 2 is a cochlear implant system 500 including internal/implanted section 100 including stimulator 110 and associated stimulating electrode array 111 implanted into the cochlea of user 50, and external section 120, being (in the case of a cochlear implant system), a speech/data processor for converting input audio signals into electrical control signals. Also shown in Figure 2 is diagnostic system 200 which receives neural response data from the implant system 500 for therapeutic diagnosis and prosthesis customisation.

Detecting these signal artefacts, or even the electrical stimulation signals directly, for diagnostic purposes, unfortunately requires electrically isolated, mains powered, computer driven equipment and electrodes for placement on the recipient's head. The operational complexity, cost and bulk of this equipment limits the use and availability of this equipment to clinics and specialist medical centres only.

SUMMARY

According to one aspect of the present invention, there is provided a method of testing a medical implant implanted in a user of a medical implant system comprising the medical implant and an external part, the method comprising: generating at the external part, a test signal for application to the user via one of, a pair of electrodes connected to the external part and applied to the user, or via the medical implant; receiving at the external part, a measurement signal via the other of the pair of electrodes or the medical implant, in response to the test signal; and generating an output from the external part, representative of the received measurement signal.

hi one form, the method further comprises prompting the user or assistant, to insert the pair of electrodes into one of an accessory output or an accessory input of the external part.

In one form, the medical implant system is a cochlear implant system and the external part is a processor and the medical implant is a cochlear implant.

In one form, the accessory input is a microphone input and the accessory output is a headphone output.

In one form, the method further comprises prompting the user or an assistant to connect the pair of electrodes to one of the microphone input or the headphone output. In one form, the method further comprises instructing the user or the assistant where to place the pair of electrodes on the user.

In one form, the output generated from the external part comprises a diagnosis of a state of the medical implant.

In one form, the output generated from the external part comprises data that may be further processed to provide a diagnosis of a state of the medical implant.

In one form, the step of providing the diagnosis of the state of the medical implant comprises comparing the measurement signal with a comparison signal associated with a particular state of the medical implant.

In one form, the method only executes after a preset command sequence is input.

According to second aspect of the present invention, there is provided a method of testing a medical implant implanted in a user of a medical implant system comprising the medical implant and an external part, the method comprising: generating at the external part, a test signal for application to the user via a pair of electrodes connected to the external part and applied to the user; measuring at the medical implant, a measurement signal resulting from the application of the test signal to the user; transmitting the measurement signal from the medical implant to the external part; receiving at the external part, the measurement signal; and generating an output from the external part, representative of the received measurement signal.

According to a third aspect of the present invention, there is provided a method of testing a medical implant implanted in a user of a medical implant system comprising the medical implant and an external part, the method comprising: generating at the external part, a test signal; transmitting the test signal to the medical implant; applying the test signal to the user via the medical implant; measuring via a pair of electrodes applied to the user and connected to the external part, a measurement signal resulting from the application of the test signal to the user; receiving at the external part, via the connected pair of electrodes, the measurement signal; and generating an output from the external part, representative of the received measurement signal.

According to a fourth aspect of the present invention, there is provided a medical implant system for testing a medical implant, the medical implant system comprising: the medical implant; and an external part comprising a test signal generator for generating a test signal for application to the user via one of, a pair of electrodes connected to the external part and applied to the user, or via the medical implant; a receiver for receiving a measurement signal via the other of the pair of electrodes or the medical implant, in response to the test signal; and an output generator for generating an output from the external part, representative of the received measurement signal.

In one form, the medical implant system is a cochlear implant system and the medical implant is a cochlear implant and the external part is a processor.

According to a fifth aspect of the present invention, there is provided a medical implant system for testing a medical implant implanted in a user of the medical implant system, the medical implant system comprising: the medical implant; and an external part comprising: a test signal generator for generating a test signal for application to the user via a pair of electrodes connected to the external part and applied to the user; a receiver for receiving a measured signal measured by the medical implant, resulting from the application of the test signal to the user; and an output generator for generating an output from the external part, representative of the received measurement signal. In one form, the medical implant system is a cochlear implant system and the medical implant is a cochlear implant and the external part is a processor.

In one form, the cochlear implant comprises a stimulator and an array of electrodes, and wherein the measurement signal is measured by the array of electrodes.

According to a sixth aspect of the present invention, there is provided a medical implant system for testing a medical implant implanted in a user of the medical implant system, the medical implant system comprising: the medical implant; and an external part comprising: a test signal generator for generating a test signal for application to the user via the medical implant; a receiver for receiving a measured signal measured by a pair of electrodes applied to the user, resulting from the application of the test signal to the user by the medical implant; and an output generator for generating an output from the external part, representative of the received measurement signal.

In one form, the cochlear implant comprises a stimulator and an array of electrodes and the test signal is applied to the user via the array of electrodes.

According to a further aspect of the present invention, there is provided a medical implant system for testing a medical implant, the medical implant comprising: the medical implant; and an external part having stored thereon, instructions for causing the medical implant system to perform the method of any one of the aspects of the present invention.

According to another aspect of the present invention, there is provided a method of testing a medical implant implanted within a user, the method comprising: externally applying an electric current to tissue around the implanted medical implant; and measuring one or more electrical characteristics of one or more elements of the implanted medical implant resulting from the applied electric current.

In one form, the medical implant is a cochlear implant and the electric current is applied to the user's head.

According to yet a further aspect of the present invention, there is provided a machine readable medium containing instructions to cause a machine to perform the method of any one of the previous aspects of the present invention.

In one form, the machine is a cochlear implant system.

According to a further aspect of the present invention, there is provided a kit comprising the machine readable medium, at least one pair of electrodes for use in the system of any one of the previous aspects of the present invention; and a set of instructions for the use of the system.

In one form, the at least one electrode pair comprises circuitry for processing at least a portion of signals transmitted through the at least one electrode pair.

DRAWINGS The various aspects of the present invention will now be described in more detail with reference to the following figures in which:

Figure IA - shows an internal section of a prior art cochlear implant;

Figure IB - shows an external section of a prior art cochlear implant;

Figure 1C - shows the external section of Figure IB with optional accessories; Figure 2 - shows a typical prior art system for performing integrity testing on a convention medical implant;

Figure 3 - shows a system according to one aspect of the present invention in a first mode of operation;

Figure 4 - shows the system of Figure 3 in operation; Figure 5 - shows an arrangement of an inserted cochlear implant in normal operation being tested;

Figure 6 - shows a graph of electrode voltage v electrode pair number for the arrangement of

Figure 5; Figure 7 - shows an arrangement of a damaged cochlear implant inserted in a user;

Figure 8 - shows a graph of electrode voltage v electrode pair number for the arrangement of

Figure 7;

Figure 9 - shows another arrangement of a cochlear implant with another fault;

Figure 10 - shows a graph of a simulated sinusoidal signal; Figure 11 - shows the simulated sinusoidal signal of Figure 10 with random noise components;

Figure 12 - shows a graph of an average of 50 "noisy" sinusoids;

Figure 13 - shows a system according to one aspect of the present invention in a second mode of operation;

Figure 14 - shows a system block diagram of one exemplary application the present invention; and

Figure 15 - shows an example of one type of kit for sale for use with existing cochlear implant systems to use one aspect of the present invention;

Figure 16 - shows an example of one type of external electrode system with a reversible connector; Figure 17 - shows an alternative arrangement to that shown in Figure 16; and

Figure 18 - shows a further alternative arrangement of the electrode set shown in Figures 16 and

17.

DETAILED DESCRIPTION

Figure 3 shows an arrangement according to one aspect of the present invention. In Figure 3, there is shown a medical implant system, in this example, a cochlear implant system 500 including an external part of the medical implant system 120, being the speech/data processor for converting input sound into control data for supply to the medical implant 100 and in the case of a cochlear implant system, internal part/implanted control section or stimulator 110. Stimulator 110 generates electrical stimulation signals for stimulating the nerves of the user in the cochlea, by way of a further part of the medical implant system, in this case, by stimulating electrodes in electrode array 111. These stimulation signals are generated in response to the data transmitted by processor 120. In this aspect of the invention, external electrodes 151 and 152 are applied to the user's head 50, and electrically connected to the external part or processor 120, in one form, via an existing accessory output such as headphone outlet 122. In this embodiment, a small current in the form of a test signal, is applied to the two external electrodes 151, 152, for example, an AC coupled, 500Hz sinusoidal signal of for example, one hundred microamps.

The resulting current induced in the tissue about the implanted internal part 110, 111, results in various electrical effects in the implanted internal part 110 and electrode array 111 that can be measured as a measurement signal as will be described in more detail below. In one aspect, these electrical effects in the form of the measurement signal are provided to the external part or processor 120 by the medical implant 100, and an output is generated by the external part or processor 120, controlled by software installed in the processor 120. This output is representative of the received measurement signal.

The results of the various measurements are then provided to the user at 300. This can be done in a variety of ways, including simply providing raw data through a display or other interface means for further analysis by the user (e.g. using a look-up table provided in an instruction booklet), providing raw data for uploading on the internet for further processing, or for access by a healthcare professional, or for other wired or wireless transfer of the data. Using the neural response measuring circuitry of the implant to measure signals applied to the user's head 50, may be used to test the integrity of the neural response circuits.

In one aspect, the processor 120 can process the data and provide a system diagnosis by various means 300, including video display, spoken audio output, audio output in the form of various beeps or tones, and visual outputs such as combinations of flashing lights.

Testing may be undertaken at the user's discretion by executing instructions that guide and instruct the participant (whether it be the user himself or an assistant) into positioning the set of independently moveable electrodes 151, 152 on the head 50 of the user and to connect these to the processor 120 by cables 153 and 154.

In one embodiment, the system could be set up so that testing begins only when a control or command sequence that is entered into the user's prosthesis by the user or assistant, matches a sequence previously stored within the prosthesis by the user's health care professional. This not only prevents inadvertent use, it allows health care professionals to restrict diagnostic use by unsuitable persons.

Once testing is initiated, the prosthesis executes software commands that cause test signals to be conveyed to and/or from the external electrodes 151,152 applied to the user (in one form in the case of a cochlear implant system, applied to the user's head 50), and the electrode array 111 of the medical implant 100 of the cochlear implant system 500. The subsequent automatic measurement and analysis of the amplitude and phase of these signals, relative to that of the software defined characteristics of the applied electrical test signals and other factors, as the positioning of the external electrodes is changed, reveals much about the placement and integrity of the implant, its electrode systems and that of intervening body tissue. Such analysis may reveal whether there are any faults in the medical implant itself, or in the placement of the medical implant within the patient or user (e.g. dislodgement of the implant or of a part of the implant such as the electrode). Any of these problems or faults can affect the integrity of the medical implant system as a whole.

Figure 4 demonstrates the means whereby measurement and analysis of the voltage developed across each adjacent pair of stimulating electrodes of an intra-cochlear electrode array can be used to confirm its correct placement within the cochlea.

Common as well as uncommon anatomical and biological factors can affect electrical conduction pathways invivo. As one example, a condition of otospongiosis, which can increase the electrical conductivity of cochlea bone mass, is assumed for the following example use of one aspect of the invention, since actual voltage gradients across body tissue more closely resemble those depicted in Figure 4. It will be understood that Figures 4 to 8 have been simplified for clarity and are neither anatomically accurate nor to scale.

In Figure 4 two external electrodes 151 and 152 are placed on the skin surface of an implant user's head 50 and connected to an alternating source of electric current supplied from the external part or processor 120 of the prosthesis as a test signal. Several curved lines 2, joining points of equal voltage potential, graphically illustrate the voltage gradient that is manifested across intervening body tissue as a result of the alternating electric current that is made to flow between electrodes 151 and 152. Sensitive voltage measuring circuitry located within the implant 100, such as the type employed in current art cochlear implants to sense the neural activity that occurs in response to applied stimulation, is operated so as to sequentially measure (as a measurement signal) the voltage across adjacent pairs of electrodes of the intra-cochlear electrode array 111. If at one particular moment in time the alternating voltage developed at electrode 152 is more positive than its counter electrode 151, the voltage measured between electrodes E6 and E7 will show that electrode E7 is more positive than electrode E6. Similarly during the same moment in time, electrode E16 will be more positive than electrode E17. Since the path between electrodes E13 and E12 lies parallel to that of the equipotential lines, the voltage between electrodes E13 and E12 is minimal.

It is in this way that an analysis of the distribution of the voltages developed across intra-cochlear array electrodes, or any other electrode for that matter, resulting from electric current applied to known locations on the user's head 50 can be used to determine electrode positioning and the integrity of its electrical connection with the electronics systems of implanted prosthesis.

Figures 5, 6, 7 and 8 illustrate how the distribution of electrode voltages changes when an intra- cochlear electrode array is partially displaced and withdrawn from its usual position within the cochlear due to a severe head impact.

Figure 6 illustrates the distribution of electrode voltage measurements across electrode pair numbers that might be obtained from a correctly inserted intra-cochlear electrode array 111 as shown in Figure 5, using the method described above.

Figure 7 shows a cochlear implant electrode array 111 that has been partially withdrawn and uncoiled as a result of a minor impact the user's head 50.

hi contrast to Figure 6, Figure 8 illustrates the distribution of electrode voltage measurements that might be obtained from the intra-cochlear electrode array 111 shown in Figure 7. It can be seen that these voltage distributions are quite distinct from each other and can be used to differentiate between a normal state and abnormal state of the electrode array 111.

While the text and accompanying illustrations refer to instantaneous voltage measurements, persons skilled in the art will appreciate that non-instantaneous, alternating voltage measurements that relate both the phase and amplitude of the electrode voltage to that of the phase and amplitude of the current applied to the external electrodes will yield the same information.

In one embodiment, the alternating current applied as the test signal, applied to a user's head 50 is applied at a frequency of around 10,000 Hertz. This frequency is intended to take advantage of the frequency response of the neural response sensing system of the implant 110, while the current level that is applied is chosen to remain safe for the user and within the limits set by regulatory authority safety requirements. This current need not be supplied as a continuous sinusoid but may be applied as multiple bursts of any type of repeatable waveform morphology that allows it to be distinguished from noise. It will however, be appreciated that any other frequency within the responsive range and data acquisition rate limits of a specific prosthesis system incorporating the invention may be used. Typically this could encompass frequencies between 100 and 50,000 Hz and repetition rates of 1 to 1000 Hz.

It will be appreciated that Figures 6 and 8 do not depict actual measurement data. These were manually created to illustrate one method that may be used to determine electrode position. In practice, individual differences in the anatomical morphology and tissue impedance of each user will add some variability to the plots. The effects of such variability are largely removed when measurement results acquired over a long period of time, including when the prosthesis status has been positively established by some independent means, are compared in a manner that allows changes to be readily detected.

In one form, the invention may also be used to capture and store characteristics related to the distribution of both internal and external electrode voltages soon after implantation when other means is available to confirm correct electrode placement and implant operation. By doing so, subtle changes can be detected by comparing subsequent electrode voltage measurements with those made initially, when the implant system was known to be functioning correctly.

In another example, as illustrated in Figure 9, a test is conducted to detect a further type of fault in the medical implant 100. In this example, undue strain applied from a pair of hand manipulated forceps to the intra-cochlear array electrode 111 cable during insertion has damaged the array cable's insulation and weakened one of its platinum wire conductors. Whilst no fault is evident during post-operative testing, slow ingress of body fluid during the passage of several months causes the cable to expand such that the tension force applied to the weakened conductor causes it to break. In addition to severing the connection to its associated electrode, the exposed broken end connected to the implanted stimulator 110 comes into contact with the body fluid that has infiltrated the cable, effectively providing an ionic conduction pathway to surrounding tissue, instead of the tissue surrounding its assigned electrode.

The subsequent loss in the user's hearing ability is noticeable but not extreme. Without the invention and faced with the prospect of a 2-day trip to their hearing clinic and the possibility of learning that their hearing loss is the result of normal, post-operative physiological variation, the recipient decides to ignore the change, only to suffer a lifetime of substandard hearing.

Conversely, if this user had access to the present invention, they could obtain a reliable diagnosis of the fault and discuss this and their options for remedial action over the telephone with the health care professional, without having to leave home.

The detection and diagnosis of this example fault condition is described below in more detail with reference to Figure 9.

Shown in Figure 9 is the head 50 of the user implanted with a cochlear hearing prosthesis 100 with attached induction coil 112 for wireless conveyance of power and data as will be understood by the person skilled in the art. The cochlea 5 contains an array 111 of more than 20 stimulating electrodes which is connected to the stimulator 110 via an insulated cable 114. One of the wires 115 in the cable 114 has become broken or severed as shown in Figure 9. Two external electrodes 151, 152 connect with the external speech processor 120 (not shown).

hi the above described integrity fault, the speech processor 120 incorporating this aspect of the present invention and under control of its stored software, supplies as a test signal, a low frequency electric current at about 500 Hz and of a few hundred microamps (for example) in magnitude to the electrodes 151, 152.

At the same time the external part, or speech processor 120, causes the medical implant 100 to progressively sense the voltage that is developed across the various stimulating electrodes as a measurement signal. Since the broken electrode wire is coupled to body fluid at position 115, the voltage manifested by the current applied to the recipient's head at external electrodes 151 and 152 results in a higher voltage between 115 and 111, than that which would be measured between adjacent electrodes at 111.

Further confirmation as to the nature of the fault is gained by progressively measuring the 500 Hz voltage between all pairs of intra-cochlear electrodes. The speech processor 120, in detecting such a large difference in the voltage from the broken wire 115 as compared with the voltage across all other electrodes allows the nature of the discrepancy to be automatically determined.

Whilst the current applied to external electrodes 151 and 152 remains at a safe level of a few hundred micro amps, repetitive measurements, synchronised to the generation of the 500 Hz signal by the speech processor 120 and averaged over many measurements, allows extraneous noise or interference that is uncorrelated with the 500Hz and measurement timing to be significantly minimised. Such signal averaging is achieved by synchronising the timing many sets of multiple skin voltage measurements to repeatedly coincide with the delivery of current to implanted electrodes of interest. By averaging the result of each measurement in a set, with that of its counterparts from all other measurement sets or vectors, the amplitude of any uncorrelated noise is diminished.

In this way target signals many times lower in amplitude than any noise that may otherwise overwhelm them can be recovered by averaging the results of a thousand or more sets of measurement vectors.

This ability to average time- vectored signals is further demonstrated by figures 10, 11 and 12. Created using spreadsheet tools, these figures demonstrate how a series of vectors representing a sinusoidal signal (see Figure 10) are corrupted by the addition of unwanted noisy signals as shown by Figure 11. In this case, the noisy signal was simulated by adding pseudo randomly generated values with an amplitude that is 4 times the peak value of the sinusoid, as shown in Figure 11. hi averaging across 50 sets of simulated measurement vectors containing sinusoid and noise components, a recognisable form of the sinusoid emerges as demonstrated in Figure 12. The same repetitive measurement and signal averaging techniques are applied to diagnostic signal measurements in the reverse direction. This is when low-level alternating electric currents are supplied from the headphone output circuit to the skin of the user's head for detection and measurement by the pre-existing neural response detection system of the implanted part as is described in more detail below, with reference to Figure 13.

In one embodiment, automatic fault diagnosis may be used to automatically compare actual measurement data with stored data representing various predicted measurement outcomes from likely fault scenarios. The most likely fault scenario is determined as the one who's predicted measurements best match the measurement data. Fuzzy logic and selective ranking and weighting as to the significance of the various measurement parameters and combinations of such are employed in determining the best match of parameters.

In Figure 13, there is shown a further embodiment in which electrical signals in the external electrodes 151, 152 are generated as the measurement signal, in response to neural stimulation signals applied to the user via the implant 100 and specifically, via electrode array 111 as the test signal. These electrical signals are in one form, amplified and extended in duration by electronics associated with the accessory cable and electrode system before being supplied to an accessory input such as the microphone input of the user's speech processor 120. Subsequent digitisation, data processing and analysis by the processor allow diagnostic data regarding the status of the implant system 100 to be produced. Measuring voltage on the surface of an implant user's head allows the neural stimulation current delivery circuits on the implant system to be tested for connectivity and functionality for example.

In current art cochlear implant hearing prostheses; bursts of oscillatory electric current of several megahertz in frequency, are supplied to an external induction coil that is arranged to be in close proximity to a similarly dimensioned coil associated with the implanted part.

The alternating current that is subsequently induced within the implanted coil powers the implanted part and supplies data that controls the operation of the implant and the delivery of electric current to the neural stimulation electrodes 111.

In addition to the supply of electric current as a means to evoke sensations of hearing, some cochlear implants apply low values of charge balanced, pulsatile current to their electrode systems that while remaining below the value necessary to evoke hearing sensations, allow the implant system to measure impedance as well as other characteristics of their electrode systems and their associated body tissue interface.

It is this same pulsatile current delivery system that this invention, in one embodiment, re-uses for self-diagnostic integrity testing purposes. Whilst the pulsatile voltage that subsequently appears on the skin of a user's head maybe just a few microvolts, those skilled in the art will appreciate that viable measurements of such low voltages can be achieved in the presence of several volts of unwanted noise or spurious signals using well-known signal averaging techniques as described above.

The measurement and analysis of implanted electrode voltages while current is applied to the external electrodes allows the status of the electrode system, neural response sensing circuits and data telemetry circuitry to be determined.

Conversely, by measuring and analysing the voltage developed across the external electrodes while the neural stimulation circuits are active and supplying current to the implanted electrodes, allows the operational status of the stimulation circuits and electrodes to be determined. Since much of the electrode wiring is common to both stimulation delivery and neural response sensing circuits, many measurement ambiguities can be eliminated by analysing the combined data of both types of measurements. A failed current delivery output transistor could for example be distinguished from a broken electrode wire if the voltage sensed from the electrode associated with the failed device was normal.

In another form, the measurement signal may comprise Evoked Auditory Brainstem Response (EABR) signals or cortical potentials. In this application, a pre-amp may be provided that plugs into the input of the processor 120 either directly, or incorporated in the electrode pair.

Written instructions that may be included with the product user manual instruct the user in connecting the cables 153,154 to their speech processor 120 and positioning the electrodes 151, 152 at various locations on the skin of their head. Other instructions inform the user as to what speech processor controls should then be operated in order to initiate the various types of integrity tests to be undertaken together with any electrode repositioning that may be necessary to facilitate the tests. The following is an illustrative example of what might be contained in the user manual of an implanted hearing prosthesis that incorporates this, or aspects of this, invention. Example Step-by-Step Instructions:

(a) To begin self-testing press the program select button for at least 10 seconds. Should you wish to stop self-testing at any time and return to normal listening mode, press the program button again, (b) Once a flashing "A" is displayed by the prosthesis LCD remove any audio accessories connected to the prosthesis accessory socket. (c) Insert the plug connected to the self-test electrode cable.

(d) Moisten each of the two self-test electrodes with drinking water and place these on the users head either side of the forehead.

(e) After about 10 seconds your processor will display a steady non-flashing letter "A" and a number between 0 and 9. As this is the result of self-test "A" you should remove the self test electrodes and record this result for later reference.

(f) Press the volume increase button on the prosthesis once and the display will change to a flashing "B" to indicate it is ready to begin self-test "B".

(While self-tests should be conducted in alphabetical order, you can use the volume increase-decrease buttons to repeat or skip tests.) (g) Remoisten the electrodes and place one electrode under the chin and the other, after wetting your hair, on the crown of your head.

(h) After about 10 seconds your processor will display a steady non-flashing letter "B" and a number between 0 and 6. As this is the result of self-test "B" you should remove the self- test electrodes and record this result for later reference. (i) Remoisten the electrodes and place one electrode in the centre of the forehead and the other on the bare skin behind the ear, nearest to your implant.

(j) After about 10 seconds your processor will display a steady non-flashing letter "C" and a number between 0 and 9. As this is the result of self-test "C" you should remove the self- test electrodes and record this result for later reference.

The remaining tests are conducted in a similar manner until all are complete. The following tables are an example of what might be contained in the user instructions to assist the user in interpreting the results of self-tests A B and C, and in conveying these results to their hearing healthcare professional by telephone etc.

Operation of the relevant control settings of the speech processor 120, causes it to execute various sets of electrically stored instructions that may for example cause the implant to deliver a particular regime of electro-neural stimulation, the result of which may then be verified by the speech processor test control software using either a response from the implanted user, telemetered data related to any evoked action potential of the stimulated neural cells, or the voltage signal developed across the electrodes located on the recipient head, as conveyed to and measured by the speech processor electronics.

Figure 14 shows a system block diagram of the system of one example of the invention. Shown is the system 500 with internal part or medical implant 100, comprising stimulator 110, and array of electrodes 111, and external part or processor 120. Processor 120 includes signal conditioner and AJD converter 124, sound data processing and encoder block 125, wireless data transceiver 126, central processing unit (CPU) 127, non- volatile data memory 128, volatile data memory 129, user control switches 130, visual display system 131, auxiliary audio input and A/D converter circuitry 123 and headphone output and D/A converter 122. An example of such a system may be seen in the Freedom Cochlear Implant Hearing Prosthesis system by Cochlear Limited.

Wireless data transceiver 126 provides control data as a test signal to medical implant 100 for use in generating excitation signals for use in one or more aspects of the present invention. Transceiver 126 also receives data from medical implant 100 relating to the measurement signal as voltage measurement data.

External electrodes 151 and 152 are connected via cables 153 and 154, to the input of the processor 120 headphone output 122 and/or audio input 123 ports. These may be connected directly to the ports, or may be connected via a change over switch arrangement 140. It should be noted that while a double pole, change over switch 140 is shown in Figure 14, one skilled in the art will appreciate the change over function between supplying current to the external electrodes and measuring current or voltage from these electrodes can be implemented in a variety of ways including the use of two independent sets of cabled electrodes.

In this example, one electrode set for use in supplying signals to the skin, would have its connections wired so as to make contact with the headphone output terminals of the prosthesis connector. The other electrode set for use in sensing skin voltage, would have its connector wired so as to make contact with the microphone input terminals of the prosthesis connector. In an exemplary example of the invention, a peak hold circuit comprising both passive and active electronic circuitry extends the duration of incoming skin voltage signals so as to overcome the bandwidth limitations of the microphone input circuitry and facilitate skin voltage measurements associated with the delivery of short duration stimulus current pulses lasting just a few tens of microseconds.

Appropriately low and safe levels of electrical signal can be supplied from the user's speech processor 120 to electrodes 151,152 on the user's head for detection and measurement by the implanted part of their prosthesis. Resulting measurement data is conveyed to the speech processor 120 for processing and analysis via its wireless telemetry communications system.

The measurement data conveyed from each integrity test is in one embodiment, automatically analysed by the user's prosthesis and presented to the user, their carer, guardian or healthcare professional by way of various visual and or audio cues emitted by their prosthesis. These cues may consist of audible tones, beeps or electronically synthesised speech, or alternatively or in conjunction with various illumination or activation of indicator lights or other visual display devices such as LCDs. A printed look-up table and or other interpretive information contained in a user manual allow the user to interpret the meaning and significance of the applied tests.

Conversely, the diagnostic test information can be conveyed by cable or wireless means to a nearby PC which, if fitted with appropriate software, can present a plain text and or spoken diagnosis locally and or convey it via the internet or other communications medium to a remote PC and possibly the user's health care professional.

In one form, the measurement signal is compared with a comparison signal such as equivalent signals previously taken and associated with a particular state of the medical implant, such as broken wires or displacement as previously discussed.

Unlike current art implantable hearing prosthesis that don't allow the user to self diagnose, the various aspects of this invention allow many types of prosthesis integrity tests to be conducted by the user, an assistant such as their guardian or carer, in the comfort and convenience of their own home. Given the reported frequency of post-operative user complaints or observations, this home testing may well reduce overall costs to the user and the community by eliminating some of the unnecessary clinic visits that currently occur.

It will be appreciated that, while the various aspects of the present invention may apply to systems that are electrically and/or mechanically modified to adapt the various aspects of the present invention, one aspect of the present invention provides a particular advantage in that it makes use of existing hardware and data processing capabilities in a diagnostic role that is substantially different from, and non-analogous to, the function for which such existing arrangements of current art devices have been provided to fulfil. In other words, this aspect of the present invention allows an existing device to provide additional functionality, simply by using its existing hardware in a new way.

This aspect provides a particular advantage in providing a practically feasible system, with an inexpensive electrode system in combination with existing hardware to provide user accessible diagnostics that are attractive to manufacturers and users alike, avoiding the extra size, weight and cost of the extra hardware that would otherwise be necessary to undertake the described prosthesis system diagnostics.

While additional electronics and other hardware for diagnostic use might be incorporated into existing cochlear implant systems, the adverse aesthetics, discomfort and economics associated with the extra size, weight and cost of this hardware would seem to be sufficiently unattractive to manufacturers and users as to possibly account for the reason why such diagnostics are not already incorporated in current art devices. This is understandable given the necessarily small size, weight and prominence of these head worn devices. It should also be borne in mind that the majority of users may never need, be capable of using, nor desire the use of such diagnostics. For those users who do encounter problems or misadventure with their prosthesis however, this invention could provide significant advantage.

As described above, it will be appreciated that one or more of the various aspects of the present invention may be implemented in an existing implant, without any mechanical or electrical modification. The existing circuitry, inputs, outputs and processing functionality can be used to advantage to implement the various instructions and additional processing as directed by a piece of software downloaded onto the existing microprocessor of the implant processor. Combined with a set of suitable electrodes, a current device can be turned into a self-diagnostic system. To this end, a kit consisting of, or containing, one or more of a machine-readable medium containing instructions to cause a machine (for example the microprocessor of the processor and/or the processor itself) to carry out the various aspects of the invention, an instruction manual (which in one form, could be provided in electronic form itself such as on a compact disk, memory stick or other memory device/machine readable medium); and a set of suitable electrodes for use with the system. Alternatively, each could be sold separately. For example, a user of an existing hearing system could simply purchase the software for carrying out the various aspect of the invention, as well as a pair of suitable electrodes, if the user does not already own such a pair. Figure 15 shows an example of a kit 500 containing the machine readable medium 501 having instructions thereon for loading onto the microprocessor of the implant system processor, an instruction manual possibly provided as either or both in booklet form 503 and compact disc form 502, and one or more sets of suitable electrodes 150. Examples of electrodes suitable for this application include the intra-cochlea electrode arrays and extra cochlea electrodes used by the majority of present day cochlear implant hearing prostheses such as the Cochlear Limited, Freedom Implant. The instructions may comprise text, graphics and or images provided as hardcopy and/or in a form suitable for electronic display.

In another alternative, new medical implant systems could be sold with the diagnostic software pre-loaded, and include the instructions and/or electrode set.

In yet a further alternative, implant systems with processors could be especially designed and produced with dedicated mechanical and/or electrical/electronic features to implement the various aspects of the invention. For example, a processor could be provided with one or more additional dedicated input/output ports for the electrode set.

In a further alternative, as shown in Figure 16, an external electrode system may be provided with a reversible connector for allowing a user to connect the external electrodes to either the prosthesis audio output or input circuits, depending on the relative alignment electrode and prosthesis connectors.

In Figure 16, two medical grade, stainless steel metal discs 152 and 153, are arranged to have one metal face of each disc exposed for contact with the user's skin, and the opposing face, covered by an electrically insulating layer that allows finger pressure to be applied in order to secure the position of the electrode on a users head during use. Each disc electrode is connected via electrostatically screened and polymer covered, coaxial cable 155, to a connector 156, that is configured to connect with an audio signal connector on the prosthesis. By reversing the relative alignment between adjoining connectors in the manner shown by the arrow, a user is able to connect the external electrodes to either the audio input or audio output circuits of the prosthesis.

Figure 17 shows an alternative arrangement whereby the external electrode connector 156 has been enlarged to accommodate additional signal enhancement circuitry and one or more user accessible control switches 157 to control one or more aspects of the diagnostic method described herein.

Figure 18 shows how two press-button user operated switches 158 and 159, may be accommodated within the insulated face of the external electrodes to allow the user to control various aspects of diagnostic procedure, such as to signal the prosthesis when the electrodes are correctly positioned or when for example to begin the next diagnostic test. An electrode pair may also be provided incorporating two or more of the features shown in Figures 16 to 18, in any combination.

The various aspects of the present invention may be used in relation to any type of electrode array, including straight and curved, peri-modiolar electrodes, short/basilar electrodes, as well as electrode arrays with or without stylets.

The various aspects of the present invention are also applicable to other implantable electrode arrays, including ABI, FES, SCS and PABI.

Throughout the specification and the claims that follow, unless the context requires otherwise, the words "comprise" and "include" and variations such as "comprising" and "including" will be understood to imply the inclusion of a stated integer or group of integers, but not the exclusion of any other integer or group of integers.

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement of any form of suggestion that such prior art forms part of the common general knowledge.

Claims

CLAIMS:

1. A method of testing a medical implant implanted in a user of a medical implant system comprising the medical implant and an external part, the method comprising: generating at the external part, a test signal for application to the user via one of, a pair of electrodes connected to the external part and applied to the user, or via the medical implant; receiving at the external part, a measurement signal via the other of the pair of electrodes or the medical implant, in response to the test signal; and generating an output from the external part, representative of the received measurement signal.

2. A method as claimed in claim 1, wherein the method further comprises prompting the user or an assistant, to connect the pair of electrodes to one of an accessory output or an accessory input of the external part.

3. A method as claimed in claim 2 wherein the medical implant system is a cochlear implant system and the external part is a processor and the medical implant is a cochlear implant.

4. A method as claimed in claim 3 wherein the accessory input is a microphone input and the accessory output is a headphone output.

5. A method as claimed in claim 4 further comprising prompting the user or the assistant to connect the pair of electrodes to one of the microphone input or the headphone output.

6. A method as claimed in claim 5 further comprising instructing the user or the assistant where to place the pair of electrodes on the user.

7. A method as claimed in claim 1 wherein the output generated from the external part comprises a diagnosis of a state of the medical implant.

8. A method as claimed in claim 1 wherein the output generated from the external part comprises data that may be further processed to provide a diagnosis of a state of the medical implant.

9. A method as claimed in claims 7 or 8 wherein the step of providing the diagnosis of the state of the medical implant comprises comparing the measurement signal with a comparison signal associated with a particular state of the medical implant.

10. A method as claimed in any one of claims 1 to 8 wherein the method only executes after a preset command sequence is input.

11. A method of testing a medical implant implanted in a user of a medical implant system comprising the medical implant and an external part, the method comprising: generating at the external part, a test signal for application to the user via a pair of electrodes connected to the external part and applied to the user; measuring at the medical implant, a measurement signal resulting from the application of the test signal to the user; transmitting the measurement signal from the medical implant to the external part; receiving at the external part, the measurement signal; and generating an output from the external part, representative of the received measurement signal.

12. A method of testing a medical implant implanted in a user of a medical implant system comprising the medical implant and an external part, the method comprising: generating at the external part, a test signal; transmitting the test signal to the medical implant; applying the test signal to the user via the medical implant; measuring via a pair of electrodes applied to the user and connected to the external part, a measurement signal resulting from the application of the test signal to the user; receiving at the external part, via the connected pair of electrodes, the measurement signal; and generating an output from the external part, representative of the received measurement signal.

13. A medical implant system for testing a medical implant, the medical implant system comprising: the medical implant; and an external part comprising a test signal generator for generating a test signal for application to the user via one of, a pair of electrodes connected to the external part and applied to the user, or the medical implant; a receiver for receiving a measurement signal via the other of the pair of electrodes or the medical implant, in response to the test signal; and an output generator for generating an output from the external part, representative of the received measurement signal.

14. A medical implant system as claimed in claim 13 wherein the medical implant system is a cochlear implant system and the medical implant is a cochlear implant and the external part is a processor.

15. A medical implant system for testing a medical implant implanted in a user of the medical implant system, the medical implant system comprising: the medical implant; and an external part comprising: a test signal generator for generating a test signal for application to the user via a pair of electrodes connected to the external part and applied to the user; a receiver for receiving a measurement signal measured by the medical implant, resulting from the application of the test signal to the user; and an output generator for generating an output from the external part, representative of the received measurement signal.

16. A medical implant system as claimed in claim 15 wherein the medical implant system is a cochlear implant system and the medical implant is a cochlear implant and the external part is a processor.

17. A medical implant system as claimed in claim 16 wherein the cochlear implant comprises a stimulator and an array of electrodes, and wherein the measurement signal is measured by the array of electrodes.

18. A medical implant system for testing a medical implant implanted in a user of the medical implant system, the medical implant system comprising: the medical implant; and an external part comprising: a test signal generator for generating a test signal for application to the user via the medical implant; a receiver for receiving a measurement signal measured by a pair of electrodes applied to the user, resulting from the application of the test signal to the user by the medical implant; and an output generator for generating an output from the external part, representative of the received measurement signal.

19. A medical implant system as claimed in claim 18 wherein the medical implant system is a cochlear implant system and the medical implant is a cochlear implant and the external part is a processor.

20. A medical implant system as claimed in claim 19 wherein the cochlear implant comprises a stimulator and an array of electrodes and the test signal is applied to the user via the array of electrodes.

21. A medical implant system for testing a medical implant, the medical implant comprising: the medical implant; and an external part having stored thereon, instructions for causing the medical implant system to perform the method of any one of claims 1 to 12.

22. A method of testing a medical implant implanted within a user, the method comprising: externally applying an electric current to tissue around the implanted medical implant; and measuring one or more electrical characteristics of one or more elements of the implanted medical implant resulting from the applied electric current.

23. A method as claimed in claim 22 wherein the medical implant is a cochlear implant and the electric current is applied to the user's head.

24. A machine readable medium containing instructions to cause a machine to perform the method of any one of claims 1 to 12, 22 or 23.

25. A machine readable medium as claimed in claim 24 wherein the machine is a cochlear implant system.

26. A kit comprising the machine readable medium as claimed in claim 24, at least one pair of electrodes for use in the system as claimed in any one of claims 13 to 21 ; and a set of instructions for the use of the system as claimed in any one of claims 13 to 21.

27. A kit as claimed in claim 26 wherein the at least one electrode pair comprises circuitry for processing at least a portion of signals transmitted through the at least one electrode pair.