The present disclosure relates to neurostimulator implant devices, and more particularly to a system and method that uses multi-electrode stimulation provided by a neurostimulator implant device to elicit electrically-evoked compound action potentials. Such an evoked compound action potential (ECAP) provides valuable objective feedback information useful in setting the stimulation parameters associated with the neurostimulator implant device.
Traditional methods used to elicit the electrically-evoked compound action potential, or ECAP, deliver stimulation to a single electrode contact. There are cases where such application of a stimulus to a single electrode contact do not evoke a suitable action potential. The present disclosure provides an improved system and method for obtaining the ECAP through application of the stimulus to multiple electrodes. The present disclosure may be used in many different kinds of neurostimulator devices, but will be described in terms of a cochlear implant device.
Electrical stimulation of predetermined locations within the cochlea of the human ear through an intra-cochlear electrode array is described, e.g., in U.S. Pat. No. 4,400,590. The electrode array shown in the '590 patent comprises a plurality of exposed electrode pairs spaced along and imbedded in a resilient curved base for implantation in accordance with a method of surgical implantation, e.g., as described in U.S. Pat. No. 3,751,615. The system described in the '590 patent receives audio signals, i.e., sound waves, at a signal processor (or speech processor) located outside the body of a hearing impaired patient. The speech processor converts the received audio signals into modulated RF data signals that are transmitted by a cable connection through the patient's skin to an implanted multi-channel intracochlear electrode array. The modulated RF signals are demodulated into analog signals and are applied to selected ones of the plurality of exposed electrode pairs in the intra-cochlear electrode so as to electrically stimulate predetermined locations of the auditory nerve within the cochlea.
U.S. Pat. No. 5,938,691, incorporated herein by reference, shows an improved multi-channel cochlear stimulation system employing an implanted cochlear stimulator (ICS) and an externally wearable speech processor (SP). The speech processor employs a headpiece that is placed adjacent to the ear of the patient, which receives audio signals and transmits the audio signals back to the speech processor. The speech processor receives and processes the audio signals and generates data indicative of the audio signals for transcutaneous transmission to the implantable cochlear stimulator. The implantable cochlear stimulator receives the transmission from the speech processor and applies stimulation signals to a plurality of cochlea stimulating channels, each having a pair of electrodes in an electrode array associated therewith. Each of the cochlea stimulating channels uses a capacitor to couple the electrodes of the electrode array.
Other improved features of a cochlear implant system are taught, e.g., in U.S. Pat. Nos. 5,626,629; 6,067,474; 6,157,861; 6,195,585; 6,205,360; 6,219,580; 6,249,704; 6,289,247; 6,295,467; and 6,415,185; each of which patents is also incorporated herein by reference.
The implantable cochlear stimulators described in the '629, '474, '861 and '580 patents are also able to selectively control the pulse width of stimulating pulses that are applied through the electrode array to the cochlea, and the frequency at which the stimulating pulses are applied.
One of the problems encountered when using a cochlear implant device, or many other type of neurostimulator devices, is “fitting” the device to a particular patient. Fitting involves setting the stimulation parameters, e.g., the amplitude, pulse width and frequency of the stimulation pulses to a level that is efficacious and comfortable for that patient. In the past, such “fitting” has been a very subjective process, requiring constant feedback from the patient. Some patients, however, e.g., old patients and extremely young patients, are not able to provide meaningful subjective feedback. Hence, clinicians are constantly looking for improved ways to obtain objective feedback from the patient that can assist in setting the stimulation parameters.
One type of objective feedback that has been used in the past is to monitor the stapedius reflex. The implantable cochlear stimulators described in the '861 and '585 patents teach the use of the stapedius reflex (also referred to as the stapedial reflex) as a parameter for monitoring and adjusting the magnitude of the stimuli applied through the electrode array. Applicant's co-pending patent application Ser. No. 60/412,533, filed Sep. 20, 2002, incorporated herein by reference, teaches an improved way for using multi-band stimuli to obtain the Stapedial Reflex.
The new generation of cochlear implants that have the enhanced processing power, and which can provide multiple platforms for delivering electrical stimuli to the auditory nerve, including high frequency pulsitile stimulation having current pulses of controlled amplitude, width and frequency, have sometimes been referred to as a “bionic ear” implant.
As the art of cochlear stimulation has advanced to produce bionic ear implants, the implanted portion of the cochlear stimulation system, and the externally wearable processor (or speech processor) have become increasingly complicated and sophisticated. It is also noted that much of the circuitry previously employed in the externally wearable processor has been moved to the implanted portion, thereby reducing the amount of information that must be transmitted from the external wearable processor to the implanted portion. The amount of control and discretion exercisable by an audiologist in selecting the modes and methods of operation of the cochlear stimulation system have increased dramatically and it is no longer possible to fully control and customize the operation of the cochlear stimulation system through the use of, for example, switches located on the speech processor. As a result, it has become necessary to utilize an implantable cochlear stimulator fitting system to establish the operating modes and methods of the cochlear stimulation system and then to download such programming into the speech processor. One such fitting system is described in the '629 patent. Another fitting system is described in the '247 patent.
The '247 patent further highlights representative stimulation strategies that may be employed by a multichannel stimulation system. Such strategies represent the manner or technique in which the stimulation current is applied to the electrodes of an electrode array used with the stimulation system. Such stimulation strategies, all of which apply current pulses to selected electrodes, may be broadly classified as: (1) sequential or non-simultaneous (where only one electrode receives a current pulse at the same time); (2) simultaneous (where substantially all of the electrodes receive current stimuli at the same time, thereby approximating an analog signal); or (3) partially simultaneous pulsitile stimulation (where only a select grouping of the electrodes receive stimuli at the same time in accordance with a predefined pattern).
Typically, when the fitting systems described in the '629 or '247 patents are employed for multichannel stimulation systems, or when equivalent or similar fitting systems are employed, it is necessary to use directly measured threshold values and/or thresholds derived from the measurement of psycophysically-determined pseudo-comfort levels. That is, for each channel of the multichannel system, a minimum threshold level is measured, typically referred to as a “T” level, which represents the minimum stimulation current which when applied to a given electrode associated with the channel produces a sensed perception of sound at least 50% of the time. In a similar manner, an “M” level is determined for each channel, which represents a stimulation current which when applied to the given electrode produces a sensed perception of sound that is moderately loud, or comfortably loud, but not so loud that the perceived sound is uncomfortable. These “T” and “M” levels are then used by the fitting software in order to properly map sensed sound to stimulation current levels that can be perceived by the patient as sound.
Disadvantageously, determining the “T” and/or “M” levels (or other required thresholds) associated with each channel of a multichannel stimulation system is an extremely painstaking and time-intensive task. Such determinations require significant time commitments on the part of the clinician, as well as the patient. Moreover, once determined one channel at a time, such levels may not be representative of actual threshold levels that are present during real speech. That is, preliminary data indicate that thresholds set in single channel psychophysics overestimate the actual threshold required when all channels are running during live speech. Such an overestimation appears to penalize patient performance, particularly performance in noise. Hence, neural stimulation parameters which render threshold measurement unnecessary would dramatically reduce the time requirements for programming sequential and/or partially simultaneous pulsitile stimulation, as well as facilitate a higher probability of optimized programming for pediatric as well as adult populations where obtaining such measures are difficult.
As the ages of patients into which implantable cochlear stimulators are implanted decreases, it becomes increasingly more important to improve the fitting process and to minimize, or eliminate, the need to make threshold measurements. This is because very young patients, for example, two year olds, are unable to provide adequate subjective feedback to the audiologist for the audiologist to accurately “fit” the cochlear stimulation system optimally for the patient. Thus, what is needed is an improved apparatus and simplified method for fitting a speech processor where many of the threshold measurements previously required are no longer needed, or where subjective feedback from the patient is no longer needed.
As indicated, one technique that has been investigated for improving the manner in which threshold measurements are made or used is to sense the stapedius reflex of the patient in response to an applied stimulus. See, e.g., the '861 and '585 patents, previously incorporated herein by reference. An electrode that may be used to sense the stapedius reflex is described, e.g., in U.S. Pat. No. 6,208,882, also incorporated herein by reference.
When the stapedius reflex is sensed, i.e., when a stapedius reflex electrode is in place that allows the stapedius reflex to be sensed, or when other techniques are used to sense the stapedius reflex, such sensing eliminates or minimizes the need to rely solely upon subjective feedback from the patient during the fitting or adjusting process. Such subjective feedback can be highly unreliable, particularly in younger and older patients.
Traditional methods for measuring stapedial reflexes present stimuli, typically pulse trains, on a single electrode and the reflex is either directly observed by visual inspection or is inferred from a change in the impedance of the tympanic membrane.
Another technique that has been investigated for improving the manner in which threshold measurements are made is to measure an evoked compound action potential (ECAP). Such ECAP measurement is particularly useful at or near the time of implant when the patient may be under the influence of anesthesia (and therefore unavailable for subjective feedback), and at a time when it is desirable for the surgeon and other clinicians associated with the implant operation to know if the implant device is working properly. An ECAP measurement is typically made by applying a stimulus to one electrode contact while monitoring the evoked action potential on an adjacent electrode contact. That is, one electrode contact is used to apply the stimulus, and an adjacent electrode contact is used as a sensor to sense the action potential (a voltage waveform) evoked by the application of the stimulus. Advantageously, in order to make an ECAP measurement, no additional electrodes or equipment are needed, beyond the neurostimulator itself, and a means of monitoring the voltage appearing on a selected electrode contact in response to application of a stimulus on a nearby electrode contact.
Disadvantageously, there are cases where it is difficult to obtain neural response measurements, e.g., an ECAP, on a given patient. In some instances, the maximal level of comfort of the patient is reached prior to seeing the ECAP, and in others the compliance level of the neurostimulator system is reached before ECAP visualization. That is, the delivery of a stimulus pulse on a single electrode contact may fail to synchronize enough neural fibers to produce a measurable evoked response. Alternatively, the delivery of a stimulus pulse on a single electrode having sufficient amplitude to evoke an action potential may exceed the compliance limits of the neurostimulator device on a single contact.
It is thus seen that improvements are still needed in the manner in which an ECAP is obtained and used during the fitting and operation of a neurostimulator implant device, e.g., a cochlear implant system.