The present invention relates to a neurostimulation system, in particular for nerve stimulation (IVA) and/or cortical stimulation and/or deep brain stimulation (DBS), and a method for operating the same.

The present invention relates to the technical field of neurostimulation, in particular to cortical and/or deep brain stimulation. Cortical and/or deep brain Stimulation are methods for the diagnosis and therapy of neurodegenerative diseases such as inter alia the Parkinson’s disease, epilepsy, and chronic pain. Electrical stimulation by means of leads which are implanted into brain areas or regions like the subthalamic nucleus and/or the globus pallidus internus can alleviate symptoms, such as tremor symptoms of a patient suffering from a drug-resistant Parkinson’s disease. Further, the signals from a brain region or area at which the leads were implanted can be recorded and the state of the brain tissue can be determined using impedance measurements.

In this relation, neuroprosthetic devices are powerful tools to monitor, prevent and treat neural diseases, disorders and conditions by interfacing electrically with the nervous system. They are capable of recording and stimulating electrically neural activity once implanted in the nervous tissue. Currently, most neuroprosthetic technologies apply electrodes interfacing with neural tissue.

One of the major risks of active implantable medical devices refers to patient safety.

In neural implants, the electrical stimulation of the nervous tissue might lead to a degradation of the electrode and/or the nervous tissue. In particular, the surface interaction between the electrode and the nervous tissue can change over time, which may cause an increase of the impedance along such surface border between the electrode and the nervous tissue.

However, current neural stimulators do not have a feedback system which is capable of monitoring the tissue conditions and ensures that the electrodes are behaving as expected. In particular, such current neural stimulators do not provide for a continuous monitoring of the electrode and tissue conditions, i.e. during neural stimulation treatment itself. Rather, in case of presently available neural stimulators treatment parameters like the current and/or voltage of electrical pulses being applied to the nervous tissue have to be adapted, in particular increased, over time in order to be able to achieve a comparable or constant stimulation effect in the nervous tissue.

It is an object of the invention to provide a neurostimulation system which allows for monitoring and/or surveillance, in particular continuous monitoring and/or surveillance, of the stimulus application of nervous tissue, preferably before and/or during and/or after stimulation treatment, wherein such monitoring and/or surveillance can preferably be provided on basis of the tissue stimulation as proceeded during neurostimulation treatment, e.g. during deep brain stimulation, in particular on basis of a frequency analysis. Moreover, it is an object of the invention to provide a method for operating such a neurostimulation system.

The aforementioned objects are solved by the subject-matters of the independent claims. Advantageous configurations of the invention are described in dependent claims.

According to the present invention a neurostimulation system, in particular for cortical and/or deep brain stimulation, is provided which comprises at least one electrode device with at least one stimulation electrode and at least one counter electrode, a (external/internal) power source and a control unit. The stimulation electrode is configured to transmit at least one electrical pulse, preferably at least one current- controlled pulse, to the nervous tissue, wherein the counter electrode is configured to receive at least one electrical feedback pulse from the nervous tissue, based on the at least one electrical pulse transmitted by the at least one stimulation electrode, and to forward the electrical feedback pulse to the control unit.

The control unit is configured to control the power source in order to generate and provide the at least one electrical pulse, preferably the at least one current-controlled pulse, via the electrode device to the nervous tissue.

Moreover, the control unit can receive the electrical feedback pulse from the counter electrode and determine therefrom at least one current signal and/or the voltage signal. Further the control unit can apply a transfer element, preferably in form of a fast fourier transformation (FFT), to the current signal and/or the voltage signal in order to generate/calculate current spectra data and/or voltage spectra data.

Preferably, a current signal and a voltage signal are determined as well as processed, in terms of application of a FFT, by the control unit. Further, the control unit is configured to generate/calculate impedance spectroscopy data on basis of the voltage spectra data and the current spectra data, in particular by dividing the voltage spectra data by the current spectra data.

The invention is based on the idea to constantly/continuously monitor and/or surveil the conditions of the electrodes and the interfacing tissue in stimulating/electrode devices during stimulation protocols, in particular with no need of switching the neurostimulation system to an alternative operation mode or the like. For example, a pulse generator which may provide current-controlled pulses for the neurostimulation treatment does not need to be switched to potentiometric operation mode.

Rather, the current/voltage signals which can be determined/detected/extracted by the control unit during usual stimulation treatment may be utilized to monitor the electrode’s and/or the nervous tissue’s condition.

It is possible to identify changes along the surface border between the electrode device, i.e. the stimulation electrode and/or the counter electrode, and the nervous tissue to be stimulated and treated. For identification of such changes determined current and/or voltage signals are transformed into impedance spectroscopy data. On basis of these impedance spectroscopy data parameters concerning degradation of the electrodes and tissue encapsulation of the electrodes can be derived.

The power source may preferably be provided in form of a current source and/or a pulse generator in order to generate and deliver current-controlled pulses for stimulation of nervous tissue.

Preferably, the electrical pulses to be transmitted by the stimulation electrode to the nervous tissue are current-controlled pulses. Such at least one pulse can be provided in form of a monophasic or biphasic pulse(s). The at least one counter electrode provides a feedback path for the electrical pulse(s) as applied to the nervous tissue via the stimulation electrode. Hence, the at least one stimulation electrode, the nervous tissue and the at least one counter electrode provide a closed circuit loop in combination with the control unit.

The current signal and/or the voltage signal can be measured/detected/extracted in form of continuous or discontinuous, i.e. time discrete, signals. Moreover, it is also possible that current and/or voltage signals are provided as analog signals or digital signals.

In particular, the control unit is configured to process/analyse the electrical feedback pulse.

In the context of the present invention, an electrical feedback pulse can be considered as an electrical signal being passed via the nervous tissue when an electrical pulse is transmitted to the nervous tissue via the at least one stimulation electrode. Hence, the electrical feedback pulse(s) can be similar to or deviate from the corresponding electrical pulse(s) as originally applied/transmitted to the nervous tissue.

Preferably, the control unit can control the data capturing, based on the received electrical feedback pulse(s) via the at least one counter electrode, such that the current/voltage signal can represent time discrete data being sampled/captured in appropriate manner, for example with a sufficiently high frequency in comparison to an application frequency of the electrical pulse(s) via the at least one stimulation electrode.

Alternatively, the at least one counter electrode may also be substituted and/or supplemented, in particular with regard to its functional means of providing an electrical feedback pulse captured/gathered from the nervous tissue, by e.g. at least one recording electrode of the neurostimulation device or the like and preferably in combination with a correspondingly adapted configuration of the system for suitable signal processing.

In particular, the control unit can comprise a voltmeter/voltage sensor and/or a current meter/current sensor in order to determ ine/extract the current signal and/or the voltage signal from the electrical feedback pulse(s).

In case of continuous or analog signals, such signals can be pre-processed by the control unit in order to provide time discrete signals for further handling of the same. The current signal and the voltage signal to which a transfer element is applied by the control unit can also be considered as current signal data and voltage signal data, e.g. time discrete signal data.

The transfer element to be applied to the current signal and/or voltage signal can be considered as any kind of transfer algorithm for further signal processing. In this context, the control unit or a further processor being associated and/or connected with the control unit, can provide such signal processing for further analysis of the current and/or voltage signals.

Preferably, the transfer element is provided in form of a fast fourier transformation. In particular, the signals as received by the counter electrode and forwarded to/further received by the control unit are to be processed and analyzed with respect to its frequencies and/or its frequency distribution.

Dependent on the type of current and/or voltage signals being detected, e.g. in case of continuous and/or analog current/voltage signals being forwarded to the control unit, also different kinds of a fourier transformation can be utilized for (frequency) analysis.

Thus, appropriate handling of the current/voltage signals by the control unit can be provided in terms of a pre-processing, i.e. by transforming continuous signals into time discrete signals, or by applying an appropriate transfer element in order to allow for analysis of the signal frequencies.

Such frequency analysis by the transfer element generates current spectra data and/or voltage spectra data. Hence, the respective spectra data provide information about the frequencies and the frequency distribution of the current/voltage signal.

Moreover, the control unit is capable of providing/calculating/determining impedance spectroscopy data in consideration of the current/voltage spectra data. In particular, voltage spectra data are divided by current spectra data in order to arrive at corresponding data for the impedance along electrode device to the nervous tissue to be stimulated.

Preferably in comparison to standard impedance spectroscopy data of the respective electrode device, e.g. stored in a database of the control unit, changes can be identified which may indicate electrode degradation or tissue encapsulation of the electrodes. ln consequence, the neurostimulation system according to the present invention allows for a continuous observation of the system condition in cooperation with the nervous tissue and thus for a surveillance of the treatment quality and safety, in particular also during the neurostimulation treatment itself and preferably without any additional intervention on the patient.

In one preferred embodiment of the invention, the electrode device has at least one electrode comprising graphene, in particular being made of graphene or a graphene- based material or comprising a graphene(-based) coating.

In particular, graphene based materials can provide improved electrical and mechanical properties, e.g. a beneficial flexibility of the resulting electrode.

Such graphene electrodes particularly provide higher safe charge injection capacity as well as the signal-to-noise ratio/performance can be improved. Thereby, the electrode size can be reduced, even if the same amount of electrodes is maintained.

Hence, along a cross-section of the electrode device the cross-sectional area of the at least one electrode can be reduced.

Moreover, such graphene-based electrodes can provide for a safe electrical interface in aqueous environments, like in the context of neurostimulation of nervous tissue.

In summary, the safety and efficiency of the at least one electrode can be improved by also providing enhanced mechanical and structural properties of the electrode.

According to a preferred embodiment of the invention, the control unit controls the power source to generate at least one sequence of electrical pulses in the form of at least one stimulation protocol.

In particular, a stimulation protocol can be considered as a planned and pre-defined application of a sequence of/subsequent stimulation pulses.

A sequence of electrical pulses, representing a stimulation protocol, can be formed by at least two subsequent electrical pulses.

Preferably, such subsequent electrical pulses may be generated and/or applied with a specific time delay of e.g. less than 5 seconds, 10 seconds, 20 seconds or 30 seconds, or even more. In another embodiment the control unit is configured to determine deviations within the impedance spectroscopy data, in particular in comparison to standard impedance spectroscopy data, wherein such deviations indicate a change in surface interaction between a surface of the electrode device, namely of the at least one electrode, and the surrounding nervous tissue.

Standard impedance spectroscopy data can preferably be provided, e.g. stored in a database being associated with the control unit, with respect to the individual electrode device, i.e. the respective electrodes, of the neurostimulation system.

In the alternative, subsequently determined impedance spectroscopy data may be compared, at least from time to time and within appropriate time intervals, in order to determine the progress of an electrode degradation and/or a tissue encapsulation and/or a tissue degradation.

In one preferred embodiment the control unit is configured to monitor the at least one current signal and/or the at least one voltage signal with a sampling rate which is a multiple, in particular at least double, of the frequency of the electrical pulses being transmitted to the nervous tissue via the at least one electrode device.

In the alternative, the control unit may extract, e.g. in a signal pre-processing step, time discrete data from the electrical feedback pulse with a sampling rate which is a multiple, at least double, of the stimulus periodicity frequency.

The frequency of electrical pulses particularly describes the frequency of a single pulse being applied to the nervous tissue by the at least one stimulation electrode. Hence, a pause time or a time of no stimulation can be provided between single electrical pulses. Alternatively, a continuous sequence of subsequent electrical pulses can be applied to the nervous tissue, comprising a certain frequency of continuous pulse application.

In particular, an electrical feedback pulse can be received by the control unit from the counter electrode and current/voltage signals can be determined/analyzed/monitored by the control unit in accordance with the Nyquist-Shannon sampling theorem, e.g. by a voltage sensor and/or a current sensor within the control unit.

For example, assuming the stimulation pattern is periodic with a frequency of 100Hz, the sampling rate of the voltage and current signals is set be a multiple of the frequency to obtain pure harmonics of the current/voltage signals after the FFTs, but at least 200 Flz.

However, in the sense of the present invention the sampling rate may vary at least within certain tolerances, whereby sufficiently accurate signal analysis can still be provided. Hence, the sampling rate may vary by e.g. 10 to 20 %, at least temporarily, which may not be essentially harmful to the analysis and signal processing results.

Hence, the control unit can ensure an appropriate sampling for evaluation and analysis of the electrical pulses as received back from the at least one counter electrode.

In another embodiment, during the transmission and application of the at least one electrical pulse to the nervous tissue, in particular during a stimulation protocol comprising a plurality of subsequent electrical pulses, the control unit continuously monitors the current signal and/or the voltage signal based on the electrical feedback pulse as received from the at least one counter electrode.

The control unit is capable of monitoring the respective pulse and/or stimulation protocols being applied to the nervous tissue. Hence, a continuous monitoring of such temporarily limited pulse application can be provided.

Moreover, it is also possible that the control unit provides and analysis within pre defined time intervals. Such time intervals can be set e.g. by the user of the neurostimulation system, whereby the time intervals are configured in a way to ensure appropriate surveillance of the system.

According to one preferred embodiment of the invention the control unit is further configured to receive a pre-defined target current, in particular in form of a user input, forming the basis for the electrical pulses to be generated and delivered via the electrode device. In particular, the user of the neurostimulation system may provide such target current in order to ensure an appropriate neurostimulation treatment of the patient.

The neurostimulation system, i.e. the power source being controlled and/or regulated by the control unit, can provide corresponding voltages and currents in order to fulfill the pre-defined target current. Moreover, it may also be possible in accordance with the present invention that the user can pre-define further treatment parameters and details of the neurostimulation treatment in order to set up stimulation protocols.

In another embodiment, the neurostimulation system comprises a user interface device with a user input means such that a user input, in particular a pre-defined target current, can be received by the control unit.

Preferably, the user interface device of the neurostimulation system can provide input means as well as output means in order to provide for appropriate control means, like touchscreens or hardware input means, and output means, in particular visual and/acoustic output means like a monitor or speakers, for providing information to the user.

According to one embodiment the control unit controls and/or regulates the power source such that the at least one electrical pulse is generated in accordance with the pre-defined target current.

Preferably, the control-unit sets up the power source such that the pre-defined target current can be provided to the nervous tissue for neurostimulation purposes. In this context, for such control/regulation function the control unit can consider the estimated/calculated impedance along the stimulation electrode to the nervous tissue being finally stimulated.

Hence, the safety of the patient is ensured as the control unit itself can be configured to take into account the electrode/tissue degradation and/or any tissue encapsulation of the electrodes of the electrode device.

In one embodiment of the invention the at least one electrical pulse is provided by the power source in form of at least one biphasic pulse.

In particular, such biphasic pulse is generated and provided by the power sources and transmitted to the nervous tissue by the at least one stimulation electrode.

More preferably, all subsequent pulses of the stimulation protocol can comprises such biphasic pulses.

An efficient and appropriate stimulation of the nervous tissue can be provided. In another preferred embodiment the at least one electrical pulse comprises a frequency, in particular is applied to the nervous tissue with a frequency, of at least approximately 50, 100 or 1000 Hz.

In particular, a single pulse being applied to the nervous tissue can comprise such frequency of e.g. 1000 Hz.

Hence, an appropriate neurostimulation can be ensured.

In this case, the control unit can gather a current signal and/or a voltage signal with a minimum frequency of 2000 Hz, preferably more than 2000 Hz, e.g. 4000 Hz or 8000 Hz, according to the Nyquist-Shannon sampling theorem.

A further aspect of the invention refers to a method for operating a neurostimulation system according to one of the preceding claims, comprising the following steps:

- generating and delivering at least one electrical pulse, preferably at least one electrical pulse, via the at least one electrode device, in particular via the at least one stimulation electrode;

- receiving an electrical feedback pulse and determining therefrom a current signal and/or a voltage signal;

- applying a transfer element, preferably in form of fast fourier transformation, to the current signal and/or the voltage signal in order to generate current spectra data and/or voltage spectra data;

- determining the impedance spectroscopy data on basis of the voltage spectra data and the current spectra data, in particular by dividing the voltage spectra data by the current spectra data.

Preferably, the method can be controlled and/or regulated by the control unit of the neurostimulation system.

Moreover, the neurostimulation can provide the user interface device in order to allow the user to provide user input, like a pre-defined target current, and/or to receive information from the system via a user output

According to a preferred embodiment, the method further comprises the following step: - determining deviations within the impedance spectroscopy data, in particular in comparison to standard impedance spectroscopy data of the electrode device, wherein such deviations indicate a change in interaction between a surface of the electrode device, namely of the at least one electrode, and the surrounding nervous tissue.

In particular, grounds for a change of interaction may be for example a degradation of the electrode, a degradation of the nervous tissue to be stimulated/treated or a tissue encapsulation of the electrodes of the electrode device.

Hence, the changing interaction between a surface of the electrode device and the surrounding nervous tissue may particularly refer to a changing surface interaction.

Further details and advantages of the present invention shall now be disclosed in the connection with the drawings.

It is shown in:

Fig. 1 a schematic illustration of an embodiment of a neurostimulation system;

Fig. 2 a diagram of a typical impedance spectroscopy measurement according to an embodiment comprising graphene electrodes;

Fig. 3 an exemplary diagram of a measurement of an applied input current signal of the neurostimulation system according to Fig. 1;

Fig. 4 an exemplary diagram of a voltage signal/response of the neurostimulation system according to Fig. 1;

Fig. 5 an exemplary diagram of the impedance module for an embodiment comprising graphene electrodes, based on the current signal and voltage signal as shown in Fig. 3 and 4; and

Fig. 6 a schematic flow chart illustrating the method for operation of a neurostimulation system according to Fig. 1.

Fig. 1 shows an illustrative embodiment of a neurostimulation system. The neurostimulation system comprises an electrode device 110, a control unit 120 and a user interface device 130 as well as a power source 140.

The electrode device 110 is provided with at least one stimulation electrode 112 and at least one counter electrode 114.

The user interface device 130 can be provided with a user input means, for example a touchscreen means. Moreover, the user interface device can be provided with user output means 134 like monitors, speakers, warning lights or the like.

Moreover, the neurostimulation system 100 comprises a power source 140. The power source 140 can preferably be provided as a current source.

The power source 140 comprises a bidirectional connection to the control unit 120.

The control unit 120 can comprises a bidirectional connection to the electrode device 110, in particular in case of some control function means being embedded in the electrode device 110.

In particular, the control unit 120 can form a closed electrical circuit loop in combination with the stimulation electrode 112, the nervous tissue to which electrical pulses shall be applied and the counter electrode 114.

Moreover, the control unit 120 can be provided with a voltmeter/voltage sensor and/or a current meter/current sensor.

The user interface device 130 is preferably provided with a user input means 132 as well as a user output means 134.

The user interface device 130 can comprise a bidirectional connection to the control unit 120 in order to allow for exchanging user input data, being received from the user via the user input device, and for providing user output data to the patient via the output means 134.

Fig. 2 shows a diagram of a typical impedance spectroscopy measurement, according to an embodiment comprising graphene electrodes. In particular, Fig. 2 illustrates the frequency distribution with respect to the impedance and the corresponding phase in case of a graphene(-based) electrodes with a radius of 100 pm.

Accordingly, the impedance decreases when the frequency of electrical pulses is increased. Simultaneously, the phase shift is changed from higher negative values to lower negative values in case of the frequency of the electrical pulses is increased.

In Fig. 3 an exemplary diagram of an applied input current signal of the neurostimulation system 100 according to Fig. 1 is shown.

The electrical pulses being applied to the nervous tissue by the electrode device 110 are preferably provided as current-controlled pulses.

In particular, the plurality of subsequent current pulses are supplied and applied to the nervous tissue in form of biphasic pulses.

A single electrical pulse can have a positive current share and a negative current share. In particular, the positive and negative shares of single electrical pulses can be comparable.

The frequency of subsequent current pulses being applied can remain the same.

In the course of a stimulation protocol comprising multiple sequences of subsequent current pulses, the frequency of the current pulses being applied can be varied.

Fig. 4 discloses an exemplary diagram of a voltage signal/response of the neurostimulation system 100 according to Fig. 1.

In particular, the voltage response as shown in Fig. 4 refers to a voltage response of a graphene electrode with a radius of 100 pm, being further characterized by Fig. 2.

The voltage response according to Fig. 4 indicates the biphasic nature of the current pulses illustrated in Fig. 2, having positive and negative shares which are comparable, preferably equal, to one another.

The voltage response in Fig. 4 illustrates the constant frequency of the pulses being generated by the neurostimulation system 100 and applied via the electrode device 110. Fig. 5 shows a diagram of the impedance module for an embodiment comprising graphene electrodes, based on the current signal and voltage signal as exemplarily shown in Fig. 3 and Fig. 4.

In particular, Fig. 5 illustrates the frequency analysis of the current and voltage signals as shown in Fig. 3 and Fig. 4, thus after a transfer element has been applied to the measured current/voltage signals and the resulting impedance spectroscopy data have been generated/calculated.

As can be seen from Fig. 5, during neurostimulation treatment the impedance (module) decreases for higher frequencies.

Fig. 6 shows a schematic flow chart illustrating the method 200 for operation of a neurostimulation system according to Fig. 1.

In a first step electrical pulses are generated 210 by the power source 140 and delivered to the nervous tissue via the electrode device 110, i.e. via the stimulation electrode 112 of the electrode device 110.

In particular, the control unit 120 of the neurostimulation device 100 can preferably be configured to control and/or regulate the method 200.

Preferably, the power source 140 is configured as a current source in order to provide current-controlled pulses.

In a next step, electrical feedback pulses are received by the control unit 120 via the counter electrode 114, on which basis a current signal and/or a voltage signal is/are captured and determined 220.

The electrical feedback pulse as received/gathered by the counter electrode 114 from the nervous tissue is forwarded to the control unit 120 of the neurostimulation system 100.

Preferably, current and voltage signals are forwarded to and processed by the control unit 120, e.g. by a voltage sensor and/or a current sensor of the control unit 120, in order to achieve current and/or voltage signals.

The current signal and/or the voltage signal are further processed 230 in a next step. It is possible that a pre-processing can be executed by the control unit in order to appropriately prepare the received signal(s).

The signal processing 230 particularly refers to a frequency analysis of the current/voltage signals. For example, a transfer element in form of a FFT can be applied to the current signal and/or to the voltage signal.

As a result, the frequency distribution according to the measured current and/or voltage signal can be received. Such frequency information are provided in form of current spectra data and/or voltage spectra data. On basis of such frequency analysis of the current/voltage signal impedance spectroscopy data can be calculated 240.

In particular, the impedance spectroscopy data can be generated by calculating the ratio/quotient between the current spectra data and the voltage spectra data.

As a further step according to Fig. 6, the generated impedance spectroscopy data can be compared with previous impedance spectroscopy data or standard impedance spectroscopy data.

By such comparison differences and deviations may be identified which have occurred in the nervous tissue and/or along the electrodes 112; 114 of the electrode device 110.

In more detail, an electrode degradation, a degradation of the nervous tissue and/or a tissue encapsulation may cause a change of the impedance spectroscopy data.

The function of a neurostimulation system 100 as exemplarily illustrated and described in Fig. 1 to 6 can be described as follows.

Preferably, the control unit 120 can be capable of controlling 120 and/or regulating the overall process of monitoring the neurostimulation treatment. The user interface device 130 can be utilized by a user in order to request information from the control unit, e.g. about the neurostimulation treatment, being provided via the user output means 134. Moreover, a user can provide user input, e.g. pre-define a target current for the electrical pulses to be applied to the nervous tissue, via the user input mean 132.

The control unit 120 further controls and/or regulates the power source 140, being preferably embodied as a current source. Thereby the control unit 120 can control the generation of electrical pulses to be applied to the nervous tissue, for example in consideration of the target current being provided by a user of the neurostimulation system 100.

The control unit 120 can further adapt the configuration of the electrical pulses to be generated by the power source 140 on basis of signal processing, in particular on basis of the frequency analysis of the current signal and/or the voltage signal.

In particular, the control unit 120 can receive electrical feedback pulses from the electrode device 110, i.e. from the at least one counter electrode 114.

The electrical feedback pulses can result from the electrical pulses being supplied and transmitted to the nervous tissue via the at least one stimulation electrode 112 of the electrode device 110.

The electrical pulses being passed through the nervous tissue can be received/gathered by the counter electrode 114 from the nervous tissue as electrical feedback pulses.

Such electrical feedback pulses can be forwarded by the counter electrode 114 to the control unit 120. After receipt of electrical feedback pulses, the control unit 120 can execute the signal processing.

In particular, the control unit 120 can process the electrical feedback pulse to determine the current and/or voltage signal and further process the same in form of e.g. a FFT. Flence, a frequency analysis is processed, wherein frequency information are provided in form of current spectra data and/or voltage spectra data.

The spectra data can be further processed by the control unit 120 in order to achieve information about the resulting impedance being present between the electrodes 112; 114 of the electrode device 110 and the nervous tissue. The further processing by the control unit 120 can refer to the impedance spectroscopy of the current spectra data and/or the voltage spectra data, originally resulting from a measurement via the at least one counter electrode 114.

Finally, deviations and/or changes being identifiably by the control unit 120 over time in the impedance spectroscopy data can indicate decreasing quality and/or safety of the neurostimulation treatment, e.g. due to degradation of the electrodes 112; 114 and/or of the nervous tissue.

In case of such deviations/changes being identified in the impedance spectroscopy data, the control unit 120 can provide the user with a corresponding information via the user output means 134.

In summary, the present invention provides a neurostimulation system which is capable of monitoring the condition of the neurostimulation, in particular with regard to any changes of the impedance along the stimulation via the electrodes and the nervous tissue of the patient. In consequence, the safety and efficiency of the neurostimulation treatment can be ensured by the present invention.

Preferably, such beneficial monitoring and surveillance of the stimulation process can be provided before, after as well as during the neurostimulation itself, i.e. without shifting the neurostimulation system to any alternative operation mode. Rather, the monitoring of the treatment can be executed on basis of the usual neurostimulation treatment, namely the electrical pulses being applied to the nervous tissue.

Reference numerals

100 Neurostimulation system 110 Electrode device 112 Stimulation electrode

114 Counter electrode 120 Control unit 130 User interface device 132 User input means 134 User output means

140 Power/current source 200 Method for operating a neurostimulation system 210 Generating/delivering electrical pulse 220 Receiving current/voltage signal 230 Applying transfer element

240 Determining impedance spectroscopy data 250 Determining deviations