MONITORING CLOSED-LOOP NEURAL STIMULATION THERAPY

1 . An implantable neuromodulation device comprising:

a plurality of electrodes including one or more stimulus electrodes and one or more measurement electrodes;

a stimulus source configured to provide neural stimuli to be delivered via the one or more stimulus electrodes to a neural pathway of a patient in order to evoke a neural response on the neural pathway;

measurement circuitry configured to capture signal windows sensed on the neural pathway via the one or more measurement electrodes subsequent to respective neural stimuli; and

a control unit configured to:

control the stimulus source to provide a neural stimulus according to a stimulus intensity parameter;

measure an intensity of the evoked neural response in the captured signal window subsequent to the provided neural stimulus;

determine a feedback variable from the measured intensity of the evoked neural response;

implement a feedback loop by using the feedback variable to control the stimulus intensity parameter so as to maintain the feedback variable at a target value;

convert a loop variable of the feedback loop to a representation, wherein the representation is one of a time-domain representation and a frequency-domain representation; and

analyse the representation to determine a loop stability measure of the feedback loop.

2 . The device of claim 1 , wherein the loop variable comprises one of: the feedback variable; an error value between the feedback variable and the target value; and the stimulus intensity parameter.

3 . The device of claim 1 , wherein the representation is a time-domain representation.

4 . The device of claim 3 , wherein the time-domain representation is a histogram.

5 . The device of claim 4 , wherein the control unit is configured to analyse the representation by determining a measure of bimodality of the histogram as the loop stability measure.

6 . The device of claim 3 , wherein the time-domain representation is a Lissajous figure.

7 . The device of claim 6 , wherein the control unit is configured to analyse the representation by determining a measure of bilobularity of the Lissajous figure as the loop stability measure.

8 . The device of claim 3 , wherein the time-domain representation is a time series.

9 . The device of claim 8 , wherein the control unit is configured to analyse the representation using a finite impulse response filter to detect an oscillation in the time series at a predetermined frequency.

10 . The device of claim 9 , wherein the predetermined frequency is half of a frequency of the provided neural stimuli.

11 . The device of claim 8 , wherein the control unit is further configured to introduce a perturbation to the feedback loop before the converting.

12 . The device of claim 11 , wherein the perturbation is a step.

13 . The device of claim 12 , wherein the control unit is configured to analyse the representation by determining a measure of under-dampedness of the time series as the loop stability measure.

14 . The device of claim 8 , wherein the control unit is configured to analyse the representation by determining a ratio of a standard deviation of stimulus intensity noise to a therapeutic range.

15 . The device of claim 8 , wherein the control unit is configured to analyse the representation by determining a ratio of a standard deviation of noise of the feedback variable to a standard deviation of noise of the feedback variable in open-loop mode.

16 . The device of claim 1 , wherein the representation is a frequency domain representation.

17 . The device of claim 16 , wherein the representation is a Fourier spectrum of the loop variable.

18 . The device of claim 17 , wherein the control unit is configured to analyse the representation by determining the magnitude of the Fourier spectrum at a predetermined frequency as the loop stability measure.

19 . The device of claim 18 , wherein the predetermined frequency is half of a frequency of the provided neural stimuli.

20 . The device of claim 1 , wherein the control unit is further configured to take one or more actions based on the loop stability measure.

21 . The device of claim 20 , wherein the control unit is further configured to take one or more actions based on a comparison of the loop stability measure and a predetermined threshold.

22 . The device of claim 20 , wherein the one or more actions comprise one or more of: adjusting one or more program parameters of the feedback loop; issuing an alert; transitioning to an open-loop operation of the implantable neuromodulation device; and shutting down the implantable neuromodulation device.

23 . A method of determining a measure of stability of a closed-loop neural stimulation system, the method comprising:

delivering a neural stimulus to a neural pathway of a patient in order to evoke a neural response on the neural pathway, the neural stimulus being delivered according to a stimulus intensity parameter;

capturing a signal window sensed on the neural pathway subsequent to the delivered neural stimulus;

measuring an intensity of the neural response evoked by the delivered neural stimulus in the captured signal window;

determining, from the measured intensity of the evoked neural response, a feedback variable;

implementing a feedback loop by using the determined feedback variable to control the stimulus intensity parameter so as to maintain the feedback variable at a target value;

converting a loop variable of the feedback loop to a representation, wherein the representation is one of a time-domain representation and a frequency-domain representation; and

analysing the representation to determine a loop stability measure of the feedback loop.

24 . The method of claim 23 , wherein the loop variable comprises one of: the feedback variable; an error value between the feedback variable and the target value; and the stimulus intensity parameter.

25 . The method of claim 23 , wherein the representation is a time-domain representation.

26 . The method of claim 25 , wherein the time-domain representation is a histogram.

27 . The method of claim 25 , wherein the time-domain representation is a Lissajous figure.

28 . The method of claim 25 , wherein the time-domain representation is a time series.

29 . The method of claim 23 , wherein the representation is a frequency domain representation.

30 . The method of claim 23 , further comprising taking one or more actions based on the loop stability measure.

31 . A closed-loop neural stimulation system comprising:

a plurality of electrodes including one or more stimulation electrodes and one or more measurement electrodes;

a stimulus source configured to provide neural stimuli to be delivered via the one or more stimulation electrodes to a neural pathway of a patient in order to evoke neural responses on the neural pathway;

measurement circuitry configured to capture signal windows sensed on the neural pathway via the one or more measurement electrodes subsequent to respective neural stimuli;

a control unit configured to:

control the stimulus source to provide a neural stimulus according to a stimulus intensity parameter;

measure an intensity of the evoked neural response in the captured signal window subsequent to the provided neural stimulus;

determine a feedback variable from the measured intensity of the evoked neural response;

implement a feedback loop by using the feedback variable to control the stimulus intensity parameter so as to maintain the feedback variable at a target value; and

a processor configured to:

convert a loop variable of the feedback loop to a representation, wherein the representation is one of a time-domain representation and a frequency-domain representation; and

analyse the representation to determine a loop stability measure of the feedback loop.

32 . The system of claim 31 , further comprising an external device in communication with the control unit.

33 . The system of claim 32 , wherein the processor is part of the external device.

34 . The system of claim 33 , wherein the external device further comprises a display.

35 . The system of claim 34 , wherein the processor is configured to render a user interface on the display.

36 . The system of claim 35 , wherein the user interface comprises a control configured to allow a user to select the loop variable from among a plurality of candidate loop variables.

37 . The system of claim 35 , wherein the user interface comprises a control configured to allow a user to select a domain of the representation from among a plurality of candidate domains.

38 . The system of claim 35 , wherein the processor is further configured to display the loop stability measure on the user interface.

39 . The system of claim 35 , wherein the processor is further configured to display the representation on the user interface.