Neuromonitoring systems and methods
Systems, devices, and methods are described for neuromonitoring. A minimum stimulus signal required to elicit a threshold neuromuscular response is determined by delivery of stimulus signals to tissue and detection of neuromuscular responses in muscle tissue. The strength of the delivered stimulus signals is varied, for example by adjusting the current amplitude or pulse width of the signals, and muscle responses are measure, for example by detecting EMG signals. The delivered stimuli and corresponding responses are then used to determine a stimulation threshold. The stimulation threshold may be used to indicate at least one of nerve proximity and pedicle integrity.
1. A method for neuromonitoring, comprising:
delivering a first stimulus signal having a first amplitude and a second stimulus signal having a second amplitude to tissue including or adjacent to a nerve, the first amplitude being different from the second amplitude;
detecting, at one or more sensors associated with muscle tissue, a first neuromuscular response in response to the first stimulus signal and a second neuromuscular response in response to the second stimulus signal;
calculating a stimulation threshold for the nerve from the first and second stimulus signal amplitudes and the first and second neuromuscular responses, the stimulation threshold being an estimate of a minimum stimulus level required to elicit a neuromuscular response greater than or equal to a predetermined threshold, wherein the stimulation threshold is different from the first and second stimulus signal amplitudes; and
communicating to a user an indicator of the stimulation threshold to indicate at least one of nerve proximity and pedicle integrity.
2. The method of claim 1 , wherein each of the first and second neuromuscular responses is greater than or equal to the predetermined threshold.
3. The method of claim 1 , wherein one of the first and second neuromuscular responses is greater than or equal to the predetermined threshold, and the other of the first and second neuromuscular responses is less than the predetermined threshold.
4. The method of claim 1 , wherein the first and second neuromuscular responses are detected using electromyography (EMG), and the predetermined threshold corresponds to a voltage level of detected EMG signals.
5. The method of claim 1 , wherein the first and second neuromuscular responses are detected using EMG, and the predetermined threshold corresponds to a level of a correlation function calculated from detected EMG signals.
6. The method of claim 5 , further comprising cross-correlating the detected EMG signals with an EMG response template, wherein the predetermined threshold is a level of a correlation function between the detected EMG signals and the EMG response template.
7. The method of claim 1 , wherein the calculating step comprises:
calculating a linear function from the first and second stimulus signal amplitudes and the first and second neuromuscular responses; and
determining the stimulation threshold from the linear function.
8. The method of claim 1 , wherein the calculating step comprises:
calculating a curve fit from the first and second stimulus signal amplitudes and the first and second neuromuscular responses; and
determining the stimulation threshold from the curve fit.
9. The method of claim 8 , wherein the curve fit is a sigmoid function.
10. The method of claim 1 , further comprising delivering a plurality of test stimulus signals to the tissue and detecting a plurality of test neuromuscular responses, each of the plurality of test neuromuscular responses corresponding to one of the plurality of test stimulus signals.
11. The method of claim 10 , wherein each of the plurality of test stimulus signals has an amplitude that is greater than a preceding test stimulus signal.
12. The method of claim 11 , wherein the amplitudes of the plurality of test stimulus signals increase at a constant increment.
13. The method of claim 11 , wherein the amplitudes of the plurality of test stimulus signals increase at varying increments.
14. The method of claim 10 , further comprising selecting the first and second stimulus signals based on the test stimulus signals and test neuromuscular responses.
15. The method of claim 14 , further comprising calculating a curve fit using the test stimulus signals and test neuromuscular responses, and selecting the first and second stimulus signals from the curve fit.
16. The method of claim 14 , wherein the first and second stimulus signals are selected from test stimulus signals that elicit test neuromuscular responses that meet the predetermined threshold.
17. The method of claim 14 , wherein one of the first and second stimulus signals is selected from test stimulus signals that elicit test neuromuscular responses that are greater than or equal to the predetermined threshold, and the other of the first and second stimulus signals is selected from test stimulus signals that do not elicit test neuromuscular responses that are greater than or equal to the predetermined threshold.
18. The method of claim 1 , wherein the first neuromuscular response is detected before the second stimulus signal is delivered.
19. The method of claim 18 , wherein the amplitude of the second stimulus signal is adjusted based on the first neuromuscular response.
20. The method of claim 1 , wherein the second stimulus signal is delivered before the first neuromuscular response is detected.
21. The method of claim 20 , wherein delivery of the first and second stimulus signals is offset by an amount that is greater than or equal to a refractory period of the nerve and the muscle tissue.