System and methods for determining nerve proximity, direction and pathology during surgery
The present invention involves systems and methods for determining nerve proximity, nerve direction, and pathology relative to a surgical instrument based on an identified relationship between neuromuscular responses and the stimulation signal that caused the neuromuscular responses.
1. A nerve monitoring system comprising a control unit having a display and a plurality of leg muscle sensor electrodes in communication with the control unit, the nerve monitoring system being configured to (a) deliver a plurality of electrical stimulation signals so that when a surgical access instrument adapted to transmit electrical signals received from the nerve monitoring system through a stimulation electrode on the surgical access instrument is connected to the nerve monitoring system and positioned along a selected path within tissue of a patient that contains one or more motor nerves, the plurality of electrical stimulation signals are delivered to the patient tissue, (b) in response to delivery of each one of the plurality of electrical stimulation signals, receive a plurality of voltage response values detected by the plurality of leg muscle sensors coupled to different leg muscles, (c) determine the amplitude value of an unknown stimulation threshold required to evoke a threshold response from any of the leg muscles, d) enter a monitoring mode in response to determining the value of the unknown stimulation threshold, and e) as the surgical access instrument is advanced along the selected path toward a targeted intervertebral disc detect whether stimulation thresholds are changing by incrementing or decrementing the amplitude of the simulation signal delivered.
2. The system of claim 1 , wherein the value of the unknown stimulation threshold determined using the steps of bracketing the unknown stimulation threshold by determining a first stimulation amplitude that does not evoke the threshold response from any of the leg muscles and a second stimulation amplitude that does evoke the threshold response from at least one of the leg muscles, repeatedly bisecting the bracket that includes the stimulation threshold until a final bracket of a predetermined width is reached, and selecting the stimulation threshold from the final bracket.
3. The system of claim 1 , wherein the nerve monitoring system is further configured to (d) determine which range from a predetermined group of ranges the determined stimulation threshold is located in, and (e) communicate on the display an indicator corresponding to the determined range from the predetermined group of ranges as an indicator of nerve proximity to the surgical access instrument.
4. The system of claim 1 , wherein whether the stimulation amplitude is incremented or decremented is determined as a function of evoked responses from one or more of the leg muscle sensor electrodes.
5. The system of claim 1 , wherein the monitoring mode is configured to detect whether stimulation thresholds are changing as the surgical access instrument is advanced along the selected path toward the targeted intervertebral disc by incrementing or decrementing the simulation amplitude delivered by the stimulation electrode on the surgical access instrument by 0.1 mA.
6. The system of claim 2 , wherein monitoring is continued while the stimulation threshold remains in the final bracket and wherein steps (a)-(e) are repeated in response to the stimulation threshold being determined to be outside the final bracket.
7. The system of claim 6 , wherein the stimulation threshold is determined to be outside the final bracket when the threshold nerve stimulation amplitude fails to fall within upper and lower ends of the final bracket three times.
8. The system of claim 3 , wherein the indicator corresponding to the determined range from the predetermined group of ranges includes a color display.
9. The system of claim 8 , wherein the predetermined group of ranges includes three predetermined ranges and the color displayed is one of three different colors, each of the three different colors corresponding to one of the three predetermined ranges.
10. The system of claim 9 , wherein the color is one of red, yellow, and green depending on the applicable range.
11. The system of claim 1 , with further instructions that cause the display to display at least one of a name and a symbol representative of the surgical access instrument being used.
12. The system of claim 1 , wherein the threshold response is a voltage response of a predetermined peak-to-peak voltage.
13. The system of claim 12 , wherein the predetermined peak-to-peak voltage is approximately 100 μV.
14. The system of claim 1 , wherein the threshold response is a peak-to-peak voltage with peaks at a time T 1 and a time T 2 relative to a stimulation delivery time.
15. The system of claim 1 , wherein the surgical access instrument is a dilator cannula.
16. The system of claim 1 , and further comprising:
a surgical access instrument in communication with the control unit and deliverable through bodily tissue having neural structures in the selected path toward the targeted intervertebral disc of a spine, the surgical access instrument having a stimulation electrode positioned along a distal tip region to deliver a stimulation signal to the bodily tissue when the surgical access instrument is positioned in the selected path toward the targeted intervertebral disc of the spine;
a plurality of sequential dilator cannulas deliverable through the bodily tissue having neural structures in the selected path toward the targeted intervertebral disc of the spine, the plurality of sequential dilator cannulas including at least the surgical access instrument and the outer dilator cannula configured to be guided over the previously delivered surgical access instrument; and
a working corridor instrument deliverable over the outer dilator cannula of the plurality of sequential dilator cannulas toward the targeted intervertebral disc of the spine, wherein the working corridor instrument is configured to establish an operative corridor to the targeted intervertebral disc of the spine.
17. The system of claim 1 , wherein the stimulation signal amplitudes are current amplitudes.
18. The system of claim 17 , wherein the current amplitudes are measured in units of mA.