US 20090177112 A1
The present invention relates generally to an algorithm aimed at neurophysiology monitoring, and more particularly to an algorithm capable of quickly finding stimulation thresholds over multiple channels of a neurophysiology monitoring system.
1. A system for performing neurophysiologic assessments during surgery, comprising:
a stimulator configured to deliver an electrical stimulation signal to the motor cortex of a patient;
at least one sensor configured to detect neuromuscular responses evoked by the stimulation signal; and
a control unit in communication with the stimulator and the sensor, the control unit being configured to (a) direct transmission of the stimulation signal, (b) receive evoked neuromuscular response data from the sensor, (c) assess spinal cord health by identifying a relationship between the stimulation signal and the neuromuscular response, and (d) communicate the relationship between the stimulation signal and the neuromuscular response to a user via at least one of alpha-numeric indicia and audio.
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27. A system for performing neurophysiologic assessments during surgery, comprising:
a first stimulator configured to deliver a first electrical stimulation signal to the motor cortex of a patient to perform motor-evoked potential monitoring;
a second stimulator configured to deliver a second electrical stimulation signal to one or more peripheral nerves within the patient to perform at least one of somatosensory evoked potential monitoring, bone integrity assessment, nerve direction assessment, nerve pathology assessment, and neuromuscular pathway assessment;
a processor in communication with the stimulators and a plurality of sensors for detecting the response to the first and second stimulation signals, the processor being configured to (a) direct transmission of the first and second stimulation signals, (b) receive data from the sensors, (c) assess at least one of spinal cord health, bone integrity, nerve direction, nerve pathology, and neuromuscular pathway integrity by identifying a relationship between the stimulation signals and the respective responses, and (d) communicate the relationship between the stimulation signals and the responses to a user via at least one of alpha-numeric indicia and audio.
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The present invention is an International Patent Application and claims the benefit of priority from commonly owned and co-pending U.S. Provisional Patent Application Ser. No. 60/649,724, entitled “System and Methods for Monitoring Before, During and/or After Surgery,” and filed on Feb. 2, 2005, the entire contents of which is hereby expressly incorporated by reference into this disclosure as if set forth in its entirety herein. Benefit is also claimed from commonly owned and co-pending U.S. Provisional Patent Application Ser. No. 60/719,897, entitled “Multi-Channel Stimulation Threshold Detection Algorithm for Use With Neurophysiology Monitoring Systems,” and filed on Sep. 22, 2005, the entire contents of which is hereby expressly incorporated by reference into this disclosure as if set forth in its entirety herein.
I. Field of the Invention
The present invention relates generally to a system and methods for performing neurophysiologic assessments during surgery, such as assessing the health of the spinal cord via at least one of MEP and SSEP monitoring, and assessing at least one of bone integrity, nerve proximity, neuromuscular pathway, and nerve pathology (free-run and evoked) during spine surgery.
II. Discussion of the Prior Art
Surgical procedures conducted on or around the spine can be beneficial in reversing or mitigating a variety of ailments commonly suffered by patients. Despite ongoing advances in surgical methods, however, neurological impairment remains a serious concern during surgical spine procedures. The safety of the spinal cord is of paramount importance because damage to the spinal cord may have devastating results for the patient. Consequences of spinal cord damage may range from a slight loss of sensation to complete paralysis of the extremities, depending on the location and extent of damage. Assessing the spinal cord before, during and/or after surgery can provide the surgeon with valuable information on the health of the cord. Such information may allow the surgeon to initiate corrective measures if the health of the cord is compromised, thereby decreasing the chance of permanent spinal cord damage and the resulting consequences.
The spinal cord is composed of a number of nerve pathways including motor and sensory pathways. Motor pathways transmit signals from the brain to the various muscle groups of the body. Conversely, sensory pathways transmit signals from the skin and other parts of the body up to the brain. Currently, methods exist for assessing the health of the spinal cord by monitoring electrical transmission along these pathways. Degradation of an electrical signal introduced near the origin of a pathway and monitored near the end of the pathway is indicative of damage to the spinal cord.
Motor pathway monitoring may be accomplished by stimulating the motor cortex in the brain and recording the resulting EMG response of various muscles in the upper and lower extremities. This method is referred to as trans-cranial electrical motor evoked potential (tce MEP, or simply “MEP”) monitoring.
Sensory pathway monitoring may be accomplished by stimulating a peripheral nerve that enters the spinal cord below the level of surgery and recording the resulting action potentials from electrodes on the scalp or high level cervical vertebra. This method is referred to as somatosensory evoked potential (SSEP) monitoring.
While MEP and SSEP monitoring are generally effective for assessing the health of the spinal cord, data from the current methods is typically received as electrical waveforms that must first be analyzed and interpreted in order to provide meaningful data to the surgeon. Interpreting the data can be a complex and difficult task and typically requires specially trained personnel to complete it. This is disadvantageous in that it increases surgery time (additional time needed to interpret data and communicate significance to the surgeon), translates into extra expense (having extra highly trained persons in attendance), and oftentimes presents scheduling challenges because most hospitals do not retain such specially trained personnel.
Based on the foregoing, a need exists for a better system and methods for monitoring the health of the spinal cord before, during, and or after surgery, and in particular, a need for a system that has the ability to conduct MEP and SSEP monitoring while quickly presenting data to the user in a simplified yet meaningful way. A need also exists for a system for monitoring the health of the spinal cord while providing the ability to assess at least one of bone integrity, nerve proximity, neuromuscular pathway, and nerve pathology (free-run and evoked) during spine surgery.
The present invention is directed at addressing the above identified needs and overcoming, or at least improving upon, the disadvantages of the prior art.
The present invention includes a system and related methods for performing neurophysiologic assessments during surgery, such as assessing the health of the spinal cord via at least one of MEP and SSEP monitoring, and assessing at least one of bone integrity, nerve proximity, neuromuscular pathway, and nerve pathology (free-run and evoked) during spine surgery.
According to a broad aspect, the present invention includes a surgical system, comprising a control unit and a surgical instrument. The control unit has at least one of computer programmed software, firmware and hardware capable of delivering a stimulation signal, receiving and processing neuromuscular or other bioelectric responses due to the stimulation signal, and identifying a relationship between the neuromuscular response and the stimulation signal. The surgical instrument has at least one stimulation electrode in communication with the control unit (via hardwire or wireless) for transmitting a stimulation signal. The control unit is capable of assessing at least one of spinal cord health via MEP or SSEP monitoring, bone integrity, nerve proximity, and nerve pathology based on the identified relationship between a stimulation signal and a corresponding neuromuscular response.
In a further embodiment of the surgical system of the present invention, the control unit is further equipped to communicate at least one of alpha-numeric and graphical information to a user regarding at least one of MEP, SSEP, bone integrity, nerve proximity, nerve direction, and nerve pathology.
In a further embodiment of the surgical system of the present invention, the hardware employed by the control unit to provide a stimulation signal may comprise an MEP stimulator capable of delivering a range of high voltage, constant current pulses for stimulating the motor cortex through the skull, wherein the control unit assesses the health of the spinal cord based on the identified relationship between the neuromuscular response and the stimulation signal.
In a further embodiment of the surgical system of the present invention, the MEP stimulator may be communicatively linked to the control unit via wireless technology.
In a further embodiment of the surgical system of the present invention, a bite block may be used in conjunction with the MEP stimulator, wherein the bite block is communicatively linked to the system and placement of the bite block may be confirmed by the system prior to MEP stimulation.
In a further embodiment of the present invention, the hardware employed by the control unit to provide a stimulation signal may comprise a patient module capable of delivering a range of low voltage, constant current pulses for stimulating a peripheral nerve, wherein the control unit assess the health of the nerve pathways based on the identified relationship between the stimulation signal and the corresponding neuromuscular response.
In a further embodiment of the present invention, the hardware employed by the control unit to provide a stimulation signal may comprise a patient module capable of delivering a range of low voltage pulses at a constant current (or constant voltage, if desired) for stimulating a nerve, wherein the control unit determines at least one of bone integrity, nerve proximity, nerve direction, and nerve pathology based on the identified relationship between the neuromuscular response and the stimulation signal.
In a further embodiment of the surgical system of the present invention, the surgical instrument may comprise at least one of a device for forming a hole in bone (e.g. for testing pedicle integrity), a device for accessing a surgical target site, and a device for maintaining contact with a nerve during surgery.
In a further embodiment of the surgical system of the present invention, the surgical instrument comprises a screw test instrument, wherein the control unit determines the degree of electrical communication between the screw test instrument and an exiting spinal nerve root to assess whether a pedicle has been breached during at least one of pilot hole formation (e.g. via an awl), pilot hole preparation (e.g. via a tap), and screw placement (e.g. via a ball-tipped probe).
In a further embodiment of the surgical system of the present invention, the surgical instrument comprises a nerve root retractor, wherein the control unit determines nerve pathology based on the identified relationship or change in relationship between the neuromuscular response and the stimulation signal.
In a further embodiment of the surgical system of the present invention, the surgical instrument comprises a dilating instrument, wherein the control unit determines at least one of proximity and direction between a nerve and the instrument based on the identified relationship between the neuromuscular response and the stimulation signal.
In a further embodiment of the surgical system of the present invention, the dilating instrument comprises at least one of a K-wire, an obturator, a dilating cannula, and a working cannula.
In a further embodiment of the surgical system of the present invention, the surgical instrument comprises a tissue retractor assembly and the control unit determines at least one of proximity and direction between a nerve and the instrument based on the identified relationship between the neuromuscular response and the stimulation signal.
Many advantages of the present invention will be apparent to those skilled in the art with a reading of this specification in conjunction with the attached drawings, wherein like reference numerals are applied to like elements and wherein:
Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. The systems disclosed herein boast a variety of inventive features and components that warrant patent protection, both individually and in combination.
The surgical system 10 includes a control unit 12, a patient module 14, an EMG harness 16, including eight pairs of EMG electrodes 18 and a return (anode) electrode 22 coupled to the patient module 14, an MEP stimulator 21, a pair of peripheral nerve stimulation (PNS) electrodes 25 also coupled to the patient module 14, at least one pair of stimulation electrodes 23 coupled to the MEP stimulator 21, and a host of surgical accessories 32 capable of being coupled to the patient module 14 via one or more accessory cables 30. The surgical accessories 32 may include, but are not necessarily limited to, stimulation accessories (such as a screw test probe 36 and dynamic stimulation clips 42, 52), surgical access components (such as a K-wire 62, one or more dilating cannula 64, a working cannula 66, and a tissue retraction assembly 70), and neural pathology monitoring devices (such as a nerve root retractor 76).
The patient module 14 is connected via a data cable 24 to the control unit 12, and contains the electrical connections to electrodes, signal conditioning circuitry, stimulator drive and steering circuitry, and a digital communications interface to the control unit 12. In use, the control unit 12 is situated outside but close to the surgical field (such as on a cart adjacent the operating table) such that the display 26 is directed towards the surgeon for easy visualization. The patient module 14 may be located near the patient's legs or may be affixed to the end of the operating table at mid-leg level using a bedrail clamp. The position selected should be such that all EMG electrodes 18 can reach their farthest desired location without tension during the surgical procedure.
The information displayed to the user on the display 26 may include, but is not necessarily limited to, alpha-numeric and/or graphical information regarding any of the requested modes (e.g., MEP, SSEP, Twitch Test, Screw Test (Basic, Difference, Dynamic), Detection, and Nerve Retractor), myotome/EMG levels, stimulation levels, etc . . . In one embodiment, set forth by way of example only, this information may include at least some of the following components (depending on the active mode) as set forth in Table 1:
Control of the surgical system 10 is, according to one embodiment, performed by user selection of available options on the GUI display 26, which will now be described (by way of example only) with reference to
The Site Selection screen preferably sets forth a list of the modes 122 available for each spinal region. By way of example only, the Cervical and Thoracolumbar spinal regions may include the Twitch Test, Basic Screw Test, Difference Screw Test, Dynamic Screw Test, MEP Auto, and MEP Manual modes, while the Lumbar spinal region includes the Twitch Test, Basic Screw Test, Difference Screw Test, Dynamic Screw Test, MaXcess® Detection, and Nerve Retractor modes, all of which will be described in greater detail below. (Although not shown, each of the spinal regions may also include an SSEP mode, as will be described in greater detail below.) The Twitch Test mode is designed to assess the neuromuscular pathway via the so-called “train-of-four” test to ensure the neuromuscular pathway is free from muscle relaxants prior to performing neurophysiology-based testing, such as bone integrity (e.g. pedicle) testing, nerve detection, and nerve retraction. This is described in greater detail within Int'l Patent App. No. PCT/US05/36089, entitled “System and Methods for Assessing the Neuromuscular Pathway Prior to Nerve Testing,” filed Oct. 7, 2005, the entire contents of which is hereby incorporated by reference as if set forth fully herein. The Basic Screw Test, Difference Screw Test, and Dynamic Screw Test modes are designed to assess the integrity of bone (e.g. the pedicle) during all aspects of pilot hole formation (e.g., via an awl), pilot hole preparation (e.g. via a tap), and screw introduction (during and after). These modes are described in greater detail in Int'l Patent App. No. PCT/USO2/35047 entitled “System and Methods for Performing Percutaneous Pedicle Integrity Assessments,” filed on Oct. 30, 2002, and PCT/US2004/025550, entitled “System and Methods for Performing Dynamic Pedicle Integrity Assessments,” filed on Aug. 5, 2004 the entire contents of which are both hereby incorporated by reference as if set forth fully herein. The MaXcess® Detection mode is designed to detect the presence of nerves during the use of the various surgical access instruments of the surgical system 10, including the k-wire 62, dilator 64, cannula 66, retractor assembly 70. This mode is described in greater detail within Int'l Patent App. No PCT/US02/22247, entitled “System and Methods for Determining Nerve Proximity, Direction, and Pathology During Surgery,” filed on Jul. 11, 2002, the entire contents of which is hereby incorporated by reference as if set forth fully herein. The Nerve Retractor mode is designed to assess the health or pathology of a nerve before, during, and after retraction of the nerve during a surgical procedure. This mode is described in greater detail within Int'l Patent App. No. PCT/USO2/30617, entitled “System and Methods for Performing Surgical Procedures and Assessments,” filed on Sep. 25, 2002, the entire contents of which is hereby incorporated by reference as if set forth fully herein. The MEP Auto and MEP Manual modes are designed to test the motor pathway to detect potential damage to the spinal cord by stimulating the motor cortex in the brain and recording the resulting EMG response of various muscles in the upper and lower extremities. These modes will be described in greater detail below.
Before addressing the MEP and SSEP functionality of the surgical system 10 of the present invention, various general features of the surgical system 10 will be explained. In one embodiment, the surgical system 10 provides the ability to quickly and easily switch or toggle back and forth between different modes during a surgical procedure. Toggling between the various functions of the surgical system 10 may preferably be accomplished by selecting from a drop down mode menu 104, as illustrated in
The surgical system 10 also includes a Setup mode that provides a simple means for selecting and/or changing various options and parameters associated with the surgical system 10 and each the modes. In one embodiment, the display screen for each mode includes a setup tab that allows a user to access and modify the parameters for any or all of the modes.
The neuromonitoring functionality of the surgical system 10 (except SSEP, which will be described in detail below) is based on assessing the evoked response of the various muscles myotomes monitored by the surgical system 10 in relation to a stimulation signal transmitted by the system 10. This is best shown in
In a first broad aspect of the present invention, the surgical system 10 is capable of assessing the health of the spinal cord via MEP monitoring. The surgical system 10 performs the MEP function by transmitting electrical stimulation signals from the MEP stimulator 21 through the motor cortex of the brain. The stimulation signals create action potentials which travel along the spinal cord and into exiting nerve roots, evoking activity from muscles innervated by the nerves. Evoked EMG responses of the muscles are recorded by the system 10 and analyzed in relation to the stimulation signal (discussed below). Resulting data from the analysis is conveyed to the surgeon on the GUI display 26.
MEP stimulation signals are generated in the MEP stimulator 21 and delivered to the motor cortex via stimulation electrodes 23 connected to the MEP stimulator 21. Stimulation electrodes 23 may take the form of needle, corkscrew, or surface pad electrodes, among other known forms. Typically a pair of stimulation electrodes 23, one cathode and one anode, are placed on opposite sides of the skull to transcranially stimulate the motor cortex. In a preferred embodiment, a single MEP stimulation signal includes multiple electrical pulses delivered together as one group or train. Stimulation signals comprising multiple pulses are desirable when stimulating the motor cortex, such as when performing MEP, because a more reliable response can be generated as a result. Each individual pulse of the stimulation signal will cause depolarization (provided the current level is greater than or equal to the stimulation threshold). Each train of pulses (i.e. each individual stimulation signal) is preferably delivered as a series of rectangular monophasic pulses, such as that illustrated, by way of example only, in
MEP stimulator 21 is communicatively linked to the control unit 12 which commands the stimulator 21 to deliver stimulation signals (according to the predetermined parameters) at the proper time. MEP stimulator 21 may be linked to the control unit 12 with a data cable that connects in any suitable manner or protocol, including but not limited to a USB cable that plugs into a USB port 31 on MEP stimulator 21 and control unit 12. Alternatively, MEP stimulator 21 may be linked to the control unit 12 via wireless technology. By way of example only, this may be accomplished by providing each of the control unit 12 and the MEP stimulator 21 with Bluetooth transceivers, which are commercially available and commonly known in the prior art, allowing the control unit 12 to transmit stimulation commands to the MEP stimulator 21 via a robust radio link. In use, this provides flexibility in positioning the MEP stimulator 21 in relation to the control unit 12, as well as reducing the number of wires and connections required for setup. The MEP stimulator 21 may be positioned outside the sterile area but should be located such that the stimulation electrodes 23, attached to the stimulator 21, may be positioned on the patient's head without tension. By way of example only, MEP stimulator 21 may be placed on the surgical table adjacent to the patient's head. Optionally, the MEP stimulator 21 may be fashioned with a mount or hook (not shown) and hung from the surgical table, an IV pole near the patient's head, or other equipment positioned near the patient.
According to one embodiment, MEP stimulator 21 may be optionally provided with a “bite block” 130 (
In the embodiment of
In the embodiment of
In a still further embodiment, electrode 134 of bite block 130 may comprise one of MEP stimulation electrodes 23. Electrode 134 is connected, via an attachable cable 136, to MEP stimulator 21 on the same stimulation channel as the corresponding stimulation electrode 23. In this manner, transcranial stimulation of the motor cortex is achieved by sending an MEP stimulation pulse from an electrode on the top of the head to an electrode located in the mouth, as opposed to sending the stimulation signal from one side of the head to the other. In addition to providing an alternate path for the stimulation pulse, utilizing the bite block 130 as a part of the stimulation circuit ensures that the protective bite block 130 is in position prior to MEP stimulation. Additionally, the impedance tests described above may be implemented through this embodiment.
Although bite block 130 has been described above with reference to the surgical system 10, it will be appreciated as within the scope of the invention to use bite block 130 in conjunction with any device or system used for MEP stimulation. It will be further appreciated as not departing from the scope of the invention that bite block 130 may be implemented as an independent system utilizing its own electrical source or the electrical source of any system it may be used in conjunction with, for any of a variety of situations where confirming placement of a bite block may be beneficial.
A basic premise underlying the methods employed by the system 10 for MEP monitoring (as well as the other nerve monitoring functions conducted by system 10) is that neurons and nerves have characteristic threshold current levels (IThresh) at which they will depolarize, resulting in detectable muscle activity. Below this threshold current, stimulation signals will not evoke a significant EMG response. Each EMG response can be characterized by a peak-to-peak voltage of Vpp=Vmax−Vmin, shown in
To obtain Ithresh and take advantage of the useful information it provides, the system 10 identifies and measures the peak-to-peak voltage (Vpp) of each EMG response corresponding to a given stimulation current (IStim). Identifying the true Vpp of a response may be complicated by the existence of stimulation and/or noise artifacts which may create an erroneous Vpp measurement. To overcome this challenge, the surgical system 10 of the present invention may employ any number of suitable artifact rejection techniques such as those shown and described in full in the above referenced co-pending and commonly assigned PCT App. Ser. No. PCT/US2004/025550, entitled “System and Methods for Performing Dynamic Pedicle Integrity Assessments,” filed on Aug. 5, 2004. Upon measuring Vpp for each EMG response, the Vpp information is analyzed relative to the corresponding stimulation current (Istim) in order to identify the minimum stimulation current (IThresh) capable of resulting in a predetermined Vpp EMG response. According to the present invention, the determination of IThresh may be accomplished via any of a variety of suitable algorithms or techniques.
For some functions, such as (by way of example) MEP monitoring, it may be desirable to obtain Ithresh for each active channel each time the MEP function is performed. This is particularly advantageous when assessing changes in Ithresh over time as a means to detect potential problems (as opposed to detecting an Ithresh below a predetermined level determined to be safe, such as in the Screw Test modes). While Ithresh can be found for each active channel using the algorithm as described above, it requires a potentially large number of stimulations, each of which is associated with a specific time delay, which can add significantly to the response time. Done repeatedly, it could also add significantly to the overall time required to complete the surgical procedure, which may present added risk to the patient and added costs. To overcome this drawback, a preferred embodiment of the surgical system 10 boasts a multi-channel MEP threshold hunting algorithm so as to quickly determine Ithresh for each channel while minimizing the number of stimulations and thus reduce the time required to perform such determinations.
The multi-channel MEP threshold hunting algorithm reduces the number stimulations required to complete the bracketing and bisection steps when Ithresh is being found for multiple channels. The multi-channel algorithm does so by omitting stimulations for which the result is predictable from the data already acquired. When a stimulation signal is omitted, the algorithm proceeds as if the stimulation had taken place. However, instead of reporting an actual recruitment result, the reported result is inferred from previous data. This permits the algorithm to proceed to the next step immediately, without the time delay associated with a stimulation signal.
Regardless of what channel is being processed for Ithresh, each stimulation signal elicits a response from all active channels. That is to say, every channel either recruits or does not recruit in response to a stimulation signal (again, a channel is said to have recruited if a stimulation signal evokes an EMG response deemed to be significant on that channel, such as Vpp of approximately 100 uV). These recruitment results are recorded and saved for each channel. Later, when a different channel is processed for Ithresh, the saved data can be accessed and, based on that data, the algorithm may omit a stimulation signal and infer whether or not the channel would recruit at the given stimulation current.
There are two reasons the algorithm may omit a stimulation signal and report previous recruitment results. A stimulation signal may be omitted if the selected stimulation current would be a repeat of a previous stimulation. By way of example only, if a stimulation current of 100 mA was applied to determine Ithresh for one channel, and a stimulation at 100 mA is later required to determine Ithresh for another channel, the algorithm may omit the stimulation and report the previous results. If the specific stimulation current required has not previously been used, a stimulation signal may still be omitted if the results are already clear from the previous data. By way of example only, if a stimulation current of 200 mA was applied to determine Ithresh for a previous channel and the present channel did not recruit, when a stimulation at 100 mA is later required to determine Ithresh for the present channel, the algorithm may infer from the previous stimulation that the present channel will not recruit at 100 mA because it did not recruit at 200 mA. The algorithm may therefore omit the stimulation and report the previous result.
In the interest of clarity,
Once Ithresh is found for channel 1, the algorithm turns to channel 2, as illustrated in
Although the multi-channel MEP threshold hunting algorithm is described above processing channels in numerical order, it will be understood that the actual order in which channels are processed is immaterial. The channel processing order may be biased to yield the highest or lowest threshold first (discussed below) or an arbitrary processing order may be used. Furthermore, it will be understood that it is not necessary to complete the algorithm for one channel before beginning to process the next channel, provided that the intermediate state of the algorithm is retained for each channel. Channels are still processed one at a time. However, the algorithm may cycle between one or more channels, processing as few as one stimulation current for that channel before moving on to the next channel. By way of example only, the algorithm may stimulate at 100 mA while processing a first channel for Ithresh. Before stimulating at 200 mA (the next stimulation current in the bracketing phase), the algorithm may cycle to any other channel and process it for the 100 mA stimulation current (omitting the stimulation if applicable). Any or all of the channels may be processed this way before returning to the first channel to apply the next stimulation. Likewise, the algorithm need not return to the first channel to stimulate at 200 mA, but instead may select a different channel to process first at the 200 mA level. In this manner, the algorithm may advance all channels essentially together and bias the order to find the lower threshold channels first or the higher threshold channels first. By way of example only, the algorithm may stimulate at one current level and process each channel in turn at that level before advancing to the next stimulation current level. The algorithm may continue in this pattern until the channel with the lowest Ithresh is bracketed. The algorithm may then process that channel exclusively until Ithresh is determined, and then return to processing the other channels one stimulation current level at a time until the channel with the next lowest Ithresh is bracketed. This process may be repeated until Ithresh is determined for each channel in order of lowest to highest Ithresh. If Ithresh for more than one channel falls within the same bracket, the bracket may be bisected, processing each channel within that bracket in turn until it becomes clear which one has the lowest Ithresh. If it becomes more advantageous to determine the highest Ithresh first, the algorithm may continue in the bracketing state until the bracket is found for every channel and then bisect each channel in descending order.
If Ithresh cannot be confirmed, the algorithm enters the bracketing state. Rather than beginning the bracketing state from the minimum stimulation current, however, the bracketing state may begin from the previous Ithresh. The bracketing may advance up or down depending on whether Ithresh has increased or decreased. By way of example only, if the previous value of Ithresh was 400 mA, the confirmation step may stimulate at 400 mA and 375 mA. If the stimulation at 400 mA fails to evoke a significant response, it may be concluded that the Ithresh has increased and the algorithm will bracket up from 400 mA. When the algorithm enters the bracketing state, the increment used in the confirmation step (ie. 25 mA in this example) is doubled. Thus, in this example, the algorithm stimulates at 450 mA. If the channel fails to recruit at this current level, the increment is doubled again (100 mA in this example) and the algorithm stimulates at 550 mA. This process is repeated until the maximum stimulation current is reached or the channel recruits, at which time the bisection function may be performed. If, during the confirmation step, the stimulation current just below the previously determined Ithresh recruits, it may be concluded that Ithresh for that channel has decreased and the algorithm may bracket down from that value (375 mA in this case). Thus, in this example, the algorithm would double the increment to 50 mA and stimulate at 325 mA. If the channel still recruits at this stimulation current, the increment is doubled again to 100 mA such that the algorithm stimulates at 225 mA. This process is repeated until the minimum stimulation current is reached or the channel fails to recruit, at which time the algorithm may perform the bisection function. When determining Ithresh for multiple channels with previously determined Ithresh values, this technique may be performed for each channel, in turn, in any order. Again stimulations may be omitted and the algorithm may begin processing a new channel before completing the algorithm for another channel, as described above.
Although the hunting algorithm is discussed herein in terms of finding Ithresh (the lowest stimulation current that evokes a predetermined EMG response), it is contemplated that alternative stimulation thresholds may be useful in assessing the health of the spinal cord or nerve monitoring functions and may be determined by the hunting algorithm. By way of example only, the hunting algorithm may be employed by the system 10 to determine a stimulation voltage threshold, Vstimthresh. This is the lowest stimulation voltage (as opposed to the lowest stimulation current) necessary to evoke a significant EMG response, Vthresh. Bracketing, bisection and monitoring states are conducted as described above for each active channel, with brackets based on voltage being substituted for the current based brackets previously described. Moreover, although described above within the context of MEP monitoring, it will be appreciated that the algorithms described herein may also be used for determining the stimulation threshold (current or voltage) for any other EMG related functions, including but not limited to bone integrity (e.g. pedicle screw test), nerve detection, and nerve root retraction.
According to one embodiment of the present invention, the surgical system 10 may perform the MEP function in either of two modes: Automatic mode and Manual mode. In one embodiment, these MEP modes are selectable from the drop-down function menu 104 of
In use, various features and parameters of the MEP function may be controlled and/or adjusted by the operator. In one example such control may be exercised from an “MEP Auto” mode setup screen, shown by way of example only in
The polarity of the stimulation signal also has an effect on the MEP response. Positive phase stimulations may, for example, result in a lower Ithresh values for muscles on the left side of the body than the right side of the body (or vice versa). To achieve the best MEP response values for all channels, it may be desirable to include stimulation signals of both positive and negative polarity. A polarity auto-switching feature may be controlled using on/off tabs 130. When polarity auto-switching is on, stimulation signals from MEP stimulator 21 alternate between a positive phase and a negative phase. The left and right sides of the brain may respond differently to positive and negative pulses. Each stimulation signal is used twice, once as a positive phase signal and once as a negative phase signal, before the hunting algorithm advances to the next stimulation current level. By way of example, if the algorithm begins stimulations at a minimum stimulation current of 100 mA, a first stimulation signal will include 1 to 8 positive phase pulses of 100 mA. A second stimulation signal will then include the same number of negative phase pulses of the same current, 100 mA. After stimulation results have been determined for the first current level using both polarities, the algorithm will stimulate at the next current level, in this case 200 mA, first with a positive phase signal followed by the negative phase signal. The algorithm will continue in this pattern until Ithresh is determined for each channel. The order in which positive and negative phases are used is not important and may be reversed such that the first stimulation signal includes negative phase pulses and the second signal follows with positive phase pulses. When polarity auto-switching is turned off, the operator may select the polarity to be used from the MEP display screen (
The surgical system 10 includes the ability to remind the user to perform an MEP stimulation, which can be controlled from the setup screen. Using up and down control arrows 126, the operator may set and/or change a time interval for receiving stimulation reminders. After each MEP monitoring episode, the system 10 will initiate a timer corresponding to the selected time interval and, when the time has elapsed, a stimulation reminder will be activated. The stimulation reminder may include, by way of example only, any one of, or combination of, an audible tone, voice recording, screen flash, pop up window, scrolling message, or any other such alert to remind the operator to test MEP again.
EMG sensitivity controls 148 and a Free-Run status control 150 are also provided on the screen. A check mark displayed in the free-run status control 150 indicates that free-run EMG mode is activated. When free-run is activated, the surgical system 10 continuously monitors EMG electrodes 18 for spontaneous nerve activity unless another mode, such as MEP, is active. Upon completion of an MEP episode, the surgical system 10 may automatically transition into free-run EMG monitoring, in which actual EMG waveforms are continuously displayed in real-time. In doing so, the user may be alerted to any nerve activity occurring unexpectedly. An audio pick-up (not shown) may also be provided as an optional feature according to the present invention. In some cases, when a nerve is stretched or compressed, it will emit a burst or train of spontaneous nerve activity. The audio pick-up is capable of transmitting sounds representative of such activity such that the surgeon can monitor this response on audio to help him or her determine if there has been stress to the nerve.
In manual MEP mode, the user simply selects a stimulation current and the system 10 determines whether or not the selected current evokes a predetermined EMG response. The user may be alerted to a potential complication if no response is detected from an EMG channel that had previously responded to a stimulation signal of the same or lesser amplitude. In one embodiment, set forth by way of example only, the user may determine for each channel a baseline stimulation current at which the stimulation signal evokes a response. Thereafter, the user may stimulate at each baseline and determine whether the corresponding channel still responds. Alternatively, a supramaximal current may be determined at which all channels show a response. Subsequent stimulations signals may be delivered at the same supramaximal current level and should continue to evoke a response. Subsequent absence of a response may be indicative of a problem with the spinal cord.
In addition to the MEP techniques described above, or instead of these techniques, the surgical system 10 is capable of monitoring spinal cord health via any number of different manners using additional data from the MEP test. For example, the system 10 may monitor changes in amplitude of the EMG responses as an indicator of spinal cord health. The system 10 may detect changes over time of the peak-to-peak voltage (i.e. amplitude of the EMG response) relative to a given stimulation signal current (shown in
In a second broad aspect of the present invention, the surgical system 10 is capable of assessing the health of the spinal cord via SSEP monitoring. The system performs SSEP monitoring by stimulating peripheral sensory nerves that exit the spinal cord below the level of surgery and measuring the electrical action potential from electrodes located on the nervous system tract superior to the surgical target site. The neural electrical signal is then analyzed in relation to the stimulation pulse, resulting in quantitative information related to the health of the spinal cord, which is then conveyed to the surgeon via the display 26. SSEP stimulation may be conducted on the Posterior Tibial nerve with the electrical signal being recorded from any suitable location, including but not limited to the skin overlying the second cervical (C2) vertebrae or the skin of the scalp. Accordingly, a pair of peripheral nerve stimulation (PNS) electrodes 25 may be positioned on the skin above the Posterior Tibial nerve, located at the ankle, and recording electrodes 41 may be placed on the skin above the C2 vertebra. The stimulation pulse is delivered by the patient module 14, which is coupled to the PNS electrodes 25 via an accessory cable 30. Although SSEP stimulation and recording is discussed with respect to the Posterior Tibial nerve and C2 vertebra, it will be appreciated that SSEP stimulation may applied to any number of peripheral sensory nerves, including but not necessarily limited to the Ulnar and Median nerves in the wrist, as well as directly stimulating the spinal cord inferior to the expected level of potential damage. Likewise, it will be appreciated that the recording site may be located anywhere along the nervous system superior to the spinal level at risk during the procedure, including but not necessarily limited to any suitable location on the scalp.
When the peripheral sensory nerve is stimulated an electrical pulse ascends from the nerve to the spinal cord and up into the brain. Damage in the spinal cord can disrupt transmission of the signal up the cord, resulting in a weakened or delayed signal at the recording site. The surgical system 10 detects such disruptions by measuring the amplitude of the stimulation signal waveform when it reaches the recording site, as well as the latency period (time signal takes to travel from the stimulation site to the recording site). The system 10 compares amplitude measurements to a previously recorded baseline amplitude or the preceding measurement, and the difference between them is viewed on the display 26. Similarly, latency measurements are compared to a previously recorded baseline latency or the preceding measurement and the difference value is shown on the display 26. A decrease in amplitude or an increase in latency may alert the surgeon to damage in the spinal cord and corrective measures may be taken to avoid or mitigate such damage.
In a third significant aspect of the present invention, the surgical system 10 may conduct other nerve monitoring functions, including but not necessarily limited to, neuromuscular pathway assessments to ensure that muscle relaxants, paralytic agents, and/or anesthetics are no longer affecting the neuromuscular pathway (Twitch Test), bone integrity testing (e.g. Pedicle Screw Test), nerve proximity testing (Detection), and nerve pathology monitoring (Nerve Root Retraction). These additional functions have been described in detail in the NeuroVision Applications referenced above, the entire contents of which are expressly incorporated by reference as if set forth herein in their entirety, and will thus be described here only briefly. In similar fashion to the MEP function discussed above, the system 10 conducts nerve monitoring functions by electrically stimulating a nerve via one or more stimulation electrodes positioned on the surgical accessories 32, monitoring the corresponding muscle response of muscles innervated by the nerve, and analyzing the muscle response in relation to the stimulation signal to determine one of neuromuscular pathway function, bone integrity, nerve proximity, and nerve pathology. In a preferred embodiment, EMG monitoring may be conducted on the same muscle groups monitored for the MEP function, as illustrated above in Tables 2, 3, 4. In this manner, the EMG electrodes 18 need be placed only one time, prior to or at the beginning of the surgery, and may be used to monitor EMG responses for all the various functions of the system 10.
The surgical system 10 performs neuromuscular pathway (NMP) assessments (Twitch Test) by electrically stimulating a peripheral nerve via PNS electrodes 25 placed on the skin over the nerve or by direct stimulation of a spinal nerve using a surgical accessory such as balled-tipped test probe 36. Evoked responses from the muscles innervated by the stimulated nerve are detected and recorded, the results of which are analyzed and a relationship between at least two responses or a stimulation signal and a response is identified. The identified relationship provides an indication of the current state of the NMP. The identified relationship may include, but is not necessarily limited to, one or more of magnitude ratios between multiple evoked responses and the presence or absence of an evoked response relative to a given stimulation signal or signals. Details of the test indicating the state of the NMP and the relative safety of continuing on with nerve testing are conveyed to the surgeon via the screen display 26.
The surgical system 10 may test the integrity of pedicle holes (during and/or after formation) and/or screws (during and/or after introduction). The screw test probe 36 is placed in the screw hole prior to screw insertion or placed on the installed screw head and a stimulation signal is applied. The insulating character of bone will prevent the stimulation current, up to a certain amplitude, from communicating with the nerve, thus resulting in a relatively high Ithresh, as determined via the basic threshold hunting algorithm described above. However, in the event the pedicle wall has been breached by the screw or tap, the current density in the breach area will increase to the point that the stimulation current will pass through to the adjacent nerve roots and they will depolarize at a lower stimulation current, thus Ithresh will be relatively low. In an alternative embodiment, screw test probe 36 may be replaced with an electric coupling device 42, 52 which may be utilized to couple a surgical tool, such as for example, a tap member 72 or a bone awl 74, to the surgical system 10. In this manner, a stimulation signal may be passed through the surgical tool and screw testing can be performed while the tool is in use. Thus, screw testing may be performed during pilot hole formation by coupling the bone awl 74 to the surgical system 10 and during pilot hole preparation by coupling the tap 72 to the system 10. Likewise, by coupling a pedicle screw to the surgical system 10 (such as via pedicle screw instrumentation), screw testing may be performed during screw introduction.
The surgical system 10 may perform nerve proximity testing (Detection) to ensure safe and reproducible access to surgical target sites. Using the surgical access components 62-66, the system 10 detects the existence of neural structures before, during, and after the establishment of an operative corridor through (or near) any of a variety of tissues having such neural structures which, if contacted or impinged, may otherwise result in neural impairment for the patient. The surgical access components 62-66 are designed to bluntly dissect the tissue between the patient's skin and the surgical target site. Cannulae or dilators of increasing diameter, which are equipped with one or more stimulating electrodes, are advanced towards the target site until a sufficient operating corridor is established. As the cannulae or dilators are advanced to the target site electrical stimulation signals are emitted via the stimulation electrodes. The stimulation signal will stimulate nerves in close proximity to the stimulation electrode and the corresponding EMG response is monitored. As a nerve gets closer to the stimulation electrode, the stimulation current (Istim) required to evoke a muscle response decreases. Ithresh is calculated, using the basic threshold hunting algorithm described above, providing a measure of the communication between the stimulation signal and the nerve and thus giving a relative indication of the proximity between access components and nerves.
Additional and/or alternative surgical access components such as, by way of example only, a tissue retraction assembly 70 (
The surgical system 10 preferably accomplishes neural pathology monitoring via the Nerve Retractor function, specifically by determining a baseline stimulation threshold with direct contact between the nerve retractor 76 and the nerve, prior to retraction. Subsequent stimulation thresholds are determined during retraction and they are compared to the baseline threshold. Significant changes in the stimulation threshold may indicate potential trauma to the nerve caused by the retraction and are displayed to the user on the display 26. An increase in Ithresh over time is an indication that the nerve function is deteriorating and retraction should be reduced or stopped altogether to prevent permanent damage.
With reference to
The control unit 12 is configured to monitor the system status throughout its use. In the event the control unit 12 detects an aberration an error log is created in which the details of the error are described and stored to assist in later troubleshooting and system correction. To service the system 10, the error logs may be accessed directly from the control unit 12 hardware and software. In addition, error logs may be downloaded onto any of a number of suitable media to facilitate data transfer between remote locations. By way of example only, the error logs may be downloaded to a USB memory device, floppy disk, CD, or DVD. By way of further example, the error logs may be downloaded onto a network and transmitted to remote locations via the Internet or other data transfer systems.
It will be readily appreciated that various modifications may be undertaken, or certain steps or algorithms omitted or substituted, without departing from the scope of the present invention. By way of example only, several alternative methods will now be described. Rather than identifying the stimulation current threshold (IThresh) based on a predetermined VThresh it is also within the scope of the present invention to determine IThresh via linear regression. This may be accomplished via, by way of example only, the linear regression technique disclosed in commonly owned and co-pending U.S. patent application Ser. No. 09/877,713, filed Jun. 8, 200 and entitled “Relative Nerve Movement and Status Detection System and Methods,” the entire contents of which is hereby expressly incorporated by reference as if set forth in this disclosure in its entirety.
Additionally, the nerve pathology monitoring function described above may be employed for the purpose of monitoring the change, if any, in peripheral nerves during the course of the procedure. This may be accomplished by positioning additional stimulation electrodes anywhere on a surgical accessory that is likely to come in contact with a peripheral nerve during a surgical procedure. Recruitment curves or other data can be generated and assessed in the same fashion described above.
Moreover, although described with reference to the surgical system 10, it will be appreciated as within the scope of the invention to perform MEP and SSEP monitoring as described herein with any number of different neurophysiology based testing, including but not limited to the “NIM SPINE” testing system offered by Medtronic Sofamor Danek, Inc.
While this invention has been described in terms of a best mode for achieving this invention's objectives, it will be appreciated by those skilled in the art that variations may be accomplished in view of these teachings without deviating from the spirit or scope of the present invention. For example, the present invention may be implemented using any combination of computer programming software, firmware or hardware. As a preparatory step to practicing the invention or constructing an apparatus according to the invention, the computer programming code (whether software or firmware) according to the invention will typically be stored in one or more machine readable storage mediums such as fixed (hard) drives, diskettes, optical disks, magnetic tape, semiconductor memories such as ROMs, PROMs, etc., thereby making an article of manufacture in accordance with the invention. The article of manufacture containing the computer programming code is used by either executing the code directly from the storage device, by copying the code from the storage device into another storage device such as a hard disk, RAM, etc. or by transmitting the code on a network for remote execution. As can be envisioned by one of skill in the art, many different combinations of the above may be used and accordingly the present invention is not limited by the specified scope.