Surgery – Diagnostic testing – Sensitivity to electric stimulus
Reexamination Certificate
1998-12-16
2001-10-23
Schaetzle, Kennedy (Department: 3762)
Surgery
Diagnostic testing
Sensitivity to electric stimulus
C600S548000, C607S048000, C607S063000, C128S908000
Reexamination Certificate
active
06306100
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to surgical apparatus and more particularly to a neurophysiological monitoring system including a nerve integrity monitoring instrument for use in conjunction with one or more electrical stimulus probes as an intraoperative aid in defining the course of neural structures. The invention is particularly applicable for use in monitoring facial electromyographic (EMG) activity during surgeries in which a facial motor nerve is at risk due to unintentional manipulation, although it will be appreciated that the invention has broader applications and can be used in other neural monitoring procedures.
DISCUSSION OF THE PRIOR ART
Despite advances in diagnosis, microsurgical techniques, and neurotological techniques enabling more positive anatomical identification of facial nerves, loss of facial nerve function following head and neck surgery such as acoustic neuroma resection is a significant risk. Nerves are very delicate and even the best and most experienced surgeons, using the most sophisticated equipment known, encounter a considerable hazard that a nerve will be bruised, stretched or severed during an operation. Studies have shown that preservation of the facial nerve during acoustic neuroma resection may be enhanced by the use of intraoperative electrical stimulation to assist in locating nerves. Very broadly stated, the locating procedure, also known as nerve integrity monitoring, involves inserting sensing or recording electrodes directly within cranial muscles innervated or controlled by the nerve of interest. A suitable monitoring electrode is disclosed in U.S. Pat. No. 5,161,533 (to Richard L. Prass et al.), the entire disclosure of which is incorporated herein by reference.
One method of nerve localization involves the application of electrical stimulation near the area where the subject nerve is believed to be located. If the stimulation probe contacts or is reasonably near the nerve, the stimulation signal applied to the nerve is transmitted through the nerve to excite the related muscle. Excitement of the muscle causes an electrical impulse to be generated within the muscle; the impulse is transferred to the recording electrodes, thereby providing an indication to the surgeon as to the location of the nerve. Stimulation is accomplished using hand held monopolar or bipolar probes such as the Electrical Stimulus Probe disclosed in U.S. Pat. No. 4,892,105 (to Richard L. Prass), the entire disclosure of which is incorporated herein by reference. The probe of U.S. Pat. No. 4,892,105 has become known as the Prass Flush-Tip Monopolar Probe and is insulated up to the distal tip to minimize current shunting through undesired paths. An improved structure for a bipolar probe is disclosed in the provisional patent application entitled Bipolar Electrical Stimulus Probe (filed Aug. 12, 1998, application No. 60/096,243), the entire disclosure of which is also incorporated herein by reference.
Another method of nerve localization involves mechanical stimulation of the nerve of interest by various dissecting instruments. Direct physical manipulation of a motor nerve may cause the nerve to conduct a nerve impulse to its associated musculature. If those muscles are being monitored using a nerve integrity monitoring instrument, the surgeon will hear an acoustic representation of the muscle response in close temporal relationship to the antecedent mechanical stimulation. This will allow the nerve of interest to be roughly localized at the contact surface of the dissecting instrument. Prior art nerve integrity monitoring instruments (such as the Xomed® NIM-2® XL Nerve Integrity Monitor, manufactured by the assignee of the present invention) have proven to be effective for performing the basic functions associated with nerve integrity monitoring such as recording EMG activity from muscles innervated by an affected nerve and alerting a surgeon when the affected nerve is activated by application of a stimulus signal, but have significant limitations for some surgical applications and in some operating room environments. A first problem is users have noticed certain EMG measurement artifacts have a disruptive effect on monitoring and tend to cause undesirable false alarms. In particular, EMG monitoring often is performed during electrocautery in a surgical procedure, wherein powerful currents surge through and cauterize the tissue, often to devastating effect on the monitor's sensitive amplifier circuits. Electrocautery can also induce an undesired direct current (DC) offset from buildup of charge on the monitoring or sensing electrodes or within recording amplifier circuitry. A method of muting during periods of electrocautery using in-line detection of electrocautery, based upon frequency and amplitude was disclosed in Prass, et al.: “Acoustic (Loudspeaker) Facial Electromyographic Monitoring: Evoked Electromyographic Activity”, Neurosurgery 19: 392-400, 1986; and an improved method involving an inductive probe pickup was described in U.S. Pat. No. 4,934,377, entitled “Intraoperative Neuroelectrophysiological Monitoring System”, by Prass, et al., the entire disclosures of which are incorporated herein by reference.
Brief pop noise in the form of high frequency bursts (caused by spurious electromagnetic and current artifacts or when non-insulated metal instruments are accidentally brought into physical contact) may be recorded during nerve integrity monitoring. These brief artifacts may be confused for true electromyographic (muscle) responses and may lead to misinterpretation and false alarms, thereby reducing user confidence and satisfaction in nerve integrity monitoring. Maintenance of high common-mode rejection characteristics in the signal conditioning path has helped to reduce such interference, however, false alarms still occur. Any solution tending to eliminate or minimize false alarm problems would increase the accuracy and effectiveness of monitoring procedures.
Prior art nerve integrity monitoring devices incorporate a simple threshold detection method to identify significant electrical events based upon the amplitude of the signal voltage observed in the monitoring electrodes, relative to a baseline of electrical silence, a methodology having disadvantages for intraoperative nerve integrity monitoring. Use of intramuscular electrodes in close bipolar arrangement (as described in U.S. Pat. No. 5,161,533, cited above) provides adequate spatial selectivity and maintenance of high common mode rejection characteristics in the signal conditioning pathway for reduced interference by electromagnetic artifacts, but yield a compressed dynamic range of electrical voltage observed between the paired electrodes. When physically situated near one of the electrodes, a single nerve motor unit (e.g., activation of a single nerve fiber) may cause an adequate voltage deflection to be heard (by a surgeon listening to the EMG audio signal feedback) as a clear signal spike or exceeding a predetermined voltage threshold. Moreover, with close electrode spacing and bipolar amplification, recording of larger responses is frequently associated with internal signal cancellation, significantly reducing the amplitude of the observed electrical signal. The resultant compressed dynamic range is advantageous for supplying direct or raw EMG audio signal feedback to the operating surgeon, in that both large and small signal events may be clearly and comfortably heard at one volume setting, but an EMG audio signal feedback having compressed dynamic range provides limited ability to fractionate responses based upon magnitude of the response or obtain an accurate measure of signal power. Another disadvantage of prior art methodology of threshold detection is that the surgeon cannot readily distinguish or select between electrical artifacts and EMG activity.
A second problem is that the nerves of interest may frequently exhibit a variable amount of irritability during the surgical procedure, which may be caused by a disease process or by surgical manipulations suc
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