Operating Manual
58 Pages
Preview
Page 1
Operating Manual Neurosign 800 P/N 1402-23-13 Software Revision 4.1
Neurosign 800
CONTENTS Section 1: Guarantee
3
Section 2: Description
4
Section 3: Warnings and Precautions
5
Section 4: Front Panel Layout
8
Section 5: Rear Panel Layout
11
Section 6: Pre-amplifier Pod
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Section 7: Sensing Electrodes
14
Section 8: Electrode Placements
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Section 9: Preparations for Use
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Section 10: Sterilisation
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Section 11: Stimulator Probe Pod
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Section 12: Stimulation Probes
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Section 13: Using the Neurosign 800
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Section 14: Screen Menus - Set-up Procedures
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Section 15: Printer
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Section 16: Bargraph Mode
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Section 17: Waveform Mode
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Section 18: Maintenance and Servicing
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Section 19: Specifications
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The Magstim Company Limited
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Section 1:
Guarantee Equipment manufactured by The Magstim Company Limited (“The Company”) is fully guaranteed covering materials and workmanship for a period of one year from the date of shipment, excepting those items detailed below. The Company reserves the right to perform guaranteed services in its factory, at an authorised repair station, or at the customer’s installation. Reusable Stimulating Probes are guaranteed covering materials and workmanship for a period of 3 months from the date of shipment. The single use electrodes and stimulating probes are guaranteed to be free of defect upon use. Sterilisation is guaranteed subject to the packaging being undamaged and the product being within the serviceable life as stated by the sterilisation date shown on the label. The Company’s obligations under this guarantee are limited to the repair or replacement of any defective parts without charge if the defects occur during normal service. Claims for damages during shipment must be filed promptly with the transportation company. All correspondence concerning the equipment must specify both model name and/or number and serial number, as it appears on the invoice. Improper use, mishandling, tampering with, or operation of the equipment otherwise than in accordance with this Operating Manual will void this guarantee and release the Company from any further guarantee obligations. The Company will only accept responsibility for the safety, reliability and performance of the equipment if: •
modifications or repairs are carried out by persons authorised by the Company, and
•
the electrical installation of the room in which the equipment is used complies with local regulations, and
•
the equipment is used in accordance with the operating instructions.
No statement in this Guarantee limits the statutory rights of a purchaser of this equipment in the United Kingdom.
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Section 2:
Description The Neurosign 800 is an eight channel motor nerve monitor designed for use in head and neck surgeries, and monitors motor nerves by detecting the EMG generated in the muscle controlled by the nerve at risk. The signal is processed and presented to the surgeon by means of a high quality amplifier and loudspeaker, and to a neurophysiologist by means of a display. The display is controlled by a microprocessor, allowing for simple but substantial control over the format of the display and default values for many of its functions. There are two levels of performance available according to the set-up of the display; a simple moving bargraph display representing EMG activity, and a more detailed waveform display. The waveform display may be printed via a serial port. The audio path is independent of the microprocessor, giving added security in case of component failure. A stimulator is combined with the monitor so that suspect tissue, or a nerve, may be directly stimulated during the surgery. Because the Neurosign 800 stimulator has the capability of providing up to 10mA of current or 10V of voltage, care must be taken that the stimulator current is appropriate for the surgery. Direct stimulation of nerves above a setting of 2mA or 2V is not recommended. Please refer to Section 12 for further guidance on using the stimulator. The direct stimulation of nerves should be kept to a minimum in order to avoid fatigue. The monitor will detect the spontaneous activity caused by the manipulation of the nerve by the surgery, and this is often the most useful information for the surgeon. For all head and neck surgery, the stimulator should be set to the lower range. This range will supply current or voltage in the range 50µA to 5mA, or 50mV to 5V. The higher range should not be used for head and neck surgery. The stimulator may be configured during the Set-up to be either a constant current or a constant voltage source, and may have one of two ranges depending on the type of surgery, and is variable in output to accommodate a variety of surgical applications. Refer to Section 12 for guidance on the current or voltage settings which are appropriate for specific surgeries. The unit has a number of self-checking routines to ensure that it is reliable and to alert the operating room staff in the event of failure. The overall design concept has been to produce a machine which is effective in use yet simple to operate, with a minimum of training being required.
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Section 3:
Warnings and Precautions
ATTENTION_______________________________________________________________ The Neurosign 800 is designed for use during surgical procedures and must only be used by qualified personnel who have carefully studied this Operating Manual and read this section. This instrument is designed as an aid to surgeons in the identification and monitoring of motor nerves at risk during surgery. It is not a replacement for experience in surgical decisionmaking. Before use, ensure that the voltage selector is set correctly. Only use fuses of the correct type and rating (see Section 17). Serious damage may occur and a safety hazard exists if incorrect fuses are used, or the wrong voltage is selected. Micro-processor In the event of a hardware failure or software corruption, the Neurosign 800 or Hardware is designed to fail in a safe state. In this state the stimulator and audio Failure channels are disabled. Such failure is indicated by the screen going blank; a warning message may or may not be displayed. Examples of error messages are ‘RAM string error’ and ‘Memory overflow’. These failures are extremely rare, and may be caused by a dirty electrical supply or internal hardware or software failure. In the event of a micro-processor failure, switch off the monitor, wait 5 seconds, and then switch on the monitor. The monitor should start normally. If an error message remains, or the screen remains blank, the monitor should not be used for the remainder of the operation. The monitor should be switched off, disconnected from the mains (line) supply, and the preamplifier and stimulating pod cables removed from the main unit. Do not use the equipment in the presence of Flammable Anaesthetic Mixtures. Do not drop the instrument, the Stimulator Pod or the Pre-Amplifier Pod, or otherwise permit any physical damage to occur. Do not permit any liquid to be spilt such that it may enter the casing through the ventilation slots in the bottom of the casing. Do not insert objects into connector housings, holes or slots in the casing for cleaning purposes, as this may cause damage. Use a lint-free cloth damped with alcohol to clean the instrument, the Stimulator Pod and the Pre-Amplifier Pod. Store the instrument in a dry atmosphere within the temperature range of -5°C t o 30°C (see Section 18). The Stimulation Probe(s) may be autoclaved (maximum temperature 140°C), unless they are stated to be for single use only (see Sections 7, 10 and 12). The single use electrodes and stimulating probes are supplied sterile. If there The Magstim Company Limited
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are any signs of damage to the packaging, the electrodes or probes should be discarded. No attempt should be made to re-sterilise either used electrodes or probes, or electrodes or probes found to be in damaged packing. The shaft of a single use concentric, or monopolar, probe may be bent once to an angle not greater than 30°. Following bending, the probe function must be re-checked. Once bent, on no account should attempts be made to straighten the probe as this will cause it to fail. Do not attempt to bend any other Neurosign Stimulating Probes as they have not been designed to withstand bending and will be damaged. Needle electrodes should be removed from the patient by holding the heatshrink covering the end of the needle and pulling out the needle at the same angle as it was inserted. The needles should not be removed by pulling the connecting lead. This could cause damage and result in reduced performance or product life expectancy. High levels of RF, usually generated by electrocautery equipment, may interfere with the operation of the Mute Sensor if this device is not connected to the electrocautery cable. Do not connect the Mute Sensor to the Neurosign 800 if the clamp is not to be attached to the electrocautery cable, or spurious muting of the Neurosign 800 may occur. Under extreme conditions, it may be possible that the mute sensor or the pre-amplifier may be affected by RF, in which case a few segments of the bargraph may light. In this case, the offending piece of equipment should be identified and replaced; alternatively, an improvement may be achieved by moving the preamplifier pod, stimulating pod, associated cables, or the position of the Mute Sensor. Any attempt to remove the securing screws will invalidate the guarantee. There are no user-serviceable parts inside. There are high voltages present and there is the danger of electric shock if the casing is opened. The use of this instrument other than as described in Section 13 may reduce the level of protection provided by the equipment. Contact The Magstim Company Limited prior to uses not described in the brochure. Before using the instrument, check that there are no signs of physical damage to either the main unit, the Pre-Amplifier Pod, the Stimulation Probe or the needle electrodes. The Stimulation Probe must be inspected after autoclaving for any signs of physical damage or deterioration and the tip(s) checked. The use of needle electrodes when used near, or in conjunction with, electrosurgical equipment has the potential, under certain fault conditions of the electrosurgical equipment, to give rise to burns at the point of contact between the needle electrodes and tissue. It is therefore strongly recommended that the instructions for preparation and use relating to the electrosurgical equipment are strictly adhered to, and that the equipment itself is regularly checked for electrical safety and correct operation, particularly in relation to any automatic impedance measuring or current sensing safety devices. Only equipment which has an automatic cut-out feature and measures the return pad impedance should be used in The Magstim Company Limited
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conjunction with the Neurosign 800. The particular conditions which could give rise to such a potential risk are concerned with the electrosurgical unit’s return pad, often attached to the patient’s thigh. Thorough preparation of the area to ensure a reliable contact with low impedance is vital for patient safety. Because the risk is related to the return pad and its impedance with regard to the body, the risk is considered to lie exclusively with the use of monopolar electrosurgery. Bipolar electrosurgery, as its name suggests, uses its own active and return electrodes in the same forceps. It should therefore be considered whether monopolar electrosurgery is necessary when using nerve monitoring equipment. In order to reduce any possible induced current in the needle electrode leads from stray RF emitted by the electrosurgical unit, the electrode leads should be kept as far as possible from the unit. In practice, this means that the electrosurgical unit should not be positioned near the head of the patient. Cables from the electrosurgical unit, both active and return, should also be kept as far as possible from the electrode leads, especially whilst monopolar electrosurgery is being used. Consideration should therefore be given, prior to the surgery, to the balancing of the risks of the surgery to the nerve being monitored, and the possible risk of burns caused by any interaction between the electrosurgical machine and the Neurosign 800. Care should be taken to avoid the tip of the Stimulating Probe coming into contact with that of the Electrocautery Probe, as it will cause damage to the internal circuitry of the Neurosign 800 unit.
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Section 4:
Power ON/OFF
Front Panel Layout
Power indicator
Dedicated function buttons
Channel ON/OFF Preamplifier
Stimulator
Volume control Mute sensor
Most of the functions of the Neurosign 800 are accessed by pressing the switches on the front panel. Some are dedicated to a particular function - for example, the Stimulating Current or Voltage is set by dedicated buttons - or by a series of menus which interface with the display. It has been designed to minimise the number of button presses required, and with the unit’s memory facilities, many will find it unnecessary to alter the default settings once the unit has been initially set up. The unit has default options which affect various settings; these may be left at the default, changed for each procedure or during an operation, or the defaults may be changed and reused. Power ON/OFF The Power ON/OFF switch is positioned at the bottom left of the front panel, above the symbol . Ensure that the line cord is attached to the connector on the rear panel, then set the Power switch to ON so that the indicator light is illuminated. Volume Control The Volume Control is situated in the top, right-hand corner of the front panel. Turn it clockwise to increase the volume. The control, at its minimum setting, does not allow the sound to be completely silenced; this avoids the risk of the control being accidentally set to a minimum during a procedure and important responses going unheard. A setting of approximately a half of the control’s rotation is likely to be appropriate in most Operating Rooms. During use of the electrosurgical unit for monopolar cutting and coagulation, the Mute Sensor will mute all sound, irrespective of the Volume Control setting. The sound will return immediately the cautery is finished. The Magstim Company Limited
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Stimulator The cable from the Stimulator Pod should be inserted into the connector on Output the front panel. It is keyed and cannot be inserted incorrectly. Always hold the connector by its body when inserting or removing the plug, or damage may occur to the cable. Note that the Stimulator connector and the keys used to alter the Stimulating Current or Voltage are linked by a colour code on the front panel label. The arrows indicate how to increase or decrease the Stimulating Current or Voltage, this choice being provided within the set-up menus. Pressing an arrow briefly will increase or decrease the stimulating current or voltage by 0.05mA or 50mV; pressing it continuously will fast scroll through these values at a ×10 rate. The value of the Stimulating Current or Voltage is displayed on the screen. Refer to Section 12 for suggested stimulator values for specific surgeries. If there is no pre-amplifier pod connected to the main unit, the display will indicate the legend ‘PRE-AMPLIFIER NOT CONNECTED’ and a preamplifier symbol will flash at the top of the screen. When the pre-amplifier is connected, this message and the symbol will disappear. It shows that the pre-amplifier pod has been correctly connected to the main unit. Pre-amplifier The cable from the Pre-amplifier Pod should be inserted into the connector Input on the front panel. It is keyed and cannot be inserted incorrectly. In addition, the plug has a small red dot on its surface which represents the top of the connector. Always hold the connector by its body when inserting or removing the plug, or damage may occur to the cable.
Preamplifier input connector
Stimulator pod connector
Mute sensor input
Mute Sensor The cable from the Mute Sensor should be inserted into the connector on Input the front panel. The connector is a BNC type which should be locked in position by inserting the plug and turning it clockwise a quarter turn. Removal is the reverse of this procedure. Always hold the connector by its body when inserting or removing the plug, or damage may occur to the cable, and only connect the sensor if monopolar electrosurgery is to be used. Never attach the Mute Sensor to the unit and leave the sensor unattached to the electrosurgical unit’s monopolar lead, or spurious muting may occur. The Magstim Company Limited
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When the sensor detects RF in the electrosurgical unit’s cable, the unit mutes the audio signal and lights a mute symbol on the screen. EMG ON/OFF Each channel has individual ON/OFF switches. The channels are active even if the display is being altered using the menu functions, and the audio path will function normally in this state. The channels may be switched on and off at any time whilst monitoring. However, the channels must be active BEFORE menu keys are pressed; once in the menu system, it is not possible to switch channels on or off. Screen Menu These arrow-head shaped buttons are related to the messages or instructions Keys displayed on the screen, and are used to set up the display to the user’s requirements. The function of these buttons relates to the symbol on the screen, and there is considerable uniformity across the menu system. If no function is applied to a key in a menu, no symbol is displayed and pressing the key will have no effect. Main Menu Key The Main Menu key is designed as a fast track method of returning to the Main Menu, no matter which sub-menu you are in. It enables minor changes to be made to the Set-up during a procedure, whilst leaving the user confident that they can return to a monitoring status within a few seconds.
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Section 5:
Rear Panel Layout External Interface
Isolated RS232 Connection Mains Connector
Mains The standard IEC power lead should be connected before the unit is Connector switched on. The unit is OFF when the switch on the front panel is at the 0 position, and ON when at the 1 position. There are two fuses, rated at T1.25A for 100-120V operation or T630mA for 220-240V operation, next to the power lead connector. The fuses may be checked or replaced by removing the fuseholder cover. This cover also provides access to the voltage selector. Although it is unlikely that this will need to be changed, unless the instrument is shipped to another country, the mains voltage may be chosen by rotating the voltage selector and replacing the cover. A legend displays the voltage selected when the fuse cover is replaced. Do not select the wrong voltage, or replace fuses with those of a different rating, as severe damage may occur. External This connector allows the Neurosign 800 to interface with many other pieces Interface of equipment, such as the Magstim series of magnetic stimulators and computer software. Because the combined isolation of all equipment connected to the patient is critical, the Company does not recommend the connection of equipment not manufactured by the Magstim Company to this port, unless the entire system has been checked for leakage current. Equipment manufactured by the Company has been tested and complies with IEC 601-1-1.
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Section 6:
Pre-amplifier Pod
CH1
CH2
CH3
CH4
REF
CH5
CH6
CH7
CH8
NEUROSIGN 800 PRE-AMPLIFIER CAUTION: RF burns are a recognised risk of using needle electrodes in conjunction with electrosurgical equipment. The following advice is intended to minimise any risk. DO NOT use electrosurgical units which do not have safety mechanisms to determine pad impedance and cable continuity. DO ensure that the patient return pad to the electrosurgical unit is properly fixed to the patient, after thorough preparation of the skin. DO NOT allow cables from the Neurosign 800 to cross the metal base of the operating table, as this may lead to significant capacitive coupling between the cable and earth. Electrosurgical unit cables should not cross monitoring leads. DO use the minimum power necessary to cut and/or coagulate.
Care and The Pod may be cleaned using a lint-free cloth damped with alcohol. Do not Maintenance use water, as this will cause the instrument to give poor results until completely dry, and may even cause damage. Should the Pod need to be sterilised following blood contamination, a gas sterilising unit must be employed. Preparations The Neurosign 800 detects very small voltages within a number of motor for Use units. In order to ensure that as little electrical noise as possible is amplified, the instrument utilises a Pre-amplifier Pod, which should be placed on the side rail of the operating table using the attached clip or between the patient’s legs, according to the type of procedure being undertaken and the convenience of theatre staff. The cable from the Pod should be connected to the Pre-amplifier Pod Input socket on the front panel of the instrument. Electrodes Two electrodes are used to monitor each muscle group. The use of disposable needle electrodes is recommended for performance, and staff and patient safety. The two electrodes are connected to the differential amplifier inputs, +ve and -ve (colour coded red and black). Care should be taken to connect the electrode wires correctly, with each channel being kept separate. Best results will be obtained if the wires from each muscle group are twisted together. For this reason, the use of the correct electrodes is absolutely essential and it is recommended that only electrodes obtained from the The Magstim Company Limited
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Magstim Company Limited are used. The Company cannot guarantee the instrument’s performance if these electrodes are not used. The electrodes should be inserted into the muscle about 10mm apart. Do not insert vertically into the muscle since the electrode will then face straight out of the body, and will either be damaged if bent, or will be inconvenient in the management of the patient. It is recommended that the electrodes are inserted at an angle of c.30°. Once inserted, the electrodes should be taped into position to prevent accidental removal and to help in the practical management of the patient.
.
The active pairs of electrodes require a reference electrode in order to reject artifact voltages present in the body. Care must be exercised in the placement of this electrode. For head and neck surgery, the forehead or contralateral masseter using a needle electrode is recommended.
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Section 7:
Sensing Electrodes Depending on the surgery being performed, both surface and needle electrodes are suitable for use with the Neurosign 800. During surgery involving the head and neck muscles, the selectivity of needle electrodes is helpful. To use surface electrodes may lead to confusion about which nerve has been stimulated. However, in some circumstances it may be appropriate for surface electrodes to be used.
Disposal Needle electrodes should be handled with due care at all times, especially when they have been used. The electrodes are marked ‘Single use’ and must not be reused. Under no circumstances should attempts be made to resterilise these electrodes, as the materials selected for their manufacture may not resterilise effectively. They should be disposed of by following the appropriate hospital policy regarding contaminated, sharp needles. ATTENTION! The use of needle electrodes when electrosurgical units are being used presents some special problems. If the return pad used by the electrosurgical unit is not properly attached to the patient, it is possible for the electrosurgical unit’s output current to find an alternative route to ground via any patient connections, most commonly those used for monitoring purposes. When it is proposed to use needle electrodes in preference to surface devices, due care must be exercised in the placement of the electrosurgical unit’s return pad. Failure to pay sufficient attention to the quality of this connection to the patient may allow electrocautery current to flow to ground via unexpected paths, leading to the possibility of patient burns.
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Section 8:
Electrode Placements
Skull base For skull-base surgery, a variety of recording montages will be necessary surgery depending on the cranial nerves involved. The table below gives a brief synopsis of the muscles fed by the relevant nerves; certain muscles will require the use of specialised electrodes. Diagrams are shown on the following pages which illustrate the placement of electrodes for some typical head and neck applications.
Head and neck surgery CN
Nerve
Muscle
III
Oculomotor
Superior rectus, medial rectus, inferior rectus, inferior oblique
IV
Trochlear
Superior oblique
V
Trigeminal
Masseter
VI
Abducens
Lateral rectus
VII
Facial
Orbicularis oris, orbicularis oculi, frontalis
IX
Glossopharyngeal
Stylopharyngeus, soft palate
X
Vagus
Vocal cords
XI
Accessory
Sternocleidomastoid, trapezius
XII
Hypoglossal
Tongue
Reference Note that the single reference electrode should be placed on the electrode contralateral side; this provides an electrically stable muscle site and keeps the electrode away from an otherwise crowded area! Alternative sites for the reference electrode are the frontalis, sternocleidomastoid or trapezius muscles, in descending order of preference. Ideally, the reference should be as close as possible to the active electrodes, spatially in the middle, and not in a muscle which could be innervated during the surgery. If the surgery is bilateral, then this would be an indication to use either the sternocleidomastoid or the trapezius as the reference site.
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Typical 4 channel application for monitoring the facial nerve.
Channel 1: temporal
Channel 2: zygomatic
Channel 3: buccal Reference: orbicularis oris or masseter on contralateral side Channel 4: mandibular
This montage should be used during parotidectomy, acoustic neuroma, mastoidectomy, and other procedures which may involve the facial nerve. Whilst it is possible to monitor the nerve effectively using just 2 channels, 4 channels adds greater discrimination and has the benefit that the same montage can be used for all procedures involving the facial nerve. Vth Cranial If the Vth cranial nerve is involved, use an additional channel with nerve electrodes in the masseter. Ensure that no other electrodes can be involved with the masseter; for example, that the electrodes for the zygomatic branch do not veer too near the masseter, or that the reference is not also in the masseter on the ipsilateral side. Other cranial In many skull base operations, more than one cranial nerve may be nerves involved. With the number of channels available, it is simple to insert electrodes so that nerves which may be encountered can be quickly identified, even if they are not likely to be involved in the surgery. For example, identifying the Vth cranial nerve during an acoustic neuroma resection may help to speed the decision-making process.
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Radical Neck Dissection¹
Radical neck dissections are often necessary to remove tumours of the oral cavity, pharynx and neck caused by smoking. As can be seen from the above diagram, the surgical problems in removing tumours from this area are considerable. Not only has the tumour to be removed, but great care must be taken to avoid damaging the carotid or jugular arteries, or the complications may be as serious as the disease! In addition, many major nerves are in the area, and may well be involved with the tumour. In many cases where the tumour is extensive, little or no effort may be made to preserve nerve supply. Nerves at serious risk are the accessory and hypoglossal; however, the vagus, phrenic and roots C2 to C6 may be encountered, and it is important to preserve as much as possible. In tumours which are not extensive, or where the surgery is more exploratory, preservation of nerve supply becomes much more relevant. The following table suggests electrode sites to monitor all the motor nerves which could be encountered; select those which are expected to be exposed during the surgery. 1 Drawing reproduced from Gray’s Anatomy
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Nerve Hypoglossal (XII)
Muscle Undersurface and base of tongue; genioglossus, styloglossus
Facial (VII) (mandibular branch)
Orbicularis oris, mentalis
Facial (cervical branch)
Platysma
Nerve Mylohyoid (V)
Mylohyoid
Glossopharyngeal (IX)
Soft palate; stylopharyngeus
Mandibular branch of (V) Facial (on exit from stylomastoid foramen) Vagus (X)
Digastric - anterior belly Digastric - posterior belly
Accessory and C2, C3
Sternocleidomastoid
Accessory (XI) and C3, C4 Ansa cervicalis C1 to C3
Trapezius Omohyoid, sternohyoid, sternothyroid Thyrohyoid
Ansa cervicalis C1 to C2 and descending hypoglossal Phrenic
Vocal cords via recurrent laryngeal nerve
Diaphragm
Some of these are supplied only for information, so that a sensible monitoring montage can be developed for the individual patient. It is not intended that each muscle should be independently monitored! The reference electrode should be positioned close to the surgical area but on the contralateral side; the trapezius is suggested. Spinal The use of EMG monitoring at the lumbar and sacral levels offers some Applications advantages over Somatosensory recordings, in that SEPs may find a number of routes back to the brain - this can lead to normal recordings being made, but the patient may still have some nerve deficit. EMG is better defined since operating at a specific level will have an effect on a particular muscle group, although there is still patient variability. A further advantage is that setting up the monitor is much simpler than the setup required for recording SEPs. For this reason, it is important to follow the electrode montages recommended, since these have been shown to work well over the levels indicated.
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8 channels allows the Neurosign 800 to be used to monitor spinal nerve roots, especially at lumbar and sacral levels. The monitor may be used in the placement of pedicle screws, BAK implants, Graf Bands, rigid fixation, nerve root decompression, pelvic repairs, sciatic nerve preservation etc. Level L3 L4 L5 S1 S2-S4
Muscle Monitored Biceps Femoris or Adductors Quadriceps Tibialis Anterior Gastrocnemius Perineum
Stimulating If the nerve root is not to be stimulated directly, set the Stimulator to the Currents HIGH range from within the STIM menu. The range available is from 0.5mA to 10mA. Follow the table below as a guide in the use of the stimulator.
Procedure
Stim. current
Notes
Pedicle Screw
7mA
Start at 7mA; note any response
Nerve root decompression
1mA
Expect spontaneous responses
Fusion cages
7mA
Pay attention to spontaneous responses
Stimulating For most applications in spinal surgery, use the monopolar pedicle screw Probe probe; if the nerve root is likely to be stimulated directly, use the concentric probe. For the monopolar probe, a separate needle electrode is used to provide the return path. It is recommended that this be placed in the wound when required. If it is inserted before the patient is draped, keep it away from the electrosurgical unit’s return pad and the sensing electrodes; the shoulder is usually best. Pedicle Screw The great majority of pedicle screws are placed in levels L4-S1. Use all 8 channels to monitor levels L3-S1, since this will catch any patient variability. Set the stimulator to 7mA, and insert the stimulator return electrode in the wound when ready. Using the pedicle screw probe, run the probe down the pilot hole to the thinnest part of the pedicle wall essentially, run the probe up and down the hole and listen for any response. If there is no response, the screw may be placed normally. If there is some response, reduce the stimulating current to the point where the response disappears, and note this current. The lower the current, the greater attention should be given to the direction of the screw. Bear in mind that 1mA may be considered as the base stimulating current to be used in stimulating the nerve root directly. Therefore, if you obtain a
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response at 1.5mA, this should be considered as highly significant; if you obtain a response at 5mA, then this may be due to patient variability, the hole may need some redirecting, or the screw may be placed normally but with some additional surgical care. Once the screw has been placed, the current may be increased to 10mA and the screw stimulated directly. It is unlikely that any response will be obtained in so doing, as the greater surface area of the screw will dissipate the current; however, if any response is obtained, this probably indicates that the screw has breached the pedicle wall, and that some treads have broken through. Depending on the situation, it may be advisable to continue surgery in order to visually check whether the screw has broken the pedicle wall.
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