ACNS SSEP Guidelines

What is the minimum capability of the amplifier?

input signals with peak to peak amplitudes of 5 microvolts to 50 microvolts should be amplified, equal to the full range of the analog to digital (A-D) converter

• ensuring range is appropriate to the size of the signal being recorded will enhance analog to digital conversion

• allows it to process evoked potential signals

What must the amplifier bandpass be, measured at the -3 dB points?

0.1-5,000 Hz

What should common mode rejection be?

80 dB (10,000:1) at the highest sensitivity of the amplifier, when the common mode signal is applied between both inputs and neutral

What should the differential input impedance of the amplifier be?

100 megOhms (MUCH larger than kilohms)

What can the noise level of the amplifier NOT exceed?

2 microvolts rms

• bandpass of 0.1-5000 Hz

What must the time (horizontal) resolution of the system be? Also known as analysis time.

20 microseconds per data point or less

• need certain amount of data points to correctly display data - too little means not enough resolution for peaks and troughs to be displayed, too much and it will overload the computer

The averager system should allow averaging at least _____ trials (raw capability)

4,000

A mechanism where artifact-contaminated trials can be simply and quickly excluded from the averaging process is essential. This is most commonly achieved by _______.

Rejecting those trials that exceed the limits of the A-D converter, or some adjustable percentage thereof.

• usually set to 90-98% of the range of the A-D converter

Processing of the noisy data is done by Gaussian ____ to see a clear signal, which can still be inaccurate unless we _____ trials that exceed a certain upper and lower boundary or limit.

convolution; reject

In the OR, we plug our equipment into a grounded electric socket (usually the one with the green dot) since it has an established ground. This is because of Chassis ______, which is the current that flows from the equipment to the patient and can flow through paths such as the patient if not properly grounded

leakage current

Don’t use extension cords or power strips since it increases ______

Chassis leakage current

Chassis leakage should be less than ___ microamps rms with ground open in a CLINICAL setting

300

Max chassis leakage current of ____ microamps rms with ground open, for INTRAOPERATIVE settings

100

Maximum leakeage current through patient leads should be ___ microamps rms to ground, with 120 VAC applied

10 microamps rms

The maximum leakage current should be ___ microamps rms at the patient end of the cable when the _____.

50; when power line ground is disconnected

Chassis leakeage check should be performed twice a year, biannually, or semi-annually or every 6 months to prevent patient shock

semi-annually - every 6 months

What is the purpose of an isolation amplifier?

prevents (stray) current from flowing to the patient

• allows AC power to be taken from one device and fed into another without electrically connecting the two circuits

The grounding system should ensure that no voltage greater than ____ can be measured across an impedance of 1,000 ohms between any combination of exposed conductive surfaces or grounding conductors that may be in contact with patient or personnel

20 microvolts (mV) rms

What is replication?

two or more temporally independent averages

What is the criteria for replication or repeatability? (SSEPs, which are averaged potentials) - latency is ____, amplitude is ____.

1%, 15%

If data is not reproducible, do another set of averages or ____

trials

What are the steps of documentation?

• store averaged waveforms

• times of surgical events and procedures

• alerts issued

• anesthetics and drugs, and significant changes in dose

• significant changes in physiological parameters (blood pressure, temperature)

• maintain documentation along with stored waveforms

• final report filled in patient chart

What are the clinical guidelines for data collection?

Involves normative data (obtained from reference population, bell curve) - if normative data is going to be used from another lab, all collection parameters must be replicated from reference data (amplifier and stimulator settings), and a minimum of twenty neurologically intact patients must be run using test settings, and patients must fall within normal distribution

For IOM or intraoperative neuromonitoring, we cannot rely upon _____ data that is used within clinical settings - meaning that every patient is their own control, or baseline.

normative

Baseline values for IONM data are established following ____ of anesthesia and ____ of the patient, but prior to surgical intervention (ex. incision)

induction; positioning

Baseline values may need to be reestablished if changes in anesthetic medications or other physiological paramters occur during the case, but check with the ____

IP

SSEPs should be continuously performed in the operating room EXCEPT when _____ prohibits collection

electrocautery (ex. bovie)

Monitoring should begin BEFORE any surgical manipulation of the nervous system has occurred, and continue until the surgical procedure has ____.

ended (terminated)

According to ACNS, spinal cord surgery above the c6 level, we can use _____ stimulation (SSEPs) to monitor

median nerve

According to ACNS, spinal cord surgery above the c8 level (lower cervical segments) we can use _____ stimulation (SSEPs) to monitor

ulnar nerve

Surgery involving levels below the C8 segment requires SSEP monitoring to stimulation of the _____ or ______

posterior tibeal OR common peroneal

What is the utility of upper extremity SSEPs for lumbar cases?

• may detect peripheral nerve and brachial plexus ischemia or compression

• monitoring of the ulnar nerve SSEP is preferable to median nerve SSEP for this purpose since the former assesses the lower brachial plexus, which is MOST susceptible to stretch injury

What are stimulation parameters for pulse/stim type, according to IOM guidelines?

monophasic rectangular pulses delivered using either a constant voltage or constant current stimulator (constant current is best and recommended in the operating room)

What is the pulse width or duration for stimulation parameters (IOM guidelines)?

100-300 microseconds, or 0.1-0.3 ms

What is the stimulation rate for stimulation parameters, in a clinical setting?

3-5 Hz or 3.1-5.1 Hz

What is the stimulation rate for stimulation parameters, in an IONM (intraoperative) setting? MEDIAN nerve

2-8 Hz (or 2.1-8.1/sec)

What is the stimulation rate for stimulation parameters, in an IONM (intraoperative) setting? ULNAR nerve

2-10 Hz

What is the stimulation intensity for clinical settings (IOM guidelines)?

adequate to produce a muscle twitch or motor threshold (not tetany); equalizing peripheral potentials

According to IOMN guidelines, what is a motor threshold defined as?

minimum stimulis intensity necessary to elicit a twitch

According to IOMN guidelines, what is equalizing peripheral potentials defined as?

increase intensity until the peripheral responses no longer increase in amplitude, since increasing intensity even further will no longer give better responses.

• since responses for SSEPs travel up from nerves to brain, it makes sense that if the origin of the signal can’t increase anymore, anything after that won’t increase either

What is the stimulation intensity for intraoperative settings (IOM guidelines)?

equalizing peripheral potentials

What are three parameters that influence the degree of pain to the waking patient (SSEP)?

• duration of the stimulus

• current density (surface area of the stimulation electrode)

• intensity of stimulation

For duration of the stimulus, 100 microseconds is _____ painful, and 300 microseconds is _____ painful.

least; most

Large stimulators such as surface electrodes are the least painful since they have _____ current densities, while small stimulators like needle electrodes have ____ current densities, since they have smallest surface areas. This means that needle electrodes cause more pain.

lower, higher

Stimulation intensity should be decreased to _____

motor threshold

If a patient is uncomfortable in a clinical setting:

• decrease the number of averages or repetitions

• use a larger size stimulator, which uses less current density

Stimulation parameters must be within safe limits to prevent ____, so stimulation parameters ensure the safety of the patience.

tissue damage

Lowe impedance, or good contact with the skin/fully submerged in tissue, will lead to _____ patient comfort. Aim for a number of _____ kilohms or less.

greater, 5

To avoid a ground loop, place only ____ ground electrode. Ground electrodes are placed on the stimulated limb to reduce ______ artifact.

one; stimulation

In a REAL-LIFE OR setting (outside of guidelines), can we check impedance of stimulating or recording electrodes

recording only

According to IOMN guidelines, the ground is placed between the ____ of the stimulating electrode and the first recording electrode. For forearm, this is the median nerve, and for the calf, this is the PTN

cathode

Electrodes should never be ____ or taped around a limb.

wrapped

For clinical stimulation, we do ____ limb testing, while for IOMN (intraoperative) settings, we do ____ stimulation (bilateral ASYNCHRONOUS)

unilateral; interleaving

What is bilateral synchronous stimulation?

both sides are being stimulated at the same time, which can result in a unilateral problem being masked

• this produces a false negative (“everything seems OK, but it isn’t) result - the data may be unchanged, but patient wakes up with a deficit

SSEPs use _____ stimulation, and switching of the cathode and anode can lead to _____ blocking.

CATHODAL (with cathode placed more proximal); anodal

Anodal blocking does not cause (three things).

• inverted waveforms

• obligate peaks appearing earlier (latency shift)

• response to travel distally to muscles

What are the results of a failure of stimulation? (three things)

• significant increase in contact impedance (so electrodes should be making good contact with the skin)

• salt bridge (direct path for current to follow between stimulating electrodes, leading to electrodes “short-circuiting” - results from sweat, electrode jelly

• limb edema, peripheral neuropathy, variant anatomy

What is pedal edema?

accumulation of fluid in the feet and lower legs

If a distal toe twitch is not noted due to pedal edema, what two approaches do we take (both clinical and IONM).

• for clinical SSEPs, increase stimulation intensity

• for IONM SSEP, use subdermal needle electrodes for stimulation

Where do we place cathode electrode for median nerve?

between tendons of the palmaris longus and flexor carpi radialis muscles, approximately 2 cm proximal to the wrist crease

Where do we place anode electrode for median nerve?

placed 2-3 cm distal to cathode, or on dorsum of the wrist

Where do we place ground electrode for median nerve?

forearm

For median nerve, where do we see the twitch for SSEP stimulation?

abduction of the thumb (first digit) - thenar or abductor pollicus brevis

Where do we place cathode electrode for ulnar nerve?

medial to palmaris longus muscle, approximately 2 cm proximal to wrist crease

Where do we place ground electrode for ulnar nerve?

forearm

For ulnar nerve, where do we see the twitch for SSEP stimulation?

flexion of the 4th and 5th digits (hypothenar or abductor digiti minimi)

Where do we place anode electrode for ulnar nerve?

placed 2-3 cm distal to the cathode, or on the dorsum of the wrist

Alternate site for PTN is _____ (NOT popliteal fossa for CNIM, although we can use this in real life) when there is peripheral neuropathy or below the knee amputation

peroneal nerve

peroneal nerve has more _____ variability compared to posterior tibeal nerve, which is why we prefer to use PTN unless neuropathy is present

inter-subject variability

For the tibeal nerve, where do we place the cathode?

midway between the medial border of the Achilles tendon and the posterior border of the medial malleolus

For the tibeal nerve, where do we place the anode?

placed 3 cm distal to cathode

For the tibial nerve, where do we place the ground electrode?

stimulated limb (calf)

For the tibial nerve, where do we observe the twitch?

plantar flexion of the great toe, or cupping the sole of the foot, which activates the adductor hallucis muscle.

For the peroneal nerve, where do we place the cathode?

below the fibular head

For the peroneal nerve, where do we place the anode?

placed 3 cm distal to the cathode

For the peroneal nerve, where do we place the ground electrode?

stimulated limb

For the peroneal nerve, where do we observe the twitch occur?

plantar eversion of the foot, which activates the peroneus longus and peroneus brevis muscles (foot drop)

Are recording parameters the same for recording responses in children and infants?

yes

For clinical settings, what is our bandpass limitation?

30-3,000 Hz (at —6 dB per octave)

• ensure that testing is performed with the same bandpass used to acquire normative data

For intraoperative settings, what is our bandpass limitation?

30—1,000 Hz at (—3 dB)

• baselines reset if filters are changed

For clinical settings, what is our analysis time for median nerve and PTN?

40 ms for median, 60 ms for PTN

For IONM (intraoperative) settings, what is our analysis time for median nerve and PTN?

40-50 ms for median, 75-100 ms for ulnar

What is the number of averages for the clinical setting (SSEP recording parameters)?

several hundred to several thousand

• for low noise recording, two repetitions are usually adequate, while for higher noise recordings, more than two replications are often required

• sources of noise in EP: muscle activity and patient movement

What is the number of averages for the intraoperative setting (SSEP recording parameters)?

250-1000 repetitions

What is the number of channels minimum
(SSEP recording parameters)

minimum of 4

For EP, ____ volley passes under Erb’s point

propagated

N13 (_____ potential or nonpropagated) cervical potential is recorded referentially from the dorsal neck, mainly reflecting postsynaptic activity in the ____ cord.

stationary, cervical

P14 (subcortically generated _____ potential, recorded _____ from scalp electrodes. It has a widespread scalp distribution and probably reflects activity in the _____ or _____ junction

far-field, referentially. caudal medial lemniscus or cervicomedullary junction

N18 is a subcortically generated _____ potential, best recorded _____ from scalp electrodes that are (ipsilateral/contralateral) to the the stimulated nerve, AWAY from the contralateral N20. It probably reflects postsynaptic activity from sources such as ____ and ____.

far-field, referentially, ipsilateral. brainstem and maybe thalamus

N20 reflects activation in S1 OR _____. It is recorded using a ____ recording montage (derivation) to subtract widespread far-field signals, such as P14 and N18, from the superimposed primary cortical activity recorded locally over the centroparietal region (ipsilateral/contralateral) to the stimulated median nerve.

primary cortical somatosensory receiving area, bipolar. contralateral

What is N9? A ____ (non-stationary) response from the popliteal fossa

propagating

What is LP? A _____ or nonpropagated lumbar potential recorded REFERENTIALLY over the dorsal lower thoracic and upper lumbar spines, mainly reflecting postsynaptic activity in the ____ cord.

stationary, lumbar

What is P31? A subcortically generated ____ potential, which is recorded referentially from an Fpz electrode and is most likely analogous to P14 following median nerve stimulation. It probably reflects postsynaptic activity from the _____. Precedes the N34 response.

far-field, caudal medial lemniscus

What is the meaning of analogous (ex. when two potentials are analogous?)

share the same generator site

What is N34? A subcortically generated ____ potential, which is recorded referentially from an Fpz electrode and is most likely analogous to N18 following median nerve stimulation. This means that it probably reflects postsynaptic activity from MULTIPLE generator sources in the ____ and maybe _____. N34 is preceded by a small positivity

far-field. brainstem, and maybe thalamus

P37 reflects activation of the ______. It is recorded using a bipolar recording montage (derivation) to subtract widespread far-field signals from the superimposed and topographically more restricted primary cortical activity. There is considerable variability in the scalp topographic distribution of the P37 response. It is usually maximal somewhere between ____ and _____ scalp locations, (ipsilateral/contralateral) to the stimulated leg). To avoid erroneously reporting P37 as absent, it is necessary to record from both midline and ipsilateral scalp locations.

primary cortical somatosensory receiving area. bipolar derivations. midline and centroparietal, ipsilateral (due to paradoxical lateralization for LOWER extremity stimulation)

• Cpi, Cpz

Recording electrode placement uses the international ____ system because it respects the homunculus.

10-20

Near-field generators are close to recording electrodes, while far-field generators are far from recording electrodes (ex. brainstem) - true or false?

true

Cp3, Cpz, Cp4 are all in ____ lobe, or the ___central gyrus.

parietal, post-central

Erb’s point is in the angel formed by the ____ border of the clavicular head of the sternocleidomastoid muscle, 2-3 cm above the clavicle.

posterior