Troubleshooting for Multiple Modalities

Noise in SSEPs (lots of waves making it difficult to interpret the signal waveform) can mean what?

Patient may be getting light

What happens if we don’t see repeatability between traces (in other words, the trace pattern presents in one trial, but does NOT in another)?

Traces don’t constitute a signal and are thus not monitorable - which is why we need to see traces repeat across multiple trials

How can we optimize SSEP baseline data (troubleshoot - hint: think parameters), and turn non-monitorable signals into monitorable signals? Assume we have already switched all surface electrodes to needle electrodes.

• increase stimulation intensity (measured in milliamps/mA)

• increase pulse width (measured in microseconds/us)

• decrease repetition rate (which can take time - baselines need to be acquired as fast as possible, so you may have to leave it alone or do it last, after doing the other two steps first)

Detecting a 50% change in SSEPs with low amplitude (non-ideal) responses is (easy/difficult).

difficult - try to troubleshoot

Cortical waveforms are called monitorable when they come in as (peak to trough/trough to peak), while subcortical waveforms are called monitorable when they come in as (peak to trough/trough to peak)

peak to trough for cortical, trough to peak for subcortical

• if subcorticals look like they are coming in peak to trough and the name of the channel is Fpz - Mastoid (not Mastoid - Fpz, which will make it come in peak to trough), the trough might not be visible - which is why we say they are not monitorable.

When PTN signals don’t come in well, we can turn the peripheral recording site into the stimulation site instead. What is that peripheral recording site/ALTERNATE stimulation site?

popliteal fossa

Subcortical lower extremity SSEP signals are often more difficult to obtain, BUT this can be fine when…

when the lower extremity cortical signals are monitorable (which shows that the signal is clearly still traveling up from the nerves to the brain, or in this case, the cortex)

• same reasoning applies when signals don’t come in to one cortical channel but do for another - there is still proof that the signal is traveling up to the brain

What is a hematoma?

collection of clotted blood that gathers outside of a blood vessel - often indicative of trauma cases, can come in directly from the ED and thus make it difficult for the tech to obtain baseline responses

Hematomas can add ____ to the cord due to buildup of blood, affecting evoked potentials.

pressure

When signals are overly obscured with stimulation artifact, they are potentially (monitorable/not monitorable).

not monitorable

It becomes more challenging, after incision and exposure, to run evoked potentials, due to constant use of ____ (the Bovie)

electrocautery

Besides optimizing data parameters, what else can we do? Hint: for lower extremities, we can switch the stimulation site (PTN) to the peripheral recording site (pop fossa) - what can we do, and for what diagnosis might we do this?

switch from ulnar nerve to median nerve in patients with cervical myelopathy

If surgery begins posterior and ends anterior, that may be an indication that the patient’s neck is (unstable/stable)

unstable (indicative of cervical myelopathy)

for anterior and posterior cervicals, the arms of the patient are tucked to the side, meaning that we lose access to (upper/lower) extremity stimulation sites

upper extremity (ulnar nerve/potentially median nerve, as an alternate stim site)

• using the median nerve may potentially give us higher amplitude (up to 2x potentially!) responses at the SAME stim intensity, pulse width, and rep rate (parameters)

• remember, responses with higher amplitudes, and having more channels with reliable data, will help us detect any changes so much easier

Left side numbness in the patient may result in worse SSEP responses on the (left side/right side)

left side

For SSEPs, blue is baseline trace, while purple is (current/last stored) trace

last stored

BEFORE adjusting data parameters, what is important to do?

Check electrode impedance!

If you have noise in every channel (globally!), what are some steps you can take - with particular focus on the two steps you can do right at your computer?

• check impedance (an electrode may be dislodged) or adjust needle placement to move it away from localized noise sources

• check ground electrode

• if noise is more localized to a few channels, it may not be a ground electrode issue and can be fixed by optimizing parameters (specifically rep rate) and removing noise

• increase averages (gives differential amplifier more time to remove noise)

• isolate headpack wires by separating them from anesthesia wires and reduce noise

For SSEPs, noise is random while signals are ____ (in other words, same latency and amplitude/morphology)

repeatable and consistent

What can we do to troubleshoot MEPs, for technical issues?

• verify current return (that stimulation IS going through (ex. 0 / 260 v is indicative of NO current, measured in mA, going through despite stimulating at 260 v or constant voltage) - if not, needle may be dislodged

• look at whether stimulation artifact is present - if not, the entire needle may be dislodged

How can we optimize MEP parameters after ruling out technical issues?

• increase stimulation intensity (within reason - small patients shouldn’t be stimulated at super high voltages or else they will move a lot)

• adjust ISI or interstimulus interval (time between pulses) in certain extremities. For upper extremities, ISI can be decreased; for lower extremities ISI can be increased (think of the man doing a handstand) - can help even out response amplitudes bilaterally between upper and lower extremities

• reverse polarity, switching anode from the right side (normal polarity has red on the right!) to the left side

• 50-75 is the pulse width for MEPs, but since most start out at 75 msec anyway, you can’t increase it anymore

• use more pulses or increase pulse count (up to 9 for bipolar stimulation, but only 6 for LQP)

• use double train MEP stimulation to acquire more polyphasic morphology and overcome the effect of anesthetic gas (first train of 2 pulses, second train of 7 pulses, with ITI between 10-20 ms)

• adjust cortical leads (last resort step - be confident in your placement!)

MEP responses are great when they have good amplitudes and are (biphasic/polyphasic)

polyphasic

Start troubleshooting steps at (computer/patient), end them at (computer/patient),

computer, patient

Once we have ruled out technical issues and optimized MEP parameters, what can we consider?

anesthesia/vitals - ensure gas is less than 0.5 MAC, that MAP is at or over 80 mmHg, and that TOF is 4/4 twitches with limited fade (in other words, NMBA has not been administered)

• low MAP makes it super difficult to get lower extremity responses, since thoracic section of the spine is the Watershed area and affects motor/sensory function in lower limbs first

Once we have ruled out technical issues, optimized MEP parameters, AND ensured anesthetic gas MAC and vitals are ideal, what can we consider (hint: what are some of the names of the conditions that can affect MEP responses)?

patient pathology:

• muscular atrophy

• myelopathy

• neuropathy

• multiple sclerosis

• paraplegia/quadriplegia

• spinal cord lesions

• spinal cord tumors

• motor neuron diseases

For TOF, one of the first things we should check for (in case we do not have monitorable responses) is?

presence of stimulation artifact

For TOF stimulation, we use SSEP electrodes - meaning that for upper extremity stimulation, we use the (ulnar nerve/brachial plexus) and for the lower extremity stimulation, we use the (PTN/pop fossa)

ulnar nerve stimulation electrodes to stimulate hands for TOF, PTN stimulation electrodes to stimulate feet

• for PTN, recording electrode may be placed at the AH (ex. for a TOF referential montage of AH-ADM)

For TOF, If we stimulate on the left side and record from the right side, we will not see _____. In contrast, if we stimulate from the left AND record from the left as well, even if we have complete neuromuscular blockade and 0/4 twitches for TOF, we should still see ____.

stimulation artifact

For TOF, if we have increased intensity, what else can we do?

• confirm with MDA/CRNA if paralytic was introduced and what type it was (short-acting, intermediate-acting. long-acting) - if paralytic hasn’t been metabolized yet, you are likely to get 0/4 twitches

• increase stimulation intensity

• reverse polarity (for TOF, we like reverse polarity and stimulating from the distal electrode, which is farther from the brain - but we can also go from reverse to normal polarity)

• add biphasic stimulation (possible for both normal and reverse polarity) to reduce stimulation artifact

• switch to contralateral extremity - if you cannot get twitches from the right foot, switch to the left foot. Pay attention to which side the patient is showing symptoms on - do they have left/right side weakness or muscular atrophy?

• now you may have to move from computer to patient: if you can’t get a response from distal extremity: switch to a more proximal muscle! Stimulate at peroneal nerve and record from TA, or stimulate from pop fossa and record from gastroc.

• turn on SSEPs in upper/lower extremity SSEPs, go up to patient, and see if you can feel the patient twitching in the relevant extremity

How can we troubleshoot noise in EMG?

• isolate recording electrode wires from the OR bed and MDA cables (done as early on as a possible, since access to the patient can be limited when the patient is draped) - wrap cables in blue towels to isolate and insulate cables!

• check if turning off certain electronic components temporarily may be possible to allow noise source identification - OR bed is the #1 source of noise - and remove it if possible

• add Notch filter to minimize 60Hz interference (reduce noise amplitude)

EEG shows us the level of anesthetic depth. Higher frequency and higher amplitude in EEG indicate that the patient is more awake - in other words, the patient may be getting ____. Generalized noise in SSEPs, and bilateral EMG activity correlating with SSEP stimulation (showing that the patient may be feeling pain) is also indicative of the patient getting ____.

light

What do we do in case the patient is getting light (seen through global noise in SSEPs, EMG activity correlating with SSEP stimulation or even before that, and high frequency/high amplitude EEG activity)

• stop running SSEPs

• confirm with IP if necessary, but tell the MDA that patient may be getting light (which should be fixed before electrocautery or the patient may start bucking)