Neurologic Monitors

Question 1

What is electroencephalography?

Answer 1

recording of spontaneous electrical activity of the brain

Question 2

How is the data of an EEG channel presented?

Answer 2

Of the up to 32 channels, each is presented as strips of voltage vs time

Question 3

How does one interpret an EEG?

Answer 3

Simplistically changes can be described as either activation or depression.

Question 4

How does ischemia present on an EEG?

Answer 4

As slowing, then complete loss of amplitude. Anesthesia and other physiologic condictions can mimic or interfere with detection of ischemia.

Question 5

How do anesthetic agents in general affect an EEG?

Answer 5

usually produce biphasic response of activation at sub-anesthetic doses followed by dose-dependent depression

Question 6

What is the only volatile agent that produces burst suppression on EEG at clinical doses?

Answer 6

Isoflurane (1 - 2 MAC)

Question 7

What anesthetic agents are capable of producing burst suppression and electrical silence on EEG?

Answer 7

The IV agents barbiturates, etomidate, and propofol (which demonstrate a typical biphasic pattern as well at lower doses).

Question 8

How do opiates affect the EEG?

Answer 8

They cause monophasic, dose-dependent depression. At doses typically used, little change is produced.

Question 9

Do true seizures occur with high opioid doses?

Answer 9

Uncertain.

Question 10

How do benzodiazepines affect the EEG?

Answer 10

initial increased amplitude and decreased theta frequency but no electrical silence.

Question 11

How does ketamine affect the EEG?

Answer 11

activation with high amplitude theta activity at low doses followed by high amplitude sigma and low amplitude beta activities at high doses.

Question 12

What physiologic variables can affect the EEG and how?

Answer 12

Hypoxia, hypothermia, extreme hypo or hypercapnea, hypocalcemia, hepatic encephalopathy, and renal failure may all cause slowing of the EEG.

Question 13

Why would you use EEG during a neurosurgical case?

Answer 13

Can be used to detect cerebral ischemia during carotid endarterectomy and CPB; can be used for the attainment of electrical silence or burst-suppression.

Question 14

What are the limitations of an EEG?

Answer 14

only certain cortical tissues monitored; subcortical injury undetected, and regional cortical injury may go undetected, artifacts easily introduced, lack of standards or proof of efficacy.

Question 15

What are evoked potentials?

Answer 15

Recordings of the neuromuscular responses to neural stimulation.

Question 16

What are the 4 types of evoked potentials commonly used?

Answer 16

Somatosensory evoked potential
Brainstem auditory evoked potential
Visual evoked potential
Motor evoked potential

Question 17

How does ischemia present on an evoked potential?

Answer 17

Ischemia appears as an increase in post-stimulation latency (delay between EP stimulation and detection) or a decrease in amplitude (peak to trough voltage of 40%)

Question 18

What are the effects of inhalted anesthetic on SSEPs?

Answer 18

Decpress amplitude and increase latency although early components seem more resistant to their effects. 0.5 - 1.0 MAC with 60% N20 causes minimal SSEP distortion (some decrease in amplitude but no effect on latency).

Question 19

How do IV agents affect SSEPs?

Answer 19

All can increase latency and decrease amplitude except propofol which has no effect on amplitude, etomidate and ketamine increase amplitude. Fentanyl has minimal effects.

Question 20

What controversies exist with the use of SSEPs?

Answer 20

Reports of false negatives and false positives exist particularly the false negative results occurring during spinal cord surgery.

Question 21

What sites are monitored with SSEP?

Answer 21

sensory axis from peripheral nerve usually median or posterior tibial to brain.

Question 22

What sites are monitored by BAEP?

Answer 22

auditory pathway from the 8th cranial nerve to brainstem

Question 23

What sites are monitored by VAEP?

Answer 23

visual pathway including retina, optic chiasm, optic radiations, and occipital cortex

Question 24

What sites are monitored by MEPs?

Answer 24

motor cortex and descending anterior tracts as detected by hand, foot, or facial movement with myogenic MEPs or transmission of nerve impulses with neurogenic MEPs. These are used during spinal cord surgery.

Question 25

What are the anesthetic concerns with the use of MEPs?

Answer 25

Most anesthetic decrease amplitude and increase latency; the level of NMB must be kept constant (with at least 30% return of twitch height desired). Transcranial stimulation can cause hypertension or tachycardia.

Question 26

Which type of evoked potential is the most sensitive to anesthetics?

Answer 26

Visual evoked potentials. False positives are common.

Question 27

What type of evoked potential is the least sensitive to anesthetics?

Answer 27

Brain auditory evoked potential. BAEP is sensitive to direct injury of the auditory apparatus but not to hypoxia or intracranial hypertension.

Question 28

How does age affect evoked potentials?

Answer 28

The older you are the greater the increase in baseline latency and the greater the decrease in baseline amplitude.

Question 29

What is the pathway monitored by SSEPs?

Answer 29

Sensory conduction pathway. First order neuron peripheral nerve and cell body, posterior spinal cord (ipsilateral dorsal column, ipsilateral dorsal spinocerebellar tract, and contralateral ventrolateral tract). Synapses with the 2nd order neuron at the cervicomedullary region crossing contralaterally then reaching the thalamus via medial lemniscus where it meets up with 3rd order neuron through the internal capsule to the somatosensory cortex (parietal cortex for median nerve and vertex for posterior tibial nerve).

Question 30

Does using SSEPs make sense for spinal surgery in a patient with duchenne's muscular dystrophy?

Answer 30

Yes. Even though these patients have minimal response to peripheral nerve stimulation you could still proceed using those or do direct cord stimulation. Etomidate or ketamine could also be used to increase the amplitude.

Question 31

What effect does N20 have on SSEPs?

Answer 31

Decreases amplitude but does not affect latency.

Question 32

What types of neurologic monitors are available?

Answer 32

Physical exam including mental status, pupils, movement, and autonomic function. EEG Evoked potentials

Question 33

For what procedures would consider using an EEG?

Answer 33

Most commonly used during carotid endarterectomies and for achieving EEG suppression with barbiturates or hypothermia.

Question 34

Do you think EEG monitoring should be used for all carotid endarterectomies?

Answer 34

No. Only used if qualified personnel are available to interpret, if no prophylactic shunting is planned, collateral cerebral circulation were inadequate, no other means available to assess cerebral ischemia and there is a viable plan in case ischemia is detected.

Question 35

What anesthetic technique would you use with EEG monitoring?

Answer 35

Low dose potent inhaled agent, narcotics given in typical doses cause minimal change.

Question 36

What anesthetic technique would you use with SSEP, BAEP, and VEP?

Answer 36

If possible, narcotic-based technique with low dose isoflurane.

Question 37

What if depression of the EEG, decrease in the EP amplitude and increase in the EP latency occurred intraoperatively?

Answer 37

I would review the ABCs, temperature, reconfirm the level of anesthetics, and I would inform the surgeon to determine if there was a surgical cause for the change such bleeding or surgical traction.

Question 38

Does use of the SSEP mean that the wake-up test is unnecessary?

Answer 38

No. False-negatives occur albeit rarely.

Question 39

The neurophysiologist informs you that she is losing her SSEP amplitudes and that you should turn off the isoflurane. What would you do?

Answer 39

Prior to shutting off the gas, I would search for other causes for loss of amplitude. Rather than merely making the EP wave look better we should investigate the causes for its abnormal appearance.

Question 40

How would intraop EEG or EP changes affect your postop management?

Answer 40

Do quick neurolig exam (mental status, motor/sensory function, reflexes, cranial nerves), check ABCs, ABG, temp, anesthetic level appropriate, get a neuro or surgery consult.