Evoked Potential Monitoring

Question 1

What is the purpose of somatosensory evoked potential monitoring (SSEP)?

Answer 1

to evaluate the integrity of the brain or spinal cord during certain types of surgeries (spine surgery, CEA, aortic surgery, repair of intracranial aneurysms); used to detect dorsal spinal cord and or brain ischemia

Question 2

Where are somatosensory evoked potentials usually elicited (STIMULATED) for SSEPs?

Answer 2

tibial, median, or ulnar nerves… usually bilaterally

SSEP = TUM

Question 3

Where are somatosensory evoked potentials usually RECORDED from?

Answer 3

the scalp

Question 4

Where is the critical recording electrode placed to record tibial nerve evoked potentials? ulnar and median nerve?

Answer 4

tibial is placed midline

laterally for ulnar and median

Question 5

For sensory evoked potentials, the stimulating electrodes are placed _______ and the detecting electrodes are placed _______.

Answer 5

peripherally; centrally

Question 6

What can a decrease in amplitude of SSEP suggest?

Answer 6

that damage is occurring in the neural pathway being monitored

Question 7

What can an increase in latency of SSEP suggest?

Answer 7

that damage is occurring in the neural pathway being monitored

Question 8

In reference to SSEP, what does the term latency mean?

Answer 8

the time it takes for the action potential to traverse the peripheral nerves, spinal cord, and inner brain structures before arriving at the cerebral cortex

Question 9

In reference to SSEP, what does the term amplitude mean?

Answer 9

refers to the magnitude or size of the evoked potential

Question 10

Why are motor evoked potentials sometimes used in addition to SSEP during thoracic or thoracolumbar aortic aneurysm surgery?

Answer 10

paraplegia incidence is 5-40%; SSEP assesses integrity of the posterior cord ONLY; Motor evoked potentials are used to monitor the ischemia in the anterior (ventral) columns

Question 11

What is the most common technique used for motor evoked potentials?

Answer 11

stimulation over the motor cortex or the cervical spine (centrally) with sensing over the popliteal nerve (peripherally) is MOST common

Question 12

What are some physiologic factors that alter amplitude and latency of SSEPs?

Answer 12

temperature (hypo and hyperthermic)
systemic BP (hypotension below levels of cerebral autoregulation affect SSEP)
PaCO2 (changes in SSEP probably reflect changes in CBF)
PaO2 (changes in SSEP probably reflect changes in oxygen delivery to neural structures)

Question 13

Why do SSEPs only monitor the integrity of the posterior cord?

Answer 13

because action potentials elicited by peripheral nerve stimulation travel in the cuneatus and gracilis tracts, which are located in the posterior cord

Question 14

What cranial nerve integrity is monitored by brainstem auditory evoked potentials (BAEP)?

Answer 14

cranial nerve VIII

Question 15

What cranial nerve integrity is monitored by visual evoked potentials (VEP)?

Answer 15

optic nerve (II)

Question 16

What is the MOST to LEAST in regards to sensitivity to anesthetic agents? (SSEP, BAEP, VEP)

Answer 16

VEP= V for very sensitive
SSEP= S for somewhat sensitive
BAEP= B for barely sensitive

Question 17

SSEP’s monitor the integrity of what two tracts?

Answer 17

1) cuneatus
2) gracilis
* both are posterior (dorsal) cord

Question 18

Touch, pressure, and sensation are carried into the spinal cord via the _______ _______.

Answer 18

dorsal horn
*these sensations ascend on the ipsilateral side of the spinal cord before crossing over in the brainstem to the contralateral thalamus and primary sensory cortex

Question 19

SSEP’s measure intactness of the gracilus and cuneatus tracts that are considered to be the ______ route to the sensory cortex.

Answer 19

direct route

Question 20

Sensory information is also transmitted to the reticular activating system (RAS) where it percolates upwards and spreads throughout the cerebral cortex. The RAS is considered the _______ route by which sensory information reach the cortex.

Answer 20

indirect

Question 21

What is the function of the RAS?

Answer 21

maintains the awake and alert state–> shuts off while asleep; GA’s produce sedation, thereby depressing the RAS

Question 22

Complete loss of RAS is called ________.

Answer 22

coma

Question 23

SSEP’s are recorded from _____.

Answer 23

the scalp

Question 24

What is the pathway of the right tibial nerve to the sensory cortex?

Answer 24

right tibial nerve to spinal cord via dorsal horn–> right (ipsilateral “same side”) spinal cord (cuneatus and gracilis tracts)–> left cerebral cortex–> EP’s recorded from electrodes between ears near midline (over longitudinal fissure)

Question 25

What system is the gracilis and cuneatus tracts a part of?

Answer 25

dorsal-lemniscal system

Question 26

What changes in amplitude and latency suggest damage to the neural pathway being monitored?

Answer 26

increased latency and decreased amplitude

Question 27

Brainstem auditory evoked potentials are useful for monitoring CN ____.

Answer 27

VIII

Question 28

Visual evoked potentials monitor the integrity of CN ____.

Answer 28

II

Question 29

List the 3 types of evoked potentials from MOST sensitive to general anesthesia to LEAST sensitive.

Answer 29

VEP (V for VERY)–> SSEP (S for Somewhat)–> BAEP (B for Barely)
*GA increases latency and decreased amplitude (dose dependent)