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

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


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

tibial, median, or ulnar nerves... usually bilaterally


Where are somatosensory evoked potentials usually RECORDED from?

the scalp


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

tibial is placed midline
laterally for ulnar and median


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

peripherally; centrally


What can a decrease in amplitude of SSEP suggest?

that damage is occurring in the neural pathway being monitored


What can an increase in latency of SSEP suggest?

that damage is occurring in the neural pathway being monitored


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

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


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

refers to the magnitude or size of the evoked potential


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

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


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

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


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

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)


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

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


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

cranial nerve VIII


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

optic nerve (II)


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

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


SSEP's monitor the integrity of what two tracts?

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


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

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


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

direct route


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.



What is the function of the RAS?

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


Complete loss of RAS is called ________.



SSEP's are recorded from _____.

the scalp


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

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)


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

dorsal-lemniscal system


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

increased latency and decreased amplitude


Brainstem auditory evoked potentials are useful for monitoring CN ____.



Visual evoked potentials monitor the integrity of CN ____.



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

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