SomatoSensory Evoked Potentials

							SomatoSensory Evoked Potentials

Dr. Rajni C. Patel MD.
COL.. USAR. MC.
Staff Anesthesiologist Brooke Army Medical Center San Antonio TX.

Neurologic Monitors
MONITORS OF FUNCTION
Neurologic Examination
Discontinuous: Wake-up test Continuous: Local or Regional technique

Electroencephalogram (EEG)
Scalp recordings (unprocessed, processed) Direct cortical recordings

Neurologic Monitors
MONITORS OF FUNCTION
Evoked Potentials (EP s) Somatosensory
Cortical: Scalp recording; cortical recordings Subcortical: Surface recordings; Direct recordings Peripheral: Surface recordings; Direct nerve recordings Cont..

Neurologic Monitors
MONITORS OF FUNCTION Cont.. Auditory Brainstem (BAEP) Middle latency cortical (MLAEP) Visual (VEP) Direct nerve recording Motor (MEP) Electrical or Magnetic Cortical Stimulation Electrical or Magnetic spinal cord stimulation Electromyogram (myogenic potential) peripheral muscle recordings Facial nerve monitoring : active and passive Other cranial nerve monitoring

Monitors of Blood Flow
Transcranial Doppler blood flow velocity Jugular venous pO2 Near infrared spectroscopy (“ Cerebral Oximetry”)

Basic EEG patterns

Beta: accompanies mental concentration or low doses of hypnotic/barbiturates,benzo. Alpha: Recorded mainly over occipital region alert,relaxed patient whose eyes are closed Theta: Moderate Slowing: General anesthesia, some pathologic states as Delta rhythm. Delta : marked slowing: Deep Sleep, Aneth. Pathologic state: Schema,drug overdose or severe metabolic derangement

EEG patterns in Cerebral Ischemia
Cerebral Activity reduction reflects dec. O2.del. 50% of total O2. Consumed by brain
remaining 50% need for cellular integrety

O2 delivery compromised
eg. Hypoxemia, dec.. blood supply,
severe anemia, dec. in O2 Sat.

O2 diverted to maintain cellular integrity

EVOKED POTENTIALS
GENERAL THEORY: EEG records spontaneous electrical activity. Eps consists of CNS (Electrical, Auditory, Visual)

Motor electrical activity that is evoked by Sensory (Electrical, Magnetic) stimuli. SENSORY EPS
Peripheral or Cranial nerve Subcortical Cortical

MEPs (Electrical, Magnetic) applied directly and no averaging needed for they are much larger.

Signals

Initial trace action potential
measured at Erb‟s point

Trace entering Spinal cord
initial -dorsal root entry 2nd. Two mini-peaks nucleus cuneatus in the dorsal horn and the medial lemniscus in the brain stem after crossing opp. side

SIGNAL AVERAGING
The very small evoked potential (arrow)
is observed by background EEG signal.

The signal to noise ratio is improved by
the square root of the no. of epochs (events) averaged.

Signal averaging improves the ratio
by averaging out random background EEG, leaving only the evoked response, which is time locked to the stimulus.

EVOKED POTENTIAL
Described in terms of LATENCY:
Time measured from the application
of the stimulus to the point of max. amplitude. Times more than 1 peak.

Measured b/w EP peaks (interpeak
Latency) transit time b/w signal along the pathway and is clinically imp.

AMPLITUDE:
Voltage difference b/w two peaks

SSEP RECORDING:
Peripheral nerve, Spinal cord, Subcortical, and Cortical. May be recorded anywhere along the pathway Sensory information including Vibration, Proprioception and Light touch.

Cortical Responses must be monitored
and both sides should be done to rule out technical failure

Electrode Placement -Upper Extremity

Connection -Amplifier

Lower Extremity Electrode locations

Amplifier connection

Electrode Switch Box connection

Nicolet Viking 4 IOM monitor

International 10-20 System of EEG Electrode placement
International 10-20 Electrode Each by letter and No.
eg. C3, Cz, C4- Motor cortex Rt even No. Lt side odd No. Mid line „z‟.

Visual-evoked potential
Flash Stimulation Closed eye-lids normal P100 have 109ms.
After stimulation

Auditory-evoked potential recording
From Vertex (CZ) to ear lobe ipsilateral to auditory stimulation (Ai) I-- Ext.. portion of Auditory nerve II--Intracranial portion-cochlear III-- Superior olive IV-- lateral leminiscus V--medial geniculate VI-- thalamocortical radiations

SSEP Waveforms
Stimulating at Ankle and
picking response on Scalp

Stimulating at Ulnar nerve
at wrist

SSEP Recording

SSEP:Effects of Anesthetics
Drug
Thiopental Etomidate Fentanyl Diazepam Midazolam Ketamine Propofol Nitrous Oxide Volatile Anes.

Amplitude
Small/None Increased Small/None Decreased Decreased Increased None Decreased Decreased

Latency
Increased Increased Modest/No Increase Increased Increased Increased Increased No Change Increased

SSEP:Effects of Anesthetics

Effects of Anesthetics on SSEP
Other factors effecting SSEPs: Patient primary disease and physiologic state - Trauma Temperature- Hypothermia Nerve Ischemia - Neurologic and Neuromuscular disorder Hypoxia Hypotension Anemia Hypercarbia Background electrical interference (60 Hz artifact, cautery) Medication

Spine and Spinal Cord Surgery Most widely applied
kyphoscoliosis -- Harrington Rod placement Resection of Spinal cord tumors Vascular malformations Discectomies Stabilization procedure for trauma or
Degenerative Disease

Base Line

Aortic Vascular Surgery
Monitor Spinal cord function during Thoracic aorta (Aneurysm and Coarctation) Single Anterior Spinal Artery arises Mid thoracic only a few radicular branches until T8 to L1 T4 to T8 highly susceptible to ischemia No guarantee 14 min.loss of SSEP not a good outcome

Aortic Vascular Surgery
Human Study - Uncontrolled , nonrandomized 33 pts. Variable with regard to use of DAP and op. technique Results Overall Neuro deficit rate of 15% 48% SSEP change rate (16/33) no Neuro deficits in group with unchanged SSEPs 31% deficit rate in group with SSEP changes no Neuro deficit when SSEP change limited to 30min. (mean onset 17 min. after AoX)

Aortic Vascular Surgery
Conclusions SSEP monitoring can be used to judge total time of spinal cord ischemia, to assess adequacy of DAP, and to guide reimplantation of critical intercostal arteries Problems again assumes that SSEP change represents spinal cord ischemia Vs. peripheral nerve ischemia 69% false positive rate
Cunningham IV

Intracranial Aneurysm Surgery
Supraspinal portion of somatosensory pathway may be at risk Gray matter-loss rapidly-less than 15mil.100.min White matter -lower metabolic rate-slowly

Intracranial Aneurysm Surgery
Criteria: Dec. in Amplitude by 50% Inc. in Latency by 1 s. Without any obvious surgical and medical deterioration especially no change on the other side

SSEP and Neurological injury
Retractor -related
Cortical changes No changes when clipped found on closing the dura clip moved and kinked
middle cerebral art.

Carotid Endarterectomy
Not firmly established May be EEG should be a better monitor for cortical function BUT Not for Subcortical areas at risk during surgery

Coma Evaluation
Combination with BAEPs, SSEPs very helpful BAEPs and Cortical SSEPs intact, and remain intact-ultimate outcome good Cortical SSEPs absent and BAEPs present, best outcome as vegetative state both absent, brain death is very likely

Neurologic Injury During Surgery
No carefully controlled studies prove that SSEP monitoring improves outcome

Feedback is rapid and thus rapid intervention, either surgically or anesthetic

What to do if SSEP change
Increase in the latency 60% and Dec. in Amplitude 10% Amplitude more reliable and valid indicator
Remain Calm- do not panic Check ventilation, oxygenation Check body temp. Ask surgeon to stop surgical procedure Raise blood pressure to above-normal to inc.. perfusion pressure prepare to perform wake up test If this is occurring during instrumentation,passage of subliminal wires or destruction reverse the procedure and wait and then let the surgeon decide whether he should continue or not

Advantages and Limitation
Advantages:
Provides continuous monitoring of the sensory Neurologic pathways during surgery decreases the risk of accidental extubation ,awareness or embolism

Disadvantages:
No perfect correlation with the motor pathways Expensive equipment need for trained personnel invasive (needles) monitoring cortical recording attenuated by certain anesthetics