Evoked potentials in surgical neuromonitoring and effects of anesthesia

This is a slide on evoked potentials used as neuromonitoring in surgery as well as the effects of anesthesia on the evoked potentials.
ADDITIONAL TAGS:
Evoked potentials
Stimulus
Pathways monitored
Relevant surgeries
Effect of inhaled anesthetics
Brainstem auditory evoked potentials (BAEP)
Acoustic → BAEP electrode placed on scalp
Cochlea → cochlear nerve, through the cochlear nucleus (CN VIII) → superior olivary complex → lateral lemniscus → inferior colliculus in the midbrain → medial geniculate body → auditory cortex
Posterior fossa surgeries
Minimal
Motor evoked potentials (MEP)
Magnetic stimulation or transcranial electrical stimulation of the motor cortex → MEP recorded from muscles
Motor cortex
Brainstem
Descending motor pathways
Peripheral nerve
Spine
↓ amplitude and ↑ latency
Total intravenous anesthesia is preferred but can use up to 0.5 MAC
Somato- sensory evoked potentials (SSEP)
An electrical stimulus is applied to the peripheral nerve → SSEP recorded from brain or spinal cord
Upper extremity: median or ulnar nerve
Lower extremity: posterior tibial nerve
Peripheral nerve
Dorsal root ganglion
Posterior column of the spinal cord
Cardio- vascular;
Endo- vascular;
Intra- cranial;
Spine
↓ amplitude and ↑ latency
Can use MAC of 0.5-1.0 during measurements
Visual evoked potentials (VEP)
Light flash or pattern stimulus → VEP electrode on visual cortex
Retina, optic nerve (CN II), optic chiasm, optic radiations, and occipital cortex
Ophthal-mic
↓ amplitude and ↑ latency
Agent
SSEP amplitude
MEP amplitude
Comments
Isoflurane; Sevoflurane; Desflurane
↓
↓
SSEP usually recorded at less than 1 MAC, MEP less than 0.5 MAC
N2O
↓
↓
Similar to isoflurane, synergistic when combined with halogenated agents
Propofol
↓
↓
SSEP and MEP usually recorded at anesthetic doses but MEP may be lost at high doses
Barbiturates
↓
↓
Similar to propofol; limited experience with MEP
Opioids
Minimal
Minimal
SSEP and MEP usually recorded even at high doses
Etomidate
↑ at low doses - ↓ at higher doses
↑ at low doses - ↓ at higher doses
Enhancement of SSEP and MEP seen at low doses, depression at very high doses
Ketamine
Minimal, ↑ at low doses
Minimal, ↑ at low doses
Enhancement SSEP and MEP seen at low doses
Benzo- diazepines
Minimal at low doses
Minimal at low doses, prolonged ↓ at higher doses
SSEP and MEP usually recorded with small doses for amnesia
Dexmede- tomidine
Minimal
Minimal - ↓ at higher doses
SSEP and MEP usually recorded at low doses but MEP lost at higher doses
Lidocaine
Minimal
Minimal
Can be used as intravenous supplement in SSEP and MEP
Anesthesia has greatest effects on MEP and SSEP
Inhalational agents have greater effects on neuromonitoring than IV drugs