Neurophysiology Monitoring

EEG - electroencephalography

Recorded from the scalp - random continuous signal from the activity of the outer layers of the cortex

Signals 10-200 millivolts

A summation of the excitatory and inhibitory postsynaptic potentials produced in the pyramidal layer of the cerebral cortex

EEG requires about 50% of the total O₂ consumed by the brain; the other 50% goes to cellular integrity

If O₂ delivery is compromised = slowing of EEG

EEG changes in amplitude and frequency caused by anesthetic drugs/anesthetic depth

There is NO info on the subcortical brain, spinal cord, or cranial and peripheral nerves

Somatosensory Evoked Potentials (SSEPs)

Brains response to repetitive stimulus along a specific sensory nerve pathway /cranial nerve with sensory pathway

An intact neural pathway from the periphery to the cerebral sensory cortex is essential for signal generation

Signals 1, 5, or 20 microvolts

Require precise electrode positioning and signal averaging to extract specific response - application of a repeated stimulus, which is then correlated with evoked response over the corresponding area of the brain

Stimulation: median nerve, ulnar nerve, and posterior tibial nerve for predictable and reliable signals

Stimulating electrodes placed peripherally and detecting electrodes centrally

• Midline placement for tibial nerve (remember Homunculus on brain surface)
• Lateral placement for ulnar and median nerve potentials

SSEP can be used to detect localized injury

Used for dorsal column surgeries on spinal cords (sensory, dual blood supply), carotid artery, or craniotomy due to ischemia or retraction

Monitor amplitude and latency - a 50% decrease in amplitude and/or a 10% increase in latency is considered clinically significant

Opioids OK, sub-MAC, TIVA

Neuromuscular blocking drugs will improve waveform morphology by increasing the signal-to-noise ratio

Brainstem Auditory Evoked Potential (BAER)

Standardized click applied to VIII cranial nerve (auditory apparatus)

Acoustic neuroma surgery

Visual Evoked Potential (VEP)

Light stimulation of the retina via goggles

Appealing to monitor optic nerve in high-risk positions (prone), but research ongoing

Difficult to interpret

Motor Evoked Potential (MEP)

MEP evaluates descending motor pathways (ventral)

Stimulus is applied like a rapid train of four in a transcranial fashion over the motor cortex - electrodes in the muscle detect the signal

MEP has a greater magnitude

Sensitive to anesthetic agents, particularly inhalation anesthetics

Stimulus can cause patient movement, so do it when ok with the surgeon

BITE BLOCK mandatory to prevent patient biting tongue

An amplitude decrease of >50% significant

Prefer TIVA or propofol with <0.5 MAC and supplementary narcotics at a steady drip rate

NO muscle relaxants except for short duration for induction or positioning

Ketamine and etomidate, nitrous oxide, and supplement narcotics are less suppressive than propofol >120 mcg/kg/min, N₂O, and narcotics

Spontaneous Electromyography (EMG)

No intentional stimulation

Continuous recording of EMG activity in the muscle of regions innervated by nerve roots where the surgeon is working

The purpose is to detect injury to nerve roots by the procedure

• Impingement with instrument = motor activity, and the surgeon can readjust immediately

NO muscle relaxant

• Cannot detect EMG activity accurately if all four twitches are absent
• At screw testing, all four twitches must be present

Seen with spinal cord surgery - surgeon can stimulate, and tech can measure input