Evoked Potentials
Evoked potentials are electrical signals generated in response to stimuli. They include:
- Somatosensory
Electrical stimulation of the periphery with trace of the nerve impulse to the sensory cortex. SSEPs are:- Rapid
- Portable
- Non-invasive
- Reported by:
- Direction
A peak is given by a negative potential (N), a nadir by a positive potential (P). - Expected latency
How long (ms) between stimulation and detection.
- Direction
- Motor
Stimulation of the cortex with trace of impulse to a motor neuron or muscle group. - Brainstem auditory
Indications
Somatosensory potentials are used for:
- Neuroprognostication
Indications:- After 24 hours in:
- Post cardiac arrest
- TBI
- With sedatives and analgesics
- With hypothermia
- With an abnormal EEG
No prognostic utility with normal EEG
- After 24 hours in:
Contraindications
Somatosensory contraindications:
- Barbiturate coma
- Known or suspected lack of nerve integrity:
- Peripheral nerve injury
- Traumatic spinal cord injury
Motor evoked potential contraindications:
- Muscle relaxation
Anatomy
Equipment
Technique
Highly sensitive to electrical interference due to the very low (μV) voltages detected.
For neuroprognostication:
- Electrodes are placed to read activity at the:
- Dorsal root ganglion (N9)
“Erb point”. - Cervical spinal cord (N13)
- Thalamus (P18)
- Somatosensory cortex (N20)
- Dorsal root ganglion (N9)
- Median nerve stimulated at the level of the wrist
- Bilateral absence of N20 peaks is highly specific of poor neurological outcome after 24 hours