What are the recording parameters of somatosensory evoked potentials (SEPs)?

Updated: Feb 26, 2019
  • Author: Sombat Muengtaweepongsa, MD, MSc; Chief Editor: Selim R Benbadis, MD  more...
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Answer

Answer

Extraoperative SEPs typically are recorded by using standard EEG electrodes affixed with tape or collodion; electrode caps containing multiple recording electrodes also can be used. Scalp needle electrodes are not used routinely for diagnostic SEP studies but are often used for intraoperative monitoring when the patient is anesthetized.

Recording electrode impedances should be kept below 5,000 ohms and should be as uniform as possible across the electrodes to maximize common-mode rejection and minimize noise pickup. Also, placing the ground electrodes on the stimulated limb, proximal to the stimulation site, helps to reduce the electrical stimulant artifact.

Typical recording amplifier filter settings for SEPs are 5-30 Hz (low-cut or high-pass filter) to 3,000 Hz (high-cut or low pass filter). Diagnostic SEP studies should be performed using the same filter settings that were used to record the normative data.

SEPs are composed of both low and high frequencies, and filtering can be problematic. A bandpass that is too wide results in noisy SEPs, but a bandpass that is too restrictive attenuates either high- or low-frequency components, depending on the settings chosen, and distorts the SEP waveform. For example, reducing the low-frequency filter setting (low-cut, high-pass) from 30 to 5 Hz may produce a larger and clearer cortical SEP component but also may allow more low-frequency noise into the SEP waveforms.

A typical analysis time is 40 milliseconds for an upper limb SEP and 60-80 milliseconds for a lower limb SEP. During intraoperative monitoring, longer analysis times (e.g., 50 and 100 milliseconds, respectively) may be useful, as component latencies may be prolonged by hypothermia, anesthesia, and pathology. [2]

SEPs are typically not visible in the raw data recorded from surface electrodes, and signal averaging is used to extract the SEPs from the other electrical signals picked up by the recording electrodes. Online artifact rejection is used to prevent epochs with unusually high noise levels from contaminating the averages. Sedation also may be used to reduce artifacts due to muscle and patient movement. Sleep causes minor changes in the SEP waveforms.

A negativity at input 1 with respect to input 2 is usually displayed as an upward deflection in the SEP waveform, but this polarity convention is not uniform across all laboratories performing SEP recordings.


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