Electrode Placement

To record somatosensory-evoked potential data during monitoring, subdermal needle electrodes are inserted into the scalp. To acquire motor-evoked potentials, stimulating subdermal or corkscrew needle electrodes must be inserted into the scalp. The International EEG 10/20 Electrode Placement System is referenced below for needle locations. For monitoring purposes, a total of nine electrodes may be used. Six electrodes are used for recording, two electrodes are used for stimulating, and one electrode is used as a ground and may be placed in the scalp (cephalic) or on the shoulder (noncephalic). A grounding pad may be used in lieu of a needle.

The reference points and placement of the electrodes in the average adult are as follows and as depicted in the diagram below:

Reference Point	Electrode Type	Location
Fpz	Recording	10% of the distance from the bridge of the nose (nasion) to the bony knob (occipital protuberance or inion) in the back of the head
Fz	Cephalic ground	Generally between Fpz and Cz
Cz	Recording	50% of the distance between the nasion and the inion
C1	Stimulating	10% of the distance left lateral from Cz or 2 centimeters (cm) left of Cz
C2	Stimulating	10% of the distance right lateral from Cz or 2 cm right of Cz
C3 prime	Recording	20% of the distance left lateral from Cz and 2 cm posterior to Cz or 2 cm posterior and 4 cm left lateral to Cz
C4 prime	Recording	20% of the distance right lateral from Cz and 2 cm posterior to Cz or 2 cm posterior and 4 cm right lateral to Cz
A1	Recording	Left ear lobe or mastoid process (bone behind ear)
A2	Recording	Recording Right ear lobe or mastoid process (bone behind ear)
Additional subdermal needle recording electrodes are inserted in the interspinous spaces between C5-C6 (median nerve) and C7-T1 (ulnar nerve). These locations (not diagramed) represent the root entry zone (median and ulnar nerve origin) to acquire subcortical data. Subcortical data are valuable in evaluating peripheral and central conduction.

© 2011 ORIMtec