Surface electrodes placed over the back can detect evoked potentials in the lumbosacral nerve roots. This can help evaluate nerve root lesions associated with a herniated lumbar disc.
Technique:
(1) Electrolyte gel is placed over the recording sites.
(2) Recording electrodes made of 5 mm thick silver-silver chloride discs are secured to the skin. One set of electrodes is placed over the interspinous process between L3 and L4. Another set is placed over the interspinous process between T12 and L1.
(3) An electrical stimulus with frequency 5 Hz and duration 0.2 msec is delivered at the motor threshold intensity to the posterior tibial nerve at the ankle using skin electrodes.
(4) The filter bandpass was 60 to 3,000 Hz, with 500-1,000 responses averaged using a signal processor.
Evoked Potentials
Grade
normal
3
1 abnormal finding in evoked potentials
2
2 or 3 abnormal findings in evoked potentials
1
unrecordable waveform
0
Abnormal findings in the evoked potentials:
(1) prolonged latency on the affected side
(2) difference in latency between the affected vs normal side (side-to-side) >= 1 msec
(3) reduced amplitude of <= 0.2 µV
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