A methodological reappraisal of non invasive high voltage electrical stimulation of lumbosacral nerve roots.

OBJECTIVE

To describe a neurophysiological method to locate the optimal stimulation site (OSS) over the vertebral column, customized to the individual subject, to achieve maximal activation of lumbosacral roots by means of non-invasive high voltage electrical stimulation (HVES).

METHODS

OSS was located in 30 volunteers by testing different stimulation points of a surface multi-electrode array placed over the dorso-lumbar junction of the vertebral column. The dorso-ventral stimulating montage was used (Troni et al., 1996). Motor responses to root stimulation (rCMAPs) were bilaterally recorded from Vastus Medialis (VM), Tibialis Anterior (TA), Soleus (SL) and Flexor Hallucis Brevis (FHB) muscles. The direct nature of rCMAPs was tested by delivering two maximal stimuli 50 ms apart.

RESULTS

Except for a few subjects with large girth, maximal rCMAPs could be obtained from all muscles with a stimulating current intensity up to 550 V (1050 mA). Maximal double HVES excluded any reflex component in the recorded rCMAPs. The procedure was well tolerated and no side effects were observed.

CONCLUSIONS

A single maximal electric shock delivered at the proper vertebral level by means of the dorso-ventral montage is able to safely achieve synchronous, bilateral maximal activation of several roots, from L3 to S1.

SIGNIFICANCE: Maximal activation of lumbosacral roots at their origin, unattainable with magnetic stimulation, is the essential requirement for direct detection of proximal nerve conduction slowing and block in lower limbs.