Sp-SSEP may be a useful technique to monitor the femoral nerve during spine surgeries where the lumbar plexus is at risk of injury. Current methodology (proximal stimulation) for intraoperative monitoring of Sp-SSEP presents several disadvantages. Here we describe an improved set-up, using distal stimulation, that is non-invasive, simple and elicits higher Sp-SSEP amplitude. 

In 25 patients undergoing lumbar spine surgeries, Sp-SSEP (50 nerves) were elicited using two different methodologies: A. Proximal stimulation by subdermal needles placed in the groove between the Vastus Medialis and Sartorius muscles in the medial distal thigh, about 10 cm superior to the medial epicondyle of the knee; B. Distal stimulation by surface electrodes placed in the groove between the tibia and the medial gastrocnemius muscle about 15 cm above the ankle. For Sp-SSEP recording, a Cz'- Fz montage (10-20 international system) was used for both methodologies. The percentage of Sp-SSEP elicitability, the amplitude of the evoked responses and legs movement were compared between the two methodologies. 

The distal stimulation elicited Sp-SSEP in 100% of the nerves. In 5 of these nerves (10%), proximal stimulation did not elicit any reproducible response. Sp-SSEP amplitudes were 0.6±0.7 and 1.1±0.7 (mean±SD) (p<0.0001) after proximal and distal stimulation, respectively. Distal stimulation did not produce legs movement on any of the trials. Proximal stimulation produced mild (51%) or severe (49 %) legs movement on 74% of the trials.

We present an improved methodology to elicit Sp-SSEP for monitoring the femoral nerve during lumbar spine surgeries. Advantages of this methodology over the current methodology include non-invasiveness, simplicity, higher evoked responses and the absence of disturbing movement in the surgical field.