The Affect of limb positioning on Somato-Sensory Evoked potentials
Below is an example I experienced during an Anterior cervical fusion case. The PT was positioned supine and the shoulders were pulled downward to allow better lateral x-ray imaging of the lower cervical cord. After incision during the case the following SSEP were observed an noted.
The above image shows Upper somato-sensory averages. The higher amplitude waveforms are the baseline taken right after incision. The changes are seen after about 10 min. into the surgery. Notice there are peripheral nerve changes seen on R-erb and C5 as well as well as cortical response changes. This leans towards an indication that the problem may be due to an issue starting from the ulnar / median stimulator to the brachial plexus occurring before the first recording point with is the R-ERB
Below is an average taken several minutes after traction is removed form the shoulders. Notice the changes in amplitude and latency after the traction was removed that was pulling the shoulders downward.
This supports the importance of monitoring more channels which include peripheral al as well as cortical. Without the placement of and Erb point and a Cervical electrode, it would be difficult to support if the changes in waveforms concurrent with surgical manipulation or not