Functional imaging and neurophysiological assessment of spinal and brain therapeutic modulation in humans

We summarize here our experience in the neurophysiological and neuroimaging assessment of spinal and brain neuromodulation for pain relief. Techniques reviewed include somatosensory evoked potentials (SEPs), nociceptive spinal (RIII) reflexes, and positron emission tomography (PET), which have been applied both to investigate the mechanisms and to optimize the application of neurostimulation procedures. SEPs are especially useful in the preoperative assessment of patients with neuropathic pain, as they allow the establishment of the functional state of the dorsal column system. Patients with strongly abnormal SEPs due to ganglionic or preganglionic pathology are not likely to benefit from spinal (SCS) or peripheral (TENS) neurostimulation, because ascending fibers disconnected from their soma will undergo rapid degeneration and not be excitable. In the postoperative period, nociceptive spinal reflexes yield objective data concerning the effects of neurostimulation on spinal circuitry. In our experience, the best clinical results are achieved in patients with preserved preoperative SEPs, in whom neurostimulation entails profound attenuation of nociceptive reflexes.PET-scan imaging techniques have recently been used to demonstrate changes in cerebral blood flow during new neuromodulation schemes such as motor cortex stimulation for pain control (MCS). PET studies highlight the thalamus as the key structure mediating functional MCS effects. Thalamic activation would trigger a cascade of synaptic events influencing activity in other pain-related structures including the anterior cingulate gyrus, insula, and upper brainstem. The combination of clinical electrophysiology and functional neuroimaging provides insight into the mechanisms of action of neuromodulation procedures, guides clinical decision, and contributes to optimize patient selection.