The clinical and pathophysiological relevance of evoked potentials study in neuronal migration disorders

We studied pattern and flash visual (VEP) and somatosensory evoked potentials (SEP) in 23 patients with various farms of neuronal migration disorder (NMD). The in vivo diagnosis was in all cases confirmed by magnetic resonance imaging (MRI). The neurophysiological findings were generally in agreement with the distribution of alterations revealed by MRI, although in some patients EP abnormalities were greater and in others less than expected from the MRI findings. Several kinds of changes were found on EP, including global abolition or severe deterioration of components, selective loss of SEP components, VEP changes depending on spatial orientation of stimulation, asymmetric distribution of EPs, and EPs arising in non-typical areas (VEPs from centro-parietal regions, SEP N20 component with unusual anterior frontal or parieto-occipital distribution) associated with asymmetry between the two sides. The latter finding, in patients with cortical polymicrogyria and unilateral paratrigonal heterotopia, indicates extensive reorganization of the cortical representation of certain sensory modalities. In the five patients with parietal bilateral polymicrogyria there was a close correlation between clinical symptoms and signs, MRI findings and neurophysiological changes. These findings indicate that EP studies are a useful and reliable means for exploring pathophysiological changes in NMDs and for revealing CNS plasticity. EP can be useful to identify functional cortical areas that must be preserved in NMD patients undergoing surgery.