Evoked potentials in neuronal migration disorders (NMDs)

Background Neuronal migration disorders (NMDs) are heterogeneous developmental disorders of the central nervous system ranging from the severe form lyssencephaly to more benign forms such as focal heterotopia. Anatomo-functional changes, as a result of NMD, in intracortical, and thalamo-cortical connectivity are expected to result in dysfunction of conduction and processing of sensory afferent information. Methods. In order to characterise physiological changes in NMDs, pattern (checkerboard, vertical and horizontal bars) and white flash visual evoked potentials, and somatosensory evoked potential (median nerve stimulation at the wrist) were studied in 25 patients with different forms of NMD. Results. The patient with bilateral symmetrical nodular periventricular heterotopia had normal evoked responses. In the remaining patients we found various kinds of abnormalities categorized as: abolition or severe deterioration of components; selective changes related to modality of stimulation; asymmetric distribution of evoked potentials; and evoked responses arising in nontypical areas associated with significant dysymmetry between the two sides. Conclusions. These results illustrate that functional changes in NMDs can be the expression of reorganisation of thalamo-cortical projection systems or of functional changes within the cortex. Although there was good general correlation between MRI and neurophysiological findings, the functional consequences of NMDs did not exactly parallel the severity of the alterations revealed by MRI.