Down Syndrome: Neurophysiological Concepts

Introduction. Down syndrome arises from a trisomy of chromosome 21. Neurophysiological aspects of Down syndrome have not been well studied. Subjects often have delayed motor milestones, an increased risk of epilepsy, and an early onset form of Alzheimer’s disease. Methods. This report describes differences between Down syndrome individuals and neurologically normal control subjects using standard neurophysiological tests, such as motor and somatosensory evoked potentials and coherence between pairs of neurophysiological signals. Results. Subjects with Down Syndrome required a smaller voltage to elicit an equivalent motor evoked potential compared to control subjects (174V vs. 650V) and had larger cortical, but not spinal, somatosensory evoked potentials (52mV vs. 4.2mV). Both EEG-EEG and EMG-EMG coherence was higher in Down Syndrome than in control subjects. Conclusions. Because the sensory input to the nervous system is controlled between subjects, as evidenced by the consistent spinal amplitude, we believe that the increased amplitude results from supraspinal (thalamic or cortical) differences rather than spinal gating. We hypothesize that these findings represent a novel set of neurophysiological findings and may be due to an altered pattern of cortical excitability, possibly due to an increased presence of gap junctions in cortical cells.