Evidence for the specificity of intracortical inhibitory dysfunction in asymptomatic concussed athletes

Sports concussions affect thousands of individuals every year and are a major public health concern. Still, little is known about the long-term and cumulative effects of concussions on brain neurophysiology. The principal objective of this study was to investigate the long-lasting effects of multiple sports concussions on sensorimotor integration and somatosensory processing in a sample of 12 concussed athletes and 14 non-concussed athletes of similar age (mean, 23 years) and education (mean, 16 years). Right median nerve stimulation was paired with transcranial magnetic stimulation (TMS) of the left primary motor cortex to investigate sensorimotor integration with short latency afferent inhibition (SAI) and long latency afferent inhibition (LAI) at five interstimulus intervals (18, 20, 22, 100, 200 msec). Somatosensory evoked potentials (SEP) were recorded from the left centro-parietal region. We also investigated primary motor cortex inhibitory mechanisms with three TMS protocols: cortical silent period, long interval intracortical inhibition, and short interval intracortical inhibition. Motor evoked potentials were recorded from the right abductor pollicis brevis muscle. No differences were observed between groups for SAI, LAI, and SEP. However, cortical silent period duration was prolonged and long interval intracortical inhibition was enhanced in the concussed group. These findings suggest that multiple sports concussions lead to specific, long-term neurophysiological dysfunctions of intracortical inhibitory mechanisms in primary motor cortex while somatosensory processing and sensorimotor integration are spared. This study provides additional evidence for the presence of specific and stable alterations of GABA(B) receptor activity in primary motor cortex that may be of clinical value for prognosis and diagnosis.