Loss of sensory discrimination after median nerve injury and activation in the primary somatosensory cortex on functional magnetic resonance imaging

Abstract

The aim of this study was to assess the effects of median nerve injury and regeneration on neuronal activation in the somatosensory cortex by means of functional magnetic resonance (fMR) imaging and somatosensory evoked potentials (SSEPs). Ten injured male patients (mean age 26 years) were examined 15 to 58 months after a total transection of the median nerve at the wrist that was repaired with epineural sutures. Two-point discrimination was lost in Digit II-III and sensory nerve conduction displayed decreased velocity (-29%) and amplitude (-84%) in the median nerve at the wrist. The fMR images were obtained during tactile stimulation (gentle strokes) performed separately on the volar surface of either Digit II-III or Digit IV-V (eight patients: two were excluded because of movement artifacts). The SSEPs were obtained using electrical stimulation proximal to the median nerve lesion. Patients with loss of sensory discrimination after median nerve damage and regeneration had larger areas of activation in fMR imaging near the contralateral central sulcus during tactile stimulation of the injured compared with the noninjured hand. The increase relative to the unaffected hand was 43% (p < 0.02) for Digit II-III stimulation and 46% (p < 0.02) for Digit IV-V stimulation. The SSEP data showed normal latency and amplitude. The enlarged area of cortical activation may be the result of reorganization, and it may indicate that larger cortical areas are involved in the discriminatory task after a derangement of the peripheral input.