Neuroplastic changes related to pain occur at multiple levels of the human somatosensory system: A somatosensory-evoked potentials study in patients with cervical radicular pain

Abstract

Studies suggest that pain may play a major role in determining cortical rearrangements in the adult human somatosensory system. Most studies, however, have been performed under conditions whereby pain coexists with massive deafferentation (e.g., amputations). Moreover, no information is available on whether spinal and brainstem changes contribute to pain-related reorganizational processes in humans. Here we assess the relationships between pain and plasticity by recording somatosensory-evoked potentials (SEPs) in patients who complained of pain to the right thumb after a right cervical monoradiculopathy caused by compression of the sixth cervical root, but did not present with clinical or neurophysiological signs of deafferentation. Subcortical and cortical potentials evoked by stimulation of digital nerves of the right thumb and middle finger were compared with those obtained after stimulation of the left thumb and middle finger and with those obtained in a control group tested in comparable conditions. Amplitudes of spinal N13, brainstem P14, parietal N20 and P27, and frontal N30 potentials after stimulation of the painful right thumb were greater than those of the nonpainful left thumb and showed a positive correlation with magnitude of pain. This right-left asymmetry was absent after stimulation of the patients' middle fingers and in control subjects. Results suggest that chronic cervical radicular pain is associated with changes in neural activity at multiple levels of the somatosensory system. The absence of correlation between the amplitude of spinal, brainstem, and cortical components of SEPs suggests that enhancement of cortical activity is not a simple amplification of subcortical enhancement.

Figures

Fig. 1.

Somatosensory-evoked potentials to right and left digital nerve stimulation of the thumb and middle fingers in patient 2. The spinal N13, brainstem P14, corticalN20, P27, and N30potentials evoked to stimulation of the right thumb (painful) are greater in amplitude than those to stimulation of the left thumb (nonpainful). No such asymmetry was detected after stimulation of the middle, nonpainful fingers. It is relevant that the P9potential is similar between the two sides.

Fig. 2.

Somatosensory-evoked potentials to right and left digital nerve stimulation of the thumb and middle fingers in patient 9. The pattern of results is analogous to that reported in the legend of Figure 1.

Fig. 3.

SEPs to right and left digital nerve stimulation of the thumb and middle fingers in a control subject. TheN13 potential was recorded with a Cv6-AC montage. It is preceded by a P9 far-field potential reflecting the activity of the brachial plexus. Over the scalp, the N20potential recorded over the parietal electrodes (P3 and P4) contralateral to the stimulation side was preceded by a P14 potential and followed by a large P27 potential. TheN20 potential exhibited a reversed-phaseP20 potential over the frontal electrodes (F3 and F4), followed by a large negativity (N30 potential). It is worth noting that the spinal N13, brainstemP14, cortical N20, P27, and N30 potentials evoked to stimulation of the right thumb and middle finger are similar in amplitude with respect to those to stimulation of the left thumb and middle finger.

Fig. 4.

Right-to-left ratio (R/L*100) of amplitudes ofN13, P14, N20,P27, and N30 potentials obtained by stimulation of the thumb (Th) and middle finger (Mf) in patients and controls. Error bars indicate SDs. Significant comparisons are marked byasterisks.

Fig. 5.

Scatter plots showing the Spearman rank correlation between measures of pain intensity with right-to-left ratio (R/L*100) of amplitudes of N13, P14,N20, P27, and N30potentials obtained by stimulation of the thumb.