Reorganization of motor and somatosensory cortex in upper extremity amputees with phantom limb pain

Abstract

Phantom limb pain (PLP) in amputees is associated with reorganizational changes in the somatosensory system. To investigate the relationship between somatosensory and motor reorganization and phantom limb pain, we used focal transcranial magnetic stimulation (TMS) of the motor cortex and neuroelectric source imaging of the somatosensory cortex (SI) in patients with and without phantom limb pain. For transcranial magnetic stimulation, recordings were made bilaterally from the biceps brachii, zygomaticus, and depressor labii inferioris muscles. Neuroelectric source imaging of the EEG was obtained after somatosensory stimulation of the skin overlying face and hand. Patients with phantom limb pain had larger motor-evoked potentials from the biceps brachii, and the map of outputs was larger for muscles on the amputated side compared with the intact side. The optimal scalp positions for stimulation of the zygomaticus and depressor labii inferioris muscles were displaced significantly more medially (toward the missing hand representation) in patients with phantom limb pain only. Neuroelectric source imaging revealed a similar medial displacement of the dipole center for face stimulation in patients with phantom limb pain. There was a high correlation between the magnitude of the shift of the cortical representation of the mouth into the hand area in motor and somatosensory cortex and phantom limb pain. These results show enhanced plasticity in both the motor and somatosensory domains in amputees with phantom limb pain.

Fig. 1.

Relationship between the time since amputation and the hemispheric difference of the motor threshold (MT) of the arm muscle.

Fig. 2.

Superimposition of somatosensory representations of the first digit (D1) (square) and of the fifth digit (D5) (filled black triangle) of the intact hand and motor representation of the biceps brachii (large circles) on each subjects' MRI. Data from three patients with phantom limb pain (PLP, bottom) and three without PLP (non-PLP, top) are shown (patients non-PLP 1, non-PLP 2, andPLP 2 had right arm amputations; patients PLP 1, PLP 3, and non-PLP 3 had left arm amputations). The somatosensory representations of the lips on the amputated and intact sides are indicated by the white dot with the black edge. The lip representation corresponding to the intact side was projected over the representation corresponding to the amputated side (Elbert et al., 1994) and is displayed as small black dot overlaid on a larger hatched circle. Note the larger size of the biceps representations in PLP patients but not in non-PLP patients.

Fig. 3.

Superimposition of somatosensory representations of the first digit (D1) (gray filled dot) and of the fifth digit (D5) (white filled dot) of the intact hand and motor representation of the biceps brachii and depressor labii inferioris (circle outlined inblack, m bic; hatched open circle, m dep) in a patient with PLP tested in study 2 to obtain a more precise motor map using a more focal coil (Cadwell 8-shaped coil). The somatosensory representation of the lip on the amputated side is indicated by the white dot with the black border and that of the intact side is indicated by the black dot. The lip representation corresponding to the intact side was projected over the representation corresponding to the amputated side (Elbert et al., 1994) and displayed as a large filled gray circle. Note the larger size of the biceps and depressor labii inferioris representations in these patients and particularly the smaller distance between biceps and depressor labii inferioris representations in the hemisphere contralateral to the amputation.