Cortical Silent Period in Patients with Complex Regional Pain Syndrome after Conditioning Peripheral Repetitive Magnet Stimulation

In a pilot study, 13 patients suffering from complex regional pain syndrome type I (CRPS I) were stimulated with a peripherally applied repetitive magnetic stimulation (rMS). The goal of this rMS was therapeutic. We used high frequencies and suprathreshold intensities. With the idea of inducing a proprioceptive input to motor cortical areas, we measured the motor evoked potential (MEP), as well as the cortical silent period (CSP) before and after rMS. These data were compared to those from healthy subjects serving as controls. A Magstim 200 was used to induce MEP and CSP over the corresponding motor cortex, while the conditioning rMS was done with a Magstim Rapid applied to the level of affected muscles corresponding to the cervical nerve roots. First the motor threshold (MT) was determined at the „motor hot spot“ of each motor cortex, then MEP and CSP (with slight muscle precontraction) were recorded at a defined stimulation intensity. The mean MT was 44% in patients and 40% in healthy subjects. Patients showed in general a significantly smaller MEP amplitude than healthy subjects, independent of the hemisphere and if before or after rMS [F(4,11)=20.14; p<0.001]. After application of the conditioning rMS we found a significantly [* F(4,11)=5.1; p=0.01] prolonged CSP for the stimulated side only in healthy subjects (52.95 ms before vs. 73.68* ms after; 26.25 ms before vs. 39.3 ms after rMS), but not in patients (51.4 ms/57.75 ms before vs. 56.09/47.86 ms after). A surprising finding was the bilaterally reduced MEP amplitude in patients, but this fits well with former studies showing a bilaterally disturbed motor cortical excitability in CRPS patients. The prolongation of CSP only in healthy subjects led us to the assumption that there is a change in afferent input between patients and healthy subjects. This might also be caused by a disturbance in motor cortical excitability as was found with pain syndromes and CRPS I formerly. It possibly indicates a reduced ability of cortical plastic changes within the motor cortex in CRPS patients.