Unusual responses to electrocutaneous stimulation in refractory cervicobrachial pain: clues to a neuropathic pathogenesis.

Refractory cervicobrachial pain (RCBP) is a common syndrome of uncertain pathogenesis, frequently seen in an occupational context. It is characterised by widespread neck, shoulder girdle and arm pain, often of dysaesthetic quality including burning, associated with paraesthesiae, impaired perception of touch, allodynia, hyperalgesia and hyperpathia. Despite these clinical features, the syndrome has not attracted investigation with other than standard neurophysiological tests. Electrocutaneous electrical stimulation (ECS), following a well-described and validated method, was chosen as a tool to investigate the nociceptive status in RCBP. A commercially available calibrate transcutaneous electrical nerve stimulation (TENS) machine was used to determine perception threshold and pain tolerance with respect to the amplitude of current and duration of pulse. Fifteen patients with typical RCBP and ten normal volunteers were studied. The response profiles obtained were reproducible over time in both patients and controls and were able clearly to distinguish between affected and non-affected limbs. The perception threshold and pain tolerance in the unaffected limbs of patients did not differ from those in normal subjects. In the affected limbs there was reduction in pain tolerance, invariably accompanied by spread of sensation and persistence of dysaesthesiae, both induced by ECS. These results define the limbs affected by RCBP as regions of secondary hyperalgesia at the clinical level. It is suggested that neural dysfunction may be involved in the pathogenesis of RCBP, although a confident distinction between peripheral and central processes cannot be made on the basis of these findings, which call for further investigation.