Somatosensory function in patients with and without pain after traumatic peripheral nerve injury

Why traumatic injuries to the peripheral nervous system infrequently result in neuropathic pain is still unknown. The aim of this study was to examine the somatosensory system in patients with traumatic peripheral nerve injury with and without pain to try to unravel possible links to mechanisms underlying development and maintenance of pain. Eighteen patients with spontaneous ongoing pain and 16 patients without pain after unilateral partial peripheral traumatic nerve injury were studied. In the area of partial denervation and in the corresponding contralateral area perception thresholds to warmth, cold, light touch, pressure pain, cold- and heat pain were assessed as were pain intensities at suprathreshold heat pain stimulation. Comparing sides patients with pain reported allodynia to cold (p=0.03) and pressure (p=0.016) in conjunction with an increase in the perception threshold to non-painful warmth (p=0.024) on the injured side. Pain-free patients reported hypoesthesia to light touch (p=0.002), cold (p=0.039) and warmth (p=0.001) on the injured side. There were no side differences in stimulus-response functions using painful heat stimuli in any of the groups. In addition, no significant difference could be demonstrated in any sensory modality comparing side-to-side differences between the two groups. In conclusion, increased pain sensitivity to cold and pressure was found on the injured side in pain patients, pointing to hyperexcitability in the pain system, a finding not verified by a more challenging analysis of side-to-side differences between patients with and without pain.