Large and small fiber dysfunction in peripheral nerve injuries with or without spontaneous pain

Few data have been available on the functional role of small fiber damage in patients with peripheral nerve injuries with and without spontaneous pain. The aim of the present study was to investigate the function of large myelinated nerve fibers as well as small nerve fibers in a material of 60 patients with peripheral nerve injuries in upper or lower extremities, 30 patients with spontaneous pain, and 30 patients without pain. Patients were questioned about the characteristics of pain and investigated clinically with EMG/neurography and assessment of thermal thresholds in the innervation territory of the lesioned nerve as well as in the contralateral area. Sensation of touch and warmth and cold detection was significantly reduced in the injured side in both groups. There was a tendency, not significant, for heat pain thresholds to be more elevated in the affected side compared with the healthy side in the pain group only (47.8°C versus 45.1°C). There were no significant differences in thermal thresholds between the 2 groups of patients. The main finding was a high percentage of hyperphenomena (allodynia to light touch and reduced mechanical pain thresholds) in the pain group only.

Small fiber function did not significantly differ between patients with and without pain, indicating that elevated thermal thresholds alone will not reflect mechanisms responsible for the generation of pain. Hyperphenomena were present in the affected side of the pain group only.