Quantitative sensory testing in the German Research Network on Neuropathic Pain (DFNS): somatosensory abnormalities in 1236 patients with different neuropathic pain syndromes.

Neuropathic pain is accompanied by both positive and negative sensory signs. To explore the spectrum of sensory abnormalities, 1236 patients with a clinical diagnosis of neuropathic pain were assessed by quantitative sensory testing (QST) following the protocol of DFNS (German Research Network on Neuropathic Pain), using both thermal and mechanical nociceptive as well as non-nociceptive stimuli. Data distributions showed a systematic shift to hyperalgesia for nociceptive, and to hypoesthesia for non-nociceptive parameters. Across all parameters, 92% of the patients presented at least one abnormality. Thermosensory or mechanical hypoesthesia (up to 41%) was more frequent than hypoalgesia (up to 18% for mechanical stimuli). Mechanical hyperalgesias occurred more often (blunt pressure: 36%, pinprick: 29%) than thermal hyperalgesias (cold: 19%, heat: 24%), dynamic mechanical allodynia (20%), paradoxical heat sensations (18%) or enhanced wind-up (13%). Hyperesthesia was less than 5%. Every single sensory abnormality occurred in each neurological syndrome, but with different frequencies: thermal and mechanical hyperalgesias were most frequent in complex regional pain syndrome and peripheral nerve injury, allodynia in postherpetic neuralgia. In postherpetic neuralgia and in central pain, subgroups showed either mechanical hyperalgesia or mechanical hypoalgesia. The most frequent combinations of gain and loss were mixed thermal/mechanical loss without hyperalgesia (central pain and polyneuropathy), mixed loss with mechanical hyperalgesia in peripheral neuropathies, mechanical hyperalgesia without any loss in trigeminal neuralgia. Thus, somatosensory profiles with different combinations of loss and gain are shared across the major neuropathic pain syndromes. The characterization of underlying mechanisms will be needed to make a mechanism-based classification feasible.

Withdrawal reflex, skin resistance reaction and pain ratings due to electrical stimuli in man.

Simultaneous measurements of pain rating, withdrawal reflex, and skin resistance reaction with non-painful and painful electrical stimuli were performed on 15 healthy male volunteers. Eight different intensities were delivered in standardized randomized order. Each intensity appeared 10 times. There was no influence on the preceding stimulus on responses to the following stimulus. Neither skin resistance reaction nor withdrawal reflex were specific for pain in the sense that they appeared solely when pain was reported; the two reaction threshold currents were significantly smaller than the pain threshold. The reaction amplitudes, however, showed a close correlation to the intensity of sensation. The relations between all reactions measured and stimulus strength could be described best by power functions, with an exponent less than or equal to 1 if least square fits in linear scales were performed. Graphical evaluation in double logarithmic scales led to systematic errors causing higher exponents. Compound relations, like skin resistance reaction or withdrawal reflex amplitude as function of subjective estimation, could also be approximated by power functions, with parameters predictable from stimulus-reaction functions. No change in exponent was observed when subjective estimation turned from pre-pain to pain.