Mechanisms of pain in distal symmetric polyneuropathy: a combined clinical and neurophysiological study

In patients with distal symmetric polyneuropathy we assessed non-nociceptive Abeta- and nociceptive Adelta-afferents to investigate their role in the development of neuropathic pain. We screened 2240 consecutive patients with sensory disturbances and collected 150 patients with distal symmetric polyneuropathy (68 with pain and 82 without). All patients underwent the Neuropathic Pain Symptom Inventory to rate ongoing, paroxysmal and provoked pains, a standard nerve conduction study (NCS) to assess Abeta-fibre function, and laser-evoked potentials (LEPs) to assess Adelta-fibre function. Patients with pain had the same age (P>0.50), but a longer delay since symptom onset than those without (P<0.01). Whereas the LEP amplitude was significantly lower in patients with pain than in those without (P<0.0001), NCS data did not differ between groups (P>0.50). LEPs were more severely affected in patients with ongoing pain than in those with provoked pain (P<0.0001). Our findings indicate that the impairment of Abeta-fibres has no role in the development of ongoing or provoked pain. In patients with ongoing pain the severe LEP suppression and the correlation between pain intensity and LEP attenuation may indicate that this type of pain reflects damage to nociceptive axons. The partially preserved LEPs in patients with provoked pain suggest that this type of pain is related to the abnormal activity arising from partially spared and sensitised nociceptive terminals. Because clinical and neurophysiological abnormalities followed similar patterns regardless of aetiology, pain should be classified and treated on mechanism-based grounds.