Background: Pain is a common symptom in polyneuropathies (PNP), although it is still not known why some PNP are painful whereas others are painless. Increased pro-inflammatory cytokines were found in conditions resulting in exaggerated pain states in animal studies. Recently, elevated pro-inflammatory cytokine levels have also been found in the cerebrospinal fluid (CSF) of patients suffering from complex regional pain syndrome. Pro-inflammatory cytokines have been shown to induce or increase inflammatory or neuropathic pain.
Methods: Using chemiluminescent enzyme immunometric assays, we investigated cytokine levels in 36 patients with painful and painless non-inflammatory PNP in serum and CSF. Severity of PNP was measured with electroneurography (ENG). In subjects with normal results upon conventional ENG, quantitative thermotesting was performed to investigate small nerve fiber function.
Results: IL-6 and TNF-á in serum or CSF did not differ between patients with (n=18) or without (n=18) painful PNP, whereas patients with mechanical allodynia (n=5) had elevated serum TNF- á levels compared to those without allodynia. TNF- á and IL-6 serum levels were higher in patients with severe (n= 21) compared to those with mild neuropathy (n=15) and showed a positive correlation with severity of neuropathy.
Conclusions: Results suggest that nerve fiber degeneration and presence of mechanical allodynia in peripheral non-inflammatory neuropathy determine cytokine expression in serum.