Diabetes is a common cause for development of neuropathy, affecting millions of patients worldwide. The prevalence of painful diabetic neuropathy is 20%. Damage to small nerve fibers is anticipated to be crucial for the generation of pain, but it has been unclear whether tests of small fiber function may reveal a large impairment in patients with diabetic neuropathy and pain versus non-pain. The aim of the present investigation was to study whether there was a difference in large- or small fiber function in diabetic neuropathy patients with and without pain.
In this study we examined 35 patients with diabetes, investigating both large- and small nerve fibers. Large nerve fibers were examined with the use of traditional EMG/neurography, whereas small nerve fibers were examined with QST (quantitative sensory testing), thermotest (assessment of thermal thresholds). We also evaluated the autonomic nervous system using the quantitative sudomotor axon reflex test, evoked electrodermal activity test, tilt-test and RR-interval.
We found no statistically significant difference between the pain and no pain groups when we tested small nerve fibers with the thermal threshold test. There was damage to the small nerve fibers in both groups. Therefore, evaluation of small nerve fibers alone cannot predict pain.
When we investigated the results of the autonomic tests, we saw that most of the patients with a pathological autonomic test also had pathological thermal thresholds and results of EMG/neurography. It therefore seems that autonomic neuropathy most often appears in a patient who already has extensive neuropathy.