Peripheral nerve excitability measures at different target levels: the effects of aging and diabetic neuropathy

Threshold tracking testing has provided novel insights of peripheral nerve excitability in normal and pathologic conditions. However, little has been known on the nerve excitability properties of axons with different stimulation thresholds and the effects of aging and peripheral neuropathy to those. We performed multiple nerve excitability tests in normal controls divided into three age groups and in patients with diabetic neuropathy, which were recorded at three target levels (10%, 40%, and 60% of the maximum motor response amplitudes). In all the control groups, tracking at low target level shows smaller threshold change by hyperpolarizing stimuli and greater threshold change by depolarizing stimuli, suggestive of greater transient Na current. Normal elderly showed greater threshold change by hyperpolarizing pulse than younger subjects at high target level, likely reflecting decrease of axon diameters. Patients with diabetic neuropathy showed smaller threshold changes by both depolarizing and hyperpolarizing pulses ("fanning-in"), more noticeably at the lower target level, suggestive of the combined effects of membrane depolarization and greater decrease of axonal diameters in smaller fibers. Given the reported unpredictable electrical recruitment order in the diseased conditions and difference of nerve excitability measures in threshold electrotonus at different target levels, comparing threshold electrotonus values between normal and diseased axons may be problematic by comparing axons with different nerve excitability characteristics.