Early diagnosis of diabetic neuropathy using double-shock stimulation of peripheral nerves

Objective: The purpose of this study was to determine the changes in the amplitudes of a sensory nerve action potential (NAP) to a conditioning stimulus given prior to a test stimulus at 2–8 ms intervals in healthy subjects and patients with diabetes mellitus with no clinical signs of neuropathy and normal nerve conduction velocities (NCVs), to be able to diagnose peripheral neuropathy at its very early stages. Methods: NAPs in the superficial branch of the radial nerve were recorded in healthy subjects (28 women and 7 men) and type II diabetes patients without neuropathy (22 women and 12 men). Radial nerve was first stimulated with a single shock and then with double shocks at intervals of 2, 3, 4, 5, 6, 7, and 8 ms; NAP amplitudes and NAP1/NAP2 ratios were calculated in normals and diabetics. NCVs were within the normal ranges (>50 m/s) in all subjects. Results: Of the independent variables—group (control, patient), sex (male, female), and hand (right, left)—only group significantly influenced NAP amplitude; mean NAP amplitude (single shock) was significantly lower in patients than controls. NAP1/NAP2 ratios were slightly below one (facilitation) in controls; it was above one at 1–8 ms stimulus intervals (inhibition) in diabetics, which was strongest at smallest intervals, gradually decreasing, and almost disappearing as the stimulus interval approached 8 ms. Conclusions: Using double-shock stimuli, an early diagnosis of peripheral neuropathy would be possible in diabetics without clinical signs of peripheral neuropathy and exhibiting no slowing in NCV.