Small- and Large-Fiber Involvement in Early Diabetic Neuropathy: A Study With the Medial Plantar Response and Sensory Thresholds

Small- and large-fiber function in diabetic neuropathy was studied in 68 patients (mean age 45.4 ± 12.9 yr; 27 type I and 41 type II diabetics) with psychophysical tests of vibration and thermal sensation and neurophysiological measurements, including the medial plantar sensory action potential (MPSAP). Thermal sensitivity at the dorsolateral aspect of the foot (Pfizer thermal tester) correlated significantly with vibration thresholds (Somedic vibrameter) at three sites in the foot and two in the hand. Forty patients had normal sensory thresholds, but 18 of these lacked an MPSAP. Smaller groups had a single abnormal sensory threshold: 12 (18%) had an abnormal vibration threshold, and 24 (35%) had abnormal thermal sensitivity; 8 of the former group and 17 of the latter group lacked an MPSAP response. Only 8 (12%) had both abnormal vibration and thermal sensation (6 without an MPSAP). Fifteen of the 17 symptomatic patients had lost the MPSAP, but there was no consistent pattern of sensory loss. In this relatively young group of diabetics, more patients showed absent MPSAP responses than an abnormality in either sensory test on its own. The MPSAP is frequently absent in patients with no abnormalities in psychophysical tests of peripheral large-fiber function (vibration sensation) and small-fiber function (thermal sensitivity).