Absence of the sensory action potential of the medial plantar nerve: a sensitive indicator of diabetic neuropathy.

The sensory action potential of the medial plantar nerve (MPSAP), using needle recording electrodes subcutaneously, was measured in 119 patients with diabetes ((75 insulin-dependent, mean age 43.7 +/- 1.2 SEM years (range 15-73) and in 75 age-matched normal subjects (mean age 44.0 +/- 1.5 years (range 19-75)). The MPSAP was not detectable in 56 out of 119 diabetics; there was only one absent MPSAP response in the normal group, in a patient aged 71 years. Diabetics with absent MPSAP responses were significantly older, heavier and taller and had diabetes of longer duration. The amplitudes and conduction velocities in the sural, common peroneal, and tibial nerves were also significantly smaller. The amplitudes of the MPSAP, when present, correlated with height (r = -0.47(43), p less than 0.001), weight (r = -0.44(43), p less than 0.001), the distal tibial compound motor action potential (r = 0.37(43), p less than 0.01), and the common peroneal nerve conduction velocity (r = 0.32(43), p less than 0.02). The MPSAP was always absent if the sural SAP was less than 5uV but also in 14/59 patients whose sural SAP amplitudes were within 1 sd of the normal mean (greater than 10.7 uV). The MPSAP was absent at a younger age and with diabetes of shorter duration than is seen with unrecordable sural SAPs. It may therefore represent an early manifestation of neuropathy in diabetes.