Superficial radial sensory nerve potentials in immune-mediated and diabetic neuropathies

ObjectiveThe pattern of abnormal median-normal sural sensory nerve action potential (SNAP) is frequently found in acute/chronic inflammatory demyelinating polyneuropathy (AIDP/CIDP), whereas sural/radial SNAP amplitude ratio is sensitive to detect dying-back degeneration. To investigate whether radial SNAP and its amplitude ratio to median or sural SNAP provide additional particular patterns of sensory nerve involvement.MethodsSuperficial radial, median, and sural SNAPs were recorded in 63 normal subjects and in 132 patients with AIDP/CIDP (n=22), diabetic neuropathy (n=83), or other axonal polyneuropathy (n=27). Median/radial and sural/radial amplitude ratios were examined.ResultsIn normal subjects, median/radial ratio was 0.96±0.05 (mean±SEM), and sural/radial ratio was 0.50±0.03. Compared with normal controls, the median/radial ratio was lower in patients with AIDP/CIDP (0.64±0.11; P<0.001) or diabetic neuropathy (0.75±0.04; P=0.08), but similar in those with other neuropathy (0.94±0.10). The sural/radial ratio was higher in the AIDP/CIDP group (0.71±0.08; P=0.10), and lower in the diabetic (0.36±0.03; P<0.001) and other axonal neuropathy groups (0.40±0.07; P=0.08).ConclusionsAIDP/CIDP is associated with a reduced median/radial ratio and increased sural/radial ratio, probably reflecting demyelination predominant in the distal nerve terminals. Diabetic neuropathy is characterized by decreases in both median/radial and sural/radial ratios, presumably due to coexistence of carpal tunnel pathology and dying-back degeneration.SignificanceComparison of multiple SNAP amplitudes provides information about characteristic distribution patterns of sensory nerve involvement in peripheral neuropathies.