Superficial 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.
In 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).
AIDP/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.
Comparison of multiple SNAP amplitudes provides information about characteristic distribution patterns of sensory nerve involvement in peripheral neuropathies.