Correlation of EMG, CMAP and SNAP amplitude decrease in mononeuropathies with axonal loss.

We retrospectively reviewed electrodiagnostic studies from 1983 to 1994 and found 48 patients who met our criteria for mononeuropathy with axonal loss (40 ulnar, 4 peroneal, 4 radial). Appropriate diagnostic criteria required bilateral studies with a normal contralateral, sensory nerve action potential (SNAP) amplitude decrease of > 50% compared to contralateral, and/or distal compound muscle action potential (CMAP) amplitude decrease of > 40% compared to contralateral, and/or presence of denervation potentials; and sufficient electrodiagnostic investigation to rule out peripheral polyneuropathy. We conclude that in the electrodiagnosis of mononeuropathy with axonal loss: 1) a significant quantitative correlation between CMAP and SNAP amplitude percentage decrease does not exist (r = 0.274, p = 0.06), 2) SNAP amplitude percentage decrease [75.3 +/- 31.8%] is greater than CMAP amplitude percentage decrease [43.9 +/- 31.3%] (paired t-test, p = 0.0001), and 3) CMAP amplitude decrease is positively correlated with the presence of denervation potentials (Xtrend2 = 6.22, p = 0.013).