The Usefulness of Sural/Radial Amplitude Ratio in the Diagnosis of Mild Axonal Polyneuropathy

Backgrounds and objectives： In the length-dependent axonal polyneuropathy, the distal part of the longer axons are affected earlier. In cases of mild axonal polyneuropathy, nerve conduction studies are frequently normal. We theorized that a sural/radial amplitude ratio (SRAR) might be a sensitive indicator of mild polyneuropathy. We investigated whether the SRAR may be a more sensitive indicator than sural amplitude alone in the diagnosis of mild axonal polyneuropathy.Methods： We analyzed medical records and electrophysiological studies of 32 patients with mild axonal polyneuropathy and 30 normal subjects. Sural amplitudes and SRAR in patients with signs of mild axonal polyneuropathy were compared to those of normal, age-matched control subjects. Sural and radial sensory amplitudes were measured in a standard method in all subjects.Results： Thirty five patients had an average SRAR of 0.26 as compared to 0.65 for the normal subjects. An SRAR of less than 0.50 was a strong predictor of axonal polyneuropathy, with the balance of sensitivity and specificity (80%), as compared to an absolute sural amplitude, which had sensitivity of only 57%. The SRAR did not vary significantly with age.Conclusion： We conclude that the SRAR is a useful electrodiagnostic test for mild axonal polyneuropathy.