Paired stimulation study of the median nerve sensory action potential in diabetic patients

Conventional nerve conduction studies (NCS) are not sensitive to detect mild diabetic neuropathy. In order to detect subtle changes, we compared the conventional NCS with the relative refractory period (RRP) measurement of the median sensory nerve action potential by a paired stimulation method.

Subjects were 29 diabetic patients whose conventional NCS were all normal. They were divided into two groups: neurologically symptomatic and asymptomatic groups. Twenty-eight age-matched control subjects were also studied.

The RRP of the symptomatic diabetic patients (5.9 +/- 0.5 ms) and that of the asymptomatic patients (5.6 +/- 0.5 ms) was significantly longer than that of the control subjects (4.9 +/- 0.6 ms). There was no significant difference in RRP between the symptomatic and asymptomatic patients. This may be due to the fact that NCS reflects mainly large myelinated fiber function and early symptoms represent mainly thin myelinated or unmyelinated fiber function.

The RRP measurement could reveal some mild involvement of peripheral nerves undetectable by conventional NCS, even though they caused no clinical symptoms.