Properties of cutaneous afferents in diabetic neuropathy

The technique of percutaneous microneurography was used to study the response properties of 68 cutaneous afferents in 8 diabetic patients and, for comparison, of 104 afferents in 12 normal control subjects. The diabetic patients were neurologically asymptomatic or had a mild sensory neuropathy. Three afferent categories were studied in the diabetic subjects (rapidly adapting type RA and slowly adapting SAI and SAII) in response to mechanical skin stimulation and abnormal responsiveness was encountered in all categories. Distinct abnormalities were seen in about 30% of the RA and in 20% of the SAI type nerve fibres. The abnormalities consisted of the generation of only single action potentials to above threshold stimuli (RA), and of high fatiguability to repetitive stimulation (SAI). A more subtle abnormality was additionally encountered in the slowly adapting SAI and SAII groups. This consisted of low discharge rates to sustained skin indentation, with values statistically significantly different from normal. Absolute mechanical response thresholds and receptive field configurations were similar between diabetic and normal units. The conduction velocities of individual nerve fibres were measured in response to intradermal electrical stimulation. They were similar to normal, regardless of whether the afferents displayed normal or abnormal responsiveness to physiological stimulation of the receptive fields. The data indicate that different cutaneous afferent categories are affected by the disease, and are limited in their ability to transmit trains of action potentials. As a consequence, physiological stimuli may be abnormally encoded in diabetic units. Two hypotheses, one morphological and one biophysical, are proposed for explaining the present findings. The results altogether provide a physiological basis for understanding nerve fibre dysfunction and sensory disturbances in human diabetic neuropathy.