Assessment of small and large fiber function in long-term type 1 (insulin-dependent) diabetic patients with and without painful neuropathy

Twelve neural function tests (thermal discrimination thresholds, pain perception thresholds to heat and cold stimuli, vibration perception thresholds, and motor and sensory nerve conduction velocities) were assessed in the lower and upper extremities of 60 long-term type 1 diabetic patients. Thirty patients were asymptomatic (group 1) and 30 patients had painful neuropathy (group 2), predominantly originating in the distal lower limbs (group 2a; n = 20) or in the distal upper limbs (group 2b; n = 10). There were no significant differences between the groups with regard to age, duration of diabetes or glycemic control. Eleven of the 12 functions tested (6 in lower and upper limbs, respectively) were significantly diminished in both groups of diabetics as compared to age-matched control subjects. Group 2a had significant impairment in 5 of 6 parameters of the lower limbs, while in group 2b only 1 of 6 functions of the upper limbs was diminished. In the whole diabetic group, the most frequent abnormality was an elevated threshold for thermal sensation in the foot. Significant correlations between small and large fiber abnormalities were observed predominantly in the foot. Selective affection of small or large fiber functions showed different patterns in the arms and in the legs. In the upper extremities selective impairment in nerve conduction was predominant, while in the lower extremities it was thermal sensation. These findings suggest that both generalized and selective small or large fiber affection may occur in long-term type 1 diabetic patients. Dysfunction of both modalities is more severe in the lower limbs, when painful symptoms have developed in this region.