The spectrum of neuropathy in diabetes and impaired glucose tolerance.

To compare the neuropathy associated with impaired glucose tolerance (IGT) and diabetes mellitus (DM) determined by oral glucose tolerance testing (OGTT). Patients with peripheral neuropathy of unknown cause were prescribed OGTT. Duration of neuropathic symptoms, neuropathic pain, neuropathy classification, nerve conduction test results, and intraepidermal nerve fiber densities (IENFD) were compared between IGT and DM groups. Seventy-three patients completed OGTT; 41 (56%) had abnormal results. Of these 41 patients, 26 had IGT and 15 had DM. Patients with IGT had less severe neuropathy than patients with diabetes, as measured by sural nerve amplitudes (p = 0.056), sural nerve conduction velocities (p = 0.03), and distal leg IENFD (p = 0.01). Patients with IGT had predominantly small fiber neuropathy, compared to patients with DM (p = 0.05), who had more involvement of large nerve fibers. The neuropathy associated with IGT is milder than the neuropathy associated with DM. Small nerve fibers are prominently affected and may be the earliest detectable sign of neuropathy in glucose dysmetabolism. OGTT is appropriate in patients with idiopathic neuropathy.