Abnormal LDIflare but Normal Quantitative Sensory Testing and Dermal Nerve Fiber Density in Patients with Painful Diabetic Neuropathy

OBJECTIVE—Abnormal small nerve fiber function may be an early feature of diabetic neuropathy and may also underlie painful symptoms. Methods for assessing small-fiber damage include quantitative sensory testing (QST) and determining intraepidermal nerve fiber density. We recently described a reproducible physiological technique, the LDIflare, which assesses small-fiber function and thus may reflect early dysfunction before structural damage. The value of this technique in painful neuropathy was assessed by comparing it with QST and dermal nerve fiber density (NFD). RESEARCH DESIGN AND METHODS—Fifteen healthy control subjects, 10 subjects with type 2 diabetes and painful neuropathy (PFN), and 12 subjects with type 2 diabetes and painless neuropathy (PLN) were studied. LDIflare and QST were performed on the dorsum of the foot, and dermal NFD was determined. RESULTS—Results of both large- and small-fiber quantitative sensory tests were abnormal in patients with PLN but not those with PFN compared with control subjects. Dermal NFD was also significantly reduced in the PLN group compared with control subjects (205.8 ± 165.3 vs. 424.9 ± 176.3 [mean ± SD]; P = 0.003) but not in the PFN group (307.6 ± 164.5). In contrast, the LDIflare (square centimeters) was reduced in both PFN (1.59 ± 0.41) and PLN (1.51 ± 0.56) groups compared with control subjects (4.38 ± 1.4) (P < 0.001 for both). NFD correlated significantly with the LDIflare (r = 0.57, P < 0.0001). CONCLUSIONS—The LDIflare demonstrated impaired small-fiber function in patients with PFN when other assessments revealed no abnormality. We believe that this method has potential diagnostic value, particularly because it is noninvasive, has excellent reproducibility, and correlates with NFD. Furthermore, it may have an important role in assessing preventative therapies in early neuropathy.