Axonal swellings predict the degeneration of epidermal nerve fibers in painful neuropathies

Objective: To correlate the density of swellings in intraepidermal nerve fibers (IENF) with the longitudinal measurement of the epidermal innervation density in patients with painful neuropathy and to assess the predictive value of IENF swelling to progression of neuropathy. Methods: Fifteen patients with persistent pain in the feet underwent neurologic examination, nerve conduction studies, quantitative sensory examination, and skin biopsies at proximal thigh and distal leg. In all patients and in 15 healthy subjects, IENF density and swelling ratio (no. swellings/no. IENF) were quantified at distal leg. Follow-up study, including IENF density and swelling ratio quantification, was performed a mean of 19.2 months later. Double staining confocal microscope studies using anti-human protein-gene-product 9.5, anti-tubule, anti-neurofilament, and anti-synaptophysin antibodies were performed to assess specific accumulation within swellings. Ultrastructural investigation of IENF was also carried out. Results: Patients with neuropathy had lower density of IENF and higher swelling ratio than healthy subjects (p<0.01) at distal leg. At follow-up, patients showed a parallel decrease in both IENF density (p=0.02) and swelling ratio (p=0.002). However, swelling ratio remained higher (p=0.03) than in controls. Progression of neuropathy was confirmed by the decay in sural nerve sensory nerve action potential amplitude. Double immunostaining studies suggest accumulation of tubules and ubiquitin-associated proteins within swellings. Swollen and vacuolated IENF were identified in patients with neuropathy by conventional and immuno-electron microscopy. Conclusions: Increased swelling ratio predicted the decrease in IENF density in patients with painful neuropathy. Its quantification could support earlier diagnosis of sensory axonopathy.