Differences in corneal nerve fiber density and fiber length in patients with painful chronic idiopathic axonal polyneuropathy and diabetic polyneuropathy

Introduction/Aims: Corneal confocal microscopy (CCM) detects small nerve fiber loss and correlates with skin biopsy findings in diabetic neuropathy. In chronic idiopathic axonal polyneuropathy (CIAP) this correlation is unknown. Therefore, we compared CCM and skin biopsy in patients with CIAP to healthy controls, patients with painful diabetic neuropathy (PDN) and diabetics without overt neuropathy (DM). Methods: Participants with CIAP and suspected small fiber neuropathy (n = 15), PDN (n = 16), DM (n = 15), and healthy controls (n = 16) underwent skin biopsy and CCM testing. Inter-center intraclass correlation coefficients (ICC) were calculated for CCM parameters. Results: Compared with healthy controls, patients with CIAP and PDN had significantly fewer nerve fibers in the skin (IENFD: 5.7 ± 2.3, 3.0 ± 1.8, 3.9 ± 1.5 fibers/mm, all p <.05). Corneal nerve parameters in CIAP (fiber density 23.8 ± 4.9 no./mm², branch density 16.0 ± 8.8 no./mm², fiber length 13.1 ± 2.6 mm/mm²) were not different from healthy controls (24.0 ± 6.8 no./mm², 22.1 ± 9.7 no./mm², 13.5 ± 3.5 mm/mm², all p >.05). In patients with PDN, corneal nerve fiber density (17.8 ± 5.7 no./mm²) and fiber length (10.5 ± 2.7 mm/mm²) were reduced compared with healthy controls (p <.05). CCM results did not correlate with IENFD in CIAP patients. Inter-center ICC was 0.77 for fiber density and 0.87 for fiber length. Discussion: In contrast to patients with PDN, corneal nerve parameters were not decreased in patients with CIAP and small nerve fiber damage. Therefore, CCM is not a good biomarker for small nerve fiber loss in CIAP patients.