Analysis of Macrophages and Peptidergic Fibers in the Skin of Patients With Painful Diabetic Polyneuropathy

Background and Objectives

The mechanisms of pain in patients with diabetic polyneuropathy are unknown. Studies have suggested a role of inflammation and increased neuropeptides peripherally in pain generation. This study examined the possible skin markers of painful diabetic polyneuropathy (P-DPN): macrophages, substance P (SP), and calcitonin gene-related peptide (CGRP).

Methods

The participants were included from a large Danish cross-sectional clinical study of type 2 diabetes. We diagnosed definite diabetic polyneuropathy using the Toronto criteria and used the Neuropathic Pain Special Interest Group classification for defining P-DPN. We included 60 skin biopsies from patients with diabetic polyneuropathy—30 with P-DPN and 30 with nonpainful diabetic polyneuropathy (NP-DPN)—and 30 biopsies from healthy controls of similar age and sex. The biopsies were stained using PGP 9.5, IbA1, and SP and CGRP primary markers.

Results

There was increased macrophage density in patients with P-DPN (8.0%) compared with that in patients with NP-DPN (5.1%, p < 0.001), and there was increased macrophage density in patients with NP-DPN (5.1%) compared with that in healthy controls (3.1%, p < 0.001). When controlling for neuropathy severity, body mass index, age, and sex, there was still a difference in macrophage density between patients with P-DPN and patients with NP-DPN. Patients with P-DPN had higher median nerve fiber length density (274.5 and 155 mm–2 for SP and CGRP, respectively) compared with patients with NP-DPN (176 and 121 mm–2 for SP and CGRP, respectively, p = 0.009 and 0.04) and healthy controls (185.5 and 121.5 mm–2 for SP and CGRP, respectively), whereas there was no difference between patients with NP-DPN and controls without diabetes (p = 0.64 and 0.49, respectively). The difference between P-DPN and NP-DPN for SP and CGRP was significant only in female patients, although a trend was seen in male patients.

Discussion

The findings point to a possible involvement of the innate immune system in the pathogenesis of neuropathic pain in patients with DPN, although markers of activated macrophages were not measured in this study.

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