A human model of small fiber neuropathy to study wound healing

Abstract

The aim of this study was to develop a human model of acute wound healing that isolated the effects of small fiber neuropathy on the healing process. Twenty-five healthy subjects had the transient receptor vanilloid 1 agonist capsaicin and placebo creams topically applied to contralateral areas on the skin of the thigh for 48 hours. Subjects had shallow (1.2 millimeter) and deep (>3 millimeter) punch skin biopsies from each thigh on days 1 and 14. Biopsy wound healing was monitored photographically until closure. Intra-epidermal and sweat-gland nerve fiber densities were measured for each biopsy. Shallow wounds in capsaicin-treated sites healed more slowly than in placebo treated skin with biopsies taken on day 1 (P<0.001) and day 14 (P<0.001). Deep biopsies in the capsaicin and placebo areas healed at similar rates at both time points. Nerve fiber densities were reduced only in capsaicin treated regions (P<0.01). In conclusion, topical application of capsaicin causes a small fiber neuropathy and is associated with a delay in healing of shallow, but not deep wounds. This novel human model may prove valuable in the study of wound healing in patients with neuropathy.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1. Skin biopsies as a model of wound healing.

Three millimeter punch skin biopsies from the thigh with a ruler used as an internal standard measurement. The shallow biopsy (shown in subset) on the right contains pieces of hair follicles seen within the wound bed. The deep biopsy (shown in subset) is on the left and contains no remnants of adnexal structures within the wound bed.

Figure 2. Rates of shallow and deep wound healing from capsaicin and placebo treated skin.

Wound healing from day 1 (a & b) and day 14 (c & d) biopsies. Survival curves (a & c) are defined as the time from biopsy to wound closure (percent of biopsies not healed). The area of the open wound at each time point by biopsy type and treatment is shown for day 1 (b) and day 14 (d). Shallow biopsies from capsaicin treated areas healed more slowly than shallow biopsies from placebo treated areas (P<0.001 vs. placebo) on day 1 and day 14. There were no differences in capsaicin and placebo treated deep biopsies (P = 0.43 vs. placebo, day 1; P = 0.09 vs. placebo day 14). Shallow biopsies from capsaicin treated areas healed more quickly on day 14 (d) than on day 1 (b) (P = 0.03). Statistical significance is not displayed in these graphs.

Figure 3. The intra-epidermal and sweat gland nerve fiber density in capsaicin and placebo treated skin.

(a) The intra-epidermal nerve fiber density from biopsies taken on day 1 and 14 is shown (mean ± SD). *P<0.001 compared to control. (b) The sweat gland nerve fiber density from biopsies taken on day 1 and 14 is shown (mean ± SD). *P<0.01 compared to control. (c, e) Protein gene product 9.5 labeled intra-epidermal nerve fibers from day 14 biopsies in placebo (c) and capsaicin (d) treated skin. Black arrows denote intra-epidermal fibers in placebo treated skin (c), but are not seen in capsaicin treated skin (e). (d, f) Protein gene product 9.5 labeled sweat gland nerve fibers from day 14 biopsies in placebo (d) and capsaicin (f) treated skin.

Figure 4. Neurophysiologic testing in capsaicin and placebo treated skin.

The cold (a), heat (b), cold-pain (d), and heat-pain (d) detection thresholds are shown for each test visit for placebo (black circles) and capsaicin (open circles) treated regions. Cold detection thresholds were lower for every test day in the capsaicin treated side. Heat and heat-pain detection thresholds were higher for every test day on the capsaicin treated side. No significant differences were noted in cold-pain detection thresholds. Blood flow measured by laser-Doppler flowmetry is shown in (e) for placebo (black circles) and capsaicin (open circles) treated regions. Axon-reflex mediated blood flow was reduced in the capsaicin treated region compared to placebo treated region on all test days except day 28. Sweat volume measured by the quantitative sudomotor axon reflex test (QSART) is shown in (f). Sweat output was reduced in the capsaicin treated region compared to the placebo treated region on all test days except day 1. AUC = area under the curve. *P<0.05.