Sympathetic skin response in fibromyalgia patients with small fibre pathology

Objective: Fibromyalgia (FM) is a chronic disease characterised by small fibre pathology and autonomic dysfunction. Sympathetic Skin Response (SSR) can be easily obtained in clinical practice as a marker of autonomic dysfunction. This study aims to assess SSR abnormalities in small fibre pathology associated with FM. Methods: This was a retrospective, observational, single-centre, cross-sectional study including 393 patients with FM, who underwent skin biopsy from the thigh and ankle, SSR obtained with a standard electromyographic active electrode attached to the right palm and sole, and clinical evaluation. Results: SSR at the hand and foot was similarly absent in patients with both normal and abnormal skin biopsy results. Hand amplitude was reduced in patients with denervation (Wilcoxon z = 1.89, p = 0.029), while latencies were similar between groups (z = 1.12, p = 0.13). Foot latency was prolonged in patients with abnormal skin biopsy (z = 2.17, p = 0.014), while amplitudes were similar between groups (chi-square = 2.28, p = 0.21). SSR showed low predictive performance for skin biopsy results (r 2 = 0.04). Latency of SSR at the foot and Intraepidermal Nerve Fibre Density (IENFD) correlated with the score for associated symptoms. Amplitude of SSR at the foot also negatively correlated with anxiety, depression, and fatigue scores. Conclusions: SSR abnormalities did not correspond to small fibre impairment detected by skin biopsy in patients with FM. Significance: SSR and skin biopsy may capture different aspects of the complex clinical picture of FM, including psychopathological features and fatigue, and therefore represent complementary yet distinct biomarkers of the disease.