Background: Tendinopathy is a clinical diagnosis of localised tendon pain often confirmed by imaging findings. The pathophysiological cause of the pain is unknown and the sympathetic nervous system (SNS) may be implicated.
Objective: To review what is known regarding the role of the SNS in human tendinopathy.
Study selection: Published data describing sympathetic innervation or an index of sympathetic activity in human tendons were eligible for inclusion.
Data sources: Bibliographical databases (AMED, Biological Abstracts, CINAHL Plus, EMBASE, MEDLINE, Scopus, SPORTDiscus and Web of Science) were searched for relevant articles. Reference lists from included articles were screened for additional articles.
Study appraisal: Studies were scored with a quality assessment tool to identify potential sources of bias. Each question had an explicit decision rule to guide assessment.
Results: Nine case-control and four cross-sectional studies examined sympathetic innervation of tendons. There was evidence suggesting a lack of difference in sympathetic innervation of tendon proper between tendinopathy biopsies and healthy controls. In contrast, the paratendinous tissue showed evidence of increased sympathetic innervation in painful tendons. The most notable increase in SNS markers was seen in abnormal tenocytes from painful tendons. Data from two studies were suitable for meta-analysis. These heterogeneous studies revealed no difference in sympathetic innervation between painful and pain-free tendons. No studies recorded SNS activity in vivo.
Conclusion: Sympathetic innervation in painful tendons depends on tissue type. Abnormal tenocytes may have increased capacity for self-production of sympathetic neurotransmitters. Future insight may be gained by measuring global in vivo sympathetic drive in tendinopathy.