008-22 – A cross-sectional observational study exploring somatosensory phenotype and its relation to corneal nerve fibres in fibromyalgia

Fibromyalgia (FMS) is a chronic widespread pain syndrome characterised by sensory symptoms with features of neuropathic pain. Increasing evidence implicates structural and functional small nerve fibre abnormalities in FMS, but it remains unclear how this relates to symptoms and signs. The aim of this cross-sectional study was to identify small fibre pathology (SFP) in people with FMS and investigate associated sensory phenotypes.
Twenty-five FMS patients and twenty-six age and gender matched healthy controls were examined with corneal confocal microscopy (CCM) by quantifying corneal nerve fibre density (CNFD), branch density (CNBD) and fibre length (CNFL). Sensory symptoms and function were evaluated with validated questionnaires (Neuropathy Symptom Profile, Small Fibre Neuropathy Screenings List, PainDETECT, McGill visual analogue scale, Revised Fibromyalgia Impact Questionnaire) and quantitative sensory testing (QST) (German DNFS) respectively.
CNFL was used to stratify FMS participants into with and without SFP [SFP+ (n=14) and SFP- (n=11)]. Compared to healthy volunteers, patients with FMS had increased neuropathic pain scores in questionnaires (p<0.001) and decreased corneal nerve fibre length (CNFL) (p=0.019) indicative of small fibre pathology. When comparing SFP+ and SFP- no significant differences were observed in questionnaires or QST parameters. However, although not significant, dominant phenotype was observed in the SFP- group characterised by abnormal increase in mechanical pain sensitivity (marker of central sensitisation). These findings support that small fibre function is impaired in FMS with a subgroup of patients showing decrease in CCM parameters, but small fibre pathology alone cannot explain pain. Therefore, studies should not only investigate degenerative changes in nociceptor terminals but also understand the broader context as to how they may be sensitised in driving neuropathic pain.