Background The pathophysiology of fibromyalgia syndrome (FMS) remains unclear, but the current predominant hypothesis is that FMS arises from centralized hypersensitivity to pain. Recently, some studies have suggested that a subset of FMS patients may have underlying small fiber peripheral neuropathy (SFPN), a selective degeneration of the cold/pressure and nociceptive nerve fibers (Oaklander et al., 2013). This discovery has challenged the completeness of the “central hypersensitivity” hypothesis.
Objectives Here, we describe a cohort of FMS and/or diffuse unexplained pain patients who have undergone skin biopsy as part of their workup. Our goal was to identify whether these patients have underlying SFPN, and if so, whether the SFPN is idiopathic or secondary to another predisposing condition.
Methods This study is a retrospective review of patients from our medical center. Patients were diagnosed using the 2011 American College of Rheumatology FMS diagnostic criteria. Additionally, a number of patients presented with diffuse, unexplained pain which carried many of the qualities of FMS, but were not formally diagnosed with the disorder. 172 patients have a diagnosis of either FMS or diffuse pain. Of these, 66 had tingling, burning, or numbness for which they underwent skin biopsy.
These patients underwent workup for etiologies of neuropathic pain including fasting glucose, hemoglobin A1C, thyroid-stimulating hormone, vitamin D level, serum protein electrophoresis, hepatitis C, human immunodeficiency virus (HIV), and obstructive sleep apnea (OSA). Additionally, they underwent assessment to search for other conditions which might explain their pain. Patients were referred for skin biopsy whenever possible. Biopsies were analyzed using protein gene product 9.5 (PGP 9.5) immunostaining to determine the epidermal nerve fiber density (EFND). Compared to an internal set of normal controls, the bottom 5 percentile of EFND was reported as diagnostic of SFPN.
Results Overall, 43 patients (65%) had the diagnosis of FMS, while 23 patients (35%) had the diagnosis of diffuse pain. 13 patients were found to have no predisposing condition – their pain syndrome was considered to be idiopathic. 53 were found to have at least one predisposing disease.
Within the group undergoing skin biopsy that did not have predisposing factors for SFPN, 62% were found to have SFPN. Within the group of patients with an identifiable predisposing condition, 72% were found to have SFPN. Overall prevalence of SFPN was 70%. The average age at symptom onset was 35 years old (range 9 to 64 years), and the cohort was 94% female, 6% male. Prevalence of SFPN-predisposing conditions is listed on the table below.
Conclusions In our experience, SFPN is highly prevalent among patients who were diagnosed with FMS or diffuse unexplained pain.
Oaklander, A. L., Herzog, Z. D., Downs, H. M., & Klein, M. M. (2013). Objective evidence that small-fiber polyneuropathy underlies some illnesses currently labeled as fibromyalgia. Pain, 154(11), 2310–2316.
Disclosure of Interest None declared