Small Fiber Neuropathy – Fibromyalgia
A new study published in Seminars in Arthritis & Rheumatism indicates that in patients with fibromyalgia the density of small nerve fibers in the cornea is reduced, supporting an evolving concept that fibromyalgia is a small fiber neuropathy.
Fibromyalgia syndrome is a common chronic pain disorder affecting 3% to 8% of the general population. The syndrome also occurs in 20% to 40% of patients with osteoarthritis, rheumatoid arthritis, and systemic immune disorders, such as systemic lupus erythematosus and Sjogren’s syndrome.
Despite the widely accepted notion that FM is a centralized pain condition, some investigators have questioned whether neuropathic pain is involved in the syndrome. Numbness and/or tingling are present in a majority of patients with fibromyalgia and sensory changes, including to light touch, temperature, and vibration, are common.
Traditionally, the term neuropathic pain has been reserved for disorders with demonstrable pathology in the peripheral nervous system. Until recently, such pathologic changes had not been reported in patients with the syndrome.
Fibromyalgia is also considered a stress-related syndrome that affects predominantly women.
In the past few years, however, a number of investigators have reported that fibromyalgia may be linked to a small fiber neuropathy (SFN). Rather than debating whether fibromyalgia is primarily a central or neuropathic pain disorder, clinicians may benefit more from understanding that these pain categories intersect and overlap.
Thus, recent research suggests that fibromyalgia is a small fiber neuropathy and a neuropathic syndrome affecting small sensory and sympathetic nerve fibers resulting in pain, paresthesias and autonomic dysfunction.
Thus, skin biopsies of fibromyalgia patients show a reduction in intra epidermal innervation and regeneration sparing myelinated nerve fibres (N Üçeyler et al., 2013).
The cornea receives an extremely dense small fiber innervation, and the corneal confocal bio-microscopy is a new non-invasive method that can assess nerve fiber morphology and pathology.
In the Seminars in Arthritis & Rheumatism study Manuel Ramírez and colleagues, from the National Institute of Cardiology, Mexico City, Mexico, studied the cornea’s small fiber innervation in 17 female patients diagnosed with fibromyalgia. The authors demonstrate that fibromyalgia patients have thinner/smoother corneal stromal nerves and decreased corneal sub-basal nerve plexus density when compared to healthy controls.
Patients with fibromyalgia had stromal nerve thickness of 5.0 ± 1.0 µm significantly different from that of control׳s values. Patients also had decreased sub-basal plexus nerve density per square millimeter. When controls and patients were grouped together, there was an association between stromal nerve slenderness and neuropathic pain descriptors.
These results indicate that, in fibromyalgia, the small fiber neuropathy is not limited to the skin, but also present in the cornea. The authors suggest that fibromyalgia pain is sympathetically-driven and closely associated with an autonomic/sympathetic nervous system dysfunction.
According to them, in the future, a more refined and comprehensive corneal bio-microscopy may become a non-invasive diagnostic tool for small fiber neuropathy such as fibromyalgia.
In conclusion the study indicates that women suffering from fibromyalgia have thinner corneal stromal nerves and diminished sub-basal plexus nerve density when compared to healthy controls. Nerve scarcity is associated with neuropathic pain descriptors.
Thus, small fiber neuropathy may play a role in the pathogenesis of fibromyalgia pain.
Source: Seminars in Arthritis & Rheumatism.
Read more: Seminars in Arthritis & Rheumatism
Cover Image: Confocal images of corneal subbasal nerves obtained from 2 types of confocal microscope: (A) Laser scanning confocal microscope (LSCM) and (B) Slit-scanning confocal microscope (SSCM). From: Characteristics of Corneal Subbasal Nerves in Different Age Groups: An in vivo Confocal Microscopic Analysis. By Chirapapaisan C , Thongsuwan S, Chirapapaisan N, Chonpimai P, Veeraburinon, DovePress. 24 August 2021 Volume 2021:15 Pages 3563—3572. Public domain. https://www.dovepress.com/characteristics-of-corneal-subbasal-nerves-in-different-age-groups-an–peer-reviewed-fulltext-article-OPTH
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