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Are Both Small AND Large Fiber Neuropathy Present in Fibromyalgia?

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Reprinted with the kind permission of Cort Johnson and Health Rising.

We report here a surprisingly high prevalence of clinical and electrodiagnostic (EDX) abnormalities implying a demyelinating polyneuropathy, suggestive of chronic inflammatory demyelinating polyneuropathy (CIDP), in FMS. Caro et. al 2008

With three studies finding evidence of small fiber neuropathy in significant numbers of people with Fibromyalgia, the skin has become of increasing interest. (Small fiber neuropathy refers to damage to the small nerve fibers in the skin. Skin biopsies are used to determine if SFN is present).

Caro was the latest researcher to find evidence of nerve problems in FM and it turns out, he was the first as well.

From Immune Problems in the Skin

Science often moves slowly and it has in this case as well. Thirty years ago in 1984 Caro found evidence of an autoimmune process – IgG antibodies in the dermal-epidermal junction – in the skin of people with FM. Two years later he validated his findings in a blinded study. That same year Dineman found increased prevalence of Raynaud’s phenomenom, dry mouth, low complement, ANA and IgG antibodies in the skin as well. Something unusual appeared to be occurring in the skin of people with FM but progress defining it was slow.

To the Nerves….

In 1988, Sim’s study “Symptoms mimicking neurological disorders in Fibromyalgia” put a focus on the nerves. Caro reported on nerve conduction tests in FM in 2005 and then in 2008 provided the first evidence of peripheral neuropathy.

Small fiber neuropathy is kind of the rage in FM right now, but Caro’s 2008 study suggested that larger nerve neuropathy (demyelination) was also present. In this article, we take a look at that 2008 study containing 58 Fibromyalgia patients.

The Study

X. J. Caro, E. F. Winter and A. J. Dumas. A subset of fibromyalgia patients have findings suggestive of chronic inflammatory demyelinating polyneuropathy and appear to respond to IVIg. Rheumatology 2008; 47;208–211

A ‘demyelinating polyneuropathy’ was reported present when EMG and nerve conduction studies revealed nerve conduction speeds two (and often three) standard deviations below normal in two or more nerves. (Nerve signals travel more slowly in demyelinated nerves.). Caro, et. al followed standards proposed by the Inflammatory Neuropathy Cause and Treatment (INCAT) group for demyelinating polyneuropathy.

The Findings

With 76% of patients reporting paresthesias (tingling), 88% percent reporting hypaesthesia (loss of sensation – with increased loss in the lower extremities) and 90% reporting weakness, the symptom presentation in FM suggested nerve damage was present. (The ‘stocking distribution’ of nerve symptoms which involves worsening symptoms the further down the limbs one goes is highly emblematic of peripheral nerve disorders.)

Greatly increased rates of nerve symptoms as well as muscle weakness in people with FM (90%) compared to people with rheumatoid arthritis (13%) suggested FM was a far different type of disease than it’s rheumatological counterpart. (Martinez-Lavin found that 95% of FM patients as opposed to 30% of RA patients reported sensory symptoms as well).

Nerve conduction studies revealed a polyneuropathy was present in almost 50% of their FM patients. Seventy percent of the FM patients with polyneuropathy had evidence of demyelination. (Note that demyelination refers to large nerve fibers; small nerve fibers are not myelinated.) Sural nerve biopsies suggested myelin injury (segmental de and re-myelination and/or myelinated nerve fiber dropout) had occurred in the majority of those tested. (They revealed no vasculitis or amyloidosis and provided little evidence of what was causing the demyelination.)

Chronic Inflammatory Demyelinating Polyneuropathy

Comparing FM to chronic inflammatory demyelinating polyneuropathy (CIDP), Caro put FM squarely in the realm of an immune mediated neurological disorder.

CIDP is an inflammatory disorder affecting the peripheral (as opposed to the central) nervous system. Considered a chronic form of Guillain-Barre disease, CIDP is characterized by the loss of the myelin sheath in the larger nerves in the body.

The symptom presentation is quite heterogeneous and includes weakness, fatigue, numbness, tingling, pain, difficulty in walking, burning pain in the extremities, and sudden onset back or neck pain radiating down to the arms and /or legs. Loss of deep tendon reflexes (rarely increased or normal), loss of muscle mass and fasciculations (twitching) may be present. Autonomic dysfunction can cause problems standing (orthostatic intolerance, bladder, bowel and/or cardiac problems).

Left untreated about 30% of CIPD patients will end up in a wheelchair.

FM patients were determined to have a CIDP-like illness when they exhibited lower extremity stocking hypaesthesia (loss of sensation), muscle weakness in at least two extremities, and evidence of a demyelinating polyneuropathy.

Following Latov’s proposal that “most acquired demyelinating neuropathies of otherwise unknown etiology are considered to be a form of CIDP’, Caro and Winter proposed that a significant subset of FM patients have a form of CIDP.

CIDP is another disorder with decided overlaps with ME/CFS and FM. As with ME/CFS, the lack of agreed upon clinical criteria have thwarted clinical trials. It’s widely acknowledged that the criteria often used to diagnose CIDP misses many leaving many untreated (including ME/CFS and FM patients????)


Treatments include steroids, IVIG and immunosuppressive drugs such as Rituximab. Suspecting that an immune mediated nervous system disorder similar to CIPD was present Caro started IVIG treatment in 15 patients. A ‘large percentage’ reported significant improvements in pain, tender points and strength.


Since Caro’s 2008 report indicating reduced nerve conduction and possibly demyelination of the large nerves in the body, Nacir, et. al found widespread evidence of decreased nerve conduction velocity in the upper bodies of FM patients.


This study and others suggest that some people with FM could have both small and large fiber neuropathies. If you have symptoms similar to those described above, nerve conduction tests (for the large fiber neuropathy) and skin biopsies (for the small) may be warranted.

Caro and others propose that the neuropathies in Fibromyalgia are probably immune mediated and propose immune therapies for them.


About the Author:  Cort Johnson has had ME/CFS for over 30 years. The founder of Phoenix Rising and Health Rising, Cort has contributed hundreds of blogs on chronic fatigue syndrome, fibromyalgia and their allied disorders over the past 10 years. Find more of Cort’s and other bloggers’ work at Health Rising.

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One thought on “Are Both Small AND Large Fiber Neuropathy Present in Fibromyalgia?”

  1. IanH says:

    Bear in mind that carpal tunnel syndrome (nerve conduction studies) is a poorly defined pathology and surgery has very low efficacy. We have found that people with FM who have had carpal tunnel release surgery are sometimes made worse and none have had any relief. Some have their Raynauds type symptoms made worse. I think, generally that neuro/orthopaedic surgeons are very wary of suggesting carpal tunnel release surgery for people with FM where conduction studies + Raynauds is present.

    It is the conduction studies that suggest a carpal tunnel problem but if you do conduction studies in other areas than the arm-hand axis you will find the same retardation of signals in FM patients.

    And this all suggests an immune mediated pathology of which neuropathy is a consequence.

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