Fibromyalgia diagnosis may be confounded by small fiber neuropathy
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Small fiber neuropathy, a pain-inducing condition that resembles and overlaps with fibromyalgia, may be significantly under-diagnosed in the U.S., according to a presenter at the 2021 Congress of Clinical Rheumatology-West.
“The etiology and pathophysiology of fibromyalgia is not completely understood,” Khosro Farhad, MD, assistant professor of neurology at Massachusetts General Hospital, said in his presentation.
That said, Farhad noted that pain sensitivity in fibromyalgia has been shown to be associated with altered pain processing and modulation in the central nervous system. “There are also studies reporting abnormalities in the peripheral nervous system, which could also explain the pain,” he said. These abnormalities are seen in small nerve fiber function.
A body of data over the last decade or more has shown that abnormalities in small nerve fibers may be impacting a small but important proportion of fibromyalgia patients.
One study conducted in the Netherlands demonstrated a prevalence rate of 53 in 100,000 individuals had some type of small fiber neuropathy, according to Farhad. “If we assume that the U.S. has the same prevalence, we expect to have 170,000 patients with a small fiber neuropathy here,” he said.
In addition, Farhad noted that there was some evidence of small fiber pathology in 40% of patients with fibromyalgia. “If we look at the prevalence of 2% [for fibromyalgia], we expect to have 2.5 million patients in the U.S., many of them undiagnosed,” he said.
There are many unknown factors surrounding small fiber neuropathy, including the genetic factors, the autoantibodies or other autoimmune diseases that may be involved.
In the absence of a clear understanding of what is happening, the optimal way to manage small fiber neuropathy is to simply treat the symptoms. “There is a long list of labs and tests that we can do, but we do not do a million-dollar workup for everyone,” Farhad said.
A clinical exam with some lab work is often sufficient to determine the presence of small fiber neuropathy and initiate a treatment program. “The best way of treating is to address the cause, if one can be found,” Farhad said. “But many times, we cannot find the cause.”
Neuropathic pain medications, anti-depressants, anti-epileptic medications or sodium channel blockers have been used to some degree of success, according to Farhad. “These are not the best options for long-term treatment, but some patients need them,” he said.
Regarding experimental or off-label treatments, Farhad suggested that immunotherapy has been used with up to 60% response rate. In addition, steroids and intravenous immunoglobulin (IVIG) have been used to some degree of efficacy. However, given the cost and difficulty of production for IVIG, along with shortages caused by the COVID-19 pandemic, use of this approach should be judicious.
Farhad ultimately encouraged rheumatologists to keep small fiber neuropathy on the radar for diagnosis when working up patients for fibromyalgia. While treatment paradigms are far from perfect, he believes that finding the cause and addressing the pain are ultimately more beneficial strategies than misdiagnosis. “Many of these patients get a diagnosis of psychiatric disease,” he said.
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Farhad K. Implications of Small Fiber Neuropathy in Fibromyalgia. Presented at: Congress of Clinical Rheumatology-West annual symposium; September 18-21, 2021 (hybrid meeting).
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