By: Dr Alex Robber
Can small fiber neuropathy (SFN) mimic fibromyalgia?
May small neuropathy to fibers (SFN) mimic fibromyalgia? The simple response is yes, but that’s more complicated. So why does a rheumatologist think about the neuropathy of small fibers? The response to that is quite easy. A rheumatologist for fibromyalgia, a disease that causes chronic pain, brain fog, non-restorative sleep, and many other mysterious symptoms such as headaches, is referred to by many people.
Although fibromyalgia is NOT an autoimmune disease, the occurrence of small fiber neuropathy may be very similar, but CAN BE triggered by autoimmune conditions. There is a lot of controversy about fibromyalgia in the medical community. Some group thinks it is a separate entity, some don’t believe in its presence and some people are in the center somewhere.
Personally, I think that fibromyalgia potentially represents a lot of diseases that we have not yet identified. We’re going to have a hard time understanding them until we can categorize them as distinct entities, let alone identifying effective treatments.
In some cases, SFPN may really be the cause of fibromyalgia.
We’ve all been advised that fibromyalgia is an exclusion disorder but how completely did your doctor rule out other disorders that resemble fibromyalgia? I’m sure if we were lucky, we’ve all been screened for rheumatoid arthritis, lupus, hypothyroidism, anemia and perhaps Lyme disease. But I bet most of our doctors have missed one test: a biopsy of the skin for polyneuropathy with small fibers (SFPN).
SFPN is a neurological condition that causes severe chronic pain and affects body control, such as heart rate, blood pressure and sweating. Multiple studies in patients with fibromyalgia have confirmed SFPN is extremely common. Yet Anne Louise Oaklander, associate professor of neurology at Harvard Medical School and director of the Nerve Unit at Massachusetts General Hospital, does not think that SFPN is just a co-morbidity of fibromyalgia; her research suggests that SFPN in some patients may actually be the cause of fibromyalgia.
In 2013, Oaklander released a study that showed that 41 percent of patients with fibromyalgia tested positive for SFPN using a skin biopsy versus only 3 percent of healthy controls. Fibromyalgia patients with SFPN were then checked for various causes of the neurological condition and Oaklander found an interesting trend: immune system dysfunction was the most common cause of SFPN in patients with fibromyalgia.
For some patients with fibromyalgia, these findings could be a game changer because SFPN can be treated and, in some cases, cured. “It gives some of the first objective evidence of a mechanism behind some fibromyalgia cases,” said Oaklander in a press release. “The first step to finding better therapies is to recognize an underlying cause.”
What is Small Fiber Neuropathy?
The consequence of small fiber neuropathy is damage to the small, unmyelinated nerve fibers that convey pain and temperature and regulate autonomic functions such as sweating. Many of the symptoms caused by SFN include the following:
- Burning pain
- Numbness and tingling
- Pain that is out of proportion
- Unexplained itching
- Lack of sweating
- Temperature dysregulation
How to diagnose small fiber neuropathy?
The first step in diagnosing small fiber neuropathy is to take a history of treatment, review risk factors and do a thorough physical examination. Deep tendon reflexes (e.g., knee jerk reflex) are common on physical examination, and there is no lack of energy.
If you are suspected of having SFN your doctor can give you electrodiagnostic tests (EMGs). These are also measuring of nerve conduction, in colloquial terms. Limited fiber neuropathy affects limited A-delta myelinated and unmyelinated C fibres, NOT big fibres. It means that EMGs are usually negative, as they are good for finding problems that affect large fibers such as carpal tunnel syndrome.
A skin biopsy, more specifically epidermal nerve fiber density testing (ENFD), is one way to diagnose small fiber neuropathy. This technique allows for the direct visualization, quantification and morphological examination of the small sensory fibers that innervate the skin.
This procedure has an 88 percent sensitivity and a 95–97 percent accuracy. In layman’s terms, the test could miss 12 percent of small fiber neuropathy cases, but if the test is positive, there’s a 3-5 percent chance it’s a (false positive) error.
These are pretty good standards, really. A study by the European Neurological Societies Federation notes that ENFD is a reliable and effective evaluation method for SFN.
SFPN can cause a range of symptoms reported by fibromyalgia patients
SFPN may trigger a range of symptoms identified by patients with fibromyalgia, including multiple pain signs, chronic fatigue, digestive problems, brain fog, headaches and many others. Many of these fibro-patients I’ve seen may have gone over the years to see dozens of different physicians, “Oaklander said in an interview.
“What happens is that [medical providers] haven’t focused on finding what’s considered a diagnosis that unifies. Then, they’re asking the patient,’ Yeah, you have irritable intestines. Ah, you have cystitis of an interstitial type. Yeah, you got fibro. Oh, you’ve got chronic fatigue.’ They’re not saying,’ Wait a minute, how possible is it that one person would have six different problems that aren’t in any way related?
Since these [small-fibre] nerves go throughout the body, one disease can cause many, many different symptoms. Medical providers] looked at this from their own medical specialty point of view and could not see the big picture. The large picture of small-fiber polyneuropathy is that it can describe several different symptoms.
Common causes of small fiber neuropathy
When the doctor makes a diagnosis of small fiber neuropathy, the question then is whether an underlying cause is present. Approximately 50 per cent of cases of small fiber neuropathy are idiopathic, which means that doctors cannot identify an underlying cause. As a result, the other 50 percent leaves us.
Of those cases, the diabetes mellitus is the most common cause. Autoimmune diseases actually make up a relatively small proportion of cases, so it’s important to look first for other causes. MANY other causes exist, but these are some of the more common conditions that may cause minor fiber neuropathy.
- Gupta D, Harney J. Small fiber neuropathy demonstrated in pain syndromes. Poster session presented at Annual Meeting of the American Academy of Neurology; 2010 Apr 10-17; Toronto, Ontario.
- Small Fiber Neuropathy in Fibromyalgia and CFS by Adriene Dellwo via Verywell Health