Small Fiber Neuropathy: When the Burning Is There but the Tests Are Normal

TL;DR: Small fiber neuropathy (SFN) is damage to the smallest nerves in the body, the ones that carry pain, temperature, and the automatic signals that run your heart rate, gut, and sweat. It shows up as burning, tingling, or prickling, often starting in the feet and worse at night, and sometimes as lightheadedness or digestive and sweating changes. Standard nerve tests check the large fibers and come back normal in SFN, which is why so many people are told nothing is wrong. The diagnosis takes a skin biopsy or autonomic testing, and the most important step is finding the treatable cause underneath.

If you have burning or tingling that is undeniably there, and a nerve test that came back normal, and a sense that you were not quite believed, I want to start by saying: I believe you. Small nerve damage is one of the conditions that hides from the usual tests, so a normal result does not mean nothing is happening. It often means the wrong nerves were measured. Once you understand which nerves are involved and why, both the diagnosis and the plan get much clearer.

What is small fiber neuropathy?

Small fiber neuropathy is damage to the small nerve fibers, the thinly insulated and bare nerve endings that sense pain and temperature and carry the autonomic signals your body runs without thinking.² Because those fibers do two jobs, sensory and autonomic, the symptoms come in two flavors that often overlap.

The sensory symptoms are the familiar ones: burning, tingling, prickling, an electric or stabbing quality, and sometimes pain from light touch like a sock or a bedsheet. They usually begin in the feet and move upward over time, worse in the evening, though in some people the pattern is patchy and spread across the body rather than feet-first. The autonomic symptoms are less obvious and easy to miss: lightheadedness on standing, a racing heart, dry eyes and mouth, changes in sweating, and gut symptoms like early fullness or irregular digestion. When these travel together, small fiber neuropathy sits in the same neighborhood as POTS and dysautonomia, which is why the two are often evaluated together.

Why do normal nerve tests miss it?

Normal nerve tests miss small fiber neuropathy because the standard tests are built to measure different nerves. Nerve conduction studies (NCS) and electromyography (EMG) assess the large, heavily insulated fibers that control strength and vibration sense and fast reflexes. Small fiber neuropathy, by definition, affects the small fibers those tests do not read, so the results come back normal even when the small nerves are clearly damaged.

That mismatch is the single biggest reason people go years without an answer. A normal NCS or EMG gets read as "your nerves are fine," when the honest interpretation is "your large nerves are fine, and we have not yet tested the small ones." The tests that do reveal small fiber neuropathy are different: a skin punch biopsy that counts the tiny nerve endings in a small sample of skin, and autonomic tests like the quantitative sudomotor axon reflex test (QSART) that measure the sweat and heart-rate reflexes those fibers control.³ Knowing which test to order is often the whole difference between a dead end and a diagnosis.

What causes small fiber neuropathy?

Small fiber neuropathy is a pattern of damage with many possible causes, and finding the cause is what changes the outcome, since several of the common ones are treatable. The main groups are:

• Metabolic. Diabetes, prediabetes, and insulin resistance are the most common drivers, and small fiber damage can begin in the prediabetic range, before blood sugar is high enough for a diabetes diagnosis.
• Autoimmune. Sjogren's disease, celiac disease, sarcoidosis, lupus, and other immune conditions can attack the small fibers, sometimes as the first sign of the underlying disease.
• Nutritional. Vitamin B12 deficiency is a classic and correctable cause, and it links small fiber neuropathy to conditions like autoimmune gastritis that silently lower B12.
• Thyroid disease. An underactive or autoimmune thyroid can contribute.
• Post-viral illness. Small fiber neuropathy is an increasingly recognized part of long COVID and other post-infectious syndromes.⁴
• Genetic and idiopathic. Some cases trace to inherited changes in nerve sodium channels, and in a meaningful share of people no cause is found even after a thorough search.

Because the treatable causes are common, a careful workup is worth doing rather than settling for "idiopathic" too early.

How is small fiber neuropathy diagnosed?

Small fiber neuropathy is diagnosed by combining the right nerve testing with a search for the cause. The confirming tests target the small fibers directly: a skin punch biopsy from the lower leg measures the density of small nerve endings in the skin (intraepidermal nerve fiber density), which is the most established test, and autonomic testing such as QSART assesses the sweat and cardiovascular reflexes those fibers run.¹ A normal NCS or EMG does not rule the condition out, and in fact a normal large-fiber study alongside typical symptoms is part of the classic picture.

The second half of the diagnosis is the cause workup, because naming small fiber neuropathy is only useful if it points to something you can act on. That means checking glucose and insulin sensitivity (including a glucose tolerance test when the fasting numbers look fine), vitamin B12, thyroid function, and autoimmune markers, and considering a post-viral trigger. The workup is a search for the treatable thread, run in a sensible order rather than all at once.

How does Fishtown Medicine approach small fiber neuropathy?

At Fishtown Medicine, small fiber neuropathy is treated as a findable condition rather than a symptom to dismiss. The first job is to believe the symptoms and get the right nerves tested. For the confirming tests that are procedures, the skin biopsy and autonomic testing, we bring in highly qualified specialists who are in network for you, and we stay in the case, comparing notes with a network of specialists so the answer comes back right. That active coordination is a big part of how Fishtown Medicine manages problems more complex than standard primary care usually takes on, often getting you an expert opinion without a separate extra workup. From there, the work centers on the cause, because the biggest wins come from treating what is driving the nerve damage: tightening glucose and metabolic control through the Five Foundations, correcting a B12 deficiency, and evaluating and treating an autoimmune or post-viral driver.

Alongside the root-cause work, we address the day-to-day symptoms so life is livable while the underlying plan takes hold, using established options for neuropathic pain and support for the autonomic symptoms when they are present. Because small fiber neuropathy so often travels with POTS, MCAS, and connective tissue overlap, we look at the whole cluster rather than one symptom at a time. Whether you are nearby in Fishtown or Port Richmond, or coming across the Ben Franklin Bridge from Cherry Hill or Voorhees, the goal is to name what is happening, treat the cause, and take the burning seriously.

Guidance from the Clinic

"The hardest part of small fiber neuropathy is that people get told their nerves are fine when they know something is wrong. What that normal test usually means is that we checked the big nerves and not the small ones. When I hear burning feet at night, or tingling with lightheadedness and gut changes, I want to test the small fibers directly and then go looking for why, because a lot of the causes, blood sugar, B12, an autoimmune process, are things we can treat."

Related at Fishtown Medicine

• The POTS and Dysautonomia Strategy - the autonomic cluster small fiber neuropathy often joins
• The Long COVID Strategy - post-viral small fiber neuropathy and its workup
• Understanding Insulin Resistance - the most common metabolic driver
• Autoimmune Gastritis: When Low Iron Won't Resolve - a quiet cause of the B12 deficiency behind some cases
• High CRP: What an Elevated Inflammation Marker Means - the inflammation that can accompany an autoimmune cause
• Connecting Your Own Dots: POTS, MCAS, and hEDS - the overlap cluster

Scientific References

1. Devigili G, Tugnoli V, Penza P, et al. "The diagnostic criteria for small fibre neuropathy: from symptoms to neuropathology." Brain. 2008;131(Pt 7):1912-1925.
2. Terkelsen AJ, Karlsson P, Lauria G, et al. "The diagnostic challenge of small fibre neuropathy: clinical presentations, evaluations, and causes." Lancet Neurology. 2017;16(11):934-944.
3. Lauria G, Hsieh ST, Johansson O, et al. "European Federation of Neurological Societies and Peripheral Nerve Society guideline on the use of skin biopsy in the diagnosis of small fiber neuropathy." European Journal of Neurology. 2010;17(7):903-912.
4. Oaklander AL, Mills AJ, Kelley M, et al. "Peripheral Neuropathy Evaluations of Patients With Prolonged Long COVID." Neurology: Neuroimmunology and Neuroinflammation. 2022;9(3):e1146.
5. Sene D. "Small fiber neuropathy: Diagnosis, causes, and treatment." Joint Bone Spine. 2018;85(5):553-559.