Causes
Small fiber neuropathy
The commonest story is the same one every time: burning feet, a normal nerve test, and the strong impression that nobody believes you. The test was not wrong. It measures the wrong fibers.
Written by the Managing Neuropathy editorial team under the direction of Dr. Tom Biernacki, DPM, FACFAS · Clinical review pending · Last updated 6 September 2026
The short answer. Small fiber neuropathy damages the thin nerve fibers that carry pain, temperature and automatic body functions. Standard nerve conduction studies measure large fibers only, so they are typically normal — a normal result rules that type of testing in, not the condition out. Diagnosis rests on the history, the examination, and where needed a skin biopsy. A treatable cause is found in roughly half of people, which is the reason to look.
Two nerve populations, one test
Peripheral nerves are a mixed cable. Large, heavily insulated fibers carry vibration, position sense and light touch, and they conduct fast. Small fibers — thinly insulated or bare — carry pain, temperature and the autonomic signals that control sweating, blood-vessel tone and gut function. They conduct slowly.
Nerve conduction studies work by timing an electrical signal along a nerve, which means they measure the fast fibers and are effectively blind to the slow ones. So a person can have substantial small fiber loss, disabling burning pain, and an entirely normal nerve conduction study and EMG. This single fact explains an enormous amount of frustration.
What it feels like
- Burning, most often in both feet, worse in the evening and at night. Why the timing works that way is here.
- Electric, stabbing or pins-and-needles sensations arriving without warning.
- Allodynia — the bedsheet, a sock seam or a light touch reading as pain.
- Temperature confusion: feet that feel hot and are cold to touch, or the reverse.
- Autonomic symptoms: skin on the feet that has stopped sweating and become dry and cracked, dry eyes and mouth, dizziness on standing, palpitations, digestive change, bladder change.
Strength, reflexes and balance are usually preserved early, because those depend on large fibers. Weakness is not a feature of pure small fiber disease, and its presence points somewhere else.
Most cases are length-dependent — feet first, ascending slowly, symmetric. A minority are not: patchy, or involving the face, trunk or arms early. That non-length-dependent pattern raises the likelihood of an immune-mediated cause and is worth pointing out to whoever is assessing you.
How it is actually diagnosed
A clinician can test pinprick and temperature at the bedside — small-fiber functions. Reduced pinprick sensation with normal vibration and normal reflexes is the classic combination.
A small punch of skin, usually from the lower leg, examined for the density of nerve fibers in the epidermis. This is the reference test, and it measures the fibers in question directly.
Sudomotor testing and quantitative sensory testing are used in some centers to add supporting evidence.
This is where the value is, because roughly half of cases have an identifiable and sometimes treatable cause.
The causes worth chasing
Glucose, first and always. Diabetes, prediabetes and impaired glucose tolerance are the single largest identified group, and small fiber damage can precede a diabetes diagnosis by years. A normal fasting glucose does not settle it; an A1c and, in some cases, a glucose tolerance test are what settle it.
Vitamin B12 deficiency, and its mirror image, vitamin B6 excess — the latter usually from a supplement bought to help nerves. The label problem is explained here.
Thyroid disease and kidney disease, both ordinary blood tests.
Autoimmune conditions. Sjögren’s syndrome is the one most consistently linked, and dry eyes and dry mouth alongside burning feet is a combination worth mentioning out loud. Coeliac disease, sarcoidosis and lupus also appear.
Infections — HIV and hepatitis C among them.
Alcohol, and several chemotherapy agents, which commonly injure small fibers first.
Inherited causes. Uncommon but important, because a few are specifically treatable — Fabry disease and hereditary amyloidosis are the examples usually named — and because variants in sodium-channel genes are found in a subset of otherwise unexplained cases.
And in a substantial share, no cause is found. Idiopathic small fiber neuropathy is a real diagnosis. It is a reasonable conclusion after a search, and an unreasonable substitute for one.
Why the diagnosis is worth having
People sometimes ask what changes if the biopsy is positive, given there is no treatment aimed at the fibers themselves. Three things change.
It redirects the search toward causes that are found on blood tests and are sometimes reversible. It legitimises neuropathic pain treatment, since the medicines that work here are different from ordinary painkillers and are prescribed for a specific problem. And it ends the search for a psychological explanation — which, for many people, is the part that has done the most damage.
What treatment looks like
Treat the cause where one exists. Glucose control, B12 replacement, stopping a supplement carrying high-dose B6, treating thyroid disease, managing an autoimmune condition. This is the only route to genuine improvement in fiber health.
Treat the pain on its own terms. Nerve pain responds poorly to paracetamol and anti-inflammatories and reasonably well to medicines that act on nerve signalling. The options and their evidence are here.
Protect the feet anyway. Small fiber loss reduces temperature and pain sensing, which is exactly the warning system that prevents burns and unnoticed injuries — even while touch and vibration remain normal. The heating pad is as dangerous here as in any other neuropathy. The daily check applies.
Immune-directed treatment such as immunoglobulin is used in some centers for cases with an autoimmune basis. The evidence is not settled, and it belongs to a neurologist with the full picture rather than to a general recommendation.
Common questions
Is small fiber neuropathy serious?
It is rarely disabling in the way large fiber disease can be — weakness and falls are not typical features. It can be genuinely miserable, it can reduce the protective sensation that prevents foot injury, and it can be the first sign of a condition worth finding. Serious in the sense of urgent, usually not. Worth investigating, always.
Does it progress to full neuropathy?
Sometimes large fibers become involved over years, particularly where the cause continues unchecked. Many people remain small-fiber-predominant indefinitely.
Can it be reversed?
Where a cause is found and corrected early, fiber density can improve — this has been observed with glucose control and with correcting deficiencies. Where nothing correctable is found, the realistic goal is symptom control.
Is this the same as fibromyalgia?
They are different diagnoses, and there is a documented overlap: a subset of people diagnosed with fibromyalgia are found to have reduced small nerve fiber density on biopsy. If you carry a fibromyalgia diagnosis and your symptoms are concentrated in the feet with burning and temperature change, it is a fair question to raise.
Which specialist should I see?
A neurologist for the diagnostic testing and unusual patterns. A podiatrist for what the loss of temperature and pain sensing means for the safety of your feet, and for footwear and skin care.
Related reading
Burning feet at night · Neuropathy in the feet: the full picture · Numb toes: reading the pattern · Supplements and the B6 problem · Treatment options
Sources and further reading: National Institute of Neurological Disorders and Stroke (NINDS), peripheral neuropathy. American Academy of Neurology guidance on the evaluation of distal symmetric polyneuropathy, including the role of glucose testing, B12 and skin biopsy. American Diabetes Association Standards of Care, neuropathy section. National Institutes of Health Office of Dietary Supplements, vitamin B6 and vitamin B12.
This page is general education and is not a diagnosis, a treatment plan, or advice to start or stop any medicine. Clinical review by Dr. Biernacki is pending; the review date will be shown here once complete.
For our Michigan neighbors
Told your nerve
test was normal?
Pinprick, temperature and protective sensation can be examined directly, and the correctable causes can be checked. Dr. Biernacki’s Michigan practice can start that.
Everywhere else, take this page to your own clinician.
Dr. Tom Biernacki, DPM, FACFAS is a double board-certified foot and ankle surgeon (ABFAS and ABPM) and a Fellow of the American College of Foot & Ankle Surgeons. He is the founder of Balance Foot & Ankle in Howell and Bloomfield Township, Michigan, where peripheral neuropathy, diabetic foot risk and loss of protective sensation are part of his weekly clinic. His patient education videos have drawn more than 950,000 subscribers on YouTube. He writes Managing Neuropathy to put the explanation he gives in the exam room in front of the people who cannot get to one — including when a product is worth buying and when it is not.