Skin biopsy in the evaluation of small fiber neuropathy

Burning feet, strange tingling, or pain that flares for no obvious reason can be unsettling, especially when an ordinary examination finds nothing wrong. One explanation clinicians consider is small fiber neuropathy, a condition of the thinnest nerves in the body. A skin biopsy is one of the tests used to look for it. This page explains what the condition is, how it is usually evaluated, what the biopsy involves, and how it differs from the biopsy a dermatologist takes for a rash.

What small fiber neuropathy is

Peripheral nerves carry signals between the skin, muscles and the brain and spinal cord. Some of them are thick and fast, and they handle things like vibration and position sense. Others are very thin. These small fibers carry pain and temperature signals and also control automatic functions such as sweating. In small fiber neuropathy, those thin fibers are damaged.

A published review in the journal Diagnostics describes the condition as damage to the small A-delta and C fibers, and says the damage may follow metabolic, toxic, immune-mediated or genetic factors (Diagnostics review on skin punch biopsy). The same review notes that small fiber symptoms can be variable and inconsistent, which is one reason clinicians want an objective measure rather than relying on symptoms alone.

Symptoms as sources describe them

A column written for neurologists in Practical Neurology says patients usually report numbness, pins and needles (paresthesias) and pain that is often burning. Many cases resemble other neuropathies that begin in the feet and work upward. Some are patchy, come and go, and show a normal examination (Practical Neurology). That mismatch between how a person feels and what a clinician can see is exactly the gap that testing tries to close.

Symptoms alone cannot confirm the diagnosis. Burning or tingling has many causes, including problems that have nothing to do with small fibers, so the evaluation is a process of narrowing things down.

How the evaluation usually works

It starts with a clinical history and examination. The clinician asks where and when symptoms occur, checks sensation and reflexes, and looks for underlying conditions that are known to affect nerves. Standard nerve conduction studies often come back normal in this condition, because they mainly measure the large fibers (Practical Neurology). The Diagnostics review makes the same point, saying small fiber dysfunction is not typically picked up by tests designed for large-fiber neuropathy.

Several other tests may be added. Autonomic testing looks at functions the small fibers control. The review groups these into tests of heart rate control, blood pressure control and sweating, and lists quantitative sudomotor axon reflex testing and thermoregulatory sweat testing among the sweat tests. Quantitative sensory testing measures how a person perceives warm, cold or other stimuli; Practical Neurology calls it noninvasive but not entirely objective, and notes that it is not widely available. Both sources describe these as secondary or adjunctive options, with the skin biopsy as the test most often recommended as an objective measure.

What the skin biopsy involves

For this purpose the sample is usually a small punch biopsy, a procedure that uses a round blade to remove a tiny cylinder of skin. According to Practical Neurology, the punch is 3 mm across. The skin is numbed with an injection of local anesthetic, and the article says pain is limited to that injection and the wound needs only a bandage. Samples are usually taken from the leg. The Diagnostics review gives the lower leg, about 10 cm above the outer ankle bone, as the most common site, because that spot has the best reference data. Some clinicians take samples from the thigh as well, which helps show whether the nerve loss is greater farther from the trunk. A laboratory test guide from Mayo Clinic Laboratories describes the standard sample for this question as two 3 mm punches from the same side of the body, with the distal leg, mid-thigh, top of the foot or lower abdomen as possible sites (Mayo Clinic Laboratories).

The sample goes into fixative and is mailed to a laboratory. There, thin slices are stained with a marker called PGP 9.5 that highlights nerve fibers. The laboratory counts the fibers that cross into the outer layer of skin, the epidermis, and divides by the length of tissue measured. That number is the intraepidermal nerve fiber density, usually shortened to IENFD. It is compared with normal values for the person's age and sex. A density clearly below the expected range supports a diagnosis of small fiber neuropathy. Practical Neurology says results typically return in about two weeks.

How it differs from a biopsy for a rash

A biopsy taken to work out a rash usually samples the rash itself, and a pathologist looks at the pattern of inflammation or other changes in the skin. If that is the procedure you are curious about, our guide to skin biopsy, what to expect walks through the usual steps. Here the logic runs the other way. The skin is often normal-looking, the site is chosen from standard locations, and the question is about nerve endings rather than a skin disease. The skin is used as a window onto the nerves. Dermatologists and neurologists use the same basic tool for different questions, which is part of why how dermatologists diagnose skin conditions overlaps with neurology at the edges.

Who orders and reads the test

The sources we read are mostly written by and for neurologists, and in that literature the biopsy is part of a neuropathy work-up. A neurologist, or a neuromuscular specialist who focuses on nerve and muscle disease, commonly leads that work-up. Practical Neurology adds that any clinician can learn the procedure, which is quick and low risk, so the person who takes the sample is not always a specialist. The counting and interpretation happen in a laboratory with experience in the method; Mayo Clinic Laboratories refers to a neuromuscular pathologist, a pathologist who examines nerve and muscle tissue, and notes that poor fixation or handling of the sample can hinder interpretation. A pain specialist may also become involved when the main problem is persistent neuropathic pain, and a dermatologist may be the clinician who actually performs a biopsy. Who does what varies by country and by hospital.

Honest limits of the test

No test settles everything. Practical Neurology reports that there is no gold standard, and that with current normal values only about two-thirds of people with suspected small fiber neuropathy have an abnormal density. A normal biopsy therefore does not rule the condition out. An abnormal result also does not by itself prove that only small fibers are affected, since a mixed neuropathy has to be excluded by examination and nerve conduction studies.

The biopsy also rarely explains why the nerves are damaged. It may occasionally hint at conditions such as vasculitis or amyloidosis, but it cannot exclude them. Finding a cause usually means further history, blood tests and review of other conditions. Results are one piece of evidence that a clinician weighs with symptoms and examination, so your own doctor's interpretation matters more than a number on a report. Keeping notes on when symptoms occur, as suggested in our guide to preparing for a dermatology visit, gives any clinician a better starting point.

The short version

A small punch biopsy, usually from the leg, lets a laboratory count the thin nerve endings in the skin and compare them with normal values. It is among the more objective tools for small fiber neuropathy, but it has real limits and works best alongside a careful history and examination. Individual care varies, and your own doctor or specialist has the final word on what the results mean for you.