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PubMed This is a summary of 18 peer-reviewed journal articles Updated
Neurology · Small Fiber Neuropathy

Diagnostic Testing: Making Sense of Skin Biopsies, QSART, and Normal EMGs

At a Glance

Standard nerve tests like EMGs cannot detect small fiber neuropathy (SFN) because they only measure large nerves. To diagnose SFN, doctors use specialized tests like a skin punch biopsy to measure nerve fiber density and QSART tests to check autonomic nerves.

If your standard neurological tests have come back “normal” despite your pain, it is because those tests were looking at the wrong nerves. Traditional tests like Electromyography (EMG) and Nerve Conduction Studies (NCS) act like a telescope looking for large planets; they are excellent at seeing the “large-fiber” nerves that control your muscles [1]. However, they are completely blind to the “small-fiber” nerves (A-delta and C fibers) that carry pain and temperature signals [2].

To see these tiny fibers, doctors must use specialized diagnostic tools that look specifically at the skin and autonomic functions.

The Skin Punch Biopsy (IENFD)

The most direct way to diagnose SFN is a Skin Punch Biopsy [3]. This is a minimally invasive procedure where a doctor takes a tiny, 3-millimeter sample of skin—roughly the size of a matchstick head—usually from the ankle and the thigh [4][5].

The biopsy site typically heals like a minor scrape and rarely requires stitches. The sample is sent to a lab where it is treated with a special stain called PGP 9.5, which makes the nerve fibers visible under a microscope [6]. The pathologist then calculates the Intraepidermal Nerve Fiber Density (IENFD), which is simply the number of nerve endings per millimeter of skin [7].

  • What it reveals: If your fiber count is lower than the expected range for your age and sex, it provides objective, physical proof of nerve loss [8].
  • The Pattern: By comparing the ankle to the thigh, doctors can see if the damage is “length-dependent” (worse at the ankle), which is a classic sign of SFN [9].

Testing the “Automatic” Nerves (QSART)

Some small fibers control your autonomic system—things your body does without you thinking, like sweating and maintaining blood pressure. The Quantitative Sudomotor Axon Reflex Test (QSART) is used to test these specific fibers [10].

  • The Procedure: Small capsules are placed on your skin, and a mild electrical current is used to move a chemical (acetylcholine) into the skin to stimulate your sweat glands [11].
  • The Result: The test measures how much you sweat in response. A reduced or absent sweat response is objective evidence that the small autonomic nerves are not working correctly [12].

Diagnostic Criteria: When is it SFN?

Because no single test is perfect, doctors use a “triad” of information to confirm a diagnosis [13][14]:

  1. Clinical Symptoms: You have the characteristic burning, tingling, or autonomic symptoms [15].
  2. Normal Large-Fiber Tests: Your EMG and NCS are normal, ruling out large-fiber neuropathy [16].
  3. Objective Small-Fiber Proof: You have an abnormal result on a skin biopsy (IENFD), a QSART, or specialized sensory testing [15][17].

Checklist: Reading Your Lab Report

When you receive your results, ensure the following information is included to confirm a thorough evaluation:

  • [ ] Density Count: A specific number representing nerve fibers per millimeter (e.g., 4.2 fibers/mm).
  • [ ] Normative Comparison: A statement comparing your count to a “normal” range based on your age and sex [8].
  • [ ] Biopsy Sites: Confirmation that samples were taken from at least two locations (usually the distal leg and proximal thigh) [18].
  • [ ] Staining Method: Mention of PGP 9.5 or immunohistochemistry [5].
  • [ ] Pathologist Interpretation: A clear conclusion stating whether the density is “reduced” or “abnormal.”

Return to Overview

Common questions in this guide

Why was my EMG normal if I have severe nerve pain?
Standard tests like EMGs and nerve conduction studies only look at large-fiber nerves that control muscles. They cannot detect damage to the tiny small-fiber nerves that carry pain and temperature signals, which is why your results may appear normal despite experiencing pain.
How does a skin punch biopsy diagnose small fiber neuropathy?
A doctor takes a tiny skin sample, usually from the ankle and thigh, to count the number of nerve endings under a microscope. If your nerve fiber density is lower than the normal range for your age and sex, it provides physical proof of small fiber nerve damage.
Why are skin biopsies taken from both the ankle and the thigh?
Taking samples from two different locations helps doctors determine if the nerve damage is worse in the lower extremities compared to the upper legs. This specific 'length-dependent' pattern is a classic indicator of small fiber neuropathy.
What is a QSART test and why do I need it?
The Quantitative Sudomotor Axon Reflex Test (QSART) measures how much you sweat in response to a mild electrical stimulus. It is used to check for damage to the autonomic small fibers, which control involuntary bodily functions like sweating and blood pressure.
What does IENFD mean on my neuropathy lab report?
IENFD stands for Intraepidermal Nerve Fiber Density. It is the specific number of nerve endings per millimeter found in your skin biopsy sample. A reduced IENFD count compared to healthy individuals of your age and sex indicates nerve loss.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Since my EMG and NCS were normal, can we proceed with a skin punch biopsy to look at my small nerve fibers?
  2. 2.Will the skin biopsy be taken from both my ankle and my thigh to check for a length-dependent pattern?
  3. 3.Can we order a QSART to see if my autonomic symptoms, like dizziness and sweating changes, are linked to nerve damage?
  4. 4.How do my IENFD results compare to the normal range for someone of my age and sex?
  5. 5.Does my biopsy report mention the use of PGP 9.5 staining to ensure the fiber count is accurate?

Questions For You

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Related questions

Can You Have Small Fiber Neuropathy With a Normal EMG?Does Small Fiber Neuropathy Spread From Feet to Hands?What Is a QSART Sweat Test for Small Fiber Neuropathy?What Happens During a Small Fiber Neuropathy Skin Biopsy?

References

References (18)
  1. 1

    Electrodiagnostic Testing of Small Fiber Neuropathies: A Review of Existing Guidelines.

    Casanova-Molla J, Valls-Solé J

    Journal of clinical neurophysiology : official publication of the American Electroencephalographic Society 2020; (37(4)):288-293 doi:10.1097/WNP.0000000000000681.

    PMID: 33151659
  2. 2

    Intraepidermal Nerve Fiber Density as Measured by Skin Punch Biopsy as a Marker for Small Fiber Neuropathy: Application in Patients with Fibromyalgia.

    Kelley MA, Hackshaw KV

    Diagnostics (Basel, Switzerland) 2021; (11(3)) doi:10.3390/diagnostics11030536.

    PMID: 33802768
  3. 3

    Investigation of nerve fibers in the skin by biopsy: technical aspects, indications, and contribution to diagnosis of small-fiber neuropathy.

    Raicher I, Ravagnani LHC, Correa SG, et al.

    Einstein (Sao Paulo, Brazil) 2022; (20()):eMD8044 doi:10.31744/einstein_journal/2022MD8044.

    PMID: 35830153
  4. 4

    The histopathological evaluation of small fiber neuropathy in patients with vitamin B12 deficiency.

    Güneş HN, Bekircan-Kurt CE, Tan E, Erdem-Özdamar S

    Acta neurologica Belgica 2018; (118(3)):405-410 doi:10.1007/s13760-017-0847-y.

    PMID: 29052170
  5. 5

    Contribution of Skin Biopsy in Peripheral Neuropathies.

    Nolano M, Tozza S, Caporaso G, Provitera V

    Brain sciences 2020; (10(12)) doi:10.3390/brainsci10120989.

    PMID: 33333929
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    Intraepidermal Nerve Fiber Density: Diagnostic and Therapeutic Relevance in the Management of Chronic Pruritus: a Review.

    Pereira MP, Mühl S, Pogatzki-Zahn EM, et al.

    Dermatology and therapy 2016; (6(4)):509-517 doi:10.1007/s13555-016-0146-1.

    PMID: 27730494
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    Reliable Method for Estimating Nerve Fiber Density in Epidermis Using Routine Histopathologic Tissue Preparation: A Promising Diagnostic Tool for Small Fiber Neuropathy.

    Aspegren O, Pourhamidi K

    Applied immunohistochemistry & molecular morphology : AIMM 2024; (32(5)):215-221 doi:10.1097/PAI.0000000000001193.

    PMID: 38650330
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    Quantitative and qualitative normative dataset for intraepidermal nerve fibers using skin biopsy.

    Collongues N, Samama B, Schmidt-Mutter C, et al.

    PloS one 2018; (13(1)):e0191614 doi:10.1371/journal.pone.0191614.

    PMID: 29370274
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    The role of skin biopsy in differentiating small-fiber neuropathy from ganglionopathy.

    Provitera V, Gibbons CH, Wendelschafer-Crabb G, et al.

    European journal of neurology 2018; (25(6)):848-853 doi:10.1111/ene.13608.

    PMID: 29493845
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    Influence of age and gender on the function of postganglionic sympathetic sudomotor axons.

    Mal K, Abhishekh HA, Chawla MP, et al.

    The National medical journal of India 2017; (30(3)):136-138.

    PMID: 28936997
  11. 11

    Assessment of cutaneous axon-reflex responses to evaluate functional integrity of autonomic small nerve fibers.

    Hijazi MM, Buchmann SJ, Sedghi A, et al.

    Neurological sciences : official journal of the Italian Neurological Society and of the Italian Society of Clinical Neurophysiology 2020; (41(7)):1685-1696 doi:10.1007/s10072-020-04293-w.

    PMID: 32125538
  12. 12

    Autonomic Reflex Screen Test Abnormalities in Cold-Induced Sweating Syndrome Type 1.

    El-Dokla AM, Ferdous J, Ali ST, Alam K

    Journal of clinical neuromuscular disease 2017; (19(2)):76-79 doi:10.1097/CND.0000000000000184.

    PMID: 29189552
  13. 13

    Diagnostic and Screening Laboratory Tests in the Assessment of Patients With Small Fiber Neuropathy: An Evidence-Based Review-Report of the American Association of Neuromuscular and Electrodiagnostic Medicine Small Fiber Neuropathy Task Force.

    Katzberg HD, So Y, Brannagan T, et al.

    Muscle & nerve 2026; (73(6)):952-960 doi:10.1002/mus.70119.

    PMID: 41670166
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    Diagnosing small fiber neuropathy in clinical practice: a deep phenotyping study.

    Egenolf N, Zu Altenschildesche CM, Kreß L, et al.

    Therapeutic advances in neurological disorders 2021; (14()):17562864211004318 doi:10.1177/17562864211004318.

    PMID: 34335876
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    Diagnostic Criteria for Small Fiber Neuropathy.

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    Journal of clinical neuromuscular disease 2017; (18(3)):125-131 doi:10.1097/CND.0000000000000154.

    PMID: 28221302
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    Small fiber neuropathy: a disabling and underrecognized syndrome.

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    Current opinion in pulmonary medicine 2017; (23(5)):447-457 doi:10.1097/MCP.0000000000000413.

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    Advances in the Management of Small Fiber Neuropathy.

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    Neurologic clinics 2021; (39(1)):113-131 doi:10.1016/j.ncl.2020.09.006.

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  18. 18

    Blood 1-Deoxysphingolipid Levels Are Associated With Epidermal Denervation in Small Fiber Neuropathy.

    Kreß L, Meyer Zu Altenschildesche C, Egenolf N, et al.

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    PMID: 41376410

This guide to small fiber neuropathy diagnostic testing is for educational purposes only. Always consult a neurologist to discuss your symptoms and the interpretation of your specific test results.

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