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PubMed This is a summary of 22 peer-reviewed journal articles Updated
Neurology

Diagnosing Nerve Damage: Understanding Your Test Results

At a Glance

A normal EMG or nerve-conduction study does not rule out post-infectious neuralgia because these tests mainly assess large nerve fibers. Diagnosis may combine symptoms and examination with a skin biopsy that measures small nerve endings and blood tests for other causes.

One of the most difficult parts of living with post-infectious neuralgia is that your pain can be intense, yet standard medical tests often come back “normal.” This can lead to a sense of being dismissed or misunderstood. However, a “normal” result on common tests does not mean the pain is in your head; it often just means the doctor is using a tool that is looking at the wrong part of the nervous system [1][2].

To understand why your tests might be normal, you first need to understand that your body has two main types of sensory nerve fibers: large fibers and small fibers.

Large Fibers vs. Small Fibers

Your nervous system is like a complex electrical grid with different sizes of wiring:

  • Large-Fiber Nerves: These are thick, insulated (myelinated) wires. They carry information about vibration, position (knowing where your limb is in space), and muscle control [3][4].
  • Small-Fiber Nerves: These are thin wires (thinly myelinated A-delta and unmyelinated C fibers). These are the nerves that carry signals for pain, itching, and temperature [5][6].

In established post-infectious conditions like classic shingles (PHN), the damage often occurs primarily or exclusively in the small fibers [7][8]. (Note: the role of small fibers in post-COVID pain is still evolving).

Why Your EMG Might Be “Normal”

The most common tests for nerve damage are Electromyography (EMG) and Nerve Conduction Studies (NCS). While these are excellent tests, they have a major limitation: standard nerve-conduction studies primarily assess large myelinated sensory and motor fibers, while needle EMG assesses muscle denervation and motor-unit function [3][5].

If your post-infectious neuralgia is caused by Small-Fiber Neuropathy (SFN), your EMG and NCS results will almost always be normal [9][4]. A normal EMG only means no abnormality was detected in the structures tested; it does not exclude small-fiber neuropathy, early or patchy large-fiber disease, or other causes of weakness. However, it is a helpful clue—it allows your doctor to focus the search on the small fibers instead [2][10].

The Skin Punch Biopsy: Seeing the Small Fibers

Because small fibers are too tiny to be picked up by standard electrical tests, doctors use a skin punch biopsy to see them directly. This is currently considered a validated supportive test for small-fiber damage, though it should be interpreted in the clinical context and is not an absolute “gold standard” [2][11].

During this procedure, a tiny (3-millimeter) piece of skin is taken, usually from the ankle or thigh. A specialist then uses a specific stain—typically PGP 9.5—to make the nerve endings visible under a microscope [12][13]. They count how many nerve endings are present in the top layer of your skin (the epidermis). This measurement is called the Intraepidermal Nerve-Fiber Density (IENFD) [11][14].

If your IENFD is lower than expected for your age and sex, it provides objective evidence supporting small-fiber loss [2][11]. However, the biopsy can be normal in some patients with small-fiber dysfunction, and a low count does not prove that an infection caused it. A negative biopsy should not invalidate your pain.

Completeness Checklist: Your Biopsy Report

When you receive your biopsy results, check the report for these specific data points:

Feature to Check What to Look For Why It Matters
Biopsy Site Usually the distal leg (ankle) or thigh [15]. Nerve density naturally varies depending on where the sample was taken [16].
IENFD Value A number representing “fibers per millimeter” [11]. This is the actual count of your nerve endings [14].
Staining Method Should mention PGP 9.5 [12]. This is the standardized “dye” used to see the nerves [13].
Reference Range Must be adjusted for age and sex [11]. Nerve density naturally decreases as we age and is different for men and women [11].
Specimen Adequacy A note that the sample was large enough to be read. Ensures the result is reliable and not a “false negative” due to a poor sample.

Ruling Out Other “Pain Culprits”

Even if your pain started after an infection, doctors will often run blood tests to make sure there aren’t other treatable factors making your nerve pain worse. It is common for an infection to “unmask” or worsen a pre-existing nerve sensitivity [17][18].

Common lab tests include:

  • Blood Sugar (HbA1c and Glucose): To rule out diabetes or pre-diabetes, which are leading causes of small-fiber damage [19][17].
  • Vitamin B12: A deficiency in B12 can cause burning and tingling [19][20].
  • TSH (Thyroid): An underactive thyroid can contribute to nerve issues [20][18].
  • Serum Protein Electrophoresis (SPEP): This looks for abnormal proteins in the blood [19][21].

A Multimodal Diagnosis

No single test is a 100% perfect “gold standard” [2][22]. Diagnosis is a puzzle made of three pieces: your symptoms (burning, allodynia), your physical exam (how you respond to a pinprick or cold), and your test results (biopsy or labs) [1][2]. Classic PHN is usually diagnosed clinically. If your symptoms are classic for post-infectious neuralgia, your doctor may begin treatment even if diagnostic tests are normal [7][22].

Common questions in this guide

Can I have post-infectious neuralgia if my EMG and nerve-conduction study are normal?
Yes. These tests mainly assess large, insulated nerve fibers and may not detect damage or dysfunction in the small fibers that carry pain, itching, and temperature signals. A normal result does not mean the pain is imaginary or completely rule out small-fiber neuropathy.
How does a skin punch biopsy test for small-fiber damage?
A clinician removes a tiny skin sample, usually from the ankle or thigh, and stains it so nerve endings can be counted under a microscope. The result is called intraepidermal nerve-fiber density, or IENFD; a lower-than-expected value for age and sex supports small-fiber loss. A normal biopsy does not always exclude small-fiber dysfunction.
What details should be included in my nerve-fiber biopsy report?
The report should identify the biopsy site, IENFD in fibers per millimeter, staining method such as PGP 9.5, age- and sex-adjusted reference range, and whether the specimen was adequate. These details help clinicians judge whether the count is reliable and comparable with expected nerve density.
What blood tests can look for other causes of burning nerve pain?
Clinicians may check glucose or HbA1c, vitamin B12, thyroid-stimulating hormone, and serum protein electrophoresis. These tests can identify diabetes or prediabetes, vitamin deficiency, thyroid disease, or abnormal blood proteins that may contribute to nerve damage. Finding one of these conditions does not by itself prove what caused symptoms after an infection.
How is post-infectious neuralgia diagnosed when tests do not show nerve damage?
Diagnosis combines the pattern of pain, the physical examination, and test results rather than relying on one perfect test. For classic postherpetic neuralgia, clinicians often make a clinical diagnosis, and they may discuss treatment even when EMG, biopsy, or other tests are normal. The evaluation also looks for non-infectious causes that can be treated.
Does a low IENFD prove that an infection caused my nerve pain?
A low IENFD supports loss of small nerve fibers compared with age- and sex-adjusted expectations, but it does not identify the cause. Clinicians must also consider symptom timing, the physical examination, infection history, and other causes such as diabetes, vitamin B12 deficiency, thyroid disease, or abnormal blood proteins.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Since my EMG was normal, does that mean my large nerve fibers are healthy, even if I still have burning pain?
  2. 2.Can we perform a skin punch biopsy to specifically look for small-fiber neuropathy?
  3. 3.What was my specific intraepidermal nerve-fiber density (IENFD) count, and was it compared to an age- and sex-adjusted reference range?
  4. 4.Are there any other labs we should run—like a glucose tolerance test or protein electrophoresis—to rule out non-infectious causes for my pain?
  5. 5.If my skin biopsy is normal but my pain persists, what are our next steps for diagnosis?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

References

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This page is for informational purposes only and does not constitute medical advice about your post-infectious neuralgia. A neurologist or other clinician should interpret your EMG, biopsy, and laboratory results alongside your symptoms and examination.

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