Healthy Small Fiber Innervation of the Skin
Unmyelinated fibers densely populate normal epidermis.
- 5–12: Normal IENFD (distal leg) (fibers/mm, age-adjusted)
- C & Aδ: Fiber type (unmyelinated / thin myelin)
- Pain, temp, autonomic: Function (nociception & sudomotor)
- Distal leg: Biopsy site standard (10cm above lateral malleolus)
What small fibers do
Small unmyelinated fibers sense pain, temperature, and control sweat glands.
Normal density baseline
Reference values vary by age, sex, and biopsy site location.
Post-COVID Immune-Mediated Small Fiber Damage
Infection triggers immune attack or direct viral nerve injury.
- ~30–60%: Long COVID SFN prevalence (of surveyed pain cohorts)
- Autoimmune: Proposed mechanism (and/or direct viral injury)
- Weeks–months: Onset after infection (variable latency)
- Somatic + autonomic: Affected fiber classes (sensory & sudomotor)
Injury mechanisms
Inflammatory cytokines and autoantibodies may damage nerve terminals.
Progressive fiber loss
Distal fibers degenerate first in length-dependent pattern.
Skin Punch Biopsy Procedure for Nerve Fiber Counting
A small 3mm punch sample is taken from the distal leg.
- 3 mm: Punch diameter (standard biopsy tool)
- Distal leg: Biopsy location (plus proximal thigh optional)
- PGP9.5: Staining method (pan-axonal immunostain)
- 1–2 weeks: Turnaround time (specialized lab processing)
Sampling procedure
Local anesthesia, punch biopsy, sutureless closure, lab shipment.
Tissue processing
Sections stained and imaged for fiber crossing counts.
Intraepidermal Nerve Fiber Density Quantification
Fibers crossing the dermal-epidermal border are counted per mm.
- IENFD: Metric (fibers per millimeter length)
- Below 5th %ile: Diagnostic threshold (age/sex normative data)
- Linear density: Counting method (per EFNS/PNS guidelines)
- ~80–90%: Sensitivity (for confirmed SFN cases)
Counting protocol
Trained readers count fibers crossing the basement membrane line.
Reference comparison
Patient value compared against normative percentile tables.
Symptom Correlation and Potential Recovery
Lower fiber density links to burning pain and autonomic symptoms.
- Correlates: Burning/tingling pain (with lower IENFD)
- POTS-like: Autonomic overlap (symptoms in some patients)
- 6–12 mo: Partial recovery window (reported in some cohorts)
- ~0.5–1 mm/mo: Regrowth rate (approximate axon regrowth)
Pain mechanism
Damaged fibers generate abnormal firing perceived as burning pain.
Recovery trajectory
Some patients show gradual fiber regrowth and symptom easing.
Placeholder: recovery is variable and not guaranteed for all patients.