Peripheral nerve biopsy provides direct visualization of nerve pathology that EMG/NCS, antibody testing, and genetics cannot always provide. The sural nerve is the most common site (purely sensory, mostly cutaneous, well-tolerated). Modern neuropathology integrates the biopsy with clinical phenotype, electrodiagnostic findings, and molecular workup. The two cardinal patterns are axonal degeneration and demyelination, but more specific patterns (vasculitic, amyloid, leprosy, hereditary) often allow definitive diagnosis. This page covers nerve biopsy techniques and the major reaction patterns.

Nerve Biopsy Indications

• Atypical or rapidly progressive neuropathy unresponsive to standard treatment.
• Suspected vasculitic neuropathy.
• Suspected amyloid neuropathy.
• Suspected leprosy.
• Diagnostic uncertainty (paraneoplastic, infectious, hereditary).
• Inflammatory demyelinating polyneuropathy in atypical cases.
• Some hereditary neuropathies for confirmation.

Sample Acquisition and Processing

Sural Nerve Biopsy

• Surgical incision behind lateral malleolus.
• 5-10 cm segment of sural nerve.
• Sample divided for multiple analyses.

Specimen Processing

• Paraffin sections: standard histology, IHC, special stains.
• Resin-embedded toluidine blue (semi-thin sections): detailed morphology of myelinated fibers.
• Teased fiber preparations: individual fibers isolated; distinguish axonal vs demyelinating.
• Electron microscopy: unmyelinated fibers, axonal/myelin ultrastructure, amyloid, onion bulbs.
• Special stains: Congo red (amyloid), Fite stain (leprosy), Bielschowsky (axons).
• IHC: S100 (Schwann cells), NF (axons), CD3/CD20 (inflammation).

Normal Nerve Histology

• Fascicles: bundles of axons surrounded by perineurium.
• Endoneurium: connective tissue between axons within a fascicle.
• Perineurium: tight sheath around each fascicle.
• Epineurium: outer connective tissue around entire nerve, with blood vessels.
• Myelinated fibers: large (Aα, Aβ) and small (Aδ); single Schwann cell per internode.
• Unmyelinated fibers (C fibers): multiple axons share one Schwann cell.
• Normal fiber density: ~10,000 myelinated + ~30,000 unmyelinated fibers per square millimeter.

Major Reaction Patterns

Axonal Degeneration

• Reduced number of myelinated fibers.
• Wallerian-like degeneration: axonal disintegration with myelin breakdown into ovoids.
• Axonal regeneration clusters: small myelinated axons clustered together (Schwann cells producing multiple new axons).
• Variable Schwann cell preservation.
• Teased fibers: linear ovoids (Wallerian degeneration); no/minimal demyelination.
• Causes: most metabolic (diabetes), toxic, hereditary axonal, paraneoplastic.

Demyelinating Pattern

• Loss of myelin with relative preservation of axons.
• Onion bulb formations: concentric layers of Schwann cell processes around an axon (repeated demyelination and remyelination); characteristic of hereditary neuropathies (CMT1) and CIDP.
• Macrophage-associated demyelination: invading myelin debris.
• Teased fibers: segmental demyelination (segments without myelin between intact internodes); remyelination (thin myelin).
• Causes: hereditary demyelinating (CMT1, CMT4), inflammatory (CIDP, GBS), paraproteinemic.

Mixed Axonal-Demyelinating

Many neuropathies have features of both. Common in long-standing disease (axonal damage occurs after chronic demyelination, or vice versa).

Vasculitic Neuropathy

• Necrotizing vasculitis: inflammation with fibrinoid necrosis of small/medium epineurial vessels.
• Predominantly axonal pattern of nerve damage.
• Often asymmetric (mononeuritis multiplex).
• Hemosiderin from past hemorrhage at vasculitic sites.
• Causes: primary CNS vasculitis (rare in nerve), systemic vasculitis (ANCA-associated, PAN), connective tissue disease, paraneoplastic.

Inflammatory Pattern

• Endoneurial or perivascular inflammation.
• T-cell predominant in CIDP.
• Demyelinating pattern with macrophage stripping of myelin.
• Causes: CIDP, GBS, paraproteinemic neuropathies.

Amyloid Neuropathy

• Congo red apple-green birefringence: amyloid in endoneurial connective tissue.
• Often in vessel walls (perivascular) or as plaques in endoneurium.
• Predominantly axonal degeneration with small fiber loss particularly.
• Types: AL (light chain), ATTR (familial transthyretin), wild-type ATTR (senile).
• Mass spectrometry of amyloid (Mayo, others) identifies type definitively.

Hereditary Neuropathies

• CMT1 (Charcot-Marie-Tooth type 1): hypertrophic onion bulbs; demyelinating; PMP22 duplication most common.
• CMT2: axonal; MFN2, MPZ, others.
• CMTX: GJB1 (connexin-32); mixed.
• CMT4: autosomal recessive demyelinating; various genes.
• HNPP (hereditary neuropathy with liability to pressure palsies): PMP22 deletion; “tomacula” — focal sausage-shaped myelin thickenings on teased fibers.
• Hereditary ATTR amyloidosis: TTR mutations.
• Refsum disease: PHYH; phytanic acid accumulation; demyelinating.

Infectious Pattern (Leprosy)

• Mycobacterium leprae: identifiable by Fite stain.
• Granulomatous inflammation in tuberculoid leprosy.
• Foamy macrophages with bacilli in lepromatous leprosy.
• Schwann cell invasion and destruction.
• Sensory neuropathy predominantly.
• Treatment: multi-drug regimen (rifampin + dapsone + clofazimine).

Pattern Recognition

Skin Biopsy for Small Fiber Neuropathy

An alternative to nerve biopsy for evaluating small fiber neuropathy:

• Punch skin biopsy (3 mm) from distal leg.
• PGP9.5 IHC reveals intra-epidermal nerve fibers.
• Reduced density of unmyelinated nerve endings = small fiber neuropathy.
• Less invasive than nerve biopsy; can be repeated to monitor.
• Useful in diabetic, amyloid (early), idiopathic small fiber neuropathies.

🔍 Did You Know?

The recognition that hereditary transthyretin amyloidosis (hATTR) is treatable with disease-modifying therapy has transformed nerve biopsy interpretation. hATTR amyloidosis is an autosomal dominant disease caused by transthyretin (TTR) gene mutations producing a misfolded, amyloidogenic transthyretin that deposits in nerves, heart, and other tissues. The classical presentation is progressive sensorimotor and autonomic neuropathy with cardiac involvement (cardiomyopathy and arrhythmia). Until 2018, treatment was supportive only — liver transplant was the only option to remove the source of mutant TTR. The 2018 approvals of patisiran (siRNA against TTR mRNA) and inotersen (antisense oligonucleotide) changed the landscape: these drugs reduce circulating mutant TTR and dramatically improve neuropathy outcomes. Subsequent approvals (vutrisiran 2022, eplontersen 2023, acoramidis 2024) have expanded the options. The clinical implication: in any patient with progressive sensorimotor/autonomic neuropathy + cardiac features (or suggestive family history), nerve biopsy with Congo red staining and mass spectrometric typing of amyloid can identify hATTR and trigger life-changing therapy. The lesson generalizes: nerve biopsy now has therapeutic implications that didn’t exist a decade ago, and identifying treatable forms (vasculitis, hATTR, leprosy, certain hereditary neuropathies with emerging gene therapy) requires precise histopathologic diagnosis.

Pitfalls and Pearls

• Sural nerve biopsy: most common; sensory; well-tolerated.
• Process for paraffin + resin (toluidine blue) + teased fibers + EM.
• Axonal pattern: Wallerian-like + regeneration clusters; diabetes, toxic, hereditary axonal.
• Demyelinating pattern: segmental + onion bulbs; CMT1, CIDP, GBS.
• Onion bulbs: hereditary CMT1 (hypertrophic), CIDP.
• Tomacula (sausage-shaped myelin): HNPP (PMP22 deletion).
• Fibrinoid necrosis of small/medium vessels: vasculitic neuropathy.
• Congo red apple-green birefringence: amyloid.
• Mass spectrometry of amyloid: identifies type (AL, ATTR).
• hATTR amyloidosis: TTR mutations; treatable with patisiran, inotersen, vutrisiran, eplontersen, acoramidis.
• CMT subtypes: CMT1 (demyelinating, PMP22 duplication), CMT2 (axonal), CMTX (mixed, GJB1).
• Leprosy: Fite stain identifies M. leprae; multi-drug therapy.
• Skin biopsy (PGP9.5): for small fiber neuropathy; alternative to nerve biopsy.
• Teased fibers: linear ovoids (axonal); segmental demyelination (demyelinating); tomacula (HNPP).

References

1. Vallat JM, Weis J, eds. Peripheral Nerve Disorders: Pathology and Genetics. Wiley; 2014.
2. Schröder JM. Peripheral nerve biopsy: clinicopathological correlation. Acta Neuropathol Suppl. 1997;5:1-50.
3. Adams D, Gonzalez-Duarte A, O’Riordan WD, et al. Patisiran, an RNAi therapeutic, for hereditary transthyretin amyloidosis. N Engl J Med. 2018;379(1):11-21.
4. Benson MD, Waddington-Cruz M, Berk JL, et al. Inotersen treatment for patients with hereditary transthyretin amyloidosis. N Engl J Med. 2018;379(1):22-31.
5. Lacomis D. Small-fiber neuropathy. Muscle Nerve. 2002;26(2):173-188.
6. Said G. Peripheral neuropathies in vasculitis. Curr Opin Rheumatol. 1999;11(1):41-48.