Recognizing the patterns of NCS findings — axonal vs demyelinating, length-dependent vs non-length-dependent, focal vs multifocal vs generalized — translates raw measurements into syndromic diagnosis. The same NCS findings can have very different clinical meanings depending on context: reduced sural SNAP in a diabetic patient with classic distal stocking sensory loss is “diabetic neuropathy”; the same finding in a young woman with rapidly progressive ascending weakness is “GBS.” This page covers the major NCS patterns, their differential diagnoses, and the clinical context that turns NCS measurements into clinical answers.

The Five Main Patterns

  1. Length-dependent axonal (most common).
  2. Demyelinating (acquired and hereditary subtypes).
  3. Non-length-dependent (ganglionopathy).
  4. Multifocal motor neuropathy (motor-only with conduction block).
  5. Focal nerve injury (compressive neuropathies, traumatic).

Pattern 1: Length-Dependent Axonal

NCS Findings

  • Sural SNAP reduced or absent.
  • Peroneal motor amplitude reduced; CV may be slightly slow secondary to axonal loss.
  • Tibial motor amplitude reduced.
  • Upper extremity studies normal in early-mild disease; abnormal in advanced disease.
  • No conduction block, no temporal dispersion.

Differential

  • Diabetic neuropathy: most common cause; gradual onset.
  • Alcoholic neuropathy: similar pattern; nutritional deficiency contribution.
  • Chemotherapy-induced: rapid onset with chemotherapy initiation.
  • Vitamin deficiency: B12, B1 (Wernicke), vitamin E.
  • Uremic neuropathy: chronic kidney disease.
  • Hypothyroid neuropathy: rare; treatable.
  • HIV-associated: especially with antiretrovirals.
  • Drug-induced: amiodarone, isoniazid, metronidazole, others.
  • Idiopathic axonal neuropathy: ~25% of cases; diagnosis of exclusion.

Pattern 2: Demyelinating

NCS Findings (Acquired Demyelinating)

  • Prolonged distal latencies.
  • Slowed conduction velocities (often substantially below 35 m/s).
  • Prolonged F-wave latencies.
  • Conduction block at non-compressive sites.
  • Temporal dispersion.
  • SNAP amplitudes variable; sometimes reduced.

EFNS/PNS Criteria for CIDP (Simplified)

NCS findings supporting acquired demyelinating polyneuropathy:

  • Distal motor latency >125% of upper limit normal in at least 2 nerves.
  • CV reduced >30% in at least 2 nerves.
  • Prolonged F-wave latency >120% of upper limit.
  • Conduction block (drop >50% with proximal stim).
  • Temporal dispersion (>30% duration increase with proximal stim).

Differential

  • Guillain-Barré syndrome: acute (<4 weeks), monophasic, often preceded by infection.
  • CIDP: chronic (>8 weeks), relapsing or progressive.
  • Multifocal acquired demyelinating sensory and motor (MADSAM, Lewis-Sumner): asymmetric variant.
  • Paraproteinemic neuropathies: IgM-anti-MAG-related, IgG-related; often slowly progressive.
  • POEMS syndrome: with plasma cell dyscrasia, polyneuropathy, organomegaly, endocrinopathy, skin changes.
  • Hereditary demyelinating: Charcot-Marie-Tooth type 1 (uniform slowing of conduction below 38 m/s in median or ulnar); CMT4 subtypes; familial pattern.
  • Toxic demyelinating: lead, hexacarbon solvents, some chemotherapeutic agents.

Distinguishing Acquired from Hereditary Demyelinating

  • CMT1: uniform slowing across all nerves and limbs; familial.
  • CIDP / GBS / MMN: heterogeneous slowing; conduction block at random sites.
  • Family history, foot deformity (pes cavus), insidious onset support CMT.
  • Recent onset, weakness fluctuation, sensorimotor weakness pattern support acquired.

Pattern 3: Non-Length-Dependent (Ganglionopathy)

NCS Findings

  • Sensory NCS abnormal at multiple non-length-dependent sites: median, ulnar, sural reduced or absent.
  • Asymmetric.
  • Motor NCS often preserved (sensory-only pattern).
  • Length-dependent pattern NOT present.

Differential

  • Sjögren syndrome with sensory ganglionopathy: anti-Ro/SSA positive; sicca symptoms.
  • Paraneoplastic sensory neuronopathy: anti-Hu often positive; small cell lung cancer most common.
  • Idiopathic sensory ganglionopathy: diagnosis of exclusion.
  • Cisplatin / oxaliplatin chemotherapy: cumulative dose-dependent.
  • Pyridoxine toxicity: high-dose vitamin B6.
  • Hereditary sensory neuropathies: HSAN 1, others.

Pattern 4: Multifocal Motor Neuropathy (MMN)

NCS Findings

  • Conduction block in motor nerves at non-compressive sites.
  • Sensory NCS NORMAL (key feature distinguishing from CIDP).
  • Asymmetric distribution.
  • Typically upper extremity predominance.

Differential

  • MMN: anti-GM1 antibody positive in 30–80%; IVIG-responsive.
  • Multifocal CIDP (Lewis-Sumner): has sensory involvement; SNAPs reduced.
  • ALS: motor weakness without sensory; but no conduction block on NCS; positive sharp waves and fasciculations on EMG.
  • Hereditary neuropathy with liability to pressure palsies (HNPP): episodic compressive neuropathies.

Pattern 5: Focal Nerve Injury

Patterns

  • Compressive neuropathies: carpal tunnel (median at wrist), cubital tunnel (ulnar at elbow), peroneal at fibular head, tarsal tunnel.
  • Traumatic: laceration, stretch injury, compression after fracture, surgical injury.
  • Mass lesions: tumor compressing nerve, hematoma.

Patterns on NCS

  • Findings localize to the affected nerve and segment.
  • Distal to injury: reduced amplitude (Wallerian degeneration if complete; preserved if conduction block only).
  • Across the injury: slowing or conduction block at the site of pathology.
  • EMG of muscles distal to injury: denervation if axonal injury.

Specific Diagnostic Patterns

Brachial Plexopathy

  • NCS shows reduced amplitudes in multiple motor and sensory nerves distal to the affected plexus segment.
  • Lateral cord lesions: median and musculocutaneous affected.
  • Posterior cord lesions: radial and axillary affected.
  • Medial cord lesions: ulnar and median (medial branch) affected.
  • F-waves and H-reflexes preserved in pure plexopathy (DRG distal to compression).
  • EMG of paraspinal muscles can lateralize: paraspinal denervation suggests root, not plexus.

Lumbosacral Plexopathy

  • Femoral or sciatic distribution.
  • Sural and superficial peroneal sensory may be reduced.
  • EMG of paraspinal muscles can help differentiate from root lesions.

Diabetic Amyotrophy (Diabetic Lumbosacral Radiculoplexus Neuropathy)

  • Asymmetric proximal leg weakness in diabetic patient.
  • NCS shows multifocal axonal loss in upper and lower extremity nerves.
  • EMG: denervation in multiple roots and proximal muscles.
  • Imaging often unremarkable.

Mononeuritis Multiplex

  • Multiple individual nerve involvements.
  • NCS: multiple individual nerves affected, axonal pattern.
  • Differential:
    • Vasculitic neuropathy (most common cause).
    • Diabetic mononeuritis multiplex.
    • Lyme disease.
    • Sarcoid.
    • HIV.
    • Cryoglobulinemia.
  • Often associated with constitutional symptoms.

Hereditary Neuropathy with Liability to Pressure Palsies (HNPP)

  • PMP22 deletion.
  • Episodic mononeuropathies at compressive sites.
  • NCS: prolonged distal latencies at common compressive sites (median wrist, ulnar elbow, peroneal fibular head) even in asymptomatic nerves.
  • “Tomaculous neuropathy” on biopsy.

Approach to NCS Interpretation

  1. Identify the pattern: axonal vs demyelinating, length-dependent vs not, focal vs multifocal vs generalized.
  2. Consider clinical context: age, comorbidities, exposures, family history, time course.
  3. Order pattern + context = differential diagnosis.
  4. Additional testing: laboratory (diabetes, B12, autoimmune, paraprotein), EMG, imaging if compressive lesion suspected, biopsy if needed.
  5. Treatment: targeted at underlying cause; treatable causes always investigated first.

Common Treatable Neuropathies (Don’t Miss)

  • Vitamin B12 deficiency: combined neuropathy + cord involvement.
  • Thyroid dysfunction: hypothyroidism most common neuropathy cause.
  • Diabetes: glycemic control may slow progression.
  • Paraproteinemia: anti-MAG, POEMS.
  • CIDP: IVIG, steroids, plasmapheresis.
  • MMN: IVIG.
  • Vasculitic neuropathy: immunosuppression.
  • Sjögren ganglionopathy: immunotherapy.
  • HSV/zoster radiculitis: antivirals.
  • Treponemal neuropathy: antibiotics.
  • Heavy metal toxicity: chelation.

When NCS is Normal but Neuropathy Suspected

  • Small-fiber neuropathy: routine NCS normal; skin biopsy needed.
  • Very early disease: NCS may lag behind clinical findings.
  • Episodic neuropathies (HNPP, MS-like) between episodes.
  • Selectively root-only disease (sensory radiculopathy) with sensory symptoms but normal SNAP.
  • Conditions affecting unmyelinated fibers only.

🔍 Did You Know?

The distinction between “axonal” and “demyelinating” patterns on NCS is one of the most clinically consequential decisions in electrodiagnosis — it determines whether a patient receives immunotherapy (for acquired demyelinating disease like GBS or CIDP) or symptomatic management (for axonal disease). The classic teaching is that demyelinating patterns show prolonged distal latencies, slowed conduction velocities, and conduction block, while axonal patterns show preserved CV with reduced amplitude. The reality is more nuanced. Severe axonal loss produces secondary changes that look demyelinating — when the fastest-conducting axons are lost, the conduction velocity is determined by the slower surviving fibers, producing apparent slowing. Similarly, chronic demyelinating disease often has substantial secondary axonal loss, blurring the distinction. The EFNS/PNS criteria for CIDP require specific patterns of slowing at multiple sites — strict criteria that distinguish primary demyelinating from secondary changes in axonal disease. The clinical implication is that borderline cases — where you can’t tell whether axonal or demyelinating dominates — warrant immunotherapy trial in selected patients, particularly when the time course is appropriate and clinical features support inflammatory process. The lesson for electrodiagnosticians is that NCS pattern is rarely binary; for practicing neurologists, the take-home is that the EMG report should clearly state the predominant pattern and your confidence, and treatment decisions in equivocal cases should incorporate clinical features.

Pitfalls and Pearls

  • Length-dependent axonal: most common pattern; sural reduces first.
  • Demyelinating pattern: prolonged latency, slow CV, conduction block, temporal dispersion.
  • Acquired demyelinating: GBS (acute), CIDP (chronic), MMN (motor-only).
  • Hereditary demyelinating: uniform CV slowing; family history.
  • Non-length-dependent: ganglionopathy (Sjögren, paraneoplastic).
  • MMN distinguished from ALS: conduction block + anti-GM1; IVIG-responsive.
  • Focal compressive neuropathies: latency, CV slowing at specific site.
  • Brachial plexopathy vs root: paraspinal EMG to lateralize.
  • Mononeuritis multiplex: think vasculitis; do nerve biopsy.
  • HNPP: slowing at multiple compressive sites in asymptomatic nerves.
  • Always pair NCS with EMG: complete picture.
  • Don’t miss treatable causes: B12, thyroid, paraproteinemia, CIDP, MMN, vasculitis.
  • Severe axonal can look demyelinating: loss of fast fibers produces apparent slowing.
  • Borderline cases: immunotherapy trial if clinical features support.
  • Small-fiber neuropathy: NORMAL routine NCS; need skin biopsy.
  • Side-to-side comparison: useful for unilateral conditions.
  • Lab-specific norms: don’t apply textbook values blindly.

References

  1. Preston DC, Shapiro BE. Electromyography and Neuromuscular Disorders. 4th ed. Elsevier; 2021.
  2. Joint Task Force of the EFNS and the PNS. European Federation of Neurological Societies/Peripheral Nerve Society Guideline on management of chronic inflammatory demyelinating polyradiculoneuropathy. J Peripher Nerv Syst. 2010;15(1):1-9.
  3. Kimura J. Electrodiagnosis in Diseases of Nerve and Muscle. 4th ed. Oxford University Press; 2013.
  4. Asbury AK, Cornblath DR. Assessment of current diagnostic criteria for Guillain-Barré syndrome. Ann Neurol. 1990;27 Suppl:S21-S24.
  5. Aminoff MJ, ed. Aminoff’s Electrodiagnosis in Clinical Neurology. 6th ed. Elsevier; 2012.
  6. Hauer DV, Sandberg JL, Burns TM. Clinical utility of neurophysiological studies in evaluating peripheral nerve disease. Continuum (Minneap Minn). 2014;20(5 Peripheral Nervous System Disorders):1241-1262.