Residency · Residency · Physical Medicine Rehabilitation

Electrodiagnostic Approach to Polyneuropathy

Overview

Definition

Polyneuropathy is a diffuse, symmetric disorder of peripheral nerves. Typically length-dependent, affecting the longest nerves first (distal-to-proximal gradient). Presents with sensory symptoms in a "stocking-glove" distribution, progressing proximally. Motor involvement follows sensory loss in most axonal polyneuropathies.

Electrodiagnosis is essential for confirming the diagnosis, characterizing the pattern, and guiding the etiologic workup.

Epidemiology

Affects approximately 2-3% of the general population, increasing to 8% in those over 55. Diabetes mellitus is the most common cause in developed countries. Up to 30% of polyneuropathies remain idiopathic after comprehensive evaluation.

Classification Framework

By Pathology

Axonal: primary loss of axons; reduced amplitudes with relatively preserved conduction velocities. Demyelinating: primary loss of myelin; slowed conduction velocities, prolonged latencies, conduction block, temporal dispersion. Mixed: features of both axonal and demyelinating processes.

By Fiber Type

Large fiber: affects myelinated Aalpha and Abeta fibers; detected by standard NCS/EMG. Small fiber: affects thinly myelinated Adelta and unmyelinated C fibers; NOT detected by standard NCS/EMG. Mixed fiber: involves both large and small fibers.

By Distribution

Length-dependent (dying-back): most common pattern; distal-to-proximal gradient. Non-length-dependent: patchy, multifocal, or proximal; suggests inflammatory, vasculitic, or hereditary process. Polyradiculoneuropathy: involves roots and peripheral nerves (GBS, CIDP).

By Tempo

Acute: days to 4 weeks (GBS, toxic, porphyria). Subacute: 4-8 weeks (toxic, nutritional, paraneoplastic). Chronic: months to years (diabetic, hereditary, CIDP, idiopathic).

<image>Flowchart for electrodiagnostic classification of polyneuropathy showing the decision pathway from nerve conduction study findings to axonal versus demyelinating patterns, then to acquired versus hereditary etiologies, with key distinguishing electrodiagnostic features at each branch point</image>

Electrodiagnostic Protocol

Sensory NCS

Sural nerve: most important sensory nerve to study; often the first to become abnormal in length-dependent neuropathy. Median sensory (digit 2 or 3): upper extremity sensory assessment. Ulnar sensory (digit 5): upper extremity comparison. Radial sensory: may be relatively preserved in early polyneuropathy (shorter course).

Superficial peroneal sensory: complements the sural for lower extremity assessment. Reduced amplitude is the hallmark of axonal sensory neuropathy. Absent sural SNAP with preserved upper extremity SNAPs supports length-dependent pattern.

Motor NCS

Tibial nerve (recording from AH): assess distal motor latency, CMAP amplitude, F-wave. Peroneal (fibular) nerve (recording from EDB): assess conduction across the fibular head as well. Median motor (recording from APB): upper extremity motor assessment. Ulnar motor (recording from ADM): upper extremity comparison.

Include F-waves for all motor studies to assess proximal conduction. Reduced CMAP amplitude reflects motor axonal loss. Slowed conduction velocity below 70-80% of lower limit of normal suggests primary demyelination.

Needle EMG

Tibialis anterior: distal lower extremity. Medial gastrocnemius: distal lower extremity. Vastus medialis or illiopsoas: proximal lower extremity (to assess for proximal involvement). First dorsal interosseous: distal upper extremity (in more advanced cases).

Lumbar paraspinals: to evaluate for polyradiculopathy (CIDP, diabetic amyotrophy). Fibrillation potentials in distal muscles indicate active denervation. Neurogenic MUAP changes (increased amplitude, duration) indicate chronic reinnervation. Distribution of findings helps confirm length-dependent pattern.

Distinguishing Axonal from Demyelinating Neuropathy

Axonal Pattern

Reduced SNAP and CMAP amplitudes (the primary finding). Conduction velocities mildly slowed (not below 70-80% of lower limit of normal). Distal latencies mildly prolonged (not beyond 130% of upper limit of normal). F-wave latencies mildly prolonged.

No conduction block or temporal dispersion. Needle EMG: fibrillations in distal muscles, neurogenic MUAP changes. Common etiologies: diabetes, alcohol, B12 deficiency, uremia, toxic, most idiopathic.

Demyelinating Pattern

Conduction velocity slowing below 70-80% of lower limit of normal. Distal latency prolongation beyond 130% of upper limit of normal. Conduction block (>50% amplitude drop between distal and proximal stimulation). Temporal dispersion (>30% increase in CMAP duration between stimulation sites).

F-wave absence or marked prolongation. SNAP and CMAP amplitudes may be normal early or reduced with secondary axonal loss. Common etiologies: GBS, CIDP, anti-MAG neuropathy, hereditary (CMT1).

Acquired vs. Hereditary Demyelinating

Acquired Demyelinating Features

Non-uniform slowing (different degrees of slowing in different nerve segments and different nerves). Conduction block present. Temporal dispersion present. Suggests: GBS, CIDP, multifocal motor neuropathy (MMN).

Hereditary Demyelinating Features

Uniform slowing (similar degree of slowing across all segments and nerves). No conduction block. No temporal dispersion. Very slow velocities (often <20 m/s in CMT1A).

Suggests: CMT1A, CMT1B, other hereditary demyelinating neuropathies. Family history and genetic testing are confirmatory.

<image>Comparison of nerve conduction study waveforms in axonal neuropathy versus acquired demyelinating neuropathy versus hereditary demyelinating neuropathy showing characteristic differences in amplitude, conduction velocity, temporal dispersion, and conduction block patterns</image>

Common Etiologies by Electrodiagnostic Pattern

Axonal Sensorimotor (Most Common)

Diabetic distal symmetric polyneuropathy. Alcoholic neuropathy. Uremic neuropathy. Vitamin B12 deficiency. Medication-induced (chemotherapy, metformin). Idiopathic sensory-predominant polyneuropathy.

Axonal Motor-Predominant

Porphyric neuropathy (acute). Lead toxicity. Multifocal motor neuropathy (may mimic; check for conduction block). Motor neuron disease (consider if sensory studies are normal).

Axonal Sensory-Predominant

Idiopathic distal sensory neuropathy. Diabetic sensory neuropathy. Paraneoplastic sensory neuropathy (ganglionopathy pattern -- non-length-dependent). Sjogren syndrome. Cisplatin, pyridoxine toxicity.

Demyelinating Acquired

GBS (AIDP): acute, monophasic, ascending weakness. CIDP: chronic, progressive or relapsing, symmetric proximal and distal weakness. Anti-MAG neuropathy: distal acquired demyelinating symmetric neuropathy (DADS); very prolonged distal latencies. Multifocal motor neuropathy (MMN): pure motor, multifocal conduction block, anti-GM1 antibodies.

POEMS syndrome: demyelinating pattern with systemic features (polyneuropathy, organomegaly, endocrinopathy, M-protein, skin changes).

Demyelinating Hereditary

CMT1A: duplication of PMP22 gene; uniform severe slowing (<25 m/s); most common hereditary neuropathy. CMT1B: MPZ mutation. HNPP: PMP22 deletion; increased susceptibility to pressure palsies; segmental demyelination at common compression sites.

Laboratory Workup Guided by Electrodiagnosis

Initial Panel for Axonal Polyneuropathy

Fasting glucose, hemoglobin A1c, oral glucose tolerance test. Complete blood count. Comprehensive metabolic panel (renal function, liver function, electrolytes). Vitamin B12 (consider methylmalonic acid and homocysteine if borderline). TSH. Serum protein electrophoresis (SPEP) with immunofixation. ESR, CRP.

Extended Workup if Initial Panel Is Negative

Folate, vitamin B1 (thiamine), vitamin B6, copper, zinc. HIV, hepatitis B and C serologies. ANA, anti-SSA/SSB (Sjogren). Angiotensin-converting enzyme (sarcoidosis).

Heavy metal screen (lead, mercury, arsenic) if history suggests exposure. Paraneoplastic antibody panel (if subacute onset, non-length-dependent, or sensory ganglionopathy pattern). Genetic testing for CMT (if hereditary pattern suspected).

Workup for Demyelinating Pattern

All initial panel studies above. SPEP with immunofixation (MGUS, POEMS, amyloidosis). Anti-MAG antibodies (if distal predominant demyelination). Anti-GM1 antibodies (if multifocal motor neuropathy suspected).

CSF analysis (elevated protein in GBS, CIDP). Consider nerve biopsy in select cases. VEGF level, bone survey (if POEMS suspected).

Special Considerations

Small Fiber Neuropathy

Standard NCS and EMG are NORMAL (large fiber function is preserved). Clinical features: burning pain, allodynia, autonomic dysfunction. Diagnosis requires: skin punch biopsy (intraepidermal nerve fiber density), quantitative sudomotor axon reflex test (QSART), autonomic function testing. Common causes: diabetes/prediabetes, idiopathic, Sjogren, sarcoidosis, amyloidosis, Fabry disease.

Sensory Ganglionopathy (Neuronopathy)

Non-length-dependent sensory loss pattern (upper and lower extremities equally affected). Asymmetric, patchy sensory loss early; becomes diffuse. Reduced/absent SNAPs in upper and lower extremities simultaneously. Motor NCS and EMG are normal. Causes: paraneoplastic (anti-Hu), Sjogren, cisplatin, pyridoxine excess, idiopathic.

Clinical Pearls

The sural SNAP is the most valuable single nerve study for confirming polyneuropathy; always include it. A "sural-sparing" pattern (absent median/ulnar SNAPs with preserved sural SNAP) suggests acquired demyelinating neuropathy (GBS/CIDP) or ganglionopathy, not typical length-dependent neuropathy. Uniform slowing without conduction block = hereditary; non-uniform slowing with conduction block = acquired. Always check SPEP with immunofixation in any demyelinating neuropathy to screen for monoclonal gammopathy.

In diabetic patients, consider superimposed carpal tunnel syndrome or lumbosacral radiculoplexus neuropathy (diabetic amyotrophy) if findings are asymmetric. Up to 30% of polyneuropathies remain idiopathic; reassurance and symptomatic management are appropriate after thorough workup. Normal NCS does not exclude small fiber neuropathy; refer for skin biopsy if clinical suspicion is high. The extent of electrodiagnostic testing should be guided by the clinical question; a focused protocol is sufficient for straightforward diabetic neuropathy.

References

• England JD, et al. Practice Parameter: Evaluation of Distal Symmetric Polyneuropathy. Neurology. 2009;72(2):177-184.
• Donofrio PD, Albers JW. AAEM Minimonograph #34: Polyneuropathy Classification by Nerve Conduction Studies and Electromyography. Muscle Nerve. 1990;13(10):889-903.
• Preston DC, Shapiro BE. Electromyography and Neuromuscular Disorders. 4th Edition. Elsevier. 2021.
• Dimachkie MM, Barohn RJ. Guillain-Barre Syndrome and Variants. Neurol Clin. 2013;31(2):491-510.
• Joint Task Force of the EFNS and 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.

Read this lecture as Markdown