Motor vs Sensory NCS
| Feature |
Motor NCS (CMAP) |
Sensory NCS (SNAP) |
| Recording site |
Muscle |
Sensory nerve (digit or nerve trunk) |
| Amplitude reflects |
Number of motor axons + muscle fibers |
Number of sensory axons |
| Normal amplitude |
>4-5 mV (varies by nerve) |
>10-20 ΞV (varies by nerve) |
| Key clinical use |
Motor axon loss, NMJ, myopathy |
Distinguishes pre- vs postganglionic lesions |
Key NCS Parameters
| Parameter |
What It Measures |
Abnormal In |
| Amplitude |
Number of functioning axons |
Axonal loss, conduction block |
| Conduction velocity (CV) |
Speed of fastest fibers (myelination) |
Demyelination (<70-75% LLN) |
| Distal latency (DL) |
Time from distal stim to response |
Distal demyelination (>130% ULN) |
| Duration |
Synchrony of fiber conduction |
Temporal dispersion (demyelination) |
ð Board Pearl
SNAP preserved in radiculopathy because the lesion is preganglionic (DRG intact). SNAP lost in plexopathy and peripheral neuropathy (postganglionic). This is key for localization!
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NCS Criteria
| Feature |
Demyelinating |
Axonal |
| Conduction velocity |
<70-75% LLN |
Normal or mildly slow (>70% LLN) |
| Distal latency |
>130% ULN |
Normal or mildly prolonged |
| Amplitude |
May be preserved early; low late |
Low (proportional to axon loss) |
| Temporal dispersion |
Present (>30% duration increase) |
Absent |
| Conduction block |
Present (>50% amplitude drop) |
Absent |
| F-wave latency |
Prolonged or absent |
Normal or mildly prolonged |
| EMG fibrillations |
Less prominent (unless 2° axonal loss) |
Prominent |
Clinical Examples
| Demyelinating |
Axonal |
| GBS (AIDP) |
GBS (AMAN, AMSAN) |
| CIDP |
Diabetic polyneuropathy |
| CMT1 (hereditary) |
CMT2 (hereditary) |
| MMN |
Toxic neuropathies |
| Anti-MAG neuropathy |
Vasculitic neuropathy |
ð Board Pearl
Conduction block = weakness WITHOUT atrophy (axons intact but can’t conduct through demyelinated segment). Temporal dispersion indicates acquired (non-uniform) demyelination – not seen in hereditary demyelinating neuropathies like CMT1.
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Common Mononeuropathies – Entrapment Sites
| Nerve |
Site |
NCS Findings |
Clinical |
| Median |
Carpal tunnel |
Prolonged DL; slow sensory CV across wrist; compare to ulnar |
Numbness digits 1-3; thenar weakness (severe) |
| Ulnar |
Elbow (cubital tunnel) |
CV slowing across elbow (>10 m/s drop); conduction block |
Numbness digits 4-5; intrinsic hand weakness |
| Ulnar |
Wrist (Guyon’s canal) |
Abnormal dorsal ulnar cutaneous spares (branches proximal) |
Similar to elbow but dorsum hand spared |
| Peroneal |
Fibular head |
Conduction block/slowing across fibular head; superficial peroneal SNAP normal |
Foot drop; lateral leg numbness |
| Radial |
Spiral groove |
Conduction block across spiral groove |
Wrist drop; Saturday night palsy |
| Tibial |
Tarsal tunnel |
Prolonged DL; low amplitude medial/lateral plantar |
Sole numbness/burning |
Radiculopathy vs Plexopathy vs Neuropathy
| Feature |
Radiculopathy |
Plexopathy |
Mononeuropathy |
| SNAP |
Normal (preganglionic) |
Abnormal |
Abnormal |
| CMAP |
Low (if severe axon loss) |
Low |
Low |
| EMG distribution |
Myotomal (multiple nerves, one root) |
Multiple nerves/roots, one plexus region |
Single nerve distribution |
| Paraspinals |
Abnormal |
Normal |
Normal |
ð Board Pearl
SNAP normal + paraspinals abnormal = radiculopathy. SNAP abnormal + paraspinals normal = plexopathy or peripheral nerve lesion. This is the most important distinction!
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Late Responses
| Study |
Pathway |
Clinical Use |
Abnormal In |
| F-wave |
Motor nerve â anterior horn â back (antidromic-orthodromic) |
Tests proximal nerve segments, roots |
GBS (early), radiculopathy, proximal neuropathy |
| H-reflex |
Ia afferent â spinal cord â motor neuron (monosynaptic) |
Electrical ankle jerk; tests S1 root |
S1 radiculopathy, polyneuropathy |
Repetitive Nerve Stimulation (RNS)
| Finding |
Low-Frequency (2-3 Hz) |
Post-Exercise/High-Frequency |
Diagnosis |
| Decrement >10% |
Yes |
Repair of decrement |
Myasthenia gravis |
| Low baseline + increment >100% |
May decrement |
Large increment |
Lambert-Eaton |
| Low baseline + small increment |
May decrement |
20-40% increment |
Botulism |
Blink Reflex
- Tests: Trigeminal (V1 afferent) and facial (VII efferent) nerves; brainstem
- R1: Ipsilateral, oligosynaptic (pons)
- R2: Bilateral, polysynaptic (lateral medulla)
- Uses: Facial nerve lesions, trigeminal neuropathy, brainstem lesions, GBS
ð Board Pearl
F-waves test the entire motor nerve including proximal segments – prolonged or absent early in GBS when distal NCS still normal. H-reflex = S1; absent H-reflex with normal ankle jerk suggests early/mild S1 radiculopathy.
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Types of Spontaneous Activity
| Finding |
Description |
Sound |
Associated Conditions |
| Fibrillation potentials |
Spontaneous single muscle fiber discharge; biphasic/triphasic; regular |
“Rain on a tin roof” |
Denervation, inflammatory myopathy, muscular dystrophy, NMJ disorders |
| Positive sharp waves (PSWs) |
Initial positive deflection then negative; regular |
Dull “thud” |
Same as fibrillations |
| Fasciculation potentials |
Spontaneous motor unit discharge; irregular |
“Popcorn” |
ALS, radiculopathy, benign fasciculations, cramp-fasciculation syndrome |
| Myotonic discharges |
Waxing and waning frequency AND amplitude; triggered by needle movement |
“Dive bomber” |
Myotonic dystrophy, myotonia congenita, paramyotonia, hyperkalemic PP |
| Myokymic discharges |
Grouped, repetitive bursts of same MUP; semi-rhythmic |
“Marching soldiers” |
Radiation plexopathy, GBS, MS, facial myokymia (brainstem glioma) |
| Neuromyotonic discharges |
Very high frequency (150-300 Hz); decrementing; “pinging” |
“Ping” or musical |
Isaac’s syndrome (anti-VGKC/CASPR2), thymoma, post-radiation |
| Complex repetitive discharges (CRDs) |
Polyphasic; regular; abrupt start/stop; no waxing/waning |
“Jackhammer” or “motorcycle” |
Chronic denervation, chronic myopathy, radiculopathy |
| Cramp potentials |
High-frequency MUP discharge; irregular; painful |
Rumbling |
Cramps (ALS, metabolic, benign) |
ð Board Pearl
Myotonic discharge = waxing/waning (dive bomber). CRD = NO waxing/waning (jackhammer). Myokymia on EMG = consider radiation injury or GBS. Neuromyotonia = Isaac’s syndrome (continuous muscle fiber activity).
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Timeline of EMG Changes After Nerve Injury
| Time After Injury |
NCS Findings |
EMG Findings |
| Immediate (0-7 days) |
Conduction block at injury site; distal responses NORMAL |
Reduced recruitment only; NO fibrillations yet |
| Acute (7-10 days) |
Distal CMAP/SNAP drop (Wallerian degeneration complete) |
Reduced recruitment; fibs/PSWs starting (proximal muscles first) |
| Subacute (2-3 weeks) |
Low/absent distal responses |
Fibs/PSWs prominent in proximal muscles |
| Subacute (4-6 weeks) |
Low/absent distal responses |
Fibs/PSWs in distal muscles; maximal denervation |
| Early reinnervation (2-4 months) |
May see nascent CMAPs |
Nascent MUPs (small, polyphasic); fibs persist |
| Chronic reinnervation (6+ months) |
CMAP may improve |
Large, polyphasic MUPs; reduced recruitment; fibs decrease |
| Chronic stable (years) |
May be normal or low amplitude |
Giant MUPs; reduced recruitment; NO fibs (stable) |
Key Points
- Fibs appear at different times depending on distance from lesion:
- Paraspinals: ~10 days
- Proximal limb: 2-3 weeks
- Distal limb: 4-6 weeks
- Nascent MUPs = first sign of reinnervation (small, polyphasic, unstable)
- Chronic neurogenic MUPs = large amplitude, long duration, polyphasic (collateral sprouting)
- Reduced recruitment = fewer MUPs firing fast (neurogenic pattern)
ð Board Pearl
Wait 3 weeks for optimal EMG after nerve injury – fibs take time to appear. Fibs WITHOUT large MUPs = acute/ongoing denervation. Fibs WITH large MUPs = chronic with ongoing denervation. Large MUPs WITHOUT fibs = chronic stable.
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Myopathic vs Neurogenic MUPs
| Feature |
Myopathic |
Neurogenic |
| Amplitude |
Low (small) |
High (large/giant) |
| Duration |
Short |
Long |
| Phases |
Increased polyphasia |
Increased polyphasia |
| Recruitment |
Early (many small units for weak effort) |
Reduced (few units firing fast) |
| Reason |
Fewer muscle fibers per motor unit |
Collateral sprouting â larger motor units |
Recruitment Patterns
| Pattern |
Description |
Seen In |
| Normal |
Ratio ~5:1 (firing rate : number of MUPs) |
Normal |
| Reduced (neurogenic) |
Few MUPs firing rapidly (>15-20 Hz before next recruited) |
Neuropathy, radiculopathy, ALS |
| Early (myopathic) |
Many MUPs at low firing rates; full interference with weak effort |
Myopathy |
| Poor activation |
Few MUPs at low firing rates; normal MUP morphology |
UMN lesion, pain, poor effort |
ð Board Pearl
Myopathic = SNAP (Small, short, polyphasic with early recruitment). Neurogenic = LARP (Large amplitude, long duration, reduced recruitment, polyphasic). Remember: “Sick muscle = small units, sick nerve = big units.”
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Quick Recognition Table
| Disease |
Key EMG/NCS Findings |
| ALS |
Widespread fibs + fasciculations + neurogenic MUPs; NORMAL sensory NCS; multiple regions (bulbar, cervical, thoracic, lumbar) |
| Myasthenia gravis |
Decrement on RNS; increased jitter on single-fiber EMG; normal routine NCS/EMG |
| Lambert-Eaton |
Low CMAP amplitude; >100% increment post-exercise; may have mild decrement at rest |
| GBS (AIDP) |
Prolonged/absent F-waves (early); conduction block; temporal dispersion; slow CV |
| CIDP |
Same as AIDP but symmetric and chronic; uniform demyelination |
| Multifocal motor neuropathy (MMN) |
Conduction block in motor nerves ONLY; normal sensory; asymmetric |
| Myotonic dystrophy |
Myotonic discharges (dive bomber); myopathic MUPs; fibs common |
| Polymyositis/Dermatomyositis |
Fibs/PSWs + myopathic MUPs (“irritable myopathy”); CRDs; normal NCS |
| Inclusion body myositis |
Mixed myopathic AND neurogenic features; fibs; long-duration MUPs |
| Steroid myopathy |
Myopathic MUPs; NO fibrillations; normal NCS |
| Critical illness myopathy |
Low CMAP with direct muscle stimulation; fibs; myopathic MUPs |
| Radiation plexopathy |
Myokymic discharges; axonal loss pattern |
| Isaac’s syndrome |
Neuromyotonic discharges; fasciculations; doublets/triplets |
| Carpal tunnel syndrome |
Prolonged median distal latency; slow sensory CV across wrist; compare to ulnar (normal) |
ð Board Pearl
ALS = motor only (normal sensory NCS is required). MMN = motor conduction block only. IBM = mixed pattern (unique!). Radiation plexopathy = myokymia (distinguishes from tumor infiltration which doesn’t have myokymia).
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Localization Quick Reference
| Clinical Scenario |
SNAP |
Paraspinals |
Diagnosis |
| Weakness + sensory loss in one nerve |
Abnormal |
Normal |
Mononeuropathy |
| Weakness in myotome + sensory loss in dermatome |
Normal |
Abnormal |
Radiculopathy |
| Weakness/sensory loss spanning multiple nerves and roots |
Abnormal |
Normal |
Plexopathy |
| Length-dependent sensory > motor |
Abnormal (distal) |
Normal |
Polyneuropathy |
Spontaneous Activity Quick Reference
| Sound |
Finding |
Think Of |
| “Dive bomber” (waxing/waning) |
Myotonic discharge |
Myotonic dystrophy, myotonia congenita |
| “Marching soldiers” (grouped bursts) |
Myokymia |
Radiation injury, GBS |
| “Ping” (high-frequency, decrementing) |
Neuromyotonia |
Isaac’s syndrome |
| “Jackhammer” (no waxing/waning) |
CRD |
Chronic denervation/myopathy |
| “Rain on tin roof” |
Fibrillations |
Denervation, inflammatory myopathy |
Key Clinical Pearls
ð High-Yield Points
- SNAP normal = preganglionic (radiculopathy)
- Conduction block = demyelinating
- Low amplitude everywhere = axonal
- Wait 3 weeks for EMG after nerve injury
- Fibs appear proximal to distal (paraspinals first)
- Myopathic = small, short, early recruitment
- Neurogenic = large, long, reduced recruitment
- F-waves abnormal early in GBS (before distal changes)
- Myokymia = radiation plexopathy
- ALS requires normal sensory NCS
Red Flags
â ïļ Important Considerations
- Fibs in first week: Pre-existing denervation or myopathy (not new injury)
- Abnormal sensory NCS in suspected ALS: Reconsider diagnosis
- Conduction block in sensory AND motor: Think CIDP/GBS, not MMN
- Rapidly progressive with normal EMG: Consider NMJ disorder, myopathy, or too early after injury
- Myokymia without radiation history: Consider GBS, MS, or brainstem lesion
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