Nerve conduction studies (NCS) and needle electromyography (EMG) together make up the electrodiagnostic (EDX) study — an extension of the neurological exam that localizes and characterizes disorders of the peripheral nervous system: anterior horn cell, root, plexus, peripheral nerve, neuromuscular junction (NMJ), and muscle.
Normal values are laboratory- and temperature-dependent. The reference ranges below are widely used adult values (after Preston & Shapiro); always interpret against your own lab’s limits, and warm a cool limb — low temperature falsely prolongs latencies, slows conduction velocity, and increases amplitudes (rule of thumb: ~1.5–2.5 m/s slowing per °C below 32°C at the skin).
Nerve Conduction Studies (NCS)
How it is performed
- Motor study: stimulate the nerve at two (or more) points and record the compound muscle action potential (CMAP) over the belly of the target muscle (belly-tendon montage). Use supramaximal stimulation (increase current until the response no longer grows, then add ~20%).
- Sensory study: stimulate and record along a pure sensory nerve (orthodromic or antidromic) to obtain the sensory nerve action potential (SNAP).
- Measure off the same landmarks each time; keep the limb warm; mark distances accurately (CV = distance ÷ proximal-minus-distal latency).
The three parameters
- Amplitude — number of functioning axons/fibers. Low = axon loss (or conduction block proximally). CMAP in mV, SNAP in µV.
- Distal latency — slowest distal conduction; for motor it also includes NMJ transmission and muscle activation (ms).
- Conduction velocity (CV) — speed of the fastest fibers; reflects myelination (m/s); needs two stimulation sites for motor nerves.
Motor NCS — adult normal limits
| Nerve (record) | Distal latency | Amplitude (CMAP) | Conduction velocity |
|---|
| Median (APB) | ≤ 4.4 ms | ≥ 4 mV | ≥ 49 m/s |
| Ulnar (ADM) | ≤ 3.3 ms | ≥ 6 mV | ≥ 49 m/s |
| Peroneal / fibular (EDB) | ≤ 6.5 ms | ≥ 2 mV | ≥ 44 m/s |
| Tibial (AH) | ≤ 5.8 ms | ≥ 4 mV | ≥ 41 m/s |
Sensory NCS — adult normal limits
| Nerve | Peak latency | Amplitude (SNAP) | Conduction velocity |
|---|
| Median (digit II) | ≤ 3.5 ms | ≥ 20 µV | ≥ 50 m/s |
| Ulnar (digit V) | ≤ 3.1 ms | ≥ 17 µV | ≥ 50 m/s |
| Radial (snuffbox) | ≤ 2.9 ms | ≥ 15 µV | ≥ 50 m/s |
| Sural (lateral ankle) | ≤ 4.4 ms | ≥ 6 µV | ≥ 40 m/s |
SNAP and the DRG: the sensory cell body sits in the dorsal root ganglion, outside the cord. A lesion proximal to the DRG (e.g., radiculopathy) leaves the SNAP normal; a lesion at or distal to the DRG (plexopathy, peripheral neuropathy) reduces it — the key to separating radiculopathy from plexopathy.
Late Responses & Repetitive Stimulation
| Study | What it tests | Normal / key abnormality |
|---|
| F-wave | Proximal motor segment (antidromic to the anterior horn and back) | Upper limb ≤ ~31 ms, lower limb ≤ ~56 ms (height-dependent). Prolonged/absent early in acquired demyelination (GBS) and proximal lesions. |
| H-reflex | S1 reflex arc (tibial → soleus); the electrical ankle jerk | ≤ ~34 ms (height-dependent); side-to-side difference >1.5 ms abnormal. Prolonged/absent in S1 radiculopathy and proximal tibial/sciatic lesions. |
| RNS — slow (2–3 Hz) | NMJ safety factor | Decrement >10% → postsynaptic defect (myasthenia gravis). |
| RNS — fast / post-exercise | Presynaptic NMJ reserve | Increment >60–100% → presynaptic defect (LEMS, botulism). |
Needle EMG
A concentric needle samples muscle in three phases — at insertion, at rest, and during voluntary activation.
Insertional & spontaneous activity (at rest)
| Finding | Significance |
|---|
| Fibrillations & positive sharp waves | Active denervation (axon loss) or active myopathy (esp. inflammatory/necrotizing). Appear ~2–3 weeks after axon injury. |
| Fasciculation potentials | Spontaneous motor-unit discharges — prominent in motor neuron disease; also benign, and in radiculopathy/neuropathy. |
| Myotonic discharges | Waxing-and-waning “dive-bomber” sound — myotonic dystrophy, myotonia congenita, Pompe, some channelopathies. |
| Complex repetitive discharges (CRDs) | Chronic neuropathic or myopathic processes. |
| Myokymic / neuromyotonic discharges | Radiation plexopathy, demyelination, peripheral nerve hyperexcitability (Isaacs). |
| Reduced insertional activity | Fibrosis or fatty replacement (end-stage muscle). |
Motor unit potentials (MUAPs) & recruitment
| Process | MUAP morphology | Recruitment |
|---|
| Neuropathic (chronic) | Large amplitude, long duration, polyphasic (reinnervation) | Reduced — few units firing fast |
| Myopathic | Small amplitude, short duration, polyphasic | Early / rapid — many small units for little force |
| NMJ disorder | Unstable / varying MUAPs (moment-to-moment) | Normal to early |
Pathological Patterns
| Pattern | NCS | EMG |
|---|
| Axonal neuropathy | Low amplitude (CMAP/SNAP); CV and latencies relatively preserved | Fibrillations/PSWs; large, long, polyphasic MUAPs; reduced recruitment |
| Demyelinating neuropathy | Slow CV, prolonged distal latencies, prolonged/absent F-waves, conduction block & temporal dispersion (acquired) | Less denervation unless secondary axon loss |
| Myopathy | Usually normal (CMAP may be low in severe/distal myopathy; SNAPs normal) | Small, short, polyphasic MUAPs; early recruitment; ± fibs in inflammatory/necrotizing myopathy |
| Motor neuron disease | Low CMAP amplitudes; normal sensory studies | Widespread fibrillations + fasciculations; large MUAPs; reduced recruitment across multiple regions/segments |
| NMJ — postsynaptic (MG) | Normal routine NCS; decrement on slow RNS | Unstable MUAPs, ± short-duration MUAPs |
| NMJ — presynaptic (LEMS) | Low CMAP amplitudes; marked post-exercise increment | Unstable MUAPs |
| Radiculopathy | Normal SNAP (lesion proximal to DRG); may have low CMAP if severe | Denervation in a myotomal distribution incl. paraspinals |
Common Diagnoses — EDX Signatures
| Diagnosis | Hallmark findings |
|---|
| Carpal tunnel syndrome | Prolonged median sensory (then motor) distal latency; median-vs-ulnar or median-vs-radial comparison across the wrist is most sensitive. EMG of APB only in moderate-severe cases. |
| Ulnar neuropathy at the elbow | Focal CV slowing >10 m/s and/or conduction block across the elbow segment; reduced ulnar SNAP; EMG of FDI & FCU. |
| Peroneal neuropathy (fibular head) | Focal slowing/conduction block across the fibular head; EMG of tibialis anterior, sparing the short head of biceps femoris. |
| Cervical / lumbosacral radiculopathy | Normal SNAPs; needle denervation in ≥2 muscles of one myotome (incl. paraspinals) supplied by different peripheral nerves. |
| Length-dependent axonal polyneuropathy | Low/absent sural & distal SNAPs, low distal CMAPs, distal-predominant denervation (feet first). Diabetes the most common cause. |
| GBS (AIDP) | Acquired demyelination: prolonged/absent F-waves early, conduction block, temporal dispersion, prolonged distal latencies, slow CV. |
| CIDP | Same demyelinating features, chronic/relapsing, with secondary axon loss over time. |
| ALS / motor neuron disease | Normal sensory NCS; widespread active + chronic denervation across ≥3 spinal regions (bulbar/cervical/thoracic/lumbosacral). |
| Myasthenia gravis | Decrement on slow RNS; single-fiber EMG shows increased jitter/blocking (most sensitive). |
| LEMS | Low resting CMAP; >60–100% increment after brief exercise / on fast RNS. |
Needle EMG Localization — Key Muscles
| Root | Representative muscles | Peripheral nerve(s) |
|---|
| Cervical / upper limb |
| C5–C6 | Deltoid, biceps, brachioradialis | Axillary, musculocutaneous, radial |
| C6–C7 | Pronator teres, flexor carpi radialis, triceps | Median, radial |
| C8–T1 | First dorsal interosseous, abductor pollicis brevis, FDP | Ulnar, median |
| Lumbosacral / lower limb |
| L2–L4 | Vastus medialis/lateralis, iliopsoas, adductors | Femoral, obturator |
| L4–L5 | Tibialis anterior, extensor hallucis longus, gluteus medius | Peroneal/fibular, superior gluteal |
| S1–S2 | Gastrocnemius, soleus, abductor hallucis | Tibial |
Always sample paraspinal muscles and confirm a level with ≥2 muscles from different peripheral nerves sharing the same root before calling a radiculopathy.
A Practical Approach
- Is the process neuropathic, myopathic, NMJ, or anterior-horn?
- If neuropathic: axonal vs demyelinating, and focal vs generalized (sensory, motor, or both)?
- Localize: nerve, plexus, or root — use the SNAP (normal in radiculopathy) and the pattern of muscle involvement (single nerve vs myotome vs diffuse).
- Correlate with the clinical exam — EDX is an extension of the exam, not a substitute for it.
Ahmed Koriesh, MD
