EDX · EP 03 · ELECTRODIAGNOSTICS
Before You Listen
- Prerequisites: the basic neuron architecture from EDX-01 (axon, myelin, dorsal root ganglion (DRG) cell body); the instrumentation rules from EDX-02 (supramaximal stimulation, filter settings, the 32 °C / 30 °C minimum temperature rule, ~2 m/s slowing per degree Celsius of cooling); the Erlanger-Gasser fiber-diameter classification; Wallerian degeneration timing (motor 7-9 days, sensory 10-11 days).
- Runtime: 1 hour 2 minutes 30 seconds.
- Topic in one line: the five core nerve conduction study (NCS) parameters (latency, amplitude, conduction velocity, duration, area), the compound muscle action potential (CMAP) versus sensory nerve action potential (SNAP) distinction with the dorsal-root-ganglion-preservation rule that defines preganglionic versus postganglionic localization, the AANEM consensus criteria for conduction block (>50% amplitude drop with <30% duration increase, >60% threshold for the tibial nerve), the antidromic-versus-orthodromic technique trade-off, the three classic anomalous innervations (Martin-Gruber anastomosis (MGA), Riche-Cannieu anastomosis (RCA), accessory deep peroneal nerve (accessory DPN)) and the paradoxical CMAP patterns they produce, the axonal-versus-demyelinating dichotomy with the uniform-versus-non-uniform demyelination rule (uniform = hereditary, conduction block = acquired), and the inching technique with the carpal-tunnel-syndrome (CTS) comparison studies.
Vignette. A 52-year-old right-handed accountant presents with three months of progressive, asymmetric distal weakness in both hands and a foot drop on the right. Reflexes are reduced. Motor NCS show a median CMAP at the wrist of 3.2 mV with a CMAP at the elbow of 6.8 mV, with a negative-peak duration of 7.2 ms at the wrist and 7.5 ms at the elbow; ulnar motor CMAP at the wrist is 8.6 mV, at the elbow 4.1 mV, with similar duration. Median sensory studies show a normal-amplitude SNAP. F-waves are prolonged. Sural and superficial fibular SNAPs are preserved.
Which AANEM consensus criterion is met for conduction block, what is the most likely electrodiagnostic diagnosis, why are the SNAPs spared, and what single anomalous innervation must be excluded before locking in the diagnosis?
(Answer at the end of this chapter)
Section 1: The Five Parameters and Why DML Cannot Yield a Velocity
Bottom line: the five NCS parameters (latency, amplitude, conduction velocity (CV), duration, area) each test a different physical aspect of the nerve; onset latency is the standard for motor and peak latency is the standard for sensory; the distal motor latency (DML) includes nerve conduction time plus neuromuscular junction (NMJ) delay (~0.5-1.0 ms) plus muscle fiber activation, so a true CV cannot be calculated from a single distal stimulation site.
Every NCS begins with a stimulus delivered to a peripheral nerve and a response captured at a distant electrode. Five core parameters are extracted from each tracing. Latency is the time interval from stimulus to response, in milliseconds (ms). Onset latency runs from the stimulus artifact to the initial deflection from baseline; it captures the fastest-conducting fibers and is the standard for motor studies. Peak latency runs from the stimulus artifact to the negative peak of the response; it is the standard for sensory studies because the SNAP is small (microvolts (microV)) and the takeoff from baseline is harder to call than the well-defined peak. The distal motor latency (DML) is the onset latency at the most distal standard stimulation site and folds three components into one number: distal nerve conduction time, NMJ transmission time (~0.5-1.0 ms of synaptic delay), and muscle fiber depolarization time. Because the DML includes NMJ transmission and muscle fiber activation in addition to nerve conduction time, a true CV cannot be calculated from a single distal stimulation site; CV calculation requires at least two stimulation points so that the shared non-neural components cancel out in the subtraction.
Amplitude reflects how many fibers contribute to the response. CMAP amplitude is measured baseline-to-negative-peak in millivolts (mV), with normal values 2-20 mV depending on the nerve-muscle pair. SNAP amplitude is measured baseline-to-peak or peak-to-peak in microV, with normal values 6-80 microV. The thousand-fold scale difference exists because the SNAP is a direct nerve recording while the CMAP is amplified by the NMJ-and-muscle generator. A reduced CMAP has four causes worth memorizing: axonal loss, conduction block, NMJ failure, and technical errors (submaximal stimulation, active electrode (E1) off the motor point). A reduced SNAP almost always means sensory axonal loss or a postganglionic lesion, the diagnostic backbone of the next section.
Conduction velocity (CV) is the speed of action-potential propagation, calculated as CV (m/s) = Distance (mm) / (Proximal Latency − Distal Latency) (ms). The subtraction strips out the NMJ-and-muscle component. Normal motor CV is greater than or equal to 49-50 m/s in the upper extremity (UE) and greater than or equal to 40-44 m/s in the lower extremity (LE); the slower LE values reflect longer nerves, slightly cooler temperatures, and modest fiber-diameter differences. CV reflects the fastest, largest myelinated fibers and is the parameter most sensitive to demyelinating pathology. Duration is measured from initial deflection to baseline crossing and reflects the range of conduction velocities (temporal dispersion) among contributing fibers. Area under the negative peak represents total electrical charge and is more accurate than amplitude for detecting conduction block, because temporal dispersion can reduce amplitude without reducing area while true conduction block reduces both.
Normal Reference Values
The board tests specific absolute cutoffs for the most commonly performed nerve conduction studies. The table below lists standard reference values (Preston & Shapiro guidelines):
| Nerve & Study Type | Recording Site | Distal Latency | Amplitude | Conduction Velocity |
|---|---|---|---|---|
| Median Motor | Abductor pollicis brevis (APB) | ≤ 4.4 ms (onset) | ≥ 4.0 mV | ≥ 49 m/s |
| Ulnar Motor | Abductor digiti minimi (ADM) | ≤ 3.3 ms (onset) | ≥ 6.0 mV | ≥ 49 m/s |
| Fibular Motor | Extensor digitorum brevis (EDB) | ≤ 6.5 ms (onset) | ≥ 2.0 mV | ≥ 44 m/s |
| Tibial Motor | Abductor hallucis (AH) | ≤ 5.8 ms (onset) | ≥ 4.0 mV | ≥ 41 m/s |
| Median Sensory | Digit 2 (14 cm distance) | ≤ 3.5 ms (peak) | ≥ 20 μV | — |
| Ulnar Sensory | Digit 5 (14 cm distance) | ≤ 3.1 ms (peak) | ≥ 17 μV | — |
| Radial Sensory | Anatomical snuffbox (10 cm) | ≤ 2.9 ms (peak) | ≥ 15 μV | — |
| Sural Sensory | Posterior lateral calf to ankle | ≤ 4.4 ms (peak) | ≥ 6 μV | — |
| Superficial Fibular | Lateral lower leg to foot dorsum | ≤ 4.4 ms (peak) | ≥ 6 μV | — |
High Yield — The five NCS parameters
- Onset latency = standard for motor (fastest fibers).
- Peak latency = standard for sensory (more reproducible than the takeoff of a microvolt-level SNAP).
- DML = nerve conduction + ~0.5-1.0 ms NMJ + muscle activation; CV cannot be calculated from a single distal stimulation site.
- CMAP amplitude = mV (2-20 mV); reduced by axonal loss, conduction block, NMJ failure, technical error.
- SNAP amplitude = microV (6-80 microV); reduced almost only by sensory axonal loss or postganglionic lesions.
- CV (m/s) = distance / (proximal − distal latency); normal UE ≥49-50 m/s, LE ≥40-44 m/s.
- Duration = temporal dispersion marker.
- Area = total charge; better than amplitude for detecting conduction block.
Mnemonic — “DML carries luggage; you can’t time it alone”
The DML carries three pieces of luggage at once: nerve conduction, NMJ delay, and muscle activation. You can’t separate them with a single stamp. Two stamps (a proximal and a distal stimulation site) let the shared luggage cancel out in the subtraction, so the difference is pure nerve conduction time, and only then can you compute a velocity.
Antidromic vs orthodromic recording — the trade-off
The choice between antidromic and orthodromic technique for sensory studies trades amplitude resolution against artifact. Antidromic recording stimulates proximally and records distally over a digital nerve; the electrical pulse travels in the opposite direction to physiologic conduction (efferent in the sensory nerve). The recording site sits closer to receptor terminals, so the SNAP amplitude is larger and small fibers are easier to detect. The cost is motor co-stimulation artifact because the antidromic pulse activates the underlying motor nerve and the recording electrodes are over the digit’s intrinsic muscles, producing a contaminating CMAP. Orthodromic recording stimulates distally (ring electrodes on the digit) and records proximally over the nerve trunk; the pulse travels in the physiologic afferent direction. The recording site is over nerve, not muscle, so motor co-stimulation does not contaminate the trace, but the SNAP amplitude is smaller because of recording distance. Practical defaults: antidromic for median and ulnar sensory studies in CTS evaluation (the larger amplitude wins); orthodromic for sural and for studies where motor co-stimulation would obscure the trace.
Normal-value reference (adult)
| Nerve | DML cutoff | Min CMAP | CV cutoff | SNAP peak latency / amplitude |
|---|---|---|---|---|
| Median (motor) | ≤4.4 ms | ≥4 mV | ≥49 m/s | — |
| Ulnar (motor) | ≤3.6 ms | ≥6 mV | ≥49 m/s | — |
| Tibial (motor) | ≤5.8 ms | ≥3 mV | ≥41 m/s | — |
| Fibular (motor) | ≤6.5 ms | ≥2 mV | ≥41 m/s | — |
| Median (sensory) | — | — | — | ≤3.5 ms / ≥20 µV (digit II antidromic) |
| Ulnar (sensory) | — | — | — | ≤3.1 ms / ≥17 µV (digit V antidromic) |
| Sural (sensory) | — | — | — | ≤4.4 ms / ≥6 µV |
Values are reference defaults; institutional laboratory ranges supersede these when available, and age >60–70 lowers expected sural SNAP amplitude (often absent in normals).