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How to Read emg report

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EMG needle electrode motor unit action potential waveform interpretation

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how to interpret EMG report step by step fibrillation potentials motor unit action potentials

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How to Read an EMG Report

An EMG report (usually called an "EMG/NCS" report) has two main parts done together: nerve conduction studies (NCS) and needle electromyography (EMG). Reading it means working through each part systematically, then combining them into a pattern.

1. Nerve Conduction Studies (NCS) - the first half of the report

NCS stimulates a nerve electrically and records the response, either from a muscle (motor study) or from the nerve itself (sensory study).
Key numbers to check, each compared against the lab's own reference ranges (age/height/sex-matched):
  • Latency - time from stimulus to response. Prolonged latency suggests a focal slowing/demyelination (e.g., at a compression point like the carpal tunnel).
  • Amplitude (CMAP for motor, SNAP for sensory) - reflects the number of intact axons. Low amplitude = axon loss.
  • Conduction velocity - distance/time between two stimulation points. Normal is roughly 50-70 m/sec in the arms and 40-60 m/sec in the legs. Slowing suggests demyelination; near-normal velocity with low amplitude suggests axonal loss.
  • F-waves and H-reflexes - assess proximal nerve segments (useful in radiculopathy, Guillain-Barre).
General pattern reading:
  • Low amplitude + near-normal velocity = axonal loss.
  • Markedly slowed velocity/prolonged latency, conduction block, or dispersed waveform = demyelinating process.
  • Normal sensory but abnormal motor (or vice versa) helps localize to root (sensory often spared in radiculopathy since the lesion is proximal to the dorsal root ganglion) versus peripheral nerve/plexus.

2. Needle EMG - the second half

The needle electrode records muscle electrical activity in three phases:
A. At rest (insertional and spontaneous activity)
  • Insertional activity: brief activity from needle movement - normal, unless prolonged.
  • Spontaneous activity (abnormal if present at rest):
    • Fibrillation potentials / positive sharp waves - single muscle fiber discharges indicating denervation or muscle membrane irritability (myositis). Note timing: these typically take 2-3 weeks to appear after acute nerve injury, so an EMG done too early can look falsely normal.
    • Fasciculations - whole motor unit discharges, seen in motor neuron disease, radiculopathy, or benign causes.
    • Complex repetitive discharges / myotonic discharges - seen in chronic denervation or myotonic disorders respectively.
B. During mild voluntary contraction (motor unit action potential, MUAP, analysis)
  • Amplitude, duration, number of phases are assessed:
    • Large amplitude, long duration, polyphasic MUAPs = neurogenic/reinnervation pattern (chronic denervation with collateral sprouting).
    • Small amplitude, short duration, polyphasic MUAPs = myopathic pattern (fewer functioning fibers per motor unit).
C. During maximal effort (recruitment and interference pattern)
  • Reduced recruitment (fewer motor units firing faster than expected for the effort) = neurogenic loss (fewer motor units available).
  • Early/rapid recruitment with full interference pattern but low amplitude = myopathic process (more units needed for a given force because each is weak).
  • Absent MUAPs despite maximal effort = complete axonal loss or conduction block to that muscle.

3. Putting it together - the "impression"

The electromyographer synthesizes both halves to answer:
  1. Is the lesion neurogenic, myopathic, or neuromuscular junction (e.g., myasthenia, seen on repetitive stimulation)?
  2. Where is it located - single nerve (mononeuropathy, e.g., median at wrist), root (radiculopathy - paraspinal muscle involvement is a clue), plexus, or generalized polyneuropathy (distal, symmetric, length-dependent pattern)?
  3. Axonal vs demyelinating - based on amplitude vs velocity/latency findings.
  4. Acute vs chronic vs ongoing - fibrillations without large reinnervated MUAPs = acute/subacute; large polyphasic MUAPs with reduced recruitment = chronic with reinnervation; both together = chronic process with ongoing active denervation.
  • Bradley and Daroff's Neurology in Clinical Practice: NCS/EMG serve four purposes - localize the lesion, grade severity, determine acuity (acute/subacute/chronic), and determine if it's axonal or demyelinating. It also notes denervation changes may not appear on EMG until up to 4 weeks after axonal injury.
  • Pfenninger and Fowler's Procedures for Primary Care: normal motor conduction velocity is roughly 50-70 m/sec in the arms and 40-60 m/sec in the legs; reference ranges are lab-specific and must be demographically matched.
Below is a genuine EMG waveform showing an abnormal spontaneous discharge (fibrillation potential), the kind of finding you'd see flagged in the "spontaneous activity" section of a report:
Abnormal EMG wave pattern demonstrating a fibrillation potential

Practical tip for reading your own or a patient's report

Look for these labeled sections in order: (1) NCS table with latency/amplitude/velocity for each nerve tested, (2) needle EMG table listing muscles tested with columns for insertional/spontaneous activity, MUAP morphology, and recruitment, (3) a written "Impression" or "Conclusion" that states the overall pattern (e.g., "findings consistent with a chronic left C6 radiculopathy" or "moderate axonal sensorimotor polyneuropathy"). The impression is where the electromyographer's clinical correlation lives - always read it alongside the raw numbers, since EMG/NCS results must be interpreted in the context of the clinical exam.
A quick check of recent literature (meta-analyses/systematic reviews, last 2 years) didn't surface anything that changes this standard interpretive framework - it remains the accepted approach in current neuromuscular medicine.
If you have an actual report you'd like help interpreting, share the specific findings (which nerves/muscles, the numeric values, and what abnormalities are listed) and I can walk through what they mean for your specific case.
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