5.2 Needle Electromyography & MUAP Analysis

5.2 Needle Electromyography & MUAP Analysis

Principles of Needle Electromyography

Needle electromyography (EMG) evaluates the motor unit (single alpha motor neuron, its axon, neuromuscular junctions, and innervated muscle fibers). Needle EMG localizes nerve lesions (radiculopathy vs. plexopathy vs. peripheral nerve), establishes chronicity, assesses active denervation, and differentiates neurogenic from myopathic disorders.

Recording Needles & Safety

• Concentric Needle: Central active wire inside a cannula reference electrode. Provides standardized, reproducible Motor Unit Action Potential (MUAP) measurements.
• Monopolar Needle: Teflon-coated needle tip paired with a surface reference. Yields larger amplitudes and less pain.
• Safety: Caution in patients on anticoagulation or with severe thrombocytopenia. High-risk muscles for pneumothorax include serratus anterior, diaphragm, rhomboids, and paraspinals.

                   NEEDLE EMG EXAMINATION STAGES
 +----------------------------------------------------------------+
 | 1. Insertional Activity   --> Mechanical insertion of needle   |
 | 2. Spontaneous Activity --> Assessment of resting muscle       |
 | 3. MUAP Morphology      --> Minimal voluntary contraction      |
 | 4. Recruitment Pattern  --> Maximal voluntary effort           |
 +----------------------------------------------------------------+

Insertional & Spontaneous Activity Analysis

In healthy muscle, needle insertion produces a brief burst lasting <300 ms, after which the muscle is electrically silent at rest.

Insertional Activity Variations

• Increased Insertional Activity: Bursts extending >300 ms. Indicates membrane instability in active denervation, myopathies (polymyositis, myotonia), or acute injury.
• Decreased Insertional Activity: Attenuated or absent bursts. Indicates replacement of muscle by fibrous tissue, severe atrophy, or periodic paralysis attacks.

Endplate Noise vs. Endplate Spikes (Normal Findings)

Direct needle placement in the neuromuscular endplate produces normal findings:

• Endplate Noise: Low-amplitude (<50 µV), continuous negative sea-shell sound from spontaneous miniature endplate potentials (MEPPs).
• Endplate Spikes: Irregular, biphasic potentials with initial negative deflection ("sputtering" sound) from single muscle fiber irritation.

Pathological Spontaneous Activity

Fibrillation Potentials & Positive Sharp Waves (PSWs)

Fibrillations and positive sharp waves indicate active muscle fiber denervation or sarcolemmal instability.

• Pathophysiology: Loss of axonal innervation causes muscle fibers to upregulate extrajunctional acetylcholine receptors and develop spontaneous membrane oscillations.
• Morphology: Fibrillations are brief (1–5 ms), low-amplitude (20–300 µV) spikes with an initial positive deflection. PSWs are sharp positive deflections followed by a slow negative phase. Both fire metronomically (0.5–10 Hz).
• Timing Post-Injury: Fibrillations and PSWs appear 2 to 3 weeks following acute axonal injury (10–14 days in proximal paraspinals, 3–4 weeks in distal limbs).

Fasciculation Potentials

Involuntary discharges of an entire motor unit (from anterior horn cell or axon). Polyphasic or simple MUAP firing at irregular, slow intervals (0.1–2 Hz). Pathological in Amyotrophic Lateral Sclerosis (ALS) and radiculopathies; also occurs as a benign finding.

Myotonic Discharges

Spontaneous high-frequency trains of muscle fiber action potentials from channelopathies. Classic "dive-bomber" acoustic signature with waxing/waning amplitude (10 µV–1 mV) and frequency (20–150 Hz). Seen in Myotonic Dystrophy (DM1/DM2) and Myotonia Congenita.

Complex Repetitive Discharges (CRDs)

Originate from a pacemaker muscle fiber depolarizing adjacent fibers via ephaptic transmission. Abrupt onset/offset, uniform polyphasic waveform, rapid machine-like rhythm (10–100 Hz, "jackhammer" sound). Seen in chronic radiculopathy and Inclusion Body Myositis.

Motor Unit Action Potential (MUAP) Analysis

During mild contraction, MUAPs are analyzed for duration, amplitude, and phase count. During maximal effort, recruitment is evaluated.

                  MUAP MORPHOLOGY COMPARISON
  Neurogenic MUAP                      Myopathic MUAP
  Long Duration (>15 ms)               Short Duration (<5 ms)
  High Amplitude (>2-5 mV)             Low Amplitude (<0.5 mV)
  Polyphasic (>4 phases)               Polyphasic (>4 phases)

Neurogenic MUAP & Recruitment Patterns

Axonal loss leads to collateral sprouting from surviving axons, expanding motor unit territory:

• MUAP Morphology: Long duration (>15 ms), high amplitude (>2–5 mV), and polyphasic (>4 phases).
• Recruitment Pattern: Reduced recruitment. Remaining active units fire at abnormally high frequencies (>20–30 Hz) to compensate.

Myopathic MUAP & Recruitment Patterns

Loss of muscle fibers per motor unit reduces force output per unit:

• MUAP Morphology: Short duration (<5–8 ms), low amplitude (<0.5 mV), polyphasic.
• Recruitment Pattern: Early / excessive recruitment. The CNS rapidly recruits an excessive number of small myopathic units for minimal effort.