Residency · Residency · Physical Medicine Rehabilitation

Needle Electromyography: Interpretation and Technique

Overview and Indications

Role of Needle EMG

Needle electromyography (EMG) is the intramuscular component of the electrodiagnostic examination. Assesses the electrical activity of muscle fibers and motor units at rest and during voluntary contraction. Complements nerve conduction studies to localize lesions, determine chronicity, assess severity, and characterize neuromuscular pathology. Essential for evaluating radiculopathy, plexopathy, motor neuron disease, myopathy, and neuromuscular junction disorders.

Indications

Suspected radiculopathy (cervical or lumbosacral). Peripheral nerve injury (localization and prognosis). Suspected motor neuron disease (ALS). Myopathy evaluation.

Neuromuscular junction disorders. Assessment of reinnervation and recovery after nerve injury. Differentiation of neurogenic versus myopathic weakness.

Equipment and Technique

Needle Types

Concentric needle electrode: most commonly used; recording surface is the exposed tip within a beveled cannula. Monopolar needle electrode: Teflon-coated needle with exposed tip; requires a separate surface reference electrode. Concentric needles have slightly smaller recording area and produce lower amplitude MUAPs compared to monopolar. Single fiber EMG needle: specialized needle for neuromuscular junction assessment (not routine).

Patient Positioning and Preparation

Position the patient comfortably with the target muscle accessible and relaxed. Explain the procedure: mild discomfort is expected; relaxation is critical for accurate assessment. No specific preparation required (no fasting, no medication changes). Anticoagulation is NOT a contraindication; use caution in high-risk areas (paraspinals in anticoagulated patients). Avoid insertion into infected tissue or through lymphedematous limbs when possible.

Systematic Examination Approach

Select muscles based on the clinical question and NCS findings. Typical screen includes proximal and distal muscles, multiple myotomes, and multiple peripheral nerve territories. For radiculopathy: sample muscles from at least two different peripheral nerves sharing the same root, plus paraspinal muscles. For each muscle, assess: insertional activity, spontaneous activity at rest, motor unit morphology, and recruitment.

<image>Illustration of the four phases of needle EMG assessment showing insertional activity upon needle movement, spontaneous activity at rest with the muscle relaxed, motor unit action potential morphology during minimal voluntary contraction, and recruitment pattern analysis during increasing effort</image>

Insertional Activity

Normal Insertional Activity

Brief burst of electrical activity provoked by needle movement through muscle tissue. Lasts less than 300 ms after needle movement ceases. Generated by mechanical depolarization of muscle fibers by the needle.

Abnormal Insertional Activity

Increased insertional activity: prolonged bursts continuing after needle movement stops; seen early in denervation, myopathy, and inflammatory conditions. Decreased insertional activity: reduced or absent response; seen in chronic denervation with fibrosis and fatty replacement, or in severe end-stage myopathy. Increased insertional activity is often the earliest electrodiagnostic finding in acute denervation (before fibrillations appear).

Spontaneous Activity

Normal Spontaneous Activity

Endplate noise (miniature endplate potentials): low-amplitude (10-50 microvolts), monophasic negative potentials; "seashell" sound. Endplate spikes: higher-amplitude, biphasic (initial negative) potentials firing irregularly; generated by needle irritation of intramuscular nerve terminals. Both are normal findings when the needle is in the endplate zone; reposition the needle.

Abnormal Spontaneous Activity

Fibrillation Potentials

Spontaneous depolarization of a single denervated muscle fiber. Morphology: biphasic (initial positive) or triphasic, 1-5 ms duration, 20-200 microvolt amplitude. Regular firing rate (0.5-10 Hz), rhythmic "rain on a tin roof" sound. Appear 2-3 weeks after denervation (axonal degeneration must reach the muscle).

Graded 1+ to 4+ based on density and persistence. Also seen in: myopathies (inflammatory, necrotizing), neuromuscular junction disorders (severe), and upper motor neuron lesions (rarely).

Positive Sharp Waves (PSWs)

Same pathologic significance as fibrillation potentials. Morphology: initial sharp positive deflection followed by a slow negative phase. Regular firing rate similar to fibrillations. Generated when the needle tip is adjacent to the depolarizing fiber. Fibrillations and PSWs typically coexist and are graded together.

Fasciculation Potentials

Spontaneous firing of an entire motor unit. Irregular firing rate (distinguishes from voluntary MUAPs). Can be benign (benign fasciculation syndrome) or pathologic (ALS, radiculopathy, nerve compression). Cannot distinguish benign from pathologic by fasciculation morphology alone.

Complex fasciculations (polyphasic, unstable) are more concerning for pathology. Must be interpreted in context of other EMG findings.

Complex Repetitive Discharges (CRDs)

Groups of muscle fibers firing in a near-simultaneous, time-locked pattern via ephaptic transmission. Abrupt onset and cessation ("machine-like" sound). Regular firing rate, uniform morphology. Non-specific: seen in chronic neurogenic and myopathic conditions. Indicate chronicity of the process.

Myotonic Discharges

Repetitive firing of single muscle fibers with waxing and waning frequency and amplitude. Characteristic "dive bomber" or "revving motorcycle" sound. Seen in: myotonic dystrophy, myotonia congenita, paramyotonia congenita, acid maltase deficiency, hypothyroid myopathy. Also occurs in some inflammatory myopathies and hyperkalemic periodic paralysis.

Myokymic Discharges

Grouped, repetitive firing of a motor unit in a semirhythmic pattern ("marching soldiers"). Bursts of motor unit discharges at regular intervals. Seen in: radiation plexopathy (classic), GBS, multiple sclerosis, brainstem lesions. Important to distinguish radiation plexopathy (myokymia present) from tumor recurrence (myokymia absent).

Neuromyotonic Discharges

Very high frequency (150-300 Hz) decrementing bursts of motor unit potentials. "Pinging" sound. Associated with Isaacs syndrome, anti-CASPR2 antibodies, and peripheral nerve hyperexcitability.

<image>Oscilloscope tracings showing the morphology of common abnormal spontaneous activities including fibrillation potentials, positive sharp waves, fasciculation potentials, complex repetitive discharges, and myotonic discharges with their characteristic sound descriptions and clinical associations</image>

Motor Unit Action Potential (MUAP) Analysis

Normal MUAP Characteristics

Duration: 5-15 ms (varies by muscle and age). Amplitude: 200-2000 microvolts (varies by muscle). Phases: typically 2-4 phases (a phase = each baseline crossing + 1). Polyphasic MUAPs: >4 phases; up to 10-15% polyphasic MUAPs is normal in most muscles. Turns: direction changes that do not cross baseline. Stability: consistent morphology on repeated firing.

Neurogenic MUAP Changes

Increased duration: due to collateral sprouting incorporating distant muscle fibers. Increased amplitude: more muscle fibers per motor unit from reinnervation. Polyphasic MUAPs: from reinnervating sprouts with different conduction times. Unstable (variable) MUAPs: immature sprouts with unreliable transmission.

Satellite (linked) potentials: late components from slow-conducting sprouts. These changes reflect chronic reinnervation and take weeks to months to develop.

Myopathic MUAP Changes

Decreased duration: loss of muscle fibers within the motor unit. Decreased amplitude: fewer contributing muscle fibers. Increased polyphasia: variable fiber sizes and conduction times. Early recruitment (see below): more motor units activated for a given force. Short-duration, low-amplitude, polyphasic MUAPs (SLAP pattern) is the classic myopathic pattern.

Neuromuscular Junction MUAP Changes

Moment-to-moment variability in MUAP morphology (jitter and blocking). Best assessed with single fiber EMG or repetitive nerve stimulation. Standard needle EMG may show unstable MUAPs.

Recruitment Analysis

Normal Recruitment

Motor units are recruited in an orderly fashion (Henneman size principle: small units first). As force increases, firing rate of active units increases and new units are recruited. Normal onset frequency: approximately 5-10 Hz. Normal recruitment ratio: firing rate / number of MUAPs = approximately 5:1.

Reduced Recruitment (Neurogenic Pattern)

Fewer motor units available (axonal loss or conduction block). Remaining units fire at increased rates to compensate (>15-20 Hz before additional unit recruited). "Fast-firing" pattern with gaps between units. Indicates loss of motor units (neurogenic process).

Early Recruitment (Myopathic Pattern)

Motor units are present but each generates less force (fewer muscle fibers per unit). More motor units must be recruited to generate the same force. Many MUAPs firing at relatively low rates for the level of force generated. "Full" interference pattern at low force levels. Indicates myopathic process.

Decreased Activation (Central or Effort-Related)

Few motor units firing at low rates. Firing rate does not increase despite encouragement. Seen in: upper motor neuron lesions, pain inhibition, poor effort, conversion disorder. Distinguished from neurogenic reduced recruitment by the low firing rate.

Grading Spontaneous Activity

Standard Grading Scale

0: no spontaneous activity. 1+: persistent activity in at least 2 areas with transient activity in other areas. 2+: moderate amount of persistent activity in 3 or more areas. 3+: abundant spontaneous activity filling the screen in all areas. 4+: dense spontaneous activity filling the screen with no identifiable baseline in all areas.

Timing of Electrodiagnostic Abnormalities After Nerve Injury

Temporal Sequence

Day 0-3: NCS may be normal; EMG normal. Day 3-5: distal NCS amplitudes begin to decline (Wallerian degeneration progresses distally). Day 7-10: motor NCS amplitude loss complete; sensory NCS amplitude declining. Day 10-14: increased insertional activity may appear in denervated muscles.

Day 14-21: fibrillation potentials and PSWs appear in proximal muscles first. Day 21-35: fibrillations appear in more distal muscles. Weeks to months: neurogenic MUAP changes appear with reinnervation. Optimal timing for electrodiagnostic evaluation: 3-4 weeks after injury onset.

Clinical Pearls

Never perform needle EMG without first obtaining a focused history and examination; the study should be hypothesis-driven. Fibrillation potentials take 2-3 weeks to develop; a normal EMG performed too early does not exclude denervation. Paraspinal muscle fibrillations support a diagnosis of radiculopathy but can also be seen in prior surgery, spinal stenosis, and even normal aging. Fasciculation potentials in isolation are not diagnostic of ALS; they must be accompanied by other findings (fibrillations, neurogenic MUAP changes, reduced recruitment).

In myopathy, fibrillation potentials suggest active/inflammatory process (polymyositis, inclusion body myositis) versus chronic/non-inflammatory (late muscular dystrophy). Myokymic discharges in the limbs should raise suspicion for radiation plexopathy. The needle EMG is operator-dependent; proficiency requires training and ongoing experience. Always communicate findings to the patient and referring physician in the context of the clinical picture.

References

• Preston DC, Shapiro BE. Electromyography and Neuromuscular Disorders: Clinical-Electrophysiologic-Ultrasound Correlations. 4th Edition. Elsevier. 2021.
• Dumitru D, Amato AA, Zwarts MJ. Electrodiagnostic Medicine. 2nd Edition. Hanley & Belfus. 2002.
• Kimura J. Electrodiagnosis in Diseases of Nerve and Muscle: Principles and Practice. 5th Edition. Oxford University Press. 2013.
• AANEM Position Statement: Proper Performance and Interpretation of Electrodiagnostic Studies. Muscle Nerve. 2014.
• Daube JR, Rubin DI. Needle Electromyography. Muscle Nerve. 2009;39(2):244-270.

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