EDX · EP 05 · ELECTRODIAGNOSTICS
Before You Listen
- Prerequisites: basic neurophysiology of the alpha motor neuron and motor unit; Wallerian degeneration; the principle that nerve conduction studies (NCS) and needle electromyography (EMG) together form the electrodiagnostic (EDX) examination; the concept that denervated muscle fibers develop membrane supersensitivity. EDX Episodes 1 through 4 covered NCS, F-waves, H-reflexes, repetitive nerve stimulation, and single-fiber EMG.
- Runtime: 1 hour 8 minutes.
- Topic in one line: the four sequential phases of the needle examination (insertional activity, spontaneous activity at rest, motor unit action potential or MUAP morphology, recruitment), anchored by the auditory contrast of fat-frying-in-a-pan endplate spikes (initial negative deflection, normal) versus rain-on-a-tin-roof fibrillation potentials (initial positive deflection, metronomic regularity, pathological); the dive-bomber waxing-and-waning of myotonic discharges; the marching-soldiers myokymic discharges that fingerprint radiation plexopathy at 50 to 70%; the Rule of Fives for recruitment; and the four integrated diagnostic patterns.
Vignette. A 55-year-old woman presents to the EDX laboratory with three weeks of progressive proximal arm and thigh weakness. She has difficulty rising from a chair and lifting her grandchildren. Her serum creatine kinase (CK) is markedly elevated. The needle EMG of the left deltoid shows increased insertional activity, 2+ fibrillation potentials and positive sharp waves (PSWs) at rest, short-duration low-amplitude polyphasic motor unit action potentials (MUAPs) on minimal effort, and a full interference pattern with low envelope amplitude at minimal voluntary effort.
Which of the four diagnostic patterns does this represent, why are fibrillation potentials present in a non-denervated process, what single MUAP parameter is most reliable for distinguishing this from a chronic neurogenic disorder, and what is the most likely clinical diagnosis?
(Answer at the end of this chapter)
Section 1: Insertional Activity and the Resting Endplate Zone
Bottom line: the needle EMG is organized into four sequential phases (insertional activity, spontaneous activity, MUAP morphology, recruitment), and every muscle must be assessed through all four; insertional activity is the brief volley generated by needle movement through viable muscle, with increased activity (persisting after needle stops) marking the earliest sign of denervation and decreased activity marking fibrosis or fatty replacement.
The needle EMG is the second pillar of the electrodiagnostic examination, complementing nerve conduction studies (NCS). NCS evaluate the peripheral nerve through conduction velocity, amplitude, and latency. The needle examination evaluates the muscle fiber, the neuromuscular junction (NMJ), and the motor unit as an integrated functional entity. A concentric or monopolar needle electrode is inserted directly into a muscle, and the examiner systematically evaluates electrical activity through four sequential phases: (1) insertional activity, (2) spontaneous activity at rest, (3) MUAP morphology during voluntary contraction, and (4) recruitment and interference pattern during graded effort. Every muscle must be assessed through all four phases.
Two electrode types are used. Concentric needle electrodes contain the active recording surface within a beveled cannula, with the cannula serving as the reference. They record from a smaller territory and produce MUAPs of shorter duration and lower amplitude. Monopolar needle electrodes use a Teflon-coated needle with only the exposed tip as the active electrode and a separate surface electrode as the reference. Monopolar needles record from a larger territory, producing MUAPs of longer duration and higher amplitude. Normal reference values differ between electrode types.
Insertional activity is the brief burst of electrical activity generated when the needle electrode is moved through muscle tissue. The mechanical disruption of muscle fiber membranes produces a transient volley that lasts only as long as the needle is moving and ceases within a few hundred milliseconds (ms) after the needle stops. Normal insertional activity confirms that the needle is positioned within electrically viable muscle tissue.
Increased insertional activity is electrical discharge that persists beyond the period of needle movement. This indicates heightened muscle membrane irritability and is one of the earliest electrodiagnostic signs of denervation, appearing before fibrillation potentials and PSWs develop. In the acute phase of nerve injury (the first 1 to 2 weeks), a muscle may demonstrate increased insertional activity as its only abnormality. If a stem presents a patient evaluated 5 days after radicular symptom onset with increased insertional activity but no fibrillations, the correct interpretation is that denervation is in its earliest phase and the study should be repeated in 2 to 4 weeks. Increased insertional activity is also seen in inflammatory myopathies (polymyositis, dermatomyositis, inclusion body myositis), in muscular dystrophies during active fiber degeneration, and in metabolic myopathies with membrane instability.
Decreased or absent insertional activity occurs when the needle encounters less excitable tissue. Common causes: fibrosis (muscle replaced by scar tissue), fatty replacement, severe longstanding denervation with fibrofatty replacement, acute rhabdomyolysis or severe compartment syndrome (early phase), and the trivial case of needle position outside muscle (subcutaneous fat, fascia, or tendon). A muscle with extensive fibrotic replacement is unlikely to recover useful function even if the nerve supply is restored.
After insertional activity is assessed, the examiner holds the needle still and listens for spontaneous activity. In normal muscle, outside the endplate zone, there should be electrical silence. Incomplete relaxation is one of the most common sources of artifact, because volitional motor unit firing may be misinterpreted as spontaneous activity.
The endplate zone is a normal source of electrical activity. Miniature endplate potentials (MEPPs, endplate noise) arise from spontaneous quantal release of acetylcholine vesicles at the NMJ. They are monophasic negative potentials of less than 100 microvolts (uV), firing irregularly and continuously at 20 to 40 hertz (Hz), producing a hissing sound classically described as a seashell held to the ear. Endplate spikes are generated when the needle irritates terminal nerve twigs in the endplate region. They range 100 to 500 uV, are biphasic or triphasic with an initial negative deflection, and fire irregularly with a sputtering pattern that sounds like fat frying in a pan. Frequency can reach 50 Hz, but firing is inconsistent. Both are normal findings at the motor endplate zone.
High Yield — Insertional activity and the four phases
- Four phases of needle EMG (in order): insertional activity, spontaneous activity at rest, MUAP morphology, recruitment / interference pattern.
- Normal insertional activity: brief volley with needle movement, ceases when needle stops.
- Increased insertional activity: persists after needle stops; earliest sign of denervation (before fibs); also inflammatory myopathy, active dystrophy.
- Decreased insertional activity: fibrosis, fatty replacement, end-stage denervation, needle outside muscle. Important prognostic implications.
- Endplate noise (MEPPs): monophasic negative, less than 100 uV, irregular continuous firing at 20-40 Hz, “seashell” sound. NORMAL at endplate zone.
- Endplate spikes: biphasic/triphasic with initial negative deflection, 100-500 uV, irregular sputtering, “fat frying” sound. NORMAL.
- Concentric needle = smaller territory, shorter duration / lower amplitude MUAPs; monopolar needle = larger territory, longer duration / higher amplitude MUAPs.
Mnemonic — “I-S-M-R” for the four phases
Insertional, Spontaneous, MUAP morphology, Recruitment. Every muscle, every time, in this order. If a board stem skips a phase, that is a clue to which finding is being tested.