Abnormal Spontaneous Electromyographic Activity

Issues of Concern

Spontaneous Activity: Abnormal Muscle Fiber Potentials

Abnormal muscle fiber potentials represent isolated fiber discharges outside the end-plate zone. The subsequent descriptions focus on their electrophysiologic characteristics and clinical relevance. 

Fibrillation potentials

Fibrillation potentials are spontaneous action potentials generated by individual muscle fibers in the absence of innervation. Firing patterns are typically regular at rates of 0.5 to 15 Hz and may gradually slow over several seconds before ceasing. Amplitude is variable and generally correlates with fiber diameter. Fibrillation potentials produce a characteristic sound, often described as “rain on the roof,” and present in 2 distinct morphologies. The spike form is triphasic or biphasic, with an initial positive or negative deflection at the site of origin, lasting 1 to 5 ms with amplitudes of 20 to 200 μV. The positive waveform is biphasic, characterized by an initial sharp positivity followed by a prolonged negative phase, with durations ranging from 10 to 30 ms.

The density of fibrillation potentials is graded from 1+ to 4+, as follows:

• 1+: Persistent fibrillation potentials in at least 2 areas
• 2+: Moderate number of persistent fibrillation potentials in 3 or more areas
• 3+: Large number of persistent discharges present in all areas
• 4+: Profuse, widespread, persistent discharges that occupy the baseline

Spontaneous activity within the end-plate region, including endplate noise and endplate spikes, may exhibit a waveform configuration similar to fibrillation potentials. Differentiation is based on an irregular firing pattern, an initial negative deflection, and the tendency to elicit discomfort when the needle contacts the end-plate.

Fibrillation potentials arise in muscle fibers that have lost innervation, have been sectioned transversely or longitudinally, are undergoing regeneration, or have never been innervated.[3][4] In neurogenic disorders, such as radiculopathies, mononeuropathies, or motor neuron disease, axonal loss or degeneration produces denervated muscle fibers. In myopathic conditions, functional denervation occurs in individual fibers or fiber segments as they become separated from the end-plate zone due to muscle necrosis and fiber splitting.[5][6]

Complex repetitive discharges 

Complex repetitive discharges (CRDs) are action potentials generated by a group of muscle fibers that discharge spontaneously in near synchrony with a regular, repetitive pattern. CRDs originate from the spontaneous depolarization of a single fiber, followed by ephaptic spread to adjacent fibers. A variable number of neighboring fibers may subsequently depolarize in sequence until the circuit is complete, at which point the initial fiber discharges again. Therefore, each spike within a CRD represents the action potentials of individual fibers that may belong to different motor units but lie adjacent to one another.

CRDs exhibit a regular pattern with abrupt onset and cessation. Morphology is variable but typically polyphasic or serrated, with 3 to 10 spike components, amplitudes of 50 to 500 µV, and durations up to 50 ms. Frequency ranges from 3 to 40 Hz, producing a sound described as a “motorboat that misfires” or a “jackhammer.” CRDs usually occur spontaneously or manifest in response to needle movement. These patterns are nonspecific and appear in chronic or longstanding neurogenic and myopathic disorders, including chronic radiculopathies, peripheral neuropathies, and slowly progressive myopathies.

Myotonic discharges 

Myotonic discharges are action potentials generated by single muscle fibers that fire spontaneously in a prolonged manner following external excitation. These discharges result from membrane abnormalities within the affected muscle fiber.

Firing occurs with a regular rhythm, and rates vary exponentially between 40 and 100 Hz. Two distinct morphologies are described: biphasic spike potentials, characterized by an initial sharp positivity followed by a prolonged negative phase, and positive waves.

Amplitude and frequency wax and wane, producing a characteristic sound often described as a “dive-bomber.” Although slow-firing myotonic discharges may resemble fibrillation potentials morphologically, the rapid changes in frequency and amplitude, along with the distinctive auditory pattern, allow differentiation.

Myotonic discharges are typically observed in myotonic dystrophy, myotonia congenita, and paramyotonia congenita.[7][8] These patterns may also occur in other myopathies without myotonia, including hyperkalemic periodic paralysis, polymyositis, and acid maltase deficiency.[9] Occasional appearance in severe axonal disorders is possible, but these discharges are never the predominant waveform.

Spontaneous Activity: Abnormal Motor Unit Potentials

Spontaneous motor unit potentials occur when multiple fibers within a motor unit fire in the absence of voluntary activation. Key patterns and their defining EMG features are presented below. 

Fasciculation potentials 

Fasciculation potentials are spontaneous single motor unit discharges arising anywhere along the lower motor neuron, most commonly from spontaneous firing at the nerve terminal. Morphology may resemble simple motor unit action potentials (MUAPs) or present as complex, enlarged potentials when originating from a pathologic motor unit. Firing patterns are irregular, occur at low frequencies of 1 to 2 Hz, and produce characteristic sounds described as “large raindrops on a tin roof” or “popcorn.”

Fasciculations are graded from 1+ to 4+, as follows:

• 1+: Present in at least 2 areas of the muscle; frequency 2 to 10/min
• 2+: Moderate number of persistent fasciculations in multiple areas; frequency 10 to 50/min
• 3+: Large number of persistent fasciculations in all areas; frequency 50 to 100/min
• 4+: Profuse, widespread, persistent fasciculations in all areas; frequency greater than 100/min

Fasciculation potentials may be observed in healthy individuals and across a range of neuromuscular disorders. These patterns are particularly prevalent in chronic neurogenic conditions, including radiculopathies, axonal peripheral neuropathies, and anterior horn cell disorders such as amyotrophic lateral sclerosis and spinal muscular atrophy.

Doublets, triplets, and multiplets

Spontaneous MUAPs occurring in pairs are termed "doublets," whereas those appearing in groups of 3 or more are designated "triplets" and "multiplets," respectively. These patterns share the same clinical significance as fasciculation potentials, representing spontaneous depolarization of a motor unit or its axon. Doublets, triplets, and multiplets frequently accompany fasciculations, which, in this context, may be referred to as singlets. These MUAP patterns can be observed in any condition that produces fasciculations, including various neuropathic disorders, and are also prominently associated with hypocalcemic tetany.

Myokymic discharges 

Myokymic discharges are groups of spontaneously firing MUAPs that occur in repetitive burst patterns.[10] Each discharge consists of brief, repetitive firing of single MUAPs for a short duration, typically up to a few seconds at rates of 40 to 60 Hz, followed by a silent interval of 0.1 to 10 seconds. This sequence repeats consistently for each potential, producing a regular or semirhythmic pattern with a characteristic sound often described as “marching soldiers.” Myokymic discharges most commonly occur in association with radiation-induced nerve injury, chronic compressive neuropathies, or polyradiculopathies.

Myokymia manifests in peripheral nervous system lesions, particularly radiation-induced plexitis, as mentioned. However, facial myokymia is most commonly associated with brainstem lesions observed in conditions such as multiple sclerosis, pontine tumors (eg, gliomas), and vascular abnormalities, and may also develop following radiation therapy.

Understanding the anatomy of the facial nerve (cranial nerve VII) helps clarify the relationship between brainstem lesions and facial myokymia. The facial nerve nucleus resides in the lower dorsal pons, and its fibers follow a complex course, looping around the abducens nerve (cranial nerve VI) nucleus before exiting the brainstem ventrally. These fibers are considered peripheral once they leave the nucleus, even while still within the brainstem, rendering them susceptible to demyelinating, neoplastic, or other injurious processes that can produce myokymia. Facial myokymia is also observed in approximately 15% of Guillain-Barré syndrome cases, typically appearing early and resolving as recovery progresses.

Neuromyotonic Discharges

Neuromyotonic discharges consist of bursts of MUAPs originating in motor axons that fire at high frequencies of 100 to 300 Hz. These discharges occur repetitively, either in a continuous fashion or recurring decrementing bursts, and produce a characteristic “pinging” sound. The patterns begin and end abruptly, with waveform amplitudes typically waning, and are not influenced by voluntary activity.[11] Neuromyotonic discharges are observed in disorders of peripheral nerve hyperexcitability, including Isaac and Morvan syndromes, and may result from defects in potassium channels of the nerve membrane.

Cramp potentials

Cramp potentials are involuntary, repetitive MUAP discharges occurring at high frequency across a large area of muscle. These waveforms exhibit abrupt onset, rapid buildup, addition of subsequent potentials, and sudden or “sputtering” cessation, sometimes with irregular firing at 40 to 60 Hz.[12] Cramp potentials may be observed in healthy individuals when a muscle is strongly activated in a shortened position and also occur in chronic neurogenic disorders, metabolic or electrolyte disturbances, or conditions of peripheral nerve hyperexcitability.

Rest tremor

Although tremor typically manifests during voluntary movement, its occurrence at rest can complicate the interpretation of spontaneous activity on EMG. Resting tremor is characterized by synchronous bursts of MUAPs separated by brief silent intervals, as observed during EMG recording. The simultaneous firing of multiple MUAPs can obscure individual waveform morphology, often creating the appearance of increased polyphasia. Resting tremor, as observed in conditions such as Parkinson disease, may be mistaken for myokymic discharges. In contrast, myokymia involves repetitive firing of the same MUAP within a burst, whereas tremor bursts comprise distinct MUAPs. Tremor bursts also frequently exhibit fluctuating amplitude, unlike the relatively consistent amplitude observed in myokymia.

Clinical Significance

Abnormal spontaneous EMG activity occurs in various forms. Some discharges are specific to particular pathologies, while others appear across multiple conditions. Accurate recognition of different neuromuscular disorders requires the neurologist to integrate clinical findings, histological changes, and patterns observed on needle EMG.

Enhancing Healthcare Team Outcomes

Healthcare workers and nurse practitioners caring for patients with abnormal muscle activity should consult a neurologist to determine the etiology and guide management. Multiple causes may underlie abnormal muscle activity, and in some cases, EMG or muscle biopsy is warranted. Once the diagnosis is established, the interprofessional team must coordinate patient education and treatment. These neuromuscular disorders are often complex and challenging to manage, and optimal outcomes require coordinated efforts among clinicians, specialists, midlevel practitioners, nurses, and, when indicated, physical therapists.