[1]The muscle is normally electrically silent outside the end-plate zone. Therefore, abnormal spontaneous activity is considered when there is the persistence of spontaneous activity outside of the end-plate zone, usually defined as lasting more than 3 seconds.[2][3][4]

Fibrillation Potentials

Fibrillation potentials are the action potentials of single muscle fibers that are firing spontaneously in the absence of innervation. Typically, they have a regular firing pattern at rates of 0.5 to 15 Hz. May slow down gradually over several seconds before stopping. The amplitude is variable and is proportional to the fiber diameter. They sound like “rain on the roof” and have two different morphologies:

Spikes Form: Triphasic or biphasic with an initial positivity or negativity (when recorded at the site of origin); duration of 1 to 5 milliseconds and amplitude of 20 to 200 microvolts

Positive Waves Form: Biphasic, with an initial sharp positivity followed by a long-duration negative phase; duration of 10-30 milliseconds

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

• 1+: fibrillation potentials persistent in at least two areas
• 2+: moderate number of persistent fibrillation potentials in 3 or more areas
• 3+: large number of persistent discharges in all areas
• 4+: profuse, widespread, persistent discharges that fill the baseline

Spontaneous activity in the region of the end-plate (endplate noise and endplate spikes) can have a similar waveform configuration. This can be distinguished from fibrillation potentials by their irregular firing pattern, an initial negative deflection, and propensity to generate pain when the needle reaches the end-plate.

Fibrillation potentials occur in muscle fibers that have lost their innervation, have been sectioned transversely or divided longitudinally, are regenerating, or have never been innervated. In neurogenic disorders, such as radiculopathies, mononeuropathies, or motor neuron disease, loss or degeneration or axons leads to denervated muscle fibers. In myopathic diseases, functional denervation of individual or segments of muscle fibers occurs as the fiber becomes separated from the endplate zone due to muscle necrosis and fiber splitting.

Complex Repetitive Discharges (CRD) 

Complex repetitive discharges are action potentials of a group of muscle fibers that discharge spontaneously in near synchrony in a regular, repetitive fashion. They are originated by the spontaneous depolarization of a single fiber, followed by ephaptic spread to an adjacent muscle fiber. Subsequently, a variable number of neighboring muscle fibers may be depolarized in sequence until “the circuit” is complete; whereby, the initial muscle fiber discharges again. Therefore, each spike within a group in a CRD is composed of individual muscle fiber action potentials from fibers that may be part of a different motor unit, but lie adjacent to one another.

They have a regular pattern with an abrupt onset and cessation. Although their form is variable, it is usually polyphasic or serrated with 3 to 10 spike components, amplitudes from 50 to 500 microvolts, and durations up to 50 milliseconds. They have a uniform frequency of 3 to 40 Hz and a sound like “a motorboat that misfires” or a “jackhammer.” Usually, they occur spontaneously or following needle movement.

CRDs are nonspecific but occur in neurogenic and myopathic disorders that are chronic or longstanding in nature, such as chronic radiculopathies, peripheral neuropathies, and slow progressive myopathies.

Myotonic Discharges 

Myotonic discharges are action potentials of single muscle fibers that are firing spontaneously in a prolonged fashion after external excitation. These arise from an abnormality in the membrane of a muscle fiber.

They have a regular rhythm with firing rates that vary exponentially in frequency between 40 and 100 Hz. Two different morphologies are described: biphasic spike potentials with an initial sharp positivity followed by a long-duration, negative component, and positive waves.

The potentials wax and wane in amplitude and frequency, producing a characteristic sound like a “dive-bomber.” Even though slow-firing myotonic discharges can have a morphologic similarity to fibrillation potentials, the rapid rate of change in frequency and amplitude and particular sound makes the difference.

These discharges are characteristically seen in myotonic dystrophy, myotonia congenita, and paramyotonia congenita. They may occur in other myopathies without myotonia such as hyperkalemic periodic paralysis, polymyositis, and acid maltase deficiency. It can also occur in severe axonal disorders but is never the predominant waveform.

Fasciculation Potentials 

Fasciculation potentials are spontaneous single motor unit discharges, which are generated anywhere along the lower motor neuron, but usually from the spontaneous firing of the nerve terminal.

They can have the morphology of simple motor unit action potentials (MUAPs), or they can be complex and large if they represent a pathologic motor unit. The firing pattern is irregular, has a low frequency of 1 to 2 Hz, and sounds like a “large raindrops on a tin roof.”

They may occur in normal persons and many neuromuscular disorders. It is especially common in chronic neurogenic diseases such as radiculopathies, axonal peripheral neuropathies, and anterior horn cell disorders (ALS, SMA).

Myokymic Discharges 

Myokymic discharges are groups of recurring spontaneous firing MUAPs that fire in a repetitive burst pattern.

The discharge is brief, repetitive firing of single MUAPs for a short period (up to a few seconds) at a rate of 40 to 60 Hz. This is followed by a short period (0.1 to 10 seconds) of silence, with the repetition of the same sequence for a particular potential. They have a regular or semi-rhythmic pattern and a sound like “marching soldiers.”

Most commonly, these are found with radiation-induced nerve injury, chronic compressive neuropathies, or polyradiculopathies.

Neuromyotonic Discharges

Neuromyotonic discharges are a burst of MUAPs which originate in motor axons firing at high frequencies (100 to 300 Hz). These are repetitive, either continuously or in recurring decrementing bursts, and produces a “pinging” sound. They start and stop abruptly, and the amplitude of the waveforms typically wanes. It is unaffected by voluntary activity.

These are seen in disorders of peripheral nerve hyperexcitability (Isaac, Morvan syndrome). It may occur as a result of a defect in the potassium channel in the nerve membrane.

Cramp Potentials

Cramp potentials are involuntary, repetitive firing of MUAPs at a high frequency in a large area of muscle. They have an abrupt onset, rapid buildup, the addition of subsequent potentials, and a rapid or “sputtering” cessation. Sometimes irregular with a high frequency of 40 to 60 Hz.

These are seen in normal persons when a muscle is activated strongly in a shortened position. It also occurs with any chronic neurogenic disorder, in metabolic or electrolyte disorders, or peripheral nerve hyperexcitability.

There are different types of abnormal spontaneous electromyographic activity; some discharges can be seen in specific pathologies, while others can be associated with multiple diseases. Therefore, to recognize the different neuromuscular diseases, a neurologist must understand the clinical findings, histological changes, and the pattern of abnormal findings on needle EMG.[5][6][7]