EMG Pathology

EMG findings differ largely based on whether the patient has myopathy or neuropathy. Within these categories there is a large variation of EMG findings that can help further narrow the diagnosis. In cases of axonal neuropathy, EMG can be very useful in determining the time course of injury i.e. whether it is acute, subacute, or chronic. Practically speaking, when conducting the needle EMG test, we look for abnormalities in MUAP morphology during muscle contraction and spontaneous activity at rest. EMG studies require attention to how things look on the screen as well as how they sound. 

Myopathic MUAPs

In myopathy, there is loss of muscle fibers, meaning you have smaller motor units (less muscle fibers per axon). Additionally, surviving muscle fibers are usually thinner than normal, causing them to fire asynchronously.  Myopathic MUAPs morphology has the following characteristics:

• Short duration due to smaller motor units.

• Small amplitude due to smaller motor units and decreased density of muscle fibers within each unit.

• Polyphasic due to asynchronous firing of surviving injured muscle fibers.

Recruitment in myopathy is “early” and "full”. Can you think of why?

Each motor unit now has less muscle fibers, thus each motor unit is “weaker”. Our body tries to compensate for this by activating other nearby motor units to help out the weak motor unit. Many MUAPs will appear to fire almost simultaneously, occurring with very small amounts of force from the patient. In other words, you’re seeing “early full” interference (greater number of motor units active than expected for the force of contraction of that muscle).

Figure 6.1: EMG of abductor pollicis brevis (APB) muscle (testing median nerve) on a normal patient (left) and a patient with myopathy (right). Single fiber potentials are shown here to help conceptualize why MUAPs are smaller in myopathic muscle, but you would not see these single fiber potentials on a typical EMG study. The recruitment pattern at full contraction is also shown.

Neuropathic MUAPs

EMG findings differ for demyelinating and axonal neuropathies. In demyelinating neuropathy, EMG is is usually normal (except in severe conduction block, where you can see reduced recruitment). In contrast, EMG is usually abnormal in axonal lesions, anterior horn cell disorders, and radiculopathies.

When there is denervation, i.e. you lose nerves. This means you are losing entire motor units. How does our body try to compensate? There is sprouting of nearby (healthy) terminal axons, which innervate the muscle fibers that lost their parent axons earlier. Thus, these surviving motor units get larger since they now have more muscle fibers. However, the newly sprouted terminal axons are either unmyelinated or have very thin myelin sheaths, with slower conduction velocities. This difference in conduction velocities also leads to asynchrony of firing of old and newly innervated muscle fibers.

As a result, MUAP morphology has the following characteristics in axonal neuropathies, anterior horn cell disorders, and radiculopathies:

• Long duration due to larger motor units (more muscle fibers per unit).

• Higher amplitude due to unusually densely clustered muscle fibers within each motor unit.

• Polyphasic due to asynchronous firing of the old and newly innervated muscle fibers (due to immature newly sprouted terminal axons).

Recruitment in neuropathy is "decreased". Can you think of why?

Recruitment is reduced because there are fewer motor units, so there aren't as many MUAPs appearing on screen. The EMG recording basically looks less “busy” since there's less "interference" of all the MUAP waveforms, even though the patient is activating fully. This is often called "incomplete interference" and denotes reduced recruitment. It is very important to remember that sometimes, a patient may appear to have reduced recruitment but in reality might not be activating their muscle fully (e.g. due to pain or cognitive impairment). If they are not able to fully active then the results of the needle EMG should be interpreted with caution.

Figure 6.2: EMG of abductor pollicis brevis (APB) muscle (testing median nerve) on a normal patient (left) and a patient with axonal neuropathy (right). Single fiber potentials are displayed to assist with conceptualizing why the MUAP is smaller in muscle innervated by the injured nerve, but you would not see these single fiber potentials on a typical EMG study. The recruitment pattern at full contraction is also shown.

Example videos of recruitment patterns

I recommend the YouTube channel "neuromuscular teaching" and "James Burge" to see examples of normal and abnormal findings. I have included a few here.

Abnormal insertional activity

Recall from the "Normal EMG" section that insertional activity is just electrical signals from muscle reacting to insertion of the EMG needle. It can be normal if it lasts up to 300 milliseconds, but if it goes longer then it is considered increased and abnormal. Decreased insertional activity can also be abnormal.

• Increased: non-specific finding observed in many neuropathic and myopathic conditions

• Decreased: This occurs when muscle tissue is replaced by fat and connective tissue

Abnormal spontaneous activity: Part I

Spontaneous activity refers to the presence of certain electrical findings on EMG while the muscle is supposed to be at rest. When conducting EMG, it is common to stick the needle into the muscle and see spontaneous activity that is just due to the patient not being fully relaxed. However, if the patient's muscle is fully relaxed and there is still regular spontaneous activity (e.g. sharps and fibrillations discussed here), this is called abnormal spontaneous activity and is discussed below.

By far the most common types are “fibrillation potentials” (fibs) and “positive sharp waves” (PSW or sharps), and then less common “fasciculations” (fascics). Spontaneous activity can occur at the level of single muscle fibers (fibs and sharps) and/or at the level of the axon (fascics).

Let’s first talk about the clinical relevance of these findings, and then we’ll get into the details of how they look and sound on EMG.

What is the physiologic basis of fibrillations and positive sharp waves?

An unstable muscle membrane, leading to spontaneous depolarization of muscle fibers resulting in muscle fiber action potentials.

What disease states other than nerve injury can produce positive waves and fibrillation potentials? 

Inflammatory myopathies, muscular dystrophies, severe neuromuscular junction disease (especially botulism).

Which myopathies generally do not cause fibrillations? 

Steroid myopathy, most congenital and metabolic myopathies.

What is the physiologic difference between a fasciculation and a fibrillation?

Fasciculations are single, spontaneous, involuntary discharges of an individual motor unit. Fibrillations are signs of irritation of a single muscle fiber. 

What conditions or disease states are associated with fasciculations?

Motor neuron disease, radiculopathies, polyneuropathies, entrapment neuropathies, benign fasciculation syndrome. They can be benign if there are no other abnormalities on EMG/NCS or exam.

Practical question: How do you tell the difference between the presence of ABNORMAL spontaneous activity versus a patient just not being able to relax enough (also called "volitional activity")? 

When conducting EMG, it is common to stick the needle into the muscle and see persistent sharps and fibs that are just due to the patient not being fully relaxed. Findings due to incomplete relaxation are benign, since it's not due to neuropathy or myopathy. Obviously, it's important not to overcall benign findings. In abnormal spontaneous activity, the findings (fibs, sharps) occur at very regular intervals, whereas in volitional activity the fibs and sharps are irregular. As you can see, the rhythmicity of findings is just as important as MUAP morphology when interpreting EMG studies.

Example videos of abnormal spontaneous activity

The following videos were contributed by various peers and faculty.

How EMG helps determine time course of axonal injury

It is EMG that helps determine whether axonal nerve injury is acute or chronic.

MUAP morphology changes over time in axonal neuropathy

Acute denervation: In the acute phase of Wallerian degeneration the distal axon remains electrically excitable. Usually, MUAP morphology remains normal, but you can sometimes see reduced/low amplitude and duration of MUAPs (due to nascent motor units during early reinnervation). We also see reduced recruitment due to loss of motor units.

Subacute: Surviving terminal axons sprout collateral branches to reinnervate muscle fibers that previously lost their parent axons. As a result, surviving motor units become enlarged by innervating a greater number of muscle fibers. These newly formed axon sprouts are unmyelinated or thinly myelinated and therefore conduct more slowly. This results in MUAPs that are long duration (more total muscle fibers) and polyphasic (asynchronous firing of old and newly innervated fibers). MUAPs may also demonstrate instability (“jiggle”) as a result of immature endplates of the newly innervated fibers.

Chronic denervation with reinnervation: As the reinnervated units mature, there are fewer total motor units (reduced recruitment) but the surviving individual motor units are larger and produce MUAPs with increased amplitude, prolonged duration, and polyphasia.

Abnormal spontaneous activity over time

The earliest abnormal needle EMG findings following axonal nerve injury are positive sharp waves (PSWs) and fibrillation potentials. Due to the process of Wallerian degeneration, it takes between 1 and 4 weeks after a nerve injury before they appear on EMG. 

Early after their appearance, fibrillation potentials and PSWs often have relatively large amplitudes. Over time, their amplitudes generally decrease as reinnervation and muscle fiber atrophy occur. 

In general, larger-amplitude fibrillation potentials and PSWs suggest more recent denervation, whereas smaller amplitudes are more consistent with a more chronic process. However, amplitude alone cannot precisely date the timing of injury.

Abnormal spontaneous activity: Part II

There are other types of abnormal spontaneous activity that are encountered less frequently, so if your eyes are glazing over, it’s okay to skip the rest (though they do come up on the RITE and boards, and of course in clinical practice). Know that these findings exist and can help narrow the differential diagnosis.

If you're reading on...pay attention to the differences between the morphology of these discharges AND their rhythmicity! For example, is it a positive deflection or a series of spikes? Is it waxing and waning, or is it very consistently time locked? This will help understand why they sound the way they do, and also help identify what it is that you're seeing on EMG.

The following videos were contributed by various peers and faculty.

Myotonia

What is the pathophysiology? Spontaneous depolarization of single muscle fiber.

What does it look like on EMG? Positive (downward) wave or brief spike with initial positive deflection. Waxing and waning amplitude and frequency.

What does it sound like on EMG? Revving engine or dive bomber.

Clinical implications? Myotonic dystrophy, paramyotonia congenita, some inflammatory and toxic myopathies, acid maltase deficiency, etc.

Video 5: Example of myotonic discharges in a patient diagnosed with type 1 myotonic dystrophy

Complex Repetitive Discharges

What is the pathophysiology? Spontaneous depolarization of single muscle fiber with "ephaptic spread" (direct spread between muscle membranes without any neurotransmitters being involved) to adjacent denervated muscle fibers.

What does it look like on EMG? Repetitive complexes each composed of a series of time-locked spikes, with individual spikes representing different muscle fibers. Morphology of complexes are identical. High frequency, regular firing pattern. Abrupt onset/offset.

What does it sound like on EMG?  Machine gun (to me, it sounds like a generator going in the background).

Clinical implications? Usually indicative of chronic denervation in neuropathic and myopathic disorders.

Video 6: Example of complex repetitive discharges.

Myokymia

What is the pathophysiology? Spontaneous firing of single motor unit in a rhythmic, burst pattern along demyelinated nerve segments. One way to remember what  myokymia represents is to think of it as "grouped fasciculations".

What does it look like on EMG? Morphology is that of a single MUAP. Bursts of high frequency firing, interspersed with periods of very slow firing between bursts. 

What does it sound like on EMG?  Marching soldiers.

Clinical implications? Radiation-induced neuropathy /plexopathy, multiple sclerosis, Guillain-Barré Syndrome (GBS) pontine tumors, hypocalcemia.

Video 7: Example of myokymia.

EMG pathology: Self-test