Repetitive stimulation studies

We will briefly go over key concepts in repetitive stimulation EMG used for neuromuscular junction (NMJ) disorder testing. Single fiber EMG is beyond the scope of this website.

Fundamental Concepts

Please ensure you have read and understood the normal functioning of neuromuscular junctions in the "Physiology" section before reading on.

Relevant NMJ disorders: 1) myasthenia gravis, which is a POST-synaptic NMJ disorder caused by antibodies that interfere with acetylcholine receptors on the muscle end-plate and 2) Lambert-Eaton myasthenic syndrome (LEMS), which is a PRE-synaptic NMJ disorder caused by antibodies that interfere with the calcium channels in nerve endings that allow Ca2+ influx and leading to acetylcholine release.

Figure 8.1: Comparing pathophysiology of myasthenia gravis vs LEMS

Utility of repetitive stimulation studies: Studies are most sensitive for myasthenia gravis when testing muscles innervated by the facial nerve (e.g. orbicularis oculi and/or frontalis). Besides the facial nerve study, the study is also likely to be abnormal if we test the clinically affected muscle-nerve pair. Testing a proximal nerve in the affected extremity such as the spinal accessory nerve can also be useful.

In generalized myasthenia gravis, sensitivity and specificity are quite high. However, in ocular myasthenia gravis, sensitivity is low and specificity is high. In cases where clinical suspicion for ocular myasthenia is high and repetitive stimulation studies are non-confirmatory, single-fiber EMG is recommended due to higher sensitivity and specificity. *Single fiber EMG is outside of the scope of this website.

Figure 8.2: example of set-up for facial nerve "blink test" repetitive stimulation study.

Safety factor: In order for a muscle fiber to depolarize, it must reach a certain threshold of electrical stimulation also known as "end-plate potential". Normally, this threshold is low enough that most axonal action potentials reaching the motor end-plate successfully trigger a muscle action potential. This concept is called the "safety factor". However, in NMJ disorders, the axonal action potential is poorly transmitted to the muscle end-plate, resulting in sub-threshold end-plate potential and failure of muscle activation.

Figure 8.3: Graphical representation of safety factor at one muscle fiber. Abbreviations: EPP = end-plate potential, mV = millivolts, ms = milliseconds.

Repetitive nerve stimulation studies: These are performed by repeatedly stimulating a nerve and recording the successive CMAPs produced with each stimulus. The stimulation frequency and corresponding pattern of the successive CMAPs give us information about whether there is an NMJ disorder, and whether it is pre- or post-synaptic.

Low frequency studies

"Low frequency" stimulation allows us to test the effect of acetylcholine transmission on muscle action potential (CMAP). This is because at low rates of stimulation i.e. <5Hz, calcium does not progressively accumulate in the pre-synaptic nerve ending (since rate of removal is similar to rate of accumulation). As a result, there is no progressive potentiation of acetylcholine release. So, when we do "repetitive stimulation" at a rate of 2-3Hz, acetylcholine transmission remains the dominant factor affecting end-plate potential. Low frequency studies help identify the presence of an NMJ disorder; it does not help differentiate between pre- or post-synaptic disorders on its own.

Normally, at low frequency stimulation, there is a slight initial dip in CMAP amplitude and area. This is because acetylcholine is stored in small amounts close to the end plate "active zone", and these stores are easily depleted on initial stimulation. Therefore, even in normal muscle, for the first few shocks, acetylcholine release decreases with each successive stimulus. This causes a slight dip in muscle end-plate potential (EPP) that is maximal at about the fourth stimulus. 

In normal muscle, the safety factor allows this slightly low EPP to still successfully generate an action potential, and thus each CMAP has the same amplitude on repetitive stimulation. After the first few shocks, the active zone stores are quickly replenished with secondary stores so that acetylcholine release and end-plate potential recover.

In NMJ disorders, the dip from CMAP #1 through to CMAP #4 is visibly pronounced and called "decrement". This is because of decreased acetylcholine transmission resulting in sub-threshold end-plate potentials, triggering fewer muscle fiber action than normal, and leading to decremental CMAP responses. 

• Myasthenia gravis diagnostic criteria includes >10% decrement in CMAP amplitude from first to fourth response.

• However, in LEMS decrement is less obvious because the first CMAP is already quite small (due to impaired calcium channels leading to low acetylcholine release), thus the relative decrement by CMAP #4 is difficult to detect.

Figure 8.4: Train of CMAPs at 3Hz stimulation in normal muscle, myasthenia gravis, and LEMS.

High frequency studies

"High frequency" stimulation allows us to test the effect of presynaptic Ca2+ influx on muscle action potential (CMAP). This is because at high rates of stimulation i.e. >10Hz, Ca2+ progressively accumulates in the presynaptic terminus (since rate of removal is slower than rate of accumulation). We almost never do high frequency electrical stimulation (40-50 Hz) in reality because it is very painful. Instead, we simulate this by asking the patient to forcefully contract the target muscle. Presynaptic Ca2+ accumulation becomes the dominant factor affecting acetylcholine release and thus end-plate potential. High frequency studies help differentiate between pre- and post-synaptic disorders. 

Brief contraction (~10s)

Brief contraction is tested if you are suspicious that your patient has LEMS. If you are primarily concerned for MG, you can skip this test. 

In general, brief forceful muscle contraction (about 10 seconds) leads to progressive accumulation of Ca2+ in the axon terminus leading to progressive increase in acetylcholine release. This results in "post-tetanic potentiation" and increase in end-plate potential. 

• In normal muscle, this does not change the CMAP appearance since they are already maximal.

• In LEMS, the forceful contraction overcomes the dysfunctional calcium channels, leading to a huge increase in Ca2+ influx and thus, finally, acetylcholine release. This results in higher end-plate potentials, activation of many more muscle fibers, and an increment in CMAP, often by more than 100%. LEMS diagnostic criteria includes >50% increase in CMAP amplitude.

• In myasthenia gravis, the burst of extra acetylcholine release makes up for the lack of functioning acetylcholine receptors, leading to higher end-plate potential and successfully activating more muscle fibers. This results in a slightly higher amplitude of CMAP #1 and less decrement.

Figure 8.5: Train of CMAPs at 3Hz stimulation before and after 10 seconds of maximal voluntary contraction in a patient with LEMS.

Prolonged contraction (~60s)

After prolonged muscle contraction (about 60 seconds or more) the presynaptic Ca2+ concentration falls, and there is likely metabolic exhaustion. As a result, less acetylcholine is release in response to nerve stimulation. 

• In normal muscle, CMAPs appear similar to those recorded pre-exercise, since the end-plate potential is still higher than threshold.

• In myasthenia gravis, post-exercise CMAP #1 is usually smaller than the pre-exercise one, and there is increased % decrement on repetitive stimulation.

Figure 8.6: Train of CMAPs at 3Hz stimulation at rest (left), after 10 seconds of maximal voluntary contraction (middle), and 1-2 minutes after prolonged contraction (right) in a patient with myasthenia gravis.

Summary table comparing control, MG, and LEMS findings

Repetitive Stimulation Studies: Self-test