**3.4. EMG recording in peripheral nerve stimulation**

Electroneurography (ENG) assesses the function and integrity of peripheral motor nerve structures by means of sEMG recording after electrical stimulation. ENG contributes to localize, typify (axonal or demyelinating nature), and establish the course and the severity of the lesion, and is temperature sensitive [39]. On the other hand, magnetic stimulation is also a tool for the electrical stimulation of peripheral nerves and spinal roots [40].

The summation of all underlying individual muscle fiber action potentials after electrical stimulation of a peripheral nerve is called compound muscle action potential (CMAP). Abnormalities of its components are useful in the diagnostic evaluation of neurological disorders (fig 5).

The integrity of some cranial nerves can be assessed by specific tests [46]:

means of stimulating the first division of the trigeminal nerve (fig 7).

**•** Electrical elicited Blink Reflex allows the evaluation of the trigeminal-facial reflex arc by

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**Figure 7.** Blink reflex. Normal ipsilateral early (R1) with ipsilateral late (R2) responses, and contralateral R2 response

**•** Masseter reflex is elicited by a brisk tap to the lower jaw and allows assessment of the motor

However, motor nerve conduction studies have some limitations. First, selectivity is lacking in the assessment of small muscles and some nerves are not accessible. Second, the greater the intensity of the stimulation, the greater the chance of the stimulus being perceived as painful by the patient, especially during proximal stimulation in the assessment of root and plexus. In addition, stimulation is more difficult in patients who are obese, edemic, or have unusually thick or calloused skin. Third, variability due to examiner and side differences also exists [47].

*Latency* refers the time from the stimulus to the initial negative deflection from baseline. In peripheral motor nerve stimulation, the latency obtained from the stimulation of the most distal segment of the nerve is named "distal latency (DL)". DL reveals nerve conduction time from the stimulus site to the NMJ, the time delay across the NMJ, and the depolarization time across the muscle and reflects only the conduction of the fastest conducting motor fibers.

**•** Axonaldegenerationwithprimarydamageofthelargestandfastestmyelinatedfibers[44,49]. *Amplitude* is commonly measured from baseline to negative peak. It is proportional to the distance from the recording electrode to the muscle fiber. CMAP amplitude reflects only those

recorded over both orbicularis oculi muscles, by left supraorbital nerve stimulation.

component of the trigeminal nerve.

Several parameters of a CMAP can be measured:

Pathological conditions with delayed CMAPs include:

**•** Demyelinating diseases [48].

**Figure 5.** Parameters of a schematic CMAP assessed in motor nerve conduction study.

Special recording techniques are required when significantly different CMAP amplitude is recorded between two nerve segments in order to rule out the presence of anatomical variants [41-44].

On the other hand, the proximal segment of the peripheral motor nerve system can be assessed by means of determination of late responses: H-waves (elicited by subthreshold activation of muscle spindle afferents) and F-waves (elicited by supramaximal antidromic activation of motor neurons) (fig 6). Nevertheless, sensibility and specificity are limited in both tests [44,45].

**Figure 6.** Late responses. Left: F-wave evoked from supramaximal stimulation of median nerve at the wrist, recording in abductor pollicis brevis muscle. Right: H-wave recorded over the soleus muscle from submaximal stimulation of tibi‐ al nerve in the popliteal fossa.

The integrity of some cranial nerves can be assessed by specific tests [46]:

The summation of all underlying individual muscle fiber action potentials after electrical stimulation of a peripheral nerve is called compound muscle action potential (CMAP). Abnormalities of its components are useful in the diagnostic evaluation of neurological

Special recording techniques are required when significantly different CMAP amplitude is recorded between two nerve segments in order to rule out the presence of anatomical

On the other hand, the proximal segment of the peripheral motor nerve system can be assessed by means of determination of late responses: H-waves (elicited by subthreshold activation of muscle spindle afferents) and F-waves (elicited by supramaximal antidromic activation of motor neurons) (fig 6). Nevertheless, sensibility and specificity are limited in both tests [44,45].

**Figure 6.** Late responses. Left: F-wave evoked from supramaximal stimulation of median nerve at the wrist, recording in abductor pollicis brevis muscle. Right: H-wave recorded over the soleus muscle from submaximal stimulation of tibi‐

**Figure 5.** Parameters of a schematic CMAP assessed in motor nerve conduction study.

disorders (fig 5).

8 Electrodiagnosis in New Frontiers of Clinical Research

variants [41-44].

al nerve in the popliteal fossa.

**•** Electrical elicited Blink Reflex allows the evaluation of the trigeminal-facial reflex arc by means of stimulating the first division of the trigeminal nerve (fig 7).

**Figure 7.** Blink reflex. Normal ipsilateral early (R1) with ipsilateral late (R2) responses, and contralateral R2 response recorded over both orbicularis oculi muscles, by left supraorbital nerve stimulation.

**•** Masseter reflex is elicited by a brisk tap to the lower jaw and allows assessment of the motor component of the trigeminal nerve.

However, motor nerve conduction studies have some limitations. First, selectivity is lacking in the assessment of small muscles and some nerves are not accessible. Second, the greater the intensity of the stimulation, the greater the chance of the stimulus being perceived as painful by the patient, especially during proximal stimulation in the assessment of root and plexus. In addition, stimulation is more difficult in patients who are obese, edemic, or have unusually thick or calloused skin. Third, variability due to examiner and side differences also exists [47].

Several parameters of a CMAP can be measured:

*Latency* refers the time from the stimulus to the initial negative deflection from baseline. In peripheral motor nerve stimulation, the latency obtained from the stimulation of the most distal segment of the nerve is named "distal latency (DL)". DL reveals nerve conduction time from the stimulus site to the NMJ, the time delay across the NMJ, and the depolarization time across the muscle and reflects only the conduction of the fastest conducting motor fibers. Pathological conditions with delayed CMAPs include:


*Amplitude* is commonly measured from baseline to negative peak. It is proportional to the distance from the recording electrode to the muscle fiber. CMAP amplitude reflects only those depolarized fibers nearest to the recording electrode, and is the most studied outcome measurement.

*Area* is conventionally measured between the baseline and the negative peak and represents a combination of the amplitude and the duration; the calculation is performed by computerized software. Therefore, CMAP area reflects also the number and synchrony of the muscle fibers activated close to the recording electrode. "Temporal dispersion" results from the spatial distribution of the scattered motor end-plates of a MU, and depends on the individual distance and time of conduction along the muscle fibers. This phenomenon is observed with more

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*Motor conduction Velocity (MCV)* obtained from standard recording techniques reflects only the conduction of the fastest conducting fibers. The determination of true motor conduction velocity must not include the NMJ transmission and muscle depolarization times. Conduction velocity along the studied segment is usually calculated with the following formula: (distance between the proximal and distal stimulation sites) divided by (proximal latency - distal latency) [44]. MCV is dependent on internode distance and also on the total fiber diameter (axon plus myelin), since MCV increases proportionally with myelin thickness [56]. MCV increases progressively during the first 5 years of life, in relation to physiological maturation of myelinization process. Otherwise, there is a progressive and slight decrease of MCV in

MCV is an important parameter in the determination of demyelinating disease. In hereditary demyelinating neuropathies, a uniformly slowed MCV has been shown; whereas in acquired demyelinating neuropathies, slowed MCV is observed in a patchy way [44]. In reinnervation process, the slowing in conduction velocity results from the regenerating nerve fibers that

Motor evoked potential (MEP) is defined as an EMG response obtained by means of activation of the corticospinal tract by means of stimulation to the motor cortex. Transcranial Magnetic Stimulation (TMS), a painless (unlike Transcranial Electrical Stimulation) method is widely

The following components of MEP elicited by single pulse TMS, are measured to evaluate the

*Latency* refers to the time between the delivery of a TMS pulse over the scalp (area corre‐ sponding to the primary motor cortex – M1) and the appearance of MEP at the periphery. MEP latency is mainly reflective of the efficiency of conduction between the stimulated motor

*Central motor conduction time (CCT)* can be obtained by two methods of calculation. The CCT calculated by subtracting the CMAP latency obtained by stimulation of the spinal (cervical or lumbar) motor root from the latency of MEP [62] includes the time for central motor conduc‐ tion, the synaptic delay at the spinal level and time from the proximal root to the intervertebral foramen. More precise central conduction time can be calculated by use of F-wave latency [60]. On the other hand, CCT is significantly influenced by the motor system maturation [63], and

contain thinner axons and myelin sheaths and shorter internodal lengths [59].

proximal stimulation, while the distance from the recording electrodes increases [55].

relation with increase of age over 20-30 years [44,57,58].

**3.5. EMG recording in cortical stimulation**

used with this aim [60].

integrity of corticospinal pathways:

cortical area and the peripheral target muscle [61].

Abnormalities of CMAP amplitude can be observed in:


**Figure 8.** Repetitive stimulation of ulnar nerve: recording over the abductor digiti minimi muscle. A. In normal sub‐ jects, compound muscle action potential (CMAP) amplitude remains very stable. B. In a miastenia gravis (MG) patient, CMAP amplitude is normal at rest but decreases during low-rate repeated stimulation at 3 Hz. C. In a patient with Lambert–Eaton syndrome (LES), the initial CMAP amplitude is reduced. During high-rate repeated stimulation at 20 Hz, CMAP amplitude dramatically increases. NMJ: neuromuscular junction.

**•** Demyelinating lesions: Impediment to the conduction of the action potential without axonal degeneration is named conduction block (CB). No absolute expert agreement has been established in the definition criteria of CB [49]. Nevertheless, a decay of proximal CMAP amplitude/area of at least 50% has been observed in patients with nerve CB in some studies, and has even been proposed as criteria for CB. [51-53].

*Duration* is measured from the initial deflection from baseline to the first baseline cross‐ ing, but can also be measured from the initial to the terminal deflection back to baseline. It is a parameter that indicates the synchrony of the activated muscle fibers. It increases in conditions that result in slowing of some motor nerve fibers but not others (e.g., in a demyelinating lesion) [44,54].

*Area* is conventionally measured between the baseline and the negative peak and represents a combination of the amplitude and the duration; the calculation is performed by computerized software. Therefore, CMAP area reflects also the number and synchrony of the muscle fibers activated close to the recording electrode. "Temporal dispersion" results from the spatial distribution of the scattered motor end-plates of a MU, and depends on the individual distance and time of conduction along the muscle fibers. This phenomenon is observed with more proximal stimulation, while the distance from the recording electrodes increases [55].

*Motor conduction Velocity (MCV)* obtained from standard recording techniques reflects only the conduction of the fastest conducting fibers. The determination of true motor conduction velocity must not include the NMJ transmission and muscle depolarization times. Conduction velocity along the studied segment is usually calculated with the following formula: (distance between the proximal and distal stimulation sites) divided by (proximal latency - distal latency) [44]. MCV is dependent on internode distance and also on the total fiber diameter (axon plus myelin), since MCV increases proportionally with myelin thickness [56]. MCV increases progressively during the first 5 years of life, in relation to physiological maturation of myelinization process. Otherwise, there is a progressive and slight decrease of MCV in relation with increase of age over 20-30 years [44,57,58].

MCV is an important parameter in the determination of demyelinating disease. In hereditary demyelinating neuropathies, a uniformly slowed MCV has been shown; whereas in acquired demyelinating neuropathies, slowed MCV is observed in a patchy way [44]. In reinnervation process, the slowing in conduction velocity results from the regenerating nerve fibers that contain thinner axons and myelin sheaths and shorter internodal lengths [59].
