Electrophysiologicalinvestigationsare used to evaluate the conduction of electrical impulses down peripheral nerves and include nerve conduction studies (NCS) and needle electromyography

• Diagnosis (es)
• Description of disease state (old/new, static/dynamic pathophysiology)
• Localize level of nerve lesion
• Determine severity and prognosis of nerve lesion
• Differentiate upper vs. lower motor neuron lesion
• Demonstrate denervation, reinnervation, aberrant reinnervation, and motor end plate lesion

Electrodes (Figure 1)

• Stimulating electrode provides an electrical impulse to a peripheral nerve.
• Recording electrode picks up the stimulus on the other side of the nerve.
• Ground electrode: acts as a reference, against which other potentials are measured.

Figure 1. Sensory testing (orthodromic stimulation using ring electrodes) of the median innervated digit II. 

• Latency: Time taken for the stimulus to produce a response in the recording electrode. Measured in milliseconds (ms)
• Amplitude:
• Sensory: An indicator of viable axons, measured in microvolts (µV)
• Motor: Summation of the individual muscle fibre action potentials (CMAP see below). This correlates closely to the number of viable axons. Measures in millivolts (mV)
• The neurophysiological convention is that an upward deflection on the NCS graph is reported as a negative voltage.
• Conduction Velocity: Distance between the stimulating and recording electrodes divided by latency. Measured in ms^-1.

The conduction velocity and latency are influenced by:

• Myelination
• Internodal distance
• Temperature
•  Hence need for standardisation of temperature during NCS)

Age

• Reduced velocity at extremes of age

• Measures antidromic nerve conduction (proximal to distal)
• Performed by electrical stimulation of a nerve and recording the compound muscle action potential (CMAP) from surface electrodes.
• Recording electrode placed over muscle belly and the reference electrode is placed over an inert area, typically the muscle tendon. A ground electrode is placed between the stimulating and recording electrodes.
• The CMAP is the summation of the voltage response from individual muscle fibre action potentials.
• The CMAP amplitude is recorded from baseline to the negative peak (upward deflection).
• A supramaximal voltage stimulation is required to reproducibly measure the CMAP amplitude, latency and velocity.  The stimulating electrode voltage is gradually increased until there is no further elevation in CMAP amplitude. 

Figure 2. Motor testing of the median innervated abductor pollicis brevis (APB) muscle.

• May be measured either in an orthodromic or antidromic manner. This is down to the preference of the laboratory where testing occurs.
• When assessing the sensory nerve action potential (SNAP), an impulse is formed by the stimulating electrode at sensory nerve fibres and the resultant action potential is recognized by the recording electrode further along the nerve.
• A supramaximal stimulus is required to measure SNAP.
• Conventional sensory nerve conduction studies are only able to assess the 20% largest diameter sensory nerves.
• Therefore in peripheral neuropathies that affect the 80% of smaller sensory nerves, conventional sensory NCS may be normal. 

F waves (Figure 3)

-        This is a type of late motor response, first discovered in the foot, hence F.

-        When a motor nerve is stimulated an action potential is transmitted in both directions. The distal propagation is recorded as the CMAP. The proximal propagation rebounds at the anterior horn cell and results in a small delayed additional muscle depolarization. This is the F-wave.

-        The F-wave is particularly useful for assessing the proximal segment of nerves, i.e the presence of radiculopathy.

-        The ratio of the F-wave between a proximally located stimulus and that of a distally placed stimulus can be used to further determine the site of pathology in a nerve segment.

Figure 3. Physiology of the F-wave.

H reflexes Figure 4)

The H reflex is basically an electrophysiologically recorded Achilles muscle stretch reflex. It is performed by stimulating the tibial nerve in the popliteal fossa. From there, the stimulus goes proximally through the reflex arc at that spinal segment, then distally from the anterior horn cell and the motor nerve. It can be recorded over the soleus or gastrocnemius muscles.

The H reflex is most commonly used to evaluate for an S1 radiculopathy or to distinguish from an L5 radiculopathy.

Figure 4. H-wave.

• Peripheral nerve pathology frequently affects myelin or axons. Often there is a combination of demyelination and axonal loss.
• The hallmark of focal demyelination is a conduction block and prolonged latency, which is typically seen in neurapraxia.

o    In demyelination the speed of nerve impulse transmission is decreased,

• Axonal loss as occurs in axonotmesis, a more severe form of nerve injury, is seen as a reduction in CMAP and SNAP amplitude.

In summary:

1.      NCS assess peripheral nerves.

2.      NCS are affected by extremes of age. For results to be reproducible the test should be performed at a standardized temperature.

3.      The 3 critical factors in evaluating NCS are; latency, velocity and amplitude:

a.      Latency and velocity are mainly affected by the speed of impulse transmission along an axon.

b.      The amplitude is a reflection of the number of viable axons.

EMG

• Electromyography (EMG) records electrical activity in muscle. Recordings are made from a concentric needle electrode that is inserted in to muscle.
• Motor unit activity at rest and on attempted contraction of the muscle is recorded
• Needle EMG is used to assess both nerve and muscle function.
• A small-diameter monopolar pin or coaxial needle is placed into a muscle to evaluate insertional activity, resting activity, voluntary recruitment, morphology, and size of motor units, as well as motor unit recruitment.
• The needle electrode examination provides valuable information about the electrical characteristics of individual muscle fibers and motor units, as well as the integrity and innervation of muscle fibers.
• This test can be uncomfortable for the patient.
• Normal resting muscle is silent and shows no muscle activity and a characteristic pattern on voluntary contraction. However, during EMG muscle testing it is difficult for the patient to completely relax the muscle being tested. The activity generated by incomplete muscle relaxation can be differentiated from abnormal spontaneous activity

Abnormal spontaneous activity(Figure 6):

-        Fibrillations

o    Following an acute nerve transection, nerve fibres degenerate from the site of the lesion distally. Muscle fibres remain viable, but after about a week become hypersensitive due to the accumulation of acethylcholine. As a result they discharge spontaneously. The EMG needle detects this spontaneous discharge as   single fibre discharge or fibrillation.

o    Seen in neurogenic disorders, inflammatory conditions and muscular dystrophy.

-        Fasciculations

o    Are larger and more complex than a fibrillation.

o    May be visible clinically.

o    Occur in motor neurone disease, radiculopathy, neuropathy and thyroid disease.

o    May be benign. Benign fasciculations are not associated with denervation changes but pathologic fasciculations usually are.

-        Myotonia

o    Discharge is provoked by gently stimulating the muscle.

o    There is a varying frequency and amplitude that produces a characteristic noise described as the “dive-bomber” sound.

-        Neuromyotonia

-        Myokinia

The pattern of motor unit recruitment is affected by the force of muscle contractility. This has to be considered by the electromyographer when assessing motor unit recruitment.

-        Reduced recruitment

May be caused by reduced contraction due to pain inhibition or lack of patient compliance, an upper motor neurone lesion, motor conduction block or partial denervation.

-        Early recruitment

In primary muscle disease the recruitment pattern is described as early. Even at low force contraction, a full recruitment pattern is seen.

-        Motor unit potentials

The amplitude, duration and number of phases may be measured to assess muscle denervation and the presence of re-innervation following nerve injury.

-        Tremor

EMG is used in conjunction with NCS to make diagnoses of peripheral nerve lesions, neuromuscular junction disorders, motor neurone disease and primary muscle disease.  Its strength lies in its ability to identify which particular muscle or muscle groups are pathological and then deduce from the spontaneous activity and motor recruitment pattern, the likely causes for the noticed abnormality.

Figure  6 Abnormal spontaneous activity. (A) Fibrillations (*) and positive sharp waves (**) in an acutely denervated hand muscle. (B) Single, doublet, triplet, and multiple motor unit neuromyotonic discharges. Bursts of discharge are irregular in frequency and the intra-burst frequency of discharge is up to 200 Hz. (C) Fasciculations in the tongue in a patient with5 amyotrophic lateral sclerosis. The single discharges are irregular and occur on a background of ongoing EMG activity caused by poor relaxation. (D) Myotonic discharges in a patient with dystrophia myotonica. There is a characteristic waxing and waning in frequency.

Insertional Activity: Needle is inserted into muscle or moved within muscle, there is a single burst of activity that usually lasts 300 to 500 ms; thought to result from mechanical stimulation or injury of the muscle fibers

Rest Activity: Differentiates neuropathic muscle atrophy from myopathic atrophy

Fibrillations: These are action potentials that arise spontaneously from single muscle fibers; usually occur rhythmically and are though to be due oscillations of the resting membrane potential in denervated muscles. Appears 3 - 5 weeks after the nerve lesion. Preceded by Sharp P waves

Potentials: - number of phases (? action potentials); indicates coll ateral axonal sprouting; polyphasic = > 4 phases

Figure 7. Positive sharp waves of Denervation 

Figure 8. Denervation fibrillation potentials

·         Immediately after nerve section, EMG will be normal, although there will be no muscle response after stimulation of the nerve proximal to the nerve injury (CMAP)

·         Within Between 5 and 14 days positive sharp waves consistent with denervation

·         At between 15 and 30 days, spontaneous denervation fibrillation potentials are present if denervation fibrillation potentials are not present by the end of the secondweek this is a good prognostic sign.

·         Evidence of reinnervation is when highly polyphasic motor unit potentials are detected at attempts at voluntary activity

·         Denervation fibrillations in a muscle only tell you that the muscle is not innervated. It does not determine whether the injury is 2 ^nd 3 ^rd or 4 ^th degree.

·         Reinnervation potentials by the same token can be restored after regeneration of only a few motor fibres and does not necessarily mean a good return to voluntary motor control