ELECTROMYOGRAPHY AND NERVE CONDUCTION STUDIES
ELECTROMYOGRAPHY AND NERVE CONDUCTION STUDIES
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| Electromyography-Nerve conduction studies |
KEY POINTS
1. Nerve conduction studies (NCS) and needle electromyography (EMG) are the most useful diagnostic tests in determining the presence, type, severity, and chronicity of a suspected neuromuscular disorder.
2. Although the information collected during NCS and needle EMG testing is more objective and quantitative than that obtained by a standard clinical examination, there are many important technical factors that contribute to the collection of accurate data. Selection of a reputable or accredited laboratory and experienced electrodiagnostic consultant will help ensure the most accurate data and reliable interpretation.
3. Sensory NCS are the most useful way to distinguish preganglionic (radiculopathy) from postganglionic (peripheral neuropathy or plexopathy) processes that cause numbness.
4. NCS can help distinguish between demyelinating and axonal neuropathies. Demyelinating neuropathies have moderate to severe slowing of the nerve conduction velocities with relatively preserved compound muscle action potential (CMAP)/sensory nerve action potential (SNAP) amplitudes. On the other hand, axonal neuropathies have reduced CMAP/SNAP amplitudes with relatively preserved conduction velocities.
5. On EMG, the motor unit action potentials (MUAPs) in neurogenic disorders have characteristic large amplitudes with long durations and reduced recruitment. The MUAPs in myopathic disorders have small amplitudes with short durations and early recruitment.
¤ What is an EMG?
It is a term used in two ways:
• As a general description of the combination of NCS and needle EMG testing.
• More specifically, to describe the needle electrode recording performed to assess the function of motor units.
¤ What is an NCS?
NCS may be performed on either sensory or motor nerves. From the skin surface, a brief electrical stimulus is applied to the nerve of interest, and the evoked electrical signal is recorded distally from another point in the nerve in the case of a sensory nerve or from the innervated muscle in the case of a motor nerve; the evoked responses are known as SNAP or CMAP, respectively. Characteristics of the evoked action potentials such as amplitude, onset and/or peak latency, and conduction velocity provide information about axon and myelin components of the tested nerve.
¤ Some other types of electrodiagnostic tests :
• Repetitive stimulation studies are utilized for the evaluation of the neuromuscular junction (e.g., in myasthenia gravis).
NCS may be performed on either sensory or motor nerves. From the skin surface, a brief electrical stimulus is applied to the nerve of interest, and the evoked electrical signal is recorded distally from another point in the nerve in the case of a sensory nerve or from the innervated muscle in the case of a motor nerve; the evoked responses are known as SNAP or CMAP, respectively. Characteristics of the evoked action potentials such as amplitude, onset and/or peak latency, and conduction velocity provide information about axon and myelin components of the tested nerve.
¤ Some other types of electrodiagnostic tests :
• Repetitive stimulation studies are utilized for the evaluation of the neuromuscular junction (e.g., in myasthenia gravis).
• Somatosensory evoked potentials are used to evaluate conduction within the spinal cord and brain.
• Other less frequently used tests include single-fiber EMG, motor-evoked potentials, and nerve root stimulation.
¤ What is a motor unit?
A motor unit includes the motor neuron found within the anterior horn of the spinal cord, its axon, the neuromuscular junction, and the associated muscle fibers supplied by the axon. The electrodiagnostic physician can utilize a combination of needle EMG, NCS, repetitive stimulation, and other electrophysiologic tests to assess the individual components of the motor unit.
¤ What are the clinical indications for ordering an EMG ?
An EMG should be ordered to determine the localization and severity of a suspected neuromuscular disorder. The
NCS and needle EMG are almost always performed together in reputable laboratories. Testing can distinguish between neurogenic (nerve or neuron-related), myopathic (muscle-related), and neuromuscular junction disorders.
A motor unit includes the motor neuron found within the anterior horn of the spinal cord, its axon, the neuromuscular junction, and the associated muscle fibers supplied by the axon. The electrodiagnostic physician can utilize a combination of needle EMG, NCS, repetitive stimulation, and other electrophysiologic tests to assess the individual components of the motor unit.
¤ What are the clinical indications for ordering an EMG ?
An EMG should be ordered to determine the localization and severity of a suspected neuromuscular disorder. The
NCS and needle EMG are almost always performed together in reputable laboratories. Testing can distinguish between neurogenic (nerve or neuron-related), myopathic (muscle-related), and neuromuscular junction disorders.
In neurogenic conditions, testing can often distinguish between disorders primarily affecting myelin (i.e., demyelinating neuropathies) and axonal or neuronal disorders. An EMG is generally not useful in the assessment of brain or spinal cord disease. In order to get the most useful information from the test, the requesting physician should indicate his or her clinical question or concern as specifically as possible. It is not usually necessary to request particular procedures (e.g., NCS, repetitive stimulation, needle EMG) as a qualified electrodiagnostic consultant can decide what is needed to answer the clinical question.
¤ What are some common disorders of nerve?
Functionally, the peripheral nervous system starts in the vicinity of the spinal neural foramen where the sensory and motor fibers join. At its most proximal level, peripheral nervous system injury in the form of radiculopathy is caused by
an injury to a nerve root due to structural disease (e.g., herniated disc or trauma), inflammation, or other causes.
Plexus involvement by disease or injury may occur in the upper (brachial plexus) or lower extremity (lumbar or lumbosacral plexopathy).
¤ What are some common disorders of nerve?
Functionally, the peripheral nervous system starts in the vicinity of the spinal neural foramen where the sensory and motor fibers join. At its most proximal level, peripheral nervous system injury in the form of radiculopathy is caused by
an injury to a nerve root due to structural disease (e.g., herniated disc or trauma), inflammation, or other causes.
Plexus involvement by disease or injury may occur in the upper (brachial plexus) or lower extremity (lumbar or lumbosacral plexopathy).
Peripheral nerve conditions can be acquired or genetically mediated. Examples of genetic conditions include the hereditary sensory and motor neuropathies (e.g., Charcot-Marie-Tooth disease). Acquired peripheral neuropathies can stem from conditions such as diabetes, toxins (medications or other exogenous substances), inflammatory disorders, or metabolic disturbances.
Focal neural entrapment can be seen in carpal tunnel syndrome (CTS), cubital tunnel syndrome, or tarsal tunnel syndrome, and few others.
¤ What are the three main types of nerve injury ?
Nerves sustain a gradient of injury, which was originally defined by Seddon:
1. Neurapraxia is the functional loss of conduction without anatomic change of the axon, usually due to focal demyelination. With remyelination, conduction returns to normal.
2. In axonotmesis, the axonal continuity is lost. With its loss, Wallerian degeneration occurs in the distal segment. Recovery, which is frequently incomplete, occurs as a result of axonal regrowth at a rate of 1- 3 mm/day in otherwise healthy individuals.
3. Neurotmesis results from separation of the entire nerve, including its supporting connective tissue. Regeneration frequently does not occur. Nerves with this degree of trauma may need surgical attention for recovery to occur.
¤ Do these types of nerve injuries occur together ?
Neurapraxia and axonotmesis commonly occur as a result of the same injury. When compression is relieved from the involved segment of the nerve, two periods of healing typically occur. One is relatively immediate, from hours to weeks, as the neurapraxia resolves. A second period of healing, from weeks to months, may occur as a result of axonal regrowth.
¤ What is an innervation ratio?
For each motor axon, there are a variable number of associated muscle fibers. Depending on the specific requirement of control, the ratio may be quite low or extremely high. The innervation ratio of the extraocular muscles is typically 1: 3, owing to the fine control required for binocular vision. Conversely, the innervation ratio of the gastrocnemius can be as high as 1: 2000 since most movements involving the plantar flexors of the ankle are relatively large motions requiring more force than accuracy.
¤ What are some common disorders of muscle?
Muscle disease can be acquired or genetically mediated. Acquired muscle diseases such as inflammatory myopathies are most often proximal and symmetrical in distribution. A notable exception is inclusion body myositis, which has a predilection for the quadriceps and finger flexors and may be quite asymmetrical. Genetic myopathies or muscular dystrophies may also demonstrate specific patterns of muscle group involvement that may aid in diagnosis. In general, the degree of clinical weakness in a particular muscle correlates well with the severity of the findings on needle EMG examination.
¤ Do these types of nerve injuries occur together ?
Neurapraxia and axonotmesis commonly occur as a result of the same injury. When compression is relieved from the involved segment of the nerve, two periods of healing typically occur. One is relatively immediate, from hours to weeks, as the neurapraxia resolves. A second period of healing, from weeks to months, may occur as a result of axonal regrowth.
¤ What is an innervation ratio?
For each motor axon, there are a variable number of associated muscle fibers. Depending on the specific requirement of control, the ratio may be quite low or extremely high. The innervation ratio of the extraocular muscles is typically 1: 3, owing to the fine control required for binocular vision. Conversely, the innervation ratio of the gastrocnemius can be as high as 1: 2000 since most movements involving the plantar flexors of the ankle are relatively large motions requiring more force than accuracy.
¤ What are some common disorders of muscle?
Muscle disease can be acquired or genetically mediated. Acquired muscle diseases such as inflammatory myopathies are most often proximal and symmetrical in distribution. A notable exception is inclusion body myositis, which has a predilection for the quadriceps and finger flexors and may be quite asymmetrical. Genetic myopathies or muscular dystrophies may also demonstrate specific patterns of muscle group involvement that may aid in diagnosis. In general, the degree of clinical weakness in a particular muscle correlates well with the severity of the findings on needle EMG examination.
¤ What are the components of needle EMG evaluation?
Insertional/spontaneous activity: An EMG needle inserted into a normal muscle should evoke brief electrical discharges of muscle fibers. Increased or prolonged electrical activity may indicate abnormalities of the muscle fibers or the nerves supplying them. Fibrillations, positive sharp waves, and complex repetitive discharges are electrical signals that represent abnormal spontaneous firing of muscle fibers due to nerve or muscle damage.
There should be no spontaneous activity in a healthy relaxed muscle. Motor unit analysis: When a patient slightly contracts a muscle, MUAPs can be recorded. The parameters of interest include the amplitude, duration, number of phases, and firing pattern of the MUAPs. Assessment of these parameters occurs in real time and is generally subjective. The quality of the interpretation depends on the skill and experience of the electromyographer, technical recording conditions, and patient cooperation.
Recruitment: When a patient contracts a muscle more forcefully, a large number of MUAPs can be recorded. How “full” this pattern of MUAPs is reflects the underlying health of the motor units and the ability of the patient to “recruit” available motor units. In myopathic conditions, recruitment may be “early” because myopathic motor units generate less force than healthy ones. In neurogenic conditions, recruitment may be reduced as a result of axon or neuron loss.
A fasciculation potential is an involuntary firing of an entire motor unit, that is, single motor neuron and all its innervated muscle fibers. This is seen as a large electrical spike on needle EMG recording of a relaxed muscle. It is sometimes clinically visible in the patient as a brief, irregular twitch of the muscle. This can often be seen in normal individuals; however, if in excess, it may be a sign of a motor nerve or motor neuron disorder.
A fibrillation potential is an involuntary contraction of a single muscle fiber that usually indicates denervation or muscle damage. Unlike a fasciculation, a fibrillation usually does not cause clinically visible muscle movement.
Positive sharp wave potentials are similar to fibrillation potentials in that they represent abnormal muscle fiber firing from nerve or muscle damage. They are identified by their initial positive deflection from the baseline as opposed to the initial negative deflection of a fibrillation potential.
¤ How do normal EMG findings compare with the findings seen in a denervated muscle (neurogenic disorder) ?
The fibrillations and positive sharp waves are not seen in acutely denervated muscles until 7 to 14 days after the onset of axonal degeneration. Full reinnervation of denervated muscle, resulting in large, polyphasic MUAPs may take 3 to 4 months or more. In patients with reinnervation after nerve injury, muscles may be clinically strong and yet be very abnormal on needle EMG testing.
¤ How do normal EMG findings compare with the findings seen in a myopathic disorder ?
The weaker a patient with myopathy is, the more likely the needle EMG findings will be abnormal. In patients with very mild weakness or those with steroid myopathy, the needle EMG recording may appear normal.
¤ Is nerve conduction velocity the same throughout the length of a nerve ?
Nerve conduction velocities vary among nerves and along their lengths. Normally, proximal nerve conduction is faster than distal nerve conduction because of the increased temperature and larger diameter of the proximal nerve segments. For example, median nerve conduction velocity from wrist to palm should be faster than from palm to finger.
¤ Why is temperature recorded during the course of an electrodiagnostic examination ?
Nerve conduction velocities drop by 1.5 to 2.5 m/s per °C reduction in both sensory and motor nerves. These changes can be significant. Failure to warm the limb to a standard temperature, usually 30°C for the leg and 32°C for the arm, can result in false-positive studies, leading to a misdiagnosis.
¤ What is the F-wave? How is it clinicaly useful?
The F-wave is a delayed motor potential recorded by stimulating a motor nerve in the distal extremity. As the electrical impulse travels backward along the nerve to the spinal cord, a small population of anterior horn cells is stimulated, resulting in small motor action potentials that can be recorded from the associated muscle. Abnormal F-waves can indicate proximal nerve disease such as radiculopathy or plexopathy. Absent F-waves are also early findings in Guillain–Barré syndrome.
¤ How is the sensory portion of the peripheral nervous system tested ?
Sensory NCS are the primary means to test the integrity of the sensory nerves. The amplitude of SNAP, its point of onset, and its peak can be compared with standardized normal values and with those from the opposite extremity.
Sensory NCS are only abnormal in lesions distal to the dorsal root ganglia where sensory neurons reside.
Abnormal SNAPs can be an important way of distinguishing between peripheral neuropathies or plexopathies and radiculopathies. In the latter, SNAPs are usually normal even when a patient complains of numbness. Using the complementary information obtained from needle EMG examination, an electromyographer can further localize the lesion to a particular spinal nerve root, portion of the plexus, or a particular peripheral nerve.
¤ How can a demyelinating peripheral neuropathy and an axonal peripheral neuropathy be differentiated by NCS and needle EMG?
Axonal loss and demyelination rarely occur in strict isolation, but some electrodiagnostic features may indicate relatively more damage to myelin versus axons. The features of demyelinating neuropathies include moderate to severe slowing of conduction velocity, temporal dispersion of evoked sensory or motor action potentials, conduction block, and prolonged distal latencies.
Sensory NCS are the primary means to test the integrity of the sensory nerves. The amplitude of SNAP, its point of onset, and its peak can be compared with standardized normal values and with those from the opposite extremity.
Sensory NCS are only abnormal in lesions distal to the dorsal root ganglia where sensory neurons reside.
Abnormal SNAPs can be an important way of distinguishing between peripheral neuropathies or plexopathies and radiculopathies. In the latter, SNAPs are usually normal even when a patient complains of numbness. Using the complementary information obtained from needle EMG examination, an electromyographer can further localize the lesion to a particular spinal nerve root, portion of the plexus, or a particular peripheral nerve.
¤ How can a demyelinating peripheral neuropathy and an axonal peripheral neuropathy be differentiated by NCS and needle EMG?
Axonal loss and demyelination rarely occur in strict isolation, but some electrodiagnostic features may indicate relatively more damage to myelin versus axons. The features of demyelinating neuropathies include moderate to severe slowing of conduction velocity, temporal dispersion of evoked sensory or motor action potentials, conduction block, and prolonged distal latencies.
Axonal neuropathies show milder slowing of nerve conduction, with generally low sensory and motor amplitudes on NCS. The needle EMG shows denervation abnormalities early in axonal neuropathies and only later in demyelinating neuropathies when axons are secondarily affected.
¤ Which systemic diseases cause predominantly a demyelinating peripheral neuropathy ?
In most rheumatologic conditions where in neuropathy is present, the axons are primarily affected, though myelin is rarely completely normal. Demyelination may predominate in a few disorders, such as acute inflammatory demyelinating polyneuropathy (Guillain–Barré syndrome), chronic inflammatory demyelinating polyneuropathy, multifocal motor neuropathy, anti-myelin-associated glycoprotein antibody syndrome, and other paraproteinemias, and some hereditary neuropathies, such as Charcot–Marie–Tooth disease type 1A.
¤ Which systemic diseases cause predominantly a demyelinating peripheral neuropathy ?
In most rheumatologic conditions where in neuropathy is present, the axons are primarily affected, though myelin is rarely completely normal. Demyelination may predominate in a few disorders, such as acute inflammatory demyelinating polyneuropathy (Guillain–Barré syndrome), chronic inflammatory demyelinating polyneuropathy, multifocal motor neuropathy, anti-myelin-associated glycoprotein antibody syndrome, and other paraproteinemias, and some hereditary neuropathies, such as Charcot–Marie–Tooth disease type 1A.
¤ How is EMG/NCS used in diagnosing CTS? Ulnar nerve entrapment at the elbow (cubital tunnel syndrome)
CTS or compressive median neuropathy at the wrist is the most common entrapment neuropathy, affecting 1%
of the population. CTS may show segmental nerve conduction slowing across the wrist. SNAP latencies of the median nerve are delayed most often, but with increasing severity, motor latencies can be affected. Denervation of the thenar muscles seen on needle EMG indicates moderate to severe CTS. Clinical correlation is recommended for mild CTS as sometimes NCS/EMG studies are normal despite classic symptoms of hand pain/numbness in a median nerve distribution.
CTS or compressive median neuropathy at the wrist is the most common entrapment neuropathy, affecting 1%
of the population. CTS may show segmental nerve conduction slowing across the wrist. SNAP latencies of the median nerve are delayed most often, but with increasing severity, motor latencies can be affected. Denervation of the thenar muscles seen on needle EMG indicates moderate to severe CTS. Clinical correlation is recommended for mild CTS as sometimes NCS/EMG studies are normal despite classic symptoms of hand pain/numbness in a median nerve distribution.
In cubital tunnel syndrome, the ulnar nerve is compressed at the elbow resulting in motor or sensory nerve conduction slowing. Needle EMG examination may identify denervation in the ulnar-innervated muscles of the hand and forearm. The ulnar nerve can also be compressed at the wrist.
