Conventional Electrodiagnosis (eSlide 8.3)

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  Strength-duration curve (SDC) and nerve excitability tests: An SDC study may show the earliest objective sign of denervation. It reveals an abnormality when a nerve is injured for more than 72 hours. If reinnervation occurs, it can be characterized by a shift of the curve to the left, with a fall in the chronaxie and the appearance of a plateau. The earliest evidence of nerve degeneration after injury is the failure to respond to electrical stimulation. In addition, abnormal nerve excitability presents 3-5 days after a nerve lesion.

  
  
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  Nerve conduction studies (NCSs): Nerve conduction velocity (NCV) is the propagation speed of an action potential along nerve fibers. It is often affected by myelination of the nerve. Besides the conduction velocity itself, the amplitude of the compound muscle action potential (CMAP) and sensory nerve action potential (SNAP) reflects the axonal numbers of the tested nerve. The reducing amplitude of CMAP or SNAP observed during NCSs always represents a neuropathy associated with axonal degeneration. The NCS mainly contributes in the assessment of peripheral neuropathy, entrapment neuropathy, and peripheral nerve injury. It is the most common method of objective and quantitative testing of neural functions.

  
  
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  Repetitive nerve stimulation test (Jolly test): This test is frequently used in screening of neuromuscular junction diseases. The function of the neuromuscular junction can be indirectly assessed by repetitive stimulation of a motor nerve with a recording electrode placed over the appropriate muscle. It is helpful in diagnosing neuromuscular junction disorders such as myasthenia gravis, myasthenic syndrome (Lambert-Eaton syndrome), and botulism. The significant decremental response at lower rate (2–10 Hz) stimulation in myasthenia gravis is more than 10%. The significant incremental response at a higher rate (20–50 Hz) stimulation in myasthenic syndrome is more than 50%.

  
  
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  Long latency reflex studies: Long latency reflex studies include those of Hoffmann (H) reflexes, which are considered to originate from spinal reflexes and are induced by both electrical stimulation of afferent nerve (Aα) fibers in the mixed nerve to the muscle and activation of motor neurons to the muscle through a monosynaptic connection in the spinal cord. These reflexes often appear in the S1 spinal nerve pathway and are commonly used in the diagnosis of S1 radiculopathy. The F wave is a compound action potential that is evoked intermittently from a muscle by a supramaximal electrical stimulus to the nerve. It is induced by antidromic activation of motor neurons. During the test, the F wave always requires 10-20 stimuli and is selected by minimal or mean conduction latency. Blink reflexes are compound muscle action potentials that are evoked from the orbicularis oculi muscles and that present as a result of brief electrical or mechanical stimuli to the cutaneous area, innervated by the supraorbital branch of the trigeminal nerve. They always require bilateral muscle recordings and present as an ipsilateral monosynaptic R1 wave response and a multisynaptic R2 wave response, accompanied by a contralateral R2 wave. Deep tendon reflexes are reflexes that occur in response to a muscle tendon being tapped briskly. Much like a stretch reflex, a tendon reflex is the contraction of a muscle in response to the stretching of muscle spindles, which triggers the receptors that lie parallel to extrafusal muscle fibers.

  
  
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  Electromyography (EMG): By inserting a needle electrode into the muscle under examination, muscular activities and motor unit action potentials (MUAPs) can be identified in an oscilloscope as different diagnostic patterns. Four steps should always be observed during an EMG examination:

  
  
  
  
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    Insertion of the needle electrode: Prolonged insertional activities are frequently encountered in denervation, myositis, and myotonia, whereas shortened activities are seen in areas of muscle atrophy, fatty degeneration, and myofibrosis.

    
    
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    Muscle at rest: Normal muscle will exhibit electrical silence during this step. Abnormal conditions include spontaneous activities, such as fibrillation, positive sharp waves, fasciculation, myokymia, myotonic discharges, and complex repetitive discharges (CRDs).

    
    
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    Muscle during volitional contraction: This step may show the shape and figure of individual MUAPs. The parameters of an MUAP, such as amplitude, phase, turn, and duration, can be measured. This is important for the determination of acute or chronic neuropathy and the measurement of disease progression and treatment outcomes.

    
    
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    Muscle during maximal contraction: This step reveals interference or recruitment in muscle activation and reflects both the quantity and firing rate of neurons. It is an important test for differentiating between neuropathy and myopathy.

  
  
  
  
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  Evoked potential (EP) studies: These studies include somatosensory evoked potentials (SSEPs), visual evoked potentials (VEPs), brainstem auditory evoked potentials (BAEPs), and event-related potentials (ERPs). They are recorded at the scalp over the cortices or at various sites along sensory pathways and are elicited by stimulating the corresponding receptors, which could be the skin, eye, ear, or nerves themselves. EP studies show the physiologic integrity of sensory pathways and are able to detect abnormalities that may not be clinically obvious. Motor evoked potential (MEP) studies, developed in recent decades, have been used to assess the physiologic integrity of motor pathways in the central nervous system by stimulating the motor cortex beneath the scalp. Clinically, EP studies are useful in determining central nerve conduction status and monitoring patients during surgery.

Advanced Electrodiagnosis (eSlide 8.4)

New advances that have been brought to electrodiagnosis include advanced EMG, different techniques of NCS, magnetoencephalography (MEG), and transcranial direct current stimulation (tDCS). Needle EMG is invasive. However, it is essential and irreplaceable in the diagnosis of neuromuscular diseases. Surface EMG (eSlide 8.5) is noninvasive and has been widely used in various aspects of rehabilitation medicine, including rhythmic and involuntary movement monitoring, muscle reeducation, motor control, biofeedback, gait and motion analysis, and kinematic measurement. Although single fiber EMG has been in use for almost half a century, it remains the most sensitive clinical method for studying the stability of neuromuscular transmission and functional integrity of peripheral nerves. Stimulated single fiber EMG (eSlides 8.6 and 8.7) has been modified for use in testing paralyzed muscles, unconscious patients, uncooperative patients, pediatric patients, and patients with involuntary movements. Other advanced electrodiagnostic methods, such as quantitative EMG, refractory period studies, decomposition EMG, quantitative sensory tests, and power spectrum analysis, have been in use for years for special purposes.

Transcranial Magnetic Stimulation (eSlide 8.8)

MEP with transcranial magnetic stimulation (TMS) and deep brain stimulation are the most commonly used electrodiagnosis methods in recent years. TMS is a noninvasive method that excites neurons in the brain. A weak current is induced in the tissue by rapidly changing magnetic fields, through a process called electromagnetic induction. For diagnostic purposes, clinical uses of MEP with TMS may serve as a tool for the evaluation of central motor pathways using measurements of central motor conduction time and estimates of spinal cord motor conduction velocity (eSlide 8.9) . For therapeutic purposes, MEP with TMS can be used as a prognostic indicator for motor recovery of central nervous system diseases, such as stroke, traumatic brain injury, or aphasia. It can also be used in the measurement of functional integrity and as potential treatment for many neurologic and psychological conditions, such as migraines, Parkinson disease, dystonia, tinnitus, neurogenic pain, drug addiction, schizophrenia, obsessive–compulsive disorder, Tourette syndrome, autism, bipolar disorder, and major depressive disorder (eSlide 8.10) . Among MEP applications, TMS is commonly used because of its advantages of noninvasiveness, with painless but deep penetration of nerve tissues. The magnetic pulse can easily pass through highly electroresistant tissues, such as the skull. It can also be applied in the study of deep-seated peripheral nerves.

Magnetoencephalography (eSlide 8.11)

MEG is a noninvasive technique for investigating human brain activity. It allows the measurements of ongoing brain activity on a millisecond basis and shows where the brain activity is produced. MEG signals are obtained directly from neuronal electrical activity and are able to show absolute neuronal activity, whereas functional magnetic resonance imaging (MRI) signals show only relative neuronal activity. MEG can also electrophysiologically provide a more accurate spatial location of neural activities than electroencephalography (EEG). Current equipment, which includes up to several hundreds of whole-head channels, may accurately detect cortical and subcortical activities. Apart from evaluating physiologic activity, MEG may also be used for assessing many conditions, such as epilepsy, dementia, movement disorders, stroke, and learning disabilities, as well as in fetal studies and precise cortical delimitation prior to tumor or lesion resection. It has been a great advance in the diagnostic approaches in neurosciences.

Future Prospective of Electrodiagnosis (eSlide 8.12)

Electrodiagnosis has become increasingly important in rehabilitation medicine and in many applications, including clinical and biomedical diagnosis, neuroprosthetics or rehabilitation devices, brain-computer interfaces, and neuromodulations. The future of electrodiagnosis requires more neurotechnological assistance to develop new equipment and techniques for the tests and methods to apply them in clinical diagnosis and therapeutic medicine.