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EEG is an important tool in pediatric neurology and EEG abnormalities occur in many different disorders. EEG abnormalities are nonspecific and do not make a specific diagnosis. However, the EEG is especially useful in the diagnosis, differential diagnosis, classification, and management of seizures and epileptic syndromes. Once the diagnosis of epilepsy is established, specific EEG patterns help to define specific epileptic syndromes.

The methodology for performing standard nerve conduction studies has been established by identifying the most helpful and consistent physiological data obtainable while being constrained by a variety of technical and practical limitations. Nerve stimulation occurs underneath the negatively charged anode of the applied stimulator and simultaneous hyperpolarization of the nerve occurs beneath the positively charged cathode. Referential or bipolar recording techniques are used for all types of measurements. Sensory conduction studies can be performed either antidromically or orthodromically, although, for technical reasons, the former are usually preferred; the recorded sensory nerve action potential is made up of the simultaneous depolarization of all of the cutaneous sensory axons. In motor studies, the compound motor action potential is recorded from the motor point of the muscle of interest and represents the depolarization of the underlying muscle fibers rather than the nerve itself and is, thus, of considerably greater amplitude and duration. F-waves and H-reflexes represent the two most commonly evaluated forms of late responses and assist with assessing the entire length of the neurons, from spinal cord to distal muscle.

Nerve conduction studies and their interpretation are subject to a variety of factors. First, technical factors including submaximal stimulation, environmental electrical noise, inaccurate placement of the recording electrodes, and stimulus artifact can substantially interfere with accurate recording of nerve and muscle responses. Second, physiological factors, such as the effects of body height and age, can cause profound variation in all nerve conduction parameters, and studies require interpretation keeping these individual variations in mind. Another physiological factor is temperature, in which cooling can produce a variety of changes, including slowing of conduction velocity and increase of response amplitude. Anatomic factors are also important, the most common being the Martin-Gruber anastomosis, usually presenting with a reduction in response amplitude with proximal stimulation of the ulnar nerve, and the second most common being the presence of an accessory peroneal nerve. Paying constant attention to all of these details is a critical element to the accurate performance and interpretation of nerve conduction studies.

Needle EMG remains an important part of the evaluation of the peripheral nervous system and can assist substantially in characterizing a variety of disease states. The needle electrode examination generally consists of three parts: evaluation of spontaneous activity, evaluation of motor unit potential morphology, and evaluation of motor unit potential recruitment. Abnormal spontaneous activity includes the commonly observed fibrillation potential and positive sharp wave and the less frequent myotonic, myokymic, neuromyotonic, and complex repetitive discharges. Neurogenic disease produces prolonged duration and increased amplitude of motor unit potentials with reduced recruitment, whereas myopathic disease generally produces low-amplitude, short-duration motor unit potentials, with normal or early recruitment. An understanding of the basic mechanisms of these disease changes can greatly aid in the interpretation of EMG data. Finally, the distribution of abnormalities across muscles can also aid in diagnosis, especially in the assessment of radiculopathy and plexopathies.

Nerves of both the upper and lower extremities are frequently injured for a variety of reasons. In the arms, median neuropathy at the wrist is by far the most common disorder; ulnar neuropathy also occurs with a relatively high frequency. Other mononeuropathies affecting the upper extremities, including anterior and posterior interosseous neuropathies, and musculocutaneous neuropathies, are very infrequent. In the legs, peroneal neuropathy at the fibular neck and lateral femoral cutaneous neuropathy are the most common disorders, and other focal neuropathies, such as tibial neuropathy at the ankle (tarsal tunnel syndrome) are often sought but rarely identified. Although all mononeuropathies can be preliminarily diagnosed by history and clinical examination, neurophysiological testing provides a key component to confirming the diagnosis and assisting with treatment planning.

As a class, polyneuropathies are some of the most commonly presented disorders to a neuromuscular clinician. Polyneuropathies generally are of one of two forms, axonal or demyelinating. This classification is often helpful in identification of potential causes. Axonal polyneuropathies are most commonly associated with diabetes, alcohol, or toxins, including side effects of medication. Demyelinating polyneuropathies, as a whole, are much less common and can be acute or chronic, and idiopafhic or associated with a monoclonal gammopathy. Finally, hereditary neuropathies, including the various forms of Charcot-Marie-Tooth disease, have similar but somewhat distinct characteristics compared with acquired forms of polyneuropathy. Nerve conduction studies, and, to some extent, needle eletromyography (EMG), can assist with the evaluation of polyneuropathies, and can help to shorten the differential diagnosis regarding potential causes.

Although radiculopathies and motor neuron disorders have vastly different underlying mechanisms and clinical presentations, the electrodiagnostic examination for these disorders demonstrate remarkable similarities. In all of these conditions, the electrophysiological examination reflects motor neuron injury with virtually no involvement of sensory neurons, even in the face of substantial sensory symptoms, as may occur in radiculopathy. The needle examination is a critical part of electrophysiological testing, helping to define the extent and severity of the abnormality, often more effectively than is possible with nerve conduction studies alone. Late responses, including F-waves and the H-reflexes, can also assist in the diagnosis of radiculopathy to some extent. The electrodiagnostic evaluation of motor neuron disease requires special consideration and care.

Brachial and lumbosacral plexopathies represent a heterogeneous group of disorders including traumatic injury as well as infiltrating and inflammatory lesions. The anatomy of both regions is complex, creating challenges in their evaluation; electrophysiological testing is a key tool in the assessment of plexopathies. Accurate electrodiagnosis requires a comfortable knowledge of the relevant plexus anatomy and applicable nerve conduction techniques, as well as recognition of the underlying pathophysiological processes affecting these regions. The sensory nerve conduction studies are used to differentiate lesions of the plexus from radiculopathies, which they may closely resemble clinically and electrophysiologically. Motor nerve conduction studies, late responses, and needle EMG, in conjunction with the sensory studies, allow precise localization and often provide clues to the underlying pathophysiology of a given lesion.

Electrophysiological testing of the cranial nerves remains an important and perhaps under utilized area of neurophysiological evaluation. Motor responses from individual branches of the facial nerve can be obtained to assist in the diagnosis and prognosis of facial neuropathies. In blink reflex testing, the first division of the trigeminal nerve is stimulated and responses from the orbicularis oculi are obtained. This form of testing can be used for a variety of purposes including helping to localize disorders of the Vth and VIIth cranial nerves, assisting in the evaluation of Guillain-Barré syndrome, and even in the assessment of brainstem disorders. It is also useful in the assessment of hemifacial spasm. The masseter reflex and master silent period can also be assessed when clinically indicated. Although less commonly pursued, other cranial nerves can also be evaluated electrophysiologically, including the XIth and XIIth cranial nerves. Assessment of these cranial nerves can be important in assessing other diseases, including amyotrophic lateral sclerosis.

The myopathic disorders represent a heterogeneous group of diseases with a variety of causes. Although electrodiagnostic testing rarely allows an entirely specific diagnosis to be made, such testing can be extremely helpful in first confirming the presence of myopathy and therefore helping to appropriately categorize it. Standard motor nerve conduction studies generally do not demonstrate substantial abnormalities, except occasional reductions in compound motor potential amplitude in severe cases or where predominantly distal disease is present. Needle EMG remains the most important part of neurophysiological examination. In most myopathic conditions, spontaneous activity is increased, although it is most prominently increased in inflammatory or necrotizing myopathic processes. The presence of myotonic discharges can be very helpful in limiting the differential diagnosis. Complex repetitive discharges, which are present in myopathic conditions, are nonspecific. Motor unit potentials in myopathy are generally of low-amplitude and short duration, except in very chronic conditions, in which the motor unit potentials can actually become of long duration and high amplitude. Finally, it is critical to remember that an entirely normal EMG does not exclude the presence of myopathy.