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Electrodiagnostic Tests Electrodiagnosis is the field of study that uses electrical technology to study the neurophysiology of the human body. Several separate tests comprise the field, including neurodiagnostics (NDS), electromyography (EMG), and evoked potentials (EPs). Neurophysiology Electrical signals are generated in the brain and pass through the spinal cord and out into the peripheral nervous system. These signals are carried down the nerve to the synaptic cleft where a chemical release of acetylcholine crosses the synaptic cleft to create an electrical discharge in the muscle. This electrical signal causes the muscle to contract. Neurodiagnostic testing bypasses the brain by delivering an electrical charge to the patient. The equipment is used to measure several aspects of the body's response to that signal to determine whether it is functioning properly. Location and degree of the injury, acuity, prognosis, and, in some cases, specific diagnosis can be determined through electrodiagnostic testing. Electromyography Electromyography (EMG) was the first electrodiagnostic test to be developed. This procedure involves placement of a needle into various muscles to record the muscle activity in various stages of muscle activity, including at rest, with minimal contraction, and with maximal muscle activity At rest, normal muscle is electrically silent. Damaged muscle tissue may result in spontaneous depolarization of individual muscle fibers. This abnormal activity can be detected during the needle examination portion of the electrodiagnostic examination. Contraindications : active cutaneous infection at the site of insertion and absolute neutropenia. Primary nerve injuries that are severe enough to create neurotmesis or axonotmesis lesions, which result in Wallerian degeneration of the nerve, demonstrate fibrillation and positive sharpwave discharges at rest. Fibrillations and positive sharp waves are pathognomonic for nerve injuries. Fasciculations represent the spontaneous firing of a single motor unit ( anterior horn cell, its nerve process, NMJ, muscle fibres innervated. Can be seen clinically. If the clinical examination and remainder of the EMG are normal, fasciculations are termed benign. Primary muscle disorders can also be detected by EMG. With minimal volitional activity, the recruitment patterns of muscle are normal. Recruitment patterns provide critical information for determining the degree of injury and prognosis. Helpful in identifying individuals who are malingering. A patient may not be fully cooperative in manual muscle testing for weakness. Determination of the recruitment pattern, however, is not dependent upon patient cooperation. Nerve Coduction Studies Important part of the complete electrodiagnostic exam. In an NCS, an electrical charge is delivered to a peripheral nerve. That charge is carried down the nerve and generates a muscle contraction. A recording electrode is placed on a muscle innervated by that nerve, and information about the impulse can be recorded, including the latency or time to get from stimulus to recording The distance traveled and the nerve conduction velocity (NCV) can also be computed. These measures are a sensitive indicator of nerve damage and look specifically at the integrity of the myelination of the nerve. Factors which affect the recording are : Age, height,skin temperature technical errors like improper placement of the electrode and not achieving supramaximal stimulation of the nerve The amplitude of the muscle contraction can be determined and compared to the initial size of the signal, providing information about the number of neurons that are functioning within the nerve. By stimulating the nerve in various places along its course, the practitioner can isolate a specific site of injury. Changes in amplitude often signal an axonal injury Focal slowing of more than 10 m/s in a short segment is considered significant. Direct relationship exists between nerve diameter and conduction velocity. Larger myelinated fibers (Ia) are the ones measured in nerve conduction studies. The smaller fibers (Ib, II, III) are not recorded with standard nerve conduction techniques. Therefore, a small fiber neuropathy may not be detectable by NCV. Distal latency and conduction velocity measurements are helpful in evaluating the speed of conduction along the distal and mid-portions of a peripheral nerve, respectively. F wave latency - evaluating conduction along proximal segment of motor nerve if the distal segment is normal. Evoked Potential Evoked potentials are electrical signals generated by the nervous system in response to sensory stimuli. Evoked potential testing is the application of electrodiagnostic testing to the central nervous system (CNS). These tests are useful clinically to demonstrate abnormal sensory function when the neurologic examination results do not reveal abnormalities reveal clinically unsuspected pathology when demyelinating diseases are suggested determine the anatomic distribution of a disease process monitor a patient's progress or deterioration objectively over time Somatosensory EPs (SSEPs) Visual EPs (VEPs) Brainstem auditory EPs (BSAEPs Recording electrodes may be either surface electrodes or small subdermal needles placed anywhere along the neuropathway, including the spine and scalp. SSEPs SSEPs are the most widely used evoked potential test. Stimulation occurs at the extremity with recordings on the scalp near the sensory cortex. This technique may be used to locate the level of the injury at the nerve root, spinal cord, or brain. SSEPs have been helpful in determining the degree and prognosis in CNS insults, such as spinal cord trauma and strokes SSEPs can also be used to localize root level injuries in cervical and lumbar radiculopathies and demyelinating diseases like multiple sclerosis, Used to determine the level of coma and evaluate for brain death. Intraoperative monitoring of patients undergoing neurosurgical procedures. VEP With VEPs, the practitioner uses a photoelectric checkerboard pattern flash to stimulate the optic nerve. This pattern is then recorded on the cortex, arriving at the occiput, near the visual centers. This usually takes 100 milliseconds to arrive and is referred to as the P100. Injuries along the optic nerve, including demyelination, result in a delay of the latency loss of the amplitude of the signal, similar to the results in NCVs. A P100 at greater than 102 milliseconds is considered beyond the reference range in most laboratories Multiple sclerosis - abnormalities in the VEP are often the first indicator of the disease process. Optic chiasm tumors also produce recordings suggesting abnormalities. Visual acuity may be determined in infants with suspected visual disturbances Brainstem Auditory Evoked Response An auditory click is used to stimulate the cochlear nerve. The response is than recorded over the cortex. Because of the multiple signal generators in the brainstem, injuries along this nerve can suggest localization of the lesion to various sites along the nerve route. BSAEPs may be used to help evaluate for Brain death Demyelinating disease Midbrain tumors.