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Nerve Conduction Velocity
A nerve conduction velocity test, also called a nerve conduction study, measures how quickly electrical impulses move along a nerve. It is often done at the same time as an electromyogram, in order to exclude or detect muscle disorders.
A healthy nerve conducts signals with greater speed and strength than a damaged nerve. The speed of nerve conduction is influenced by the myelin sheath—the insulating coating that surrounds the nerve.
Most neuropathies are caused by damage to the nerve’s axon rather than damage to the myelin sheath surrounding the nerve. The nerve conduction velocity test is used to distinguish between true nerve disorders (such as Charcot-Marie-Tooth disease) and conditions where muscles are affected by nerve injury (such as carpal tunnel syndrome).
This test is used to diagnose nerve damage or dysfunction and confirm a particular diagnosis. It can usually differentiate injury to the nerve fiber (axon) from injury to the myelin sheath surrounding the nerve, which is useful in diagnostic and therapeutic strategies.
During the test, flat electrodes are placed on the skin at intervals over the nerve that is being examined. A low intensity electric current is introduced to stimulate the nerves.
The velocity at which the resulting electric impulses are transmitted through the nerves is determined when images of the impulses are projected on an oscilloscope or computer screen. If a response is much slower than normal, damage to the myelin sheath is implied. If the nerve’s response to stimulation by the current is decreased but with a relatively normal speed of conduction, damage to the nerve axon is implied.
There is generally minimal discomfort with the test because the electrical stimulus is small and usually is minimally felt by the patient.
Evoked potentials are used to measure the electrical activity in certain areas of the brain and spinal cord. Electrical activity is produced by stimulation of specific sensory nerve pathways. These tests are used in combination with other diagnostic tests to assist in the diagnosis of multiple sclerosis (MS) and other disorders.
Evoked potentials test and record how quickly and completely the nerve signals reach the brain. Evoked potentials are used because they can indicate problems along nerve pathways that are too subtle to show up during a neurologic examination or to be noticed by the person. The disruption may not even be visible on MRI exam.
Balance Testing – VNG
There are different aspects of the balance system which can malfunction. In some people the problem is an irreversible injury to the nerves which serve the “gyroscope” of the inner ear, called the vestibular system. Fortunately, the brain has a remarkable ability to recognize when one of the vestibular systems is not working correctly. At first a person with a vestibular “weakness”, or vestibulopathy, feels disoriented and maybe sick to the stomach. With time the brain usually learns to disregard this faulty information. In other words, the brain “compensates” for the problem.
Videonystagmography (VNG) refers to a series of tests that evaluate the contribution of the three systems that keep us in balance, vestibular (inner ear), visual, and the somatosensory (input from contact with the surfaces we are walking on). The brain combines the information from each of these three systems and keeps us in balance as we walk, turn our head, read, drive, etc.
Our window into the balance system are the eyes. Goggles are worn while a series of simple tasks, such as following the movement of a light on a bar, changing head and body positions are performed. A separate portion of the test stimulates the vestibular system directly by delivering warm then cool air into each ear. Information from the tests is combined with other clinical information to determine if the balance disorder is ear related or not. Then a strategy for treating the disorder is formulated.
The brain’s electrical activity fluctuates from second to second, but routine EEGs provide only a 20- to 40-minute sample of this activity. If epilepsy waves occur in your brain only once every 3 or 4 hours, or if they happen only after an hour of sleep, for instance, a routine EEG will usually be normal. Then the doctor may want to see a longer recording that includes prolonged periods when you are both awake and asleep. This kind of recording is called an ambulatory EEG. (“Ambulatory” [AM-byew-lah-TOR-ee] means able to walk around.)
An ambulatory EEG, on the other hand, can record up to 72 hours of EEG activity with a special recorder that is slightly larger than a portable cassette player. This recorder allows you to go about your normal routine while the EEG is being recorded.
The EEG has been used for many years and is considered a safe procedure. The test causes no discomfort. The electrodes only record activity and do not produce any electrical current.
Electroencephalography (EEG) is the recording of electrical activity along the scalp produced by the firing of neurons within the brain. In clinical contexts, EEG refers to the recording of the brain’s spontaneous electrical activity over a short period of time, usually 20–40 minutes, as recorded from multiple electrodes placed on the scalp. In neurology, the main diagnostic application of EEG is in the case of epilepsy, as epileptic activity can create clear abnormalities on a standard EEG study. A secondary clinical use of EEG is in the diagnosis of coma, encephalopathies, and brain death. EEG used to be a first-line method for the diagnosis of tumors, stroke and other focal brain disorders, but this use has decreased with the advent of anatomical imaging techniques such as MRI and CT.
Derivatives of the EEG technique include evoked potentials (EP), which involves averaging the EEG activity time-locked to the presentation of a stimulus of some sort (visual, somatosensory, or auditory). Event-related potentials (ERPs) refer to averaged EEG responses that are time-locked to more complex processing of stimuli; this technique is used in cognitive science, cognitive psychology, and psychophysiological research.
American Imaging also provides musculoskeletal and soft-tissue ultrasound testing for the cervical, thoracic, and lumbar areas, along with extremities of shoulders, elbows, wrists, knees, and ankles.