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disappears instantaneously when the offending vessel is moved off the facial nerve. Observing this muscle response therefore helps the surgeon identify which blood vessel is causing the symptoms of hemifacial spasm and makes it possible for the surgeon to he assured that the facial is identified. Normally, the abnormal muscle response will disappear instantaneously when the offending vessel is lifted off the facial nerve and will reappear if the vessel is allowed to fall back on the facial nerve. This makes it possible to accurately identify the vessel that is causing the symptoms. It is often seen that the prolonged EMG activity that follows the initial component at 10-ms latency disappears as the operation proceeds, even before the dura mater is opened, and only a brief potential with a latency of 10 ms remains when the MVD procedure begins. The small unsynchronized EMG potentials that were present at the end of the operation in this case are indications of a slight (temporary) injury to the facial nerve.
In some patients, particularly patients who have had hemifacial spasm symptoms for a relatively short time, it may he necessary to activate the abnormal muscle response by stimulating at a high rate. The initial component of EMG activity elicited during monitoring of the abnormal muscle response in hemifacial spasm follows a rapid rate of stimulation (at least up to 50 pps). Such rapid stimulation rates activate the abnormal muscle response so that its amplitude increases. Stimulation at these rates may also reactivate the activity that follows the initial 10-ms component, when such activity has disappeared.
BSAER
The origins of BSAER can be traced to the animal experiments of the 19th century. In 1875, Caton first reported that there was electrical activity in the brain, which could be demonstrated by evoked potentials in the rabbit. He was also the first to notice the unexpected spontaneous ossilations of the baseline that were later to become known as the electrocorticogram.
Danilevsky working in Russia, studied the spontaneous electrical activity of the brain in dogs in or around 1877. Essentially, he rediscovered evoked potentials in the dog rather than in the rabbit. Unlike Caton, he was aware of the auditory potentials. Between 1883 and 1891 Fleischl von Marxow, Beck, and Gotch and Horsley actively pursued the recording of electrical activity of the brain in a variety of experimental animals, but it was Pravdich-Neminsky in 1913 who first photographed the record of an animal electroencephalogram with a string galvanometer.
Until more sophisticated equipment was designed, researchers were limited in their range of activity. In the early part of the 20th century, electrophysiologists were working under very primitive conditions owing to the lack of sophisticated equipment. It was not until 1920 that an electrical amplifier was used in a physiological experiment by Forbes and Thacher; the cathode-ray oscilloscope was not in general use until 1922. In the l930s, oscilloscopic images (Continued on page 120)
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