Advantages of Digital Electroencephalography in Clinical Veterinary Medicine Part 1Vet Neurol Neurosurg J. January 2001;3(1):1.
Full Text Article (Reviews/Notes)
This is the first of a series on digital electroencephalography. The present discussion deals with:
1. Changing electrode montages for display after recording is completed (so-called "remontaging"). (To read about the principles of forming electrode derivations and montages visit Holliday and Williams, VNN, 1999.)
2. Changing frequency limits after recording is completed.
1. Changing Electrode Montages After Recording ("remontaging").
Veterinary EEG recording typically is performed under certain limitations. Because of the cost of equipment, most veterinary laboratories have had polygraphs with only 6 or 8 EEG channels. By comparison, thirty-two channels are now used in routine recordings from human patients and even more in some specialized studies. In addition to monetary limits, there is an inherent limit on the number of electrodes that can be placed on the head of most veterinary patients, whose calvaria are often smaller even than those of children.
If a larger number of EEG channels is available, it is possible to use more electrodes to form more derivations and thereby make possible more precise localization of EEG events. When this is impossible, it is very helpful to use multiple montages that include the desired additional derivations. We formerly used as many as 5 montages to help deal with this problem. (Holliday and Williams, VNN, 1999)
Unfortunately, using multiple montages requires additional recording time and the animal's cooperation may not be sustainable for the necessary time. Sedation can be used and has distinct advantages, (Holliday and Williams, VNN, 1999) but sedative drugs have limited durations of action that also can limit the recording period. Digital EEG eliminates these problems. It allows one to record a single period of EEG and then, using the EEG data stored in digital format in the machine, "remontage" at will at any later time, thus effectively extending the recording period without limit. Any number of standard montages can be used and one can improvise extemporaneous montages if necessary to better interpret the data in individual cases.
Remontaging consists of changing the montage of an EEG after recording is completed. This is demonstrated in Figures 1-4. These figures all show exactly the same epoch of EEG. They differ from one another only in the ways the EEG has been remontaged. Recorded from a 4 week-old foal, the epoch includes a paroxysmal discharge (PD) (specifically, repetitive spike-and-wave complexes). For more information on EEG technics see Electroencephalographic Technics at UCD-VMTH Electrophysiology Laboratory section below.
Electrophysiology Notes - Advantages of Digital Electroencephalography-1
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Electroencephalography Electrode Map
The electrode "map" shown here is that presently used for placing electroencephalography electrodes on horses at the UCD-VMTH Clinical Neurophysiology Laboratory.
Electrode designations conform, where possible, with internationally recommended terminology:
C: "central" **
Odd numbers on the left side,
Even numbers on the right side;
z: midline location.
Note: The A electrodes deviate from standard terminology in that they are located in the low temporal region, rostral to the ear canal and just above the level of the zygomatic arch, rather than being on the ear itself.
OD, OS, IC are electro-oculogram electrodes: OD, right eye; OS, left eye; IC, intercanthal, on the midline, between the medial canthi.
In the patient described herewith, the Fp electrodes were omitted because of the rather small size of the foal's head.
** We use the "C" designation because it is used widely in veterinary medicine even though it is inappropriate in all but primates. The C electrodes in higher primates are near the central sulcus of the cerebral hemispheres, hence their name. The analogous structure in the domestic species, the cruciate sulcus, is much smaller, much more rostral and is actually much closer to the frontal electrodes than to the C electrodes.
COMMENTS: Digital EEG permits greatly improved veterinary EEG results. Nevertheless, there is room for further improvement, for example: Present-day electrode montages, even with digital EEG, are limited now by two important factors: 1: space on the animal's head for more electrodes; 2. absence of recording from the more ventral regions of the hemispheres. We now have added additional electrodes in the lateral regions of the calvaria (not shown here) that seem to be helpful in more precise localization of abnormalities. Still, nasopharyngeal and/or sphenoidal electrodes are badly needed to allow recording from the lower and more rostral regions of the temporal lobes and the amygdala. We cannot truly "recognize" temporal lobe epilepsy in domestic animals and differentiate it from other epilepsies, without knowing the EEG status of such regions, interictally and ictally.
2. Changing Frequency Sensitivity to Reduce Muscle Artifact.
In domestic animals, most of the calvaria is covered by facial musculature. In EEGs recorded from such areas, high-frequency muscle artifacts sometimes abound and obscure the EEG. Sedation helps reduce the artifacts by allowing the animal to relax, but even in deep sleep, some muscle artifact can persist. Muscle potentials in the EEG tend to fall in the range of about 20 Hz to above 70 Hz. Most conventional ("hard copy") EEGs, use a high frequency limit of 70-90 Hz, therefore muscle potentials are readily recorded. Lowering the high frequency limit ("filtering") can reduce the muscle artifacts; however, when this is done the "hard copy" recording is permanently changed and the changes cannot be undone.
The high frequency limit on most digital EEG equipment is about 90 Hz, thus substantial amounts of muscle potentials will be included in the recording. Nevertheless, one can display the EEG later while applying any filter limits deemed necessary to reduce the muscle artifact. This simply affects the display of the EEG; the original data is maintained unchanged on the disk.
Changing high-frequency limits after recording.
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Electroencephalographic Technics at UCD-VMTH Electrophysiology Laboratory
In the UCD-VMTH laboratory we usually record 45-75 min of digital EEG. Recording is performed using 13-15 EEG electrodes; an additional 3 electrodes are used for the electro-oculogram (EOG) but can be used for EEG if desired. The ECG routinely is monitored from leads on the limbs (small animals) or the thorax (horses). Sedation is administered before placing electrodes. The foal whose EEG is shown herein was sedated with xylazine xxxmg/kg, IV.
Electrodes are platinum alloy needle electrodes placed subcutaneously in a routine manner, attempting to record from the dorsal and lateral surfaces of the cerebral hemispheres. For horses and medium-size or large dogs, 13 to 15 electrodes are placed and 13-19 EEG channels are recorded, plus the ECG and EOG (see Electroencephalography Electrode Mapsection above). The EEG equipment is programmed to record the EEG using the Cz (vertex) electrode** as a reference during recording. However, during recording and during interpretation of the record, the EEG can be displayed using any desired montage. During interpretation, montages are designed to take advantage of instrumental phase reversals (see Holliday and Williams, VNN, 1999) although from time to time referential montages are used.
Electrodes are connected to a Nihon-Kohden 2100 EEG system. Amplifier frequency range is 0.001 Hz to 90 Hz. For digitization, EEG signals are sampled at 200 Hz. All EEG in figures 1-4 are displayed with a time constant of 0.1 and a high frequency limit of 70 Hz, rolloff of 6 dB/octave and 12 dB/octave respectively. The high frequency limits in Fig 5 and Fig 6 were varied for demonstration purposes.
** We use the "C" designation because it is widely used in veterinary medicine even though it is inappropriate in all but primates. The C electrodes in higher primates are near the central sulcus of the cerebral hemispheres, hence their name. The analogous structure in the domestic species, the cruciate sulcus, is much more rostral and is actually much closer to the frontal electrodes than to the C electrodes.
1. Holliday, TA and Williams, DC. Interictal paroxysmal discharges in the electroencephalograms of epileptic dogs. Current Techniques in Small Animal Practice 13(3): 132-143, 1998.
2. Redding, RW and Knecht, CE. Atlas of Electroencephalography in the Dog and Cat. New York, Prager, 1984.
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