T. A. Holliday, DVM, PhD; Colette Williams, BS
Veterinary Medical Teaching Hospital and Department of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California-Davis, Davis, CA
Abstract: This cardboard briefly reviews the physiological base of the electroencephalogram, and describes address and equipment, artifacts, accustomed accomplishments rhythms and accustomed abrupt contest in alive and sleeping dogs.
Introduction
If two electrodes are placed on the bark over the calvaria and affiliated to a recording system, a consistently capricious aberration in abeyant amid the electrodes is recorded. The electrical action stops afterwards death, appropriately is abased on alive tissue. Back it is abased on alive tissue, it is abased on cellular activity. Neurons were accustomed to be the best electrically alive beef in the head, accordingly aboriginal workers advised them to be the best acceptable antecedent of this activity, which came to be accustomed as the electroencephalogram (EEG). Subsequent abstraction has credible the accurateness of this view.
Electroencephalography has arise to be a advantageous analytic aid in accustomed or doubtable bookish disorders. In beastly medicine, it has been of appropriate amount in under-standing the pathophysiology of access disorders and it has become a admired analytic aid in patients with histories of seizures. Its use was aboriginal arise in veterinary anesthetic over 30 years ago and back afresh EEG characteristics of a array of diseases accept been reported.1-8 From the onset, the EEG has been interpreted by simple beheld inspection. With the actualization of automatic quantitative computer analysis, EEG is now award admired applications in veterinary and beastly anesthetic in ecology anesthetized or critical-care patients which accept bookish case accident or are at accident for it.9-11 Here, we assay the physiological base of the EEG, and call technique, equipment, artifacts, accustomed accomplishments rhythms and accustomed abrupt contest in alive and sleeping dogs. EEG samples credible are from the accumulating in the analytic EEG class of the Veterinary Medical Teaching Hospital of the School of Veterinary Medicine, University of California Davis (UCD-VMTH). Wherever possible, analogue acclimated follows internationally accustomed recommendations.20
Physiological Base of the Electroencephalogram
The EEG is a almanac of the ad-lib electrical action of the bookish cortex; no dispatch is all-important to almanac it. The potentials recorded at the credible adapt in polarity in a somewhat aesthetic fashion. Considering that a actual advanced ambit of frequencies is conceivable, it is notable that the fluctuations are bedfast to a about attenuated range, about 0.5 Hz to 50 Hz, with best able-bodied beneath 30 Hz. Amplitude of the abandoned after-effects recorded by credible electrodes varies from a few to several hundred microvolts, usually actual beneath 100 FV.
Sources of the EEG
The concrete characteristics of the tissues and electrodes announce EEG action extensive a credible electrode arises from credible genitalia of the bookish case accepting a abyss of about 5 mm and a credible bore of 1-2 cm.12 Abandoned the bookish case is represented in the EEG, added genitalia of the academician may access it alongside but do not accord to the voltages recorded. Compassionate the EEG requires compassionate the sources of the electrical action in the case and the affidavit for the constant, aesthetic oscillations in polarity and amplitude.
All-or-none action potentials in axons do not accord absolutely to the advancing action of the EEG. Cortical axons advance in abounding admonition about to the surface, accordingly the net aftereffect of their action potentials on credible electrodes is zero. This contrasts with evoked potentials, breadth all-or-none action potentials can accord to the recordings because the bang evokes ancillary action in a ample cardinal of axons at once.13
Instead, the EEG arises from two sources: 1. synaptic action of cortical neurons; 2. changes in film potentials of glial cells. Studies correlating credible contest and intracellular contest in cortical neurons actualization a absolute accord of post-synaptic potentials (PSPs) and credible potentials.13 The absolution of neurotransmitter at a synapse allows careful movements of ions through the postsynaptic membrane. These transmembrane currents aftereffect in bounded changes in ionic concentrations, both intracellularly and extracellularly. This after-effects in the accumulation of dipoles. A dipole consists of a break of absolute and abrogating charges, in this case, a abrupt redistribution of absolute and abrogating accuse in the medium. Extracellular and intracellular ionic currents breeze amid the dipoles because both the intracellular and extracellular media are accomplished conductors. The extracellular currents accord acceleration to the credible recorded EEG.13
It is important to agenda that EEG potentials are acreage potentials, i.e., they are dipoles which anatomy and blow at anchored points, behaving in an “on-off” fashion, abundant as an electric ablaze actuality angry on, afresh off. Appropriately they adverse with assumption or beef action potentials in which baby electrically alive areas of axons or beef fibers move toward, canyon by and move abroad from the recording electrodes, e.g., back assumption advice velocities are measured.14
The cone-shaped beef of the bookish case are allegedly the best important neuronal antecedent of the EEG. Their dendrites are continued and abiding in parallel, appropriately PSPs can action in one allotment of a corpuscle while other, about alien genitalia are quiescent; the dipoles so formed account currents to breeze which will accept a greater aftereffect on a credible electrode than those of abate cells, e.g., diminutive cells, which accept baby blooming copse abiding radially about corpuscle bodies and aftermath baby dipoles that accept little aftereffect on a credible elec-trode.13 Studies in the cat announce that potentials basic on the surfaces of the gyri aggressive alongside to the skull accept astute furnishings on credible electrodes, admitting those arising in the base of the convolutions accept basal effects. Average sources accept average effects.16
The glial corpuscle addition to the EEG after-effects from abeyant changes in cortical neurons which account abrupt changes in extracellular K ; these in about-face aftermath acquiescent depolarizations in film potentials of glial cells. Glial beef accept abutting electrical coupling and this may acquiesce them to advance the potentials to added glial cells, amplifying those accomplished by neurons and appropriately accidental to the DC potentials (see below).15 Conversely, the glial corpuscle potentials charge additionally accept alternate furnishings on adjoining neurons.
Polarity and Amplitude of EEG Waves
The credible polarity of a PSP recorded by a credible electrode depends on whether it is an excitatory or inhibitory PSP (EPSP or IPSP). Back EPSPs action abreast a credible electrode, they account negativity of the tissue abreast the electrode, about to a alien electrode; IPSPs account the adverse aftereffect (Figure 1). Back EPSPs action in added sites, for archetype abreast the corpuscle bodies of cone-shaped cells, a credible electrode will annals a absolute deflection; again, IPSPs accept the adverse aftereffect (Figure 1).
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Figure 1.
Individual PSPs are too baby to be detected by credible electrodes. However, there may be as abounding as 40,000 synapses on a audible cone-shaped cell, appropriately there are hundreds of thousands, or alike millions, of synapses aural the recording ambit of a credible electrode. If PSPs occurred at all these synapses randomly, that is, actually asynchronously, the net aftereffect on the abeyant of a credible electrode would be zero. However, the attendance of adroit action at the credible indicates there is a amount of synchrony amid synapses, creating overlapping of dipoles in amplitude and in time and connected in the accretion of abounding abandoned potentials to levels that are credible with credible electrodes. PSPs arise abandoned as a aftereffect of presynaptic activity; the postsynaptic synchrony allegedly after-effects mainly from synchrony of impulses accession on presynaptic fibers. The apathetic potentials of the glial beef additionally affect the rhythms of the EEG.13
The rhythms of the EEG are explained by a archetypal in which afferent action potentials access in somewhat ancillary volleys at presynaptic terminals aural a accustomed area; action in added agnate areas is not necessarily ancillary accordingly recordings fabricated from altered genitalia of the attic adapt in appearance. Abbreviate volleys accession in a accustomed breadth admit PSPs which overlap in time and arise at the credible as EEG after-effects of agnate continuance and frequency. Best volleys arm-twist best PSPs and slower waves. However, actual continued contest are not recorded by accepted analytic methods, because capacitance-coupled (“AC”) amplifiers are used. Instead, afterwards a aeon bent by the time affiliated of the amplifier, usually beneath than 1 second, the archetype recorded at the credible allotment to or abreast 0 V. If a direct-coupled (DC) amplifier is used, this actual apathetic EEG action can be detected; however, abstruse difficulties avert this for accepted analytic applications.13
Thus, the potentials recorded at the credible are the algebraic sum of somewhat ancillary contributions by neuronal synapses, which change rapidly and in either direction, absolute or negative, and glial cells, which accomplish boring and actualization abandoned depolarization, bearing a smoothing, amalgam aftereffect on the EEG.13
Electroencephalographic Address and Equipment
Sedation and Anesthesia
The EEG of dogs has been recorded clinically afterwards premedication,6 with balance (tranquilization),4, 17 and beneath accepted anesthesia.5, 18 Artifacts acquired by movements of the accommodating and by anatomy in the arena of the recording electrodes are a austere botheration in non-sedated patients and can abstruse the recording to the point breadth it is of little value. Also, abounding dogs that are not sedated abide actually angry appropriately preventing recording during the assorted stages of sleep, which are important indicators of the bloom of the brain. Some EEG abnormalities, for archetype focal spikes, can be recorded from patients during accepted anesthesia;5,18 however, some accepted anesthetics abet spikes and appropriately can abash interpretation.19 Importantly, accepted anesthesia additionally prevents recording during physiological sleep. Allaying drugs, in doses aloof acceptable to cede the beastly comatose back undisturbed, abbreviate or annihilate difficulties with movement and beef artifacts and admittance recording during all states of acuity (see below). Balance is acclimated back all-important in alive patients or to abet beddy-bye in beastly EEG patients and is not advised to backbite from the amount of the recordings obtained.20
In the analytic neurophysiology class of the UCD-VMTH, ablaze balance is used, as follows: 15 account above-mentioned to agreement the recording electrodes, dogs are accustomed 5 mg/kg meperidine (IM or SC); 5 account above-mentioned to agreement electrodes, 0.1 mg/kg acepromazine is administered IV (in a few instances abandoned meperidine is accustomed and in a few others, no balance is administered. No concrete abstemiousness is used. Electrode adjustment requires 5-7 minutes; recording begins anon thereafter and is completed in 25-50 account in best cases. Recording is performed in a dimly lit allowance which has sound-attenuating walls. During recording, best dogs become comatose and sleep, frequently casual through all physiological stages of beddy-bye and assuming accustomed physiological transients; nevertheless, at the end of recording they are readily angry and can airing unassisted. In a few dogs, 0.2% lidocaine is injected subcutaneously about electrodes breadth assiduous beef artifacts action (~1.0 ml/electrode); additionally in dogs breadth it is adumbrated for medical reasons, atropine (0.4 mg/kg, SC) is accustomed above-mentioned to or during EEG recording. In the doses used, none of the drugs administered is accustomed to arm-twist EEG abnormalities in accustomed dogs.
Definitions: Polygraphs, Electrodes, Derivations and Montages
Polygraphs
A polygraph consists of: inputs (connectors) for assorted electrodes. The electrodes are in electrical acquaintance with the accomplished and are affiliated to amplifiers in the polygraph which afresh amplify the voltages from the electrodes. These voltages are led to pen writers in the polygraph which accomplish abiding cardboard copies of the recorded activity. Some polygraphs use heat-writing styluses rather than pen writers; there arise to be no audible advantages of one over the other. Abounding polygraphs accommodate access that admittance accomplished the EEG signals into an oscilloscope for direct affectation and/or into a computer or alluring bandage recorder, breadth they can be analyzed and/or stored.
Electrodes
To almanac the EEG, baby aggravate electrodes (e.g. Grass Instrument Co. platinum admixture subdermal electrodes) are placed in the subcutaneous tissues over the calvaria. Alternatively, baby metal deejay electrodes (Grass) can be activated to the bark credible or baby alligator clamps can be absorbed to the bark afterwards analgesic the teeth of the clamp.
Derivations
Two electrodes are affiliated to anniversary amplifier of the polygraph; the electrodes and the recording acquired accordingly are termed a “derivation”. As acclaimed earlier, anniversary electrode annal finer from abandoned a baby breadth of the academician basal it, accordingly assorted derivations charge be acclimated to ascertain differences amid academician regions. A brace of electrodes, the amplifier to which they are connected, the pen disciplinarian which annal the amplifier output, and/or the connected recording are additionally referred to generally as a “channel”.
Montages
A “montage” is the accumulated of derivations recorded at any one time. Ideally, if a acceptable cardinal of amplifiers were accessible on the polygraph, a audible montage could be devised that would admittance recording from all areas of both bookish cortices simultaneously. In best veterinary EEG laboratories the cardinal of amplifiers is limited, accordingly it is accessible to devise several montages so that recordings can be fabricated from the absolute dorsolateral breadth of anniversary cortex. The belly and ventrolateral cortical areas are aloof to accepted recordings. Derivations formed from electrodes placed belly to the cerebrum in the nasopharynx, or ventrolateral, adjoining to the sphenoid cartilage (via the orbit) abide but accept not yet accustomed accepted use in veterinary analytic patients.
Polarity Considerations and Conventions in Devising Derivations and Montages
The two electrodes aural a ancestry are referred to by assorted names anecdotic their accord to one another. Formerly, it was accepted to accredit to one as the “active” electrode and the added as the “reference” electrode alike admitting it was accustomed that both were “active”, i.e., recording electrical activity. Also, back the electrodes were affiliated to two of the grids in a exhaustion tube (“valve”), they were generally referred to as “G-1” and “G-2”. Preferred analogue now refers to one electrode as the “exploring” electrode and the added as the “reference” electrode.20
The ascribe electrodes are sometimes referred to as the ” ” and “-” electrodes. This appellation should be beat because it obscures the actuality that electrically (i.e., with account to arena potential), at any accustomed time, both electrodes may be absolute or both may be negative, or one may be absolute and the added negative.
By all-embracing convention, EEG electrodes should be affiliated to the amplifier so that if the first-named electrode of a brace becomes abrogating with account to the second, the pen will move advancement on the paper. Conversely, negativity of the added electrode of the pair, with account to the first, will account a bottomward pen movement. Positivity causes movements in the about-face directions.20
Note that both electrodes can be absolute or abrogating with account to any added abode in the universe, it is the polarity of the electrodes about to one addition that determines the administering of pen movement, a actuality that charge not be abandoned in EEG interpretation.21, 22
Montages should be advised to admittance optimal accurateness and affluence in anecdotic ambiguous and focal EEG events. Two types are used, “bipolar” montages and “common reference” montages; the closing are sometimes referred to artlessly as “reference montages”.20 Actually, both types use advertence electrodes, back it is absurd to almanac annihilation unless electrodes are affiliated to both inputs of the amplifier. (This completes the electrical circuit, which allows the accepted to canyon through one electrode, the amplifier, the added electrode and afresh through the tissue, abiding to the aboriginal electrode.) It is advised incorrect to accredit to accepted advertence montages as “monopolar” montages.20
In bipolar montages, derivations are formed by two electrodes that are accustomed to be amid over alive bookish tissue. It is recommended that the electrodes be affiliated to anatomy chains aggressive longitudinally and/or beyond over the calvaria. Anniversary electrode should arise in two adjoining channels, already as the “exploring” and already as the “reference” electrode.20 Back so arranged, a abrogating accident best affecting a accustomed electrode causes an advancement angle of the pen in the approach breadth it is the exploring electrode and a bottomward angle in a approach breadth it is the advertence electrode. This causes the pens in adjoining channels to move in adverse directions, an accident which is credible and actual accessible in localizing focal contest during interpretation. (Figure 2) The changeabout of administering is accurately referred to as an “instrumental actualization reversal”.20 It is sometimes termed “phase reversal” although this is a grossly erroneous term: the accident does not actually about-face its actualization (polarity), but artlessly appears that way as a aftereffect of the way the electrodes are affiliated to the amplifier!
Figure 2. “Instrumental actualization reversal”. Application internationally recommended conventions for abutting electrodes to amplifiers and for announcement recordings, a academic fasten causing electrode B to become added abrogating than electrodes A or C will account a bottomward angle in approach A-B and an advancement angle in B-C. Alive actualization changeabout is accessible in anecdotic the breadth of EEG events. Agenda that, all three electrodes may be either absolute or abrogating with account to “ground” potential, all that affairs is their polarity with account to one another. For example, in this case, with account to ground, electrode A could be 30 v, electrode B could be 10 V and electrode C could be 30 v.
In advertence montages, one electrode is acclimated as a advertence electrode in all derivations, the added electrode of anniversary ancestry is placed about abroad over the brain. Ideally, the advertence electrode would be placed on an electrically abeyant site; however, no such armpit exists. Furthermore, the further the advertence electrode is from the alive electrode, the added alarming do artifacts become, for example, the electrocardiogram. Therefore, the advertence electrode is usually placed on the accomplished breadth in actuality it is as alive as any of the added electrodes. Importantly, in a advertence montage, focal contest do not actualization alive actualization reversal; they can be amid abandoned by differences in amplitude. Similarly, accomplishments rhythms actualization differences in amplitude that are accompanying abandoned to the ambit amid the advertence electrode and the alive electrodes.23
Electrode placements acclimated for dogs in the UCD-VMTH are credible (Figure 3). Montages acclimated accommodate both bipolar and referential types and combinations of both (Tables 1, 2 below; or in calmly printed form).
Figure 3. Diagrams assuming the accepted locations of electrodes acclimated for recording basset EEG at the Veterinary Medical Teaching Hospital of the University of California Davis. L: left; R: right; S: amid on afterwards midline over aboveboard sinus; F: frontal; C: central; P: parietal; O: occipital; V: acme (dorsal midline); T: banausic (located anon rostral to the alien ear canal, aloof aloft the akin of the zygomatic arch). For actual baby dogs, the adjustment credible in the appropriate amount is used.
Table 1. Accepted electroencephalographic electrode montages acclimated above-mentioned to 1985 in the analytic neurophysiology class of the Veterinary Medical Teaching Hospital, University of California Davis.
See also, Table 2. Locations of electrodes on the accomplished are credible in Amount 3.
CHANNNEL
PATTERN A
PATTERN B
PATTERN C*
PATTERN D*
1.
RF-RC
RT-RC
RF-S
RT-S
2.
RC-RP
RC-CV
RC-S
RC-S
3.
RP-RO
CV-LC
RP-S
CV-S
4.
FV-CV
LC-LT
RO-S
LC-S
5.
CV-PV
RF-RP
LF-S
LT-S
6.
LF-LC
FV-CV
LC-S
RP-RF
7.
LC-LP
CV-PV
LP-S
PV-FV
8.
LP-LO
LF-LP
LO-S
LP-LF
* If electrode S is “active”, an added recording is fabricated application the PV electrode as reference.
Table 2. Accepted electroencephalographic electrode montages acclimated back 1985 in the analytic neurophysiology class of the Veterinary Medical Teaching Hospital, University of California Davis.
See also, Table 1. Locations of electrodes on the accomplished are credible in Amount 3.
CHANNEL
MONTAGE 1
MONTAGE 2
MONTAGE 3*
MONTAGE 4*
1.
LF-LC
LF-FV
LF-S
LT-S
2.
LC-LP
FV-RF
LC-S
LC-S
3.
FV-CV
LT-LC
LP-S
CV-S
4.
CV-PV
LC-CV
LT-S
RC-S
5 .
RF-RC
CV-RC
RF-S
RT-S
6.
RC-RP
RC-RT
RC-S
LP-LF
7.
LC-CV
LP-PV
RP-S
PV-FV
8.
CV-RC
PV-RP
RT-S
RP-RF
* If electrode S is “active”, an added recording is fabricated application the PV electrode as reference.
Amplifiers
It is astute to admit that in EEG, as in all analytic neurophysiology, abandoned cogwheel amplifiers are used. Cogwheel amplifiers amplify the aberration in voltages of the ascribe electrodes and it is the consequence of that aberration and the amount of addition by the amplifier that determines how far the pen moves. As acclaimed above, the administering of pen movement (up or bottomward on the page) is bent by the polarity of the electrodes about to one another.
Amplifiers should accept best acuteness in the abundance ambit of the EEG. A ambit of 0.5 to ~70 Hz is sufficient.20 Some EEG amplifiers accept a abundant college high-frequency absolute but this is not acclimated effectively, back the accomplished frequencies that can be recorded are bound to about 75-90 Hz by the apathy of the pens and pen drivers.
Additional Accessible Technics
As an aid in acceptance of artifacts associated with eye movements, it is actual accessible to almanac the electro-oculogram (EOG). This can be done by agreement a brace of electrodes adjoining to the crabbed canthus of anniversary eye or abreast the centermost and crabbed canthus of one eye and abutting them to one approach of the polygraph. These electrodes are astute to nystagmus.
If the polygraph has abundant recording channels, it is accessible to use one approach for recording the electrocardiogram, as ECG antiquity generally appears in the EEG and can be abashed with paroxysmal discharges such as epileptic spikes.
Physiological and pharmacological activation technics are accessible in eliciting EEG abnormalities in epileptic animals. Beddy-bye is the best able accessible activating abettor in epileptic dogs (personal observation). Photic stimulation, in the anatomy of a bright, ablaze light, additionally may be used, abandoned or in aggregate with chlorpromazine, to arm-twist abnormalities in the EEG of dogs.4,17 Hyperventilation, frequently acclimated to arm-twist aberancy in beastly EEG patients, cannot be acclimated calmly in analytic veterinary EEG.
A audible dosage of chlorpromazine IV (2.2 mg/kg) was credible to enhance the anticipation of recording paroxysmal discharges (PD) from epileptic dogs.4 It does not account PD in accustomed dogs, although abiding administering of phenothiazines was arise to account PD in the EEG of beastly psychiatric patients.24 Convulsant drugs accept been acclimated as activating or “provocative” agents experimentally and in beastly EEG patients;25 about difficulties in authoritative the furnishings accomplish them of bound address for analytic use in animals.
Interpretation of the Electroencephalogram
There are two methods of EEG interpretation: simple beheld assay and automated, computerized analysis. Simple beheld assay is the acceptable method. It requires study, training and experience, yet this is advantageous because it provides bigger all-embracing compassionate of the accommodating and of ache processes. For that reason, it is advised in added detail here, than is automatic analysis.
Automated EEG Analysis
Automated EEG assay cannot be actually accepted afterwards an compassionate of the attributes and assay of accepted EEG. Afterwards this background, automatic assay becomes alone a cookbook procedure. Automatic EEG assay provides a readily acquired quantitative announcement of the “amounts” of assorted frequencies that comprise EEG recordings. The accessories annal the EEG from one or added channels, at intervals takes sample periods (e.g., 5 sec) from anniversary channel, performs a abundance or breach analysis, and provides a blueprint of the assay about instantaneously. By stacking up graphs of alternating periods, it provides a concise, beheld almanac of the EEG abundance changes over time (Figure 4).27 Some accessories is able of autumn the abstracts in afterwards anatomy for statistical analysis. automatic EEG assay can be acclimated for monitor-ing surgical patients for abyss of anesthesia, bookish hypoxia or ischemia, for ecology analytical affliction patients in blackout or metabolic encephalopathy, and for assay of assertive analytic patients or alpha animals. The abstracts is artlessly a quantitative announcement of the BGR; important qualitative contest such as artifacts or PD can be hidden by automatic assay (Figure 4). Actual adult computer programs can identify, mark and calculation PD but charge be anxiously managed to abstain errors.28
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Figure 4.
Interpretation of the EEG by Beheld Inspection
The ongoing, connected voltage changes which comprise the EEG abide of accomplishments rhythms (BGR) and abrupt events; these characteristics can be focal, multlifocal or generalized.
Background Rhythms
Background rhythms accept both abundance and amplitude. Abundance is authentic as the cardinal of times per added the voltage changes in the abrogating or absolute directions. Agenda that this is not necessarily “zero crossings” but artlessly change in administering of any (predetermined) amplitude. The units of abundance are “Hertz” (Hz); the less-desirable agreement “cycles per second” or artlessly “per second” were acclimated beforehand and are still acclimated occasionally. Amplitude refers to the consequence of the changes in voltage, abstinent from aiguille negativity to aiguille positivity or carnality versa, and is best frequently bidding in microvolts.
Background rhythms of complete dogs break aural about attenuated banned (about 0.5/sec to 30/sec).26 They abide of a ascendant frequency, usually with abate amounts of added frequencies superimposed thereon. In general, lower frequencies are accompanied by college amplitudes and carnality versa.
Transient Events
Transient contest are contest that action for abrupt periods, usually abandoned a few aberrant or beneath in duration. They abide of some anatomy of EEG activity, which is audible from and is superimposed on or replaces the accomplishments rhythms. There are both accustomed and aberrant abrupt events. Importantly, abounding artifacts are abrupt and charge be acclaimed from authentic EEG transients. Paroxysmal discharges (PD) are aberrant abrupt contest that are associated with access disorders, ictally or interictally.
Regardless of the approach of interpretation, afore an EEG recorded from a accommodating can be accurately and anxiously interpreted, the analyst charge apprentice to admit EEG artifacts, accustomed states of alertness (arousal, blackout and sleep), and accustomed abrupt events.
Artifacts
With experience, abounding artifacts can be accustomed readily during interpretation; about it is actual accessible if the being assuming the EEG makes addendum on the almanac about movements or added contest empiric during recording. The best accepted artifacts encountered are those associated with beef potentials, head, limb or anatomy movements, the ECG and respiration. Beneath often, burning or mandibular movements account artifacts. (The artifacts discussed beneath are illustrated in abstracts called for the purpose and additionally are labeled in abstracts called for added purposes in adjustment to allegorize some of the aberration in appearances.) Sources of antiquity in the accessories and ambiance accept been call ahead and will not be discussed here.29
Muscle Artifact
Muscle potentials are the best accepted and generally the best alarming artifacts. They arise from the facial or masticatory anatomy and account alternate or connected about aerial abundance action that can partially or actually abstruse BGR (Figure 5). Beef potentials generally abandon or abate during the aboriginal 3-5 account of the recording aeon if allaying drugs are used. If persistent, subcutaneous bang of 1-3 ml 0.2 % lidocaine about the behind electrodes usually reduces or eliminates the problem. At times it is accessible to abate the aerial abundance absolute of the amplifiers; however, this can adapt the agreement of accelerated EEG contest such as epileptic spikes, accordingly it should be done abandoned if actually all-important and abandoned by recording for a aeon application the accepted abundance bandage width.
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Figure 5.
Solid squares represent electrode sites in use; dotted-line squares represent electrode sites not acclimated in the montage. In anniversary derivation, the solid band connects the exploring electrode and the dotted band connects the advertence electrode to the EEG approach announcement the output. Basal approach is the electrocardiogram (ECG). The case cardinal and the folio of the EEG that is reproduced arise in the lower right.
Movement Artifacts
Movements of the anatomy or its genitalia can abet artifacts (Figure 6). The best accepted of these are associated with movements of the accomplished or added anatomy parts, and respiratory movements. These artifacts allegedly arise from movements that account the electrodes or their abutting affairs to move through the ambient electromagnetic fields in the class (personal observation). Eye movements account artifacts by this agency and additionally by a actual altered accoutrement (below).
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Figure 6.
Respiratory antiquity consists of apathetic after-effects that are synchronized with movements of the anatomy and accomplished associated with afflatus and/or expiration, usually back the dog is asthmatic (Figure 7). While usually readily articular during EEG interpretation, addendum fabricated during recording are accessible in acquainted respiratory antiquity or, conversely, in acceptance that adroit apathetic action in the recording is 18-carat EEG and not antiquity (Figure 7). Accomplished movements, swallowing, and movements of the mandibles will acquaint artifacts additionally (Figure 8).
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Figure 7.
Figure 8.
Eye movements account about apathetic after-effects in the EEG by aggregate advice of the electro-oculogram (EOG) to the EEG electrodes. Recording the EOG helps in acceptance eye movement artifacts. Surprisingly, the amplitude of eye-movement artifacts in the EEG channels can be rather high, sometimes beyond the amplitude recorded by the EOG electrodes (Figure 5, 6). Some of these artifacts can be abashed with PD if the movements are not empiric or the EOG is not recorded (Figure 7). Eye movements generally are accompanied by eyelid movements (e.g., blinking) and/or with movements of the scalp, either of which may be associated with beef potentials and/or electrode/wire movement artifacts.
Electrocardiographic Artifacts
Recording the ECG in one approach of the polygraph is accessible in acquainted ECG antiquity in the EEG, breadth the QRS circuitous can be mistaken for a fasten or breadth adventitious accident of a QRS circuitous at the time of an EEG apathetic beachcomber ability be mistaken for a PD (spike-and-wave complex) (Figure 9, 10).
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Figure 9.
Figure 10.
The Accustomed electroencephalogram
Normal Accomplishments Rhythms
Background rhythms (BGR) are the ongoing, connected EEG action which forms the ambient adjoin which abrupt contest occur.20 Accomplishments rhythms in accustomed animals adapt absolutely with changes in accompaniment of beddy-bye or arousal.26 To accomplish authentic EEG interpretations these “states of vigilance” charge be recognized. Age differences abide also. The BGR of puppies are absolutely altered from those of complete animals, extensive the complete forms at 5 months of age (occasionally up to 10 months in our claimed experience).30,31 Importantly, assertive abrupt contest are accustomed during some beddy-bye stages and charge be accustomed to abstain mistaking them for PD.
When allaying drugs are used, dogs abide readily credible levels of all stages of accustomed consciousness: abounding arousal; alpha rhythm; drowsiness; non-REM sleep; accelerated eye movement (REM) sleep.26 (Non-REM beddy-bye has been referred to additionally as “slow-wave sleep”).
Full Arousal
Full activation is credible by active behavior, the beastly responding to ecology disturbances, authoritative analytic movements with the head, eyes or ears, or alike by attempting to arise from the collapsed accompaniment usually affected during recording. BGR during abounding activation are usually of actual low amplitude (< 20 mV), with college frequencies (15-25 Hz) predominating (Figure 11). Beef antiquity is accepted during arousal, generally abashing one or added channels of EEG (Figure 5, 6).
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Figure 11.
Alpha Rhythm
The alpha accent of dogs was arise to be agnate to the beastly alpha rhythm. It was recordable if the beastly was captivated in a quiet ambient and bankrupt its eyes. Typically, a aeon of abiding alpha accent had a continuance of about 8-15 sec, afterwards which it was replaced by college frequencies back the beastly opened its eyes or was aroused, or by lower frequencies as the beastly became sleepy.32-34 In our experience, dogs usually change from abounding activation to behavior and EEG rhythms that resemble the accompaniment declared by Ruckebusch as “drowsiness”.35 Although BGR during periods of credible blackout sometimes resemble the basset alpha rhythms declared earlier32-34 (e.g., Amount 8, 12) we cannot verify this because we accept not commonly activated dogs during these periods for EEG acuteness to stimulation, a all-important archetype for anecdotic the alpha rhythm.
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Figure 12.
Drowsiness
During drowsiness, dogs are collapsed with their eyes partially or actually closed; at this time they can be absolutely readily angry by basal dispatch or baby disturbances in the environment. The BGR abide of absolute 6-8 or 6-10 Hz after-effects of 10-20 FV amplitude, with superimposed accidental slower and college frequencies (Figure 12). If undisturbed during drowsiness, best sedated dogs anon access non-REM sleep.
Non-REM Sleep
During non-REM sleep, dogs are collapsed and allegedly blind of ecology disturbances; their eyes are bankrupt or about closed, and stronger stimuli are bare to agitate them than during drowsiness. BGR at this time are predominantly 2-4.
Hz with lower amplitude superimposed activity, predominantly in the 6-10 Hz range. Amplitudes during non-REM beddy-bye adapt from 15-20 FV up to 100 FV; the advanced aberration is due at atomic in allotment to variations in inter-electrode distances associated with anatomy admeasurement (Figure 11, 13). Stages of ablaze and abysmal non-REM beddy-bye are generally credible by the college amplitudes and lower frequencies during abysmal non-REM sleep.
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Figure 13.
REM Sleep
During REM sleep, dogs are collapsed with their eyes partially or actually closed. With the access of REM sleep, BGR change rapidly, accident amplitude and accretion in abundance so they resemble those of arousal. About 5 to 10 account afterwards REM beddy-bye begins, the eye movements begin; actual often, these are accompanied by movements of the feet, aperture or added anatomy genitalia and alike by delivery (Figure 13, 15).
Duration of Levels of Consciousness
Under the recording altitude acclimated in the UCD-VMTH laboratory, blackout and non-REM beddy-bye usually activate beneath than 10 account afterwards recording begins, appropriately EEG observations during activation may be rather limited. Likewise, dogs generally access REM beddy-bye 5 to 10 account afterwards non-REM beddy-bye begins, a briefer time than reported.26 This may be an aftereffect of the balance used. Also, in epileptic patients, which aggregate the majority of our EEG patients, these differences in beddy-bye latencies may be acquired by the aforementioned accoutrement which causes abbreviate abeyance of non-REM beddy-bye and short, “fragmented” sleep-wake cycles in epileptic beagles.36
Normal EEG Abrupt Events
Spindles, K-complexes and V after-effects are accustomed EEG transients that are of accomplished amplitude on the midline but are usually absolutely axiomatic in electrodes amid laterally, breadth they are balanced in amplitude and frequency. Audible asymmetries, or absences on one ancillary with attendance on the other, are apocalyptic of abnormality. The 4-6 Hz Bursts declared beneath are, in the authors’ opinions, accustomed and actualization the aforementioned characteristics as added accustomed transients.
Spindles
Sleep spindles are repetitive sinusoidal waves. They activate at somewhat lower amplitude, access and afresh abatement in amplitude appropriately giving the outline of the accident a arbor shape. Spindles are recorded generally during non-REM beddy-bye (Figure 14). Beddy-bye spindles are bilaterally balanced and of accomplished amplitude in midline derivations but are arresting in crabbed derivations as well. Spindles are recorded from some dogs during REM sleep; possibly this is a biologic aftereffect (Figure 15).
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Figure 14.
Figure 15.
K-complexes
A K-complex consists of a arbor associated with a apathetic wave; the arbor may arise before, during or afterwards the apathetic wave, appropriately there is ample aberration in the actualization of K-complexes.20,37 K-complexes action during non-REM sleep. They are of accomplished amplitude and best acutely developed on the midline, but are arresting in crabbed derivations as able-bodied (Figure 16).
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Figure 16.
Vertex Waves
Vertex aciculate transients (V waves) are responses to stimulation, usually during blackout or sleep. Best generally in beastly patients, the bang is auditory, although added stimuli can be effective.19 They can be recorded from dogs occasionally during blackout (Figure 12). V-waves can be differentiated from PD by the actuality they do not accept fasten components, action abandoned during drowsiness, and are consistently of accomplished amplitude on the midline and symmetrically distributed.
Four to Six per Added Bursts
In some dogs, rhythmic, 0.5 to 1 sec bursts of 4-6 Hz sinusoidal after-effects are recorded about the time of the alteration from blackout to non-REM sleep. These bursts, which are symmetrically broadcast with accomplished amplitude on the midline, are frequently associated with eye movements which action 1-3 sec afterwards the EEG event. In some cases the abundance of these bursts increases from access to abortion (Figure 17). The attributes of these discharges is unknown; they accept been recorded from accustomed dogs as able-bodied as from analytic patients, accordingly cannot be advised abnormal. Similar, but not identical, transients are recorded from children.37
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Figure 17.
Responses to Photic Stimulation
Photic dispatch (PS), application a aflame lamp captivated afore the eyes, is a accepted activating adjustment acclimated in recording beastly EEG. In our experience, photic dispatch causes changes in accomplishments rhythms in about 25% of accustomed dogs. These abide of the actualization of balanced sinusoidal BGR at the aforementioned abundance as the stimulation. The rhythms arise at or aural a few aberrant of the access of dispatch and abandon anon with its cessation. Bang frequencies of 2-10 Hz are best able in eliciting this alleged “photic driving” (Figure 18).4,17 It is reasonable to catechism whether these responses are in actuality the electroretinogram (ERG) or the beheld evoked acknowledgment (VEP). About these are audible responses with amplitudes of 15-50 microvolts, which beat that of the electroretinogram and the beheld evoked response, alike back the closing are recorded application arresting averaging.37 Photic active is advised to be a physiological acknowledgment in beastly patients;25 it seems acceptable this is authentic for basset photic active also.
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Figure 18.
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