Artifacts

CHAPTER 3


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Artifacts


The differentiation of true brain electrical activity from extraneous artifacts is critical to the interpretation of neonatal electroencephalogram (EEG). Traditionally, the sources of artifacts are considered in four broad categories: environment, instrumentation–patient interface, instrumentation, and physiologic potentials of noncerebral origin (Brittenham, 1990; Dworetzky et al., 2011; Kellaway and Crawley, 1964; Saunders, 1979; Scher, 1985).


The identification of artifacts in a given EEG recording is a two-step process. The first occurs during the actual recording and is dependent upon the electroneurodiagnostic technologist’s (ENDT) recognition of artifacts and their potential sources. With this recognition, it is the ENDT’s task to isolate the source and resolve the problem. If this proves impossible, the ENDT will make appropriate notations on the record and on the accompanying log to characterize the activity and suggest the source. The second step of artifact identification occurs during interpretation of the EEG. Since artifacts may mimic true brain-generated waveforms (Tables 3.1–3.5), the challenge lies with the clinical neurophysiologist to make the accurate and appropriate distinctions.


Table 3.1 Sources of Artifacts That Can Mimic EEG Focal Asymmetries












    Instrument–patient interface


      images  Asymmetric electrode placement


    Instrumentation


      images  Settings


    Physiologic


      images  Asymmetric scalp edema


      images  Skull defects






Table 3.2 Sources of Artifacts That Can Mimic Focal Slow Activity That Is Either Random or Rhythmic












    Instrument–patient interface


      images  Inadequate electrode contact


      images  Head positioning


      images  Spontaneous or passive body, limb, or head movements


      images  Pulse


      images  Respiration


    Physiologic


      images  Endogenous electrical potentials


          images  Tongue movements


          images  Eye movements






Table 3.3 Sources of Artifacts That Can Mimic Sharp Waves or Spikes That Are Either Random or Rhythmic












    Instrument–patient interface


      images  Inadequate electrode contact (“pop”)


      images  Spontaneous or passive movements


          images  Sucking, burping, hiccupping


    Physiologic


      images  Electrocardiogram (ECG or EKG)


      images  Electrooculogram (EOG)


      images  Facial electromyogram (EMG)


    Environment


      images  Infusion pumps


      images  Ventilator


      images  Extracorporeal membrane oxygenation (ECMO)


      images  Cellular telephones, radio transmitters






Table 3.4 Sources of Artifacts That Can Mimic Generalized Activity That Is Either Paroxysmal or Sustained












   Instrumentation–patient interface


      images  Spontaneous and passive movements


          images  Rocking, patting


    Instrumentation


      images  Swaying of electrode leads


    Physiologic


      images  Sweating


    Environment


      images  60-Hz interference from adjacent instruments






Table 3.5 Sources of Artifact That Can Mimic Generalized Voltage Depression












    Instrumentation


      images  Instrumentation settings


    Physiologic


      images  Diffuse scalp edema






ENVIRONMENT


Although the EEG laboratory is a relatively controlled electrical environment, the neonatal intensive care unit (NICU) is relatively uncontrolled. This is due to the wide number of instruments used to monitor or care for infants including phototherapy lights, ultrasound instrumentation, infusion instrumentation accessing the intravascular system at various sites, and extracorporeal membrane oxygenation (ECMO) pumps (Figures 3.1–3.3). Electrical artifact due to external currents from these instruments may appear in all EEG channels to the same degree or may appear focally. When all of the electrode impedances are equal, the more likely the environmental current will be expressed in all channels; unequal impedances will cause the current to be expressed to a greater degree or exclusively in those channels with the highest impedance.


Additional environmental artifacts may be created or enhanced by factors such as capacitatively induced potentials from electrode wires that sway or electrostatic potentials resulting from movement of personnel around the recording area. Malfunctions and/or improper operation of equipment such as monitors connected to the patient can also result in artifactual signal induced into the EEG machine.


Oscillatory ventilators may induce relatively high-frequency electrical artifacts compared to more traditional ventilator systems. Thus, rate alone cannot be used to exclude potentials suspected of being generated by a ventilator.


Cellular telephones and older radio transmitters can sometimes produce artifactual signals in the EEG tracing, especially if these devices are operated within a few feet of the patient.


RECORDING INSTRUMENTATION–PATIENT INTERFACE


The interface between the recording instrument and the patient is at the electrode site. Inadequate or unstable contact between the electrode surface and the skin may result in a sudden change in the junction potential and/or impedance that can produce extraneous potentials in affected channels. These may appear as single or repetitive rapid, spike-like waves with an abrupt upward initial phase (the so-called “electrode pop”) (Figures 3.4–3.7). In addition, asymmetry of the site of placement of homologous electrodes may result in significant voltage asymmetries in the EEG.


The electrode interface may also be altered by the degree to which the infant may perspire. Diffuse sweating may result in long duration potentials that initially appear as generalized or regional slow activity (Figures 3.8 and 3.9). Very slow potentials may occur due to changes caused by alterations in surface electrolyte compositions—these potentials are similar to the galvanic skin response.


Movement of the head against the bed due to respirations or other body movements may produce sharp and/or slow potentials arising from that particular electrode (Figures 3.10 and 3.11). Pulse may also cause a recorded artifact by production of movement in a region adjacent to an electrode site. The head may also be moved by mechanical devices such as a ventilator or ECMO pump (Figures 3.12 and 3.13). This is due to a mechanical, or ballistic, movement induced by the instrument—a cause of artifact from these devices different from electrical interference described earlier.


Other body movements may also alter the patient–electrode interface and result in artifacts. These include limb movements that may be random, purposeful, or associated with clinical seizures and other limb or body movements (Figures 3.14–3.18). In addition to the movements that may be caused by the infant, the infant may be moved or manipulated during comforting, feeding, medical procedures, and in the course of treatments. Movements created to comfort the infant may be particularly troublesome, such as rocking, patting, and burping the infant (Figures 3.19–3.23).


RECORDING INSTRUMENTATION


The recording instrument itself can be a source of artifacts. These may be the result of malfunction at any recording level. When analog-type EEG instruments were used they were subject to pen misalignment and excessive damping. Digital recordings may have problems relating to malfunction of the operating systems. There is also the potential for human error in the use of any recording device. Settings for EEG channels may not be uniform, electrodes may not be correctly plugged in, and montages may not be accurately selected.


NONCEREBRAL PHYSIOLOGIC POTENTIALS


Alterations in Electrical Properties of Scalp or Skull


There may be differences in impedance and volume conduction properties over various regions of the scalp due to scalp edema. The edema may be the result of transit through the birth canal, more significant birth or other trauma, placement of intravenous lines with or without extravasation of fluid, the placement of a ventriculoperitoneal shunt, or the presence of a surgical wound. If edema is diffuse, this may lead to a pattern of background activity that is low in amplitude in all regions. If the edema is regional or asymmetric, this may lead to a pattern of focal depression, suggesting a focal lesion if the edema is not noted. There may also be instances in which conductive properties are altered because of the absence of underlying skull, typically (although rarely) in the case of cranial surgery. A skull defect creates a preferential pathway for electrical current, resulting in increased amplitude of EEG activity over the affected region.


Vital Signs Monitoring


Heart rate and respiration are important sources of artifact on EEG. The electrocardiogram (ECG) in an infant may appear as a contaminant in one, some, or all of the EEG channels (Figures 3.24–3.27A,B). It may be constant or intermittent. Respirations may also appear as artifact, whether spontaneous or driven by a ventilator. These artifacts may be unilateral or bilateral, depending upon body and head position.


Movements

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Mar 8, 2018 | Posted by in PEDIATRICS | Comments Off on Artifacts
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