Artifacts of Recordings

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Artifacts of Recording

William O. Tatum, IV

Recording electrical potentials on the electroencephalogram (EEG) that do not rise from the brain represents artifact. Artifacts are most often felt to “contaminate” the EEG, but the physiologic functions that may be identified during EEG are crucial for clinical correlation. Recording a single channel of ECG is essential for proper interpretation of the EEG. The electroretinogram is a normal physiologic response of the retina to photic stimulation like lambda waves and mimics the artifactual photoelectric effect. Myogenic artifact consists of brief potentials that may occur as single motor unit potential or be present as a series of continuous myogenic potentials that obscure the underlying EEG and “contaminate” the record, limiting interpretation. Various types of artifact include inductive, electrostatic, and capacitive artifacts. Environmental artifacts may often not be readily identifiable or correctable within the confines of a “hostile” environment when performing EEG in the intensive care unit or critical care unit.

clinical correlation, electroencephalogram recording, electroretinogram, environmental artifacts, intensive care unit/critical care unit, myogenic artifact

Artifacts, Electroencephalography, Electroretinography, Intensive Care Units

Recording electrical potentials on the EEG that do not arise from the brain represents artifact. Artifact is commonly encountered, often misinterpreted, yet is an essential component for routine visual analysis of the EEG. Artifacts are most often felt to “contaminate” the EEG, but the physiologic functions that may be identified during EEG are crucial for clinical correlation. The source of artifact and the clinical context in which the generator is present determine if the features are normal or abnormal when applied to clinical use. Both physiologic and nonphysiologic (electrical) potentials may be produced. Physiologic artifacts are frequently created by face, eye, or tongue movement, myogenic, and cardiac sources. Therefore, recording a single channel of ECG is essential for proper interpretation of the EEG. These artifacts can exist in nearly every recording to some degree and may appear simple or quite complex, especially in “electrically hostile” environments such as the ICU/CCU and OR. Nonphysiologic extracerebral potentials include mechanical, motion, and environmental sources.

**FIGURE 3.1.** EEG demonstrating the presence of pulse artifact mimicking lateralized periodic discharges at the T6 derivation. Note the 1:1 relationship with the ECG and spatial field that is limited to a single electrode that suggests an artifact until proven otherwise.

The ECG should be monitored during the EEG to provide crucial information about the relationship between the heart and the brain, revealing a potential association with clinically relevant symptom derived from primary cardiac origin. The QRS complex of the ECG represents the largest voltage deflection and often confers artifact in the EEG (Figure 3.1). ECG artifact may appear simultaneously with prominent QRS complexes seen in several channels. Ballistocardiographic potentials refers to posterior predominant artifact caused by movement generated by mechanical pulsation of blood, and is time-synched to the ECG. In the example above, pulse artifact is seen. This single channel electrode artifact may mimic a periodic epileptiform discharge. It occurs when an electrode is placed near or over a pulsating artery. There is a discrete time-synched 1:1 relationship between heart rate identified on the ECG, and the periodic potential on the EEG.

**FIGURE 3.2.** Eye movement monitors demonstrating *in-phase* deflection supporting a cerebral origin and the *out-of-phase* deflection in the eye movement monitors supporting the eye as a generator.

Vertical eye blink artifact seen on the EEG is generated by the electrical potentials produced by movement of the eye. The eye, like other biological tissues (i.e., tongue), functions as an electrical dipole. There is a relative positivity of the cornea compared the electronegativity produced by the retina. The potential created is a direct current (DC) potential that is composed of higher amplitudes (in millivolts) compared to those voltage amplitudes produced by the brain (microvolts). Artifact is produced in the electrodes closest to the generator during vertical eye movements. With an eye blink, the cornea rolls up as the lid closes (Bell’s phenomenon). As a result, the electro-positivity is detected in the frontopolar (FP1/2) electrodes relative to the frontal (F3/4) electrodes to create the downward deflection of a vertical eye blink artifact. Electrodes recording above and below the eye will help to distinguish the brain as the “generator” (same polarity in peri-ocular channels) from artifact (opposite polarity in adjacent electrodes placed above and below the eye; Figure 3.2).

**FIGURE 3.3.** Artifact produced from horizontal eye movements (arrows). Note the opposite polarities produced in the lateral frontotemporal derivations in the EEG.

During the awake state, vertical eye blink artifact is present and helps identify the state of the patient. During drowsiness, slow rolling (lateral) eye movements are similarly observed. Lateral eye movements are usually easily recognized because they create phase reversals in the frontotemporal derivations that are closest to the plane of the eye movement (Figure 3.3). As a result, there is opposite polarity present in the left and right electrode derivations in the scalp EEG. When the eyes move to the left, this creates a positive phase reversal in F7 due to the electro-positivity of the corneal dipole. The homologous F8 electrode on the right side of the head demonstrates a negative phase reversal created by the electronegativity of the retina.

**FIGURE 3.4.** Eye movement monitors confirming vertical eye flutter artifact with infraorbital electrodes (electro-oculogram [EOG]) produced during intermittent photic stimulation to differentiate artifact from frontal intermittent rhythmic delta activity (FIRDA).

Detecting eye movements may also be accomplished using a single channel that connects the right upper lateral eyebrow and the left lower lateral eyebrow. However, because vertical eye movements are often the source of confusion, bilateral infraorbital electrodes referred to the ipsilateral ear as a reference may better represent the eye as a dipole and demonstrate phase reversals that are out of phase with cerebral activity when due to eye movements (Figure 3.4). Eye movement monitors may be added during the recording if difficulty separating cerebral activity from extracerebral activity occurs.

**FIGURE 3.5A.** The (physiologic) electroretinogram seen at the frontopolar (FP1/2) electrodes <50 msec after the flash associated with intermittent photic stimulation.

The electroretinogram (ERG) is a normal physiologic response of the retina to photic stimulation like lambda waves and mimics the artifactual photoelectric effect. The amplitude of the ERG is low voltage and appears in the anterior head regions. The ERG is composed of a normal a and b wave. They appear during evoked potential recording but this electrophysiologic potential generated by the retina can also be seen on standard EEG (Figure 3.5A). When it appears, it can be confused with abnormal frontal “sharp activity” or as an artifact associated with the photoelectric effect. The photoelectric effect (aka photovoltaic effect) is present in the FP electrodes, often with a spiky appearance that is time-locked to the photic flash (Figure 3.5B). The photoelectric effect is caused by photochemical reactions produced in the metal electrode and electrolytic gel. To distinguish the ERG from the photoelectric effect, covering the electrodes with a cloth will demonstrate the persistence of the ERG but eliminate the artifact associated with the photoelectric effect. Additionally, high rates of intermittent photic stimulation will fatigue the retinal response.

**FIGURE 3.5B.** The photoelectric effect (artifact) on EEG. Note the time delay in (A) and the time-locked unilateral distribution associated with the photoelectric effect in (B) from a nonadherent electrode. This separates physiologic and artifactual waveforms despite their similarity associated with photic flash.

**FIGURE 3.6.** A photomyoclonic response elicited during intermittent photic stimulation.

The photomyoclonic response is an extracerebral response obtained from the frontalis muscles of the scalp. Contraction of the frontalis muscles of the scalp produce myogenic potentials (surface EMG) that range from a single motor unit potential to a sustained series of myogenic “spikes” (Figure 3.6). The contractions are time-locked to the photic stimulation and begin and cease commensurate with the flash, though there is often a brief delay between the flash and the myogenic potentials when they appear. The principal source of confusion is one where the normal photomyoclonic response is mistaken for an abnormal photoparoxysmal response.

**FIGURE 3.7A.** Stage R (REM) sleep with prominent lateral rectus spikes associated with rapid eye movements. These “spikes” represent myogenic artifact generated by the extraoccular muscles.

Patients with rapid eye movements may demonstrate myogenic potentials that are generated by the lateral rectus muscles and appear epileptiform (Figure 3.7A). Each rapid eye movement is associated with a brief myogenic potential that may be represented by a phase reversal on eye deviation to the side produced by simultaneous contraction of eye muscles. These are most noticeable during REM sleep, though they may occur at any time where there is rapid movement of the eye, such as during reading where the eye is scanning from left to right (Figure 3.7B).