Artifacts

Artifacts

Artifacts are waveforms on an EEG caused by electrical sources other than the cortex. It is important to recognize and identify artifacts so that they are not confused with cerebral potentials. Artifacts can be generated by numerous sources, the most common of which is the body itself: the eyes, the heart, and muscle. Environmental artifacts such as the EEG machine, electrode placement or malfunction, and 60 cycle interference can also produce artifacts, particularly in “hostile” environments such as the ED, ICU and OR.

Physiologic Artifacts

Ocular Artifact The globe of the eye has an electrical potential of about 50-100 microvolts with the cornea positive and the retina negative in polarity. Artifacts of eye movement are best seen in the frontal and temporal leads closest to the eyes. Specific electrodes can be placed above and below the outer canthus and are called electrooculograms (EOG). During eye closure, the eyes move upward (Bell’s phenomenon) and there is a large bifrontal positive deflection. With lateral movements, there is a positive deflection ipsilateral to the direction eye movement and a negative deflection on the contralateral side. An easy mnemonic is “look through the key hole” or in other words, a positive phase reversal (keyhole) on a bipolar EEG is the direction to which the eye is turned.

Electroretinogram Not only does the globe have electrophysiologic characteristics; the retina itself generates potentials in response to stimulation. This finding can be manifested during photic stimulation when the bright light stimulus causes a response from the retina that can be recorded from the frontal electrodes (here at FP1 and FP2). These deflections are typically positive in the frontopolar electrodes and have a slight delay from the stimulus of between 50 to 100 milliseconds.

Electrocardiographic Artifacts The QRS complex can be recorded as a far field potential over the scalp and seen best at the A1 or A2 electrode. This signal can be particularly prominent in patients with large volume conduction. Extra systolic beats can be confused with cerebral discharges due to their apparent paroxysmal nature and sharp contour.

Cardioballistogram occur in correlation with the QRS complex as the contraction of the heart generates vibration that can be recorded by the EEG electrodes. The peak of each waveform coincides with the QRS complex. These waveforms are typically seen when the sensitivity of the EEG is increased and there are no other features to detract from their presence (i.e., when there is minimal to no cerebral activity).

Pulse artifact This artifact is usually confined to a single electrode when that electrode is placed over a superficial artery. It appears as a slow rhythmic waveform that just lags behind the QRS complex.

Electromyographic Artifacts Lateral rectus spikes are generated by single muscle units during lateral eye movement.

Muscle Artifact Frontalis muscle artifact occurs especially with a photically stimulated photomyoclonic response and with forcible eye closure or opening. Temporalis muscle artifact occurs when a patient clenches his jaw or chews. When the muscle artifact is so dense, it obscures the EEG. A tech can ask the patient to open their mouth slightly to relax the muscles and minimize the artifact.

Glossokinetic Artifacts The tip of the tongue is negative and the base of the tongue is positive. When the tongue moves there can be a diffuse or unilateral activity depending on the direction of the movements.

Galvanic Skin Responses are high amplitude slow potentials also called sweat sway. The salt bridge that develops between electrodes “shorts” or decreases their differential impedances so that the amplifier cannot differentiate between the two electrodes and the potential difference between the two electrodes approaches zero. The resulting tracing is of a relatively flat line that undulates irregularly.

Physiologic Movements Tremors and myoclonic jerks can cause diffuse or focal sporadic or rhythmic artifacts in the EEG. Here, head tremor affect the A2 (ear) reference maximally. Most physiologic movements show nearly vertical deflections on the EEG, too steep to be caused by cerebral potentials.

Nonphysiologic Artifacts: Instrumental

60 cycle interference from other electrical equipment may interfere with an EEG and produces a characteristic pattern. This exogenous electrical artifact is accentuated when electrode impedances are high (poor scalp contact). The rules of interpretation for bipolar montages allow identification of the faulty electrodes on this tracing: P3 and O2.

A 60 Hz notch filter specifically reduces the interference of activity around 60 Hz but the resultant waveforms may be misleading. This is the same page of EEG as shown on the previous page, but now the notch-filtered 60 cycle interference appears to be a continuous alpha band rhythm. Frequency filters imposed on the EEG can be useful for visualizing underlying EEG rhythms when excessive artifact is present but can themselves produce artifactual findings.

Electrode pops. This artifact is generated by tapping on each electrode to ensure the integrity and placement of each electrode at the beginning of a study. Similar artifacts can be seen if the integrity of the electrode contact is not adequate, and the electrode intermittently discharges accumulated electrical static. Unlike most cerebral potentials, electrode pops are exclusively limited to a single electrode at a time, without a surrounding electrical field.