Current Case: Summer 2024
Contributed by: Bojun Chen, MD, PhD and Nabil Khandker, MD, Department of
Neurology, The Ohio State University Wexner Medical Center, Columbus, OH, USA
Case Presentation
50-year-old right-handed female without seizure history presents to ED with acute encephalopathy/behavior changes, and high blood pressure. Her chronic medical problems include posterior reversible encephalop-athy syndrome (PRES), subarachnoid hemorrhage, poly-substance use, and hypertension. The initial differential diagnosis included seizure in the setting of hypertensive emergency, concerning for PRES. Video EEG was per-formed using standard 10-20 system electrodes to evaluate any interictal abnormalities. EEG data showed the isolated decreased amplitude at P8-O2 displayed on the bipolar double banana montage (Figure 1), but not-visualized on the common average montage (Figure 2). The red arrows indicate the point of interest (channel P8-02, P7-O1, P8-avg, and P7-avg).
Figure 1. Illustration of decreased amplitude at P8-O2 in the bipolar double banana montage. (LFF =1Hz HFF=70Hz Notch=60Hz Sensitiv-ity=7 uV/mm Timebase=15mm/sec)
Figure 2. Amplitude appears symmetric at P8 and O2 in the common average montage. (LFF =1Hz HFF=70Hz Notch=60Hz Sensitivity=7 uV/mm Timebase=15mm/sec).
Question 1: What are the arrows in the figure pointing to?
- Focal slowing
- Salt bridge artifact
- Popping artifact
- Bancaud phenomenon
Answer: (click here)
Correct answer: B. Salt bridge artifact
The arrows point to the decreased amplitude in the channel of P8-O2 present in the bipolar double banana montage, but absent at the common average montage. It is likely due to a salt bridge artifact. EEG electrodes at P8 and O2 capture similar outputs, shown on the common montage. However, the minimal potential difference from P8 to O2 is recorded, since ions move freely in between, resulting in a much lower amplitude or a rather flat signal on the bipolar double banana montage (1).
Question 2: How do you confirm the finding?
- Re-check inter-electrode impedances. If the local impedance is less than 100Ω, it likely indicates a salt bridge effect. The artifact is a result of a short circuit between the adjacent electrodes
- Remove the excess gel on top of the affected electrodes and assess for resolution of the abnormality on the repeated EEG test.
- Modify the frequency threshold setting. The morphology will remain essentially unchanged in the bipolar montage because the signals captured by adjacent electrodes cancel each other out in similar ways.
- Wipe the perspiration from the associated skin surface and maintain the exam room at a lower temperature if able.
Answer: (click here)
Correct answer: A-D. Multiple ways can be utilized to recognize and minimize the salt bridge artifacts, listed from A-D (2 and 3).
Reference 1: William Tatum, Barbara Dworetzky, Donald Schomer (2011). Artifact and recording concepts in EEG. J Clin Neurophysiol 28: 252-263.
Reference 2: Sandor Beniczky, Donald Schomer (2021). Electroencephalography: basic biophysical and technological aspects important for clinical applications. Epileptic Disorders. 22 (6): 677-715.
Reference 3: Ushtar Amin (2023). Normal variants and artifacts: Importance in EEG interpretation. Epileptic Disorders. 25 (5): 591-801
References:
- Tatum WO, Dworetzky, Schomer DL (2011). Artifact and Recording Concepts in EEG. J Clin Neurophysiol. 28: 252-263.
- Nascimento FA, Chu J, Fussner S, Krishnan V, Maheshwari A, Gavvala JR (2021). Neurostimulation EEG artifacts: VNS, RNS and DBS. Arq Neuropsiquiatr. 79 (752-753)
- Arafat T, Miron G, Strauss I, Fahoum F (2022). Electrodiagnostic artifacts due to neurostimulation for drug resistant epilepsy. Epilepsy Behav Rep. 20:100566
