A bipolar montage is a method used to record electroencephalographic (EEG) data by comparing the electrical potentials between pairs of electrodes. In this configuration, each electrode is linked with an adjacent electrode, and the voltage difference between the two is measured. The resulting EEG tracing reflects the difference in electrical potential between these paired electrodes, helping to identify specific brain activity or abnormalities, such as seizures.

How It Works:

1. Electrode Pairing

In a bipolar montage, electrodes are set up in pairs, and each pair is linked together to measure the difference in voltage between the two. The electrodes in the pair can be located in different regions of the brain. For example, in a temporal chain, electrodes like Fp2, F8, T4, T6, and O2 are paired together, with each electrode being compared to its adjacent electrode in the chain.

2. Voltage Differences

When comparing the voltage between two electrodes, the EEG shows the difference in potential. If the first electrode (e.g., Fp2) has a higher voltage than the second (e.g., F8), the tracing will show a downward deflection (negative). If the second electrode has a higher voltage, the tracing will show an upward deflection (positive).

For example, if the voltage at Fp2 is -50 µV and at F8 it is -20 µV, the difference will be -30 µV, which would result in a downward deflection on the EEG trace.

3. Phase Reversals

One of the key features of bipolar montages is the concept of phase reversals, which occur when a discharge (such as a seizure) is detected. The phase reversal is the point where the deflection in the EEG tracing switches from negative to positive or vice versa. This typically occurs at the electrode closest to the source of the electrical discharge.

Negative discharges tend to cause surrounding tracings to point toward the electrode of maximal voltage (the one closest to the discharge), while positive discharges cause the tracings to point away from the maximal voltage electrode.

4. End of Chain Phenomenon

In a bipolar montage, the first and last electrodes in each chain (e.g., Fp1 and Fp2 or O1 and O2) do not have adjacent electrodes on one side of the chain to complete a full comparison. This can result in incomplete phase reversals or missing voltage differences, a phenomenon known as the end of chain issue. This makes interpreting EEGs from these electrodes slightly more complex.

Types of Bipolar Montages

Double Banana Montage

One of the most common bipolar montages, the double banana links electrodes in two main chains: an outside temporal chain (e.g., Fp2 → F8 → T4 → T6 → O2) and an inside parasagittal chain (e.g., Fp2 → F4 → C4 → P4 → O2). The central electrodes (Fz, Cz, Pz) form a small central chain.

Other Variations

Other montages may focus on specific areas of the brain, such as the frontal, central, or occipital regions, and can be customized for specific clinical needs.

Applications

  • Epilepsy Monitoring: Bipolar montages are often used to detect epileptiform activity, including seizures, as they allow for precise localization of abnormal brain activity.
  • Brain Activity Mapping: By comparing the voltage differences across various brain regions, bipolar montages help identify regions of high or low activity and can aid in the diagnosis of conditions like sleep disorders, brain injuries, and more.