Subdermal needle electrodes must be inserted symmetrically over homologous brain areas to avoid:

In intraoperative monitoring, particularly when using subdermal needle electrodes, it is crucial to place these electrodes symmetrically over homologous brain areas. This careful placement is vital to ensure that any recorded electrical activity is representative of the underlying neural function and pathology.

When electrodes are asymmetrically placed, it can lead to artificial amplitude asymmetries in the recordings. These asymmetries may not reflect true physiological differences in brain activity but rather artifacts introduced by the uneven placement of the electrodes. Such artifacts can lead to misinterpretation of the data, potentially affecting clinical decisions made during surgery. Therefore, to achieve accurate and reliable EEG readings, maintaining symmetry in electrode placement is essential.

The other potential issues, while relevant to electrode placement, are not the primary concern in this context. Excessive high impedance might occur due to poor contact rather than electrode symmetry. Excessive bleeding is more related to the technique of insertion rather than the symmetry of the electrode placement. Common mode rejection refers to the ability of the recording system to filter out noise and does not directly involve the symmetrical placement of electrodes. Thus, placing the subdermal needle electrodes symmetrically is vital to avoid misleading data regarding amplitude, ensuring that the readings accurately reflect the underlying brain activity.