AP Gradient

The anteroposterior (AP) gradient is a characteristic of the normal awake EEG. This really consists of two gradients. One is a voltage gradient, where higher voltages tend to be seen more posteriorly, and the second is a frequency gradient, where higher frequency activity tends to be seen more anteriorly.

In the normal waking EEG, frontal activity is low voltage and high frequency, while posterior activity is higher voltage (e.g., the posterior rhythm) and lower frequency.

An AP gradient is present if, at any point in the epoch, there is a clear and persistent (at least 1 continuous minute) anterior to posterior gradient of voltages and frequencies such that lower voltage, faster frequencies are seen in anterior derivations, and higher voltage, slower frequencies are seen in posterior derivations.

When this pattern is reversed, it can suggest significant brain damage or disruption of normal brain function, often seen in situations like deep coma, severe hypoxia, or major brain injuries. The degree of reversal and the overall EEG pattern are crucial for interpretation. A mild reversal may not be as concerning as a pronounced one with other abnormal features. A reverse AP gradient is often accompanied by other abnormal patterns like low voltage background, burst suppression, or periodic discharges, further indicating severe brain dysfunction.

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