Paroxysmal fast activity (PFA) is a specific type of bursting beta frequency range activity that may be either focal or generalized (Altman and Shewmon, 1990). Other than distribution, the two forms of PFA are similar in waveform. Both forms have a sudden onset and resolution and contrast clearly with the surrounding background activity. A burst of PFA begins with the abrupt appearance of a fast, regular or irregular rhythm that usually differs from the background in its amplitude. Often, the amplitude is greater than the background activity and typically is more than 100 μV; however, occasionally it may be lower than the background activity and as low as 40 μV (Chayasirisobhon et al., 1984). Regardless of the PFA’s amplitude, the abrupt change in EEG amplitude and presence of a beta frequency rhythm are key features that lead to PFA recognition.

PFA’s waveform is the repetition of monophasic waves with a sharp contour produced by the high frequency. The frequency at the onset of a burst almost always is within the range of 10 to 30 Hz, and usually is between 15 and 25 Hz. The durations of PFA bursts vary, especially for generalized PFA (GPFA). Focal PFA (FPFA) commonly lasts 0.25 to 2 seconds, and GPFA usually lasts about 3 seconds, but may last as long as 18 seconds (Brenner and Atkinson, 1982). Most GPFA runs that last longer than 5 seconds are accompanied by a seizure; thus, the EEG subsequently also includes the very fast activity of seizure-related muscle artifact (Speckmann and Elger, 1999). Unlike FPFA, which may occur anywhere on the scalp, GPFA has a relatively consistent field with a maximum in the frontal or frontal-central regions bilaterally. The distribution of GPFA’s field often is generalized or at least including the majority of both hemispheres, but it may be limited in distribution to the bilateral frontal or frontal-central region. Unless a seizure accompanies the PFA, PFA’s rhythm remains fixed at the initial frequency or evolves with only a minor frequency decrease. The decrease, when it occurs, is rarely more than 4 Hz by the end of the burst and amount of decrease typically depends on the burst’s duration. Ictal PFA, similar to other ictal EEG patterns, has more pronounced evolution through the occurrence, which may be in amplitude, regularity, frequency, or a combination of more than one of these features. Interictal PFA typically does not include evolution in any of these features.

Sleep is the state in which PFA is most likely to occur, but it is not unusual in wakefulness. Compared to occurrence in sleep, GPFA that occurs in wakefulness is longer in duration on average and more likely to have accompanying ictal behavior. Hyperventilation and photic stimulation are not activating procedures for PFA (Drury and Henry, 1993). PFA most commonly occurs in the age range of infants to young adults.