1. General Concept of Periodicity
In EEG recordings, periodic patterns are generally considered abnormal. A normal EEG background should exhibit some level of chaos, as the neurons in the brain work asynchronously within their respective networks. When the EEG becomes overly organized, such as through periodicity, it raises concerns for abnormal synchronous firing of neurons.
Periodicity is defined by the ACNS (American Clinical Neurophysiology Society) criteria, which stipulate that for discharges to be considered periodic, they must occur for at least 6 cycles. For example:
- If the discharges are periodic at 1 per second, they must continue for at least 6 seconds.
- If the discharges occur at 2 per second, they must last at least 3 seconds.
- If the discharges occur at 3 per second, they must continue for at least 2 seconds, and so on.
It is important to note that if the interval between each discharge varies by 25-50%, the pattern is termed "quasi-periodic." However, if the variance exceeds 50%, the pattern is not truly periodic.
2. Types of Periodic Patterns
2.1 Lateralized Periodic Discharges (LPDs)
Lateralized Periodic Discharges (LPDs), also known as Periodic Lateral Epileptiform Discharges (PLEDs), are typically seen on EEG as a lateralized pattern, often with a clear focus on one hemisphere of the brain. These discharges may be borderline between periodic and quasi-periodic if the intervals between the discharges vary by more than 25%.
LPDs are of particular concern because lateralized activity is more commonly associated with seizure activity, and the presence of such periodic patterns may indicate an ongoing seizure or a predisposition for seizures.
2.2 Generalized Periodic Discharges (GPDs)
Generalized Periodic Discharges (GPDs) involve synchronous discharges that affect both hemispheres of the brain. Although GPDs are concerning, they are generally less likely to indicate seizure activity compared to lateralized periodic discharges.
Despite being less directly associated with seizures, GPDs should still be evaluated with caution, especially if they persist or become more frequent over time.
3. Quasi-Periodic Patterns
When the intervals between discharges vary by 25-50%, the pattern is described as "quasi-periodic." This is an intermediate condition, as the discharges still have a rhythm but are not strictly periodic. It is essential to identify such patterns since they can still be indicative of abnormal brain activity but may be harder to differentiate from true periodicity.
4. Clinical Significance of Periodic Patterns
Periodic patterns can be part of a seizure, but seizures require temporal or spatial evolution in the EEG pattern. If a periodic pattern persists and reaches a frequency of 2.5 Hz for at least 10 seconds, it should be classified as a seizure, which necessitates treatment.
In the clinical setting, it is crucial to monitor the EEG and recognize when periodic patterns evolve, particularly when they reach higher frequencies. If periodic discharges exceed the threshold for seizure activity, appropriate intervention should be considered.
5. Conclusion
While periodicity in EEG is often concerning and associated with abnormal brain activity, careful analysis is required to differentiate between true periodicity, quasi-periodicity, and normal brain rhythms. Recognizing the characteristics of periodic and quasi-periodic discharges, such as their timing, frequency, and evolution, is critical for accurate diagnosis and treatment planning.
Always consider the clinical context, as periodic patterns may indicate ongoing seizures or other neurological conditions that require immediate attention and intervention.