Clinical definitions of clustering:
There is no definitive clinical definition for a cluster or series of seizures. Studies examining clinically defined seizure clustering patterns have used varying empiric definitions, including two to four seizures per <48 hours; 3 seizures per 24 hours; or two generalized tonic–clonic or three complex partial seizures in 4 hours. Nonspecific definitions, such as "those having several convulsions within a day or two," have also been described. In a large randomized controlled trial of treatment for acute repetitive seizures, the condition was defined as "multiple seizures occurring with a 24 period for adults or 12 hour period for children, with a pattern distinguishable from the usual seizure pattern".
The strength of applying a clinical definition to identify seizure clustering is that it is easy to administer, with the information available from patient report or examination of diary or inpatient data. Patients can be instructed to institute treatment based on specific criteria they can easily identify, and studies of seizure clustering can stratify subjects into clusterers or nonclusterers based on diary data. Limitations include the possibility that patients with frequent seizures may meet these definitions by chance alone; alternately, for patients with infrequent seizures, two seizures may represent a cluster and be missed.
A strategy to address these potential weaknesses is to relate clustering to the individual’s unique seizure pattern, typically by defining clustering as a measurable increase over the patient’s typical seizure frequency. Investigators have considered a threefold or fourfold increase over usual seizure frequency within a 3-day period to represent seizure clustering. This approach requires sufficient follow-up so that typical seizure frequency or interseizure interval can be accurately determined, and often involves a sophisticated analysis technique.
