For many individuals, seizures are not randomly distributed in time. They cluster. A woman may go 4 weeks with no seizures and then have three in a 30-hour period. A child may awaken and have dozens of atonic seizures with head drops over 45 minutes and then not have another seizure for the rest of the day. A cluster is usually defined by the number of seizures over a period when compared to a baseline. For some patients, however, the number of seizures may be about the same as usual, but the seizures in that period are of a different severity or type.

Many clusters are the products of natural cycles such as the 24-hour circadian sleep-wake rhythm or hormonal patterns. Some people claim that the cycles of the moon or seasonal changes produce seizure clusters. Other clusters probably result from basic features of the epilepsy disorder and how the abnormal electrical activity builds up in the brain and releases its fury. We understand only some of the many biological causes of seizure clustering, and indeed of seizure occurrence in general.

The menstrual cycle is associated with seizure clusters in approximately half of women with epilepsy. Sex hormones like estrogen and progesterone can bind to nerve cells in the brain and influence chemical and electrical activity. Over the typical 28-day cycle, hormonal changes occur in a predictable sequence. (This length probably was originally related to the moon's orbit.) During ovulatory cycles (that is, menstrual cycles in which an egg is released from the ovary), seizures are most frequent just before menstruation. They also may be more frequent during the menstrual period and around the time of ovulation, about 14 days after the start of menstruation. Some patients have anovulatory cycles, in which no egg is released. For these women, seizures tend to cluster during the second half of the cycle, in the week or two before menstruation begins.

Some men may have a monthly cycle of testosterone release (and other cycles such as seasonal ones), but this not established. Rarely, adult men report a monthly pattern of seizure clusters.

Other environmental and biological cycles may cause seizure clusters. The evidence for these is soft. The lunar cycle has been associated with epilepsy for centuries, but the issue has not been adequately studied. Before modern lighting, sleep deprivation associated with a full moon may have contributed to an increase in seizures, but it is also possible that geomagnetic forces and other factors may play a role. Some people with epilepsy -- especially children and young adults with developmental disorders -- are reported to have regular clusters of increased seizure activity related to certain seasonal changes (for example, in early spring).

A phenomenon that may be called the "volcano effect" occurs in some people with epilepsy. In this group, it seems that the brain must release a certain amount of seizure activity. It is as if there is a volcano with a certain amount of lava inside. Usually it comes out slowly, but if the volcano is capped, pressure builds up until it blows and produces a more intense shower of lava. The seizure activity in these people can come out in the form of frequent minor seizures, intermittent moderate-intensity seizures, a rare big seizure, or a combination of these. Those whose seizures are all similar in strength tend to have a certain number per year. If they go for a very long interval without seizures, they are likely to have a cluster of seizures instead of the usual isolated seizure.

Early recognition of seizure clusters is critical because therapy can be given to prevent the cluster or limit its severity. Among patients with difficult-to-control complex partial seizures, those with seizure clusters are more prone to prolonged seizures (status epilepticus). When clusters involve moderate to strong seizures that are numerous or occur over a short interval, the seizures are likely to be followed by a more prolonged postictal state of tiredness, confusion, and possibly other problems such as weakness or depression. Diastat (a seizure medicine given by rectum) or Ativan (given under the tongue or swallowed) are treatments that are often used to stop clusters.