Complex sentence: Chronic cluster headaches occur continuously without any "remission" periods between cycles; there may be "high" and "low" variation in cycles, meaning the frequency and severity of attacks may change without predictability, for a period of time.
Split into simpler sentences: Chronic cluster headaches occur continuously without any "remission" periods between cycles. Chronic sufferers may have "high" and "low" variation in cycles, meaning the frequency and severity of attacks may change without predictability, for a period of time.