The Cluster Phenomenon: An Unspecific feature?

The presence of the cluster phenomenon per se is important for a correct diagnosis of cluster headache (1). There is consensus among the experts in this field that for this reason it is difficult to establish a definite diagnosis of cluster headache on clinical grounds only, during the very first bout. Already during the second bout, a temporal pattern may seem to have been establishedand a diagnostic suspicion may be verified. It has, in previous years, been a widely accepted point of view that only cluster headache has a temporal pattern with typical attacks, accumulated in cluster periods, punctuated along the time axis. Once a cluster pattern was established, so was the diagnosis. Our view on this matter has changed somewhat over the last few years. We described chronic paroxysmal hemicrania (CPH) in 1974 (2) and at an early stage felt that this was a headache in its own right (3,4). Cluster headache is characterized by certain main features (5), as shown in Table 1. Admittedly, there are considerable similarities between CPH and cluster headache (Table 1). Only as far as one of the major points is concerned is there a clear difference: the sex preponderance. There may also be a difference as far as the temporal pattern (that is, the cluster phenomenon) is concerned, but this may be a more subtle one. The similarity of the temporal pattern of even the chronic stage of CPH with that of cluster headache is considerable. Moreover, there is a prechronic or non-continuous stage in several CPH cases (4,6-lo), and this stage may be like a blueprint copy of the temporal pattern of cluster headache. Moreover, many (all?) cases of CPH in the chronic stage demonstrate a considerable fluctuation in severity (“modified cluster pattern”) along the time axis. Failure to recognize the lowgrade pain, almost without minor peaks (“attacks”) may contribute to an incorrect categorization; they may be placed in the category of non-chronic CPH (which we suspect has really happened at times in recent years).