Rare primary headaches: clinical insights.

So-called "rare" headaches, whose prevalence rate is lower than 1% or is not known at all and have been reported in only a few dozen cases to date, constitute a very heterogeneous group. Those that are best characterised from the clinical point of view can be classified into forms with prominent autonomic features and forms with sparse or no autonomic features. Among the former are trigeminal autonomic cephalalgias (TACs) and hemicrania continua, while the latter comprise classical trigeminal neuralgia, hypnic headache, primary thunderclap headache, and exploding head syndrome. The major clinical discriminating factor for the differential diagnosis of TACs is the relationship between duration and frequency of attacks: the forms in which pain is shorter lived are those with the higher frequency of daily attacks. Other aspects to be considered are the time pattern of symptoms, intensity and timing of attacks, the patient's behaviour during the attacks, the presence of any triggering factors and of the refractory period after an induced attack, and response to therapy, especially with indomethacin. Often these are little known clinical entities, which are not easily detected in clinical practice. For some of them, e. g., thunderclap headache, it is always necessary to perform instrumental tests to exclude the presence of underlying organic diseases.