2.1 Definition of Cluster Headache

The first complete description of cluster headache was made by Gerhard von Swieten, a physician to the Empress Marie Theresa [1]. Then followed many years were cluster headache was named ciliary neuralgia, erythromelalgia of the head, erythroprosopalgia of Bing, hemicranias neuralgiformis chronica, histaminic cephalalgia, Horton’s headache, Harris-Horton’s disease, migrainous neuralgia (of Harris), or petrosal neuralgia (of Gardner) [2]. The periodicity of attacks was described as a main feature in 1947 and inspired to the name cluster headache [3, 4].

The distinction between cluster headache and migraine has previously caused controversies whether they were separate entities or not, since many considered cluster headache as a subtype of migraine. Different clinical features [2, 5–7], different gender distribution (see below), and the fact that the prevalence of migraine among those with cluster headache is similar to that of migraine in the general population indicate that cluster headache is a specific subtype of headache [8].

The definition of cluster headache has changed during the years. Schiller and Ekbom based their generally accepted definitions of cluster headache on explicit diagnostic criteria [9, 10]. The first edition of the International Classification of Headache Disorders was based on these diagnostic criteria [2]. Since the first International Classification of Headache Disorders from 1988 to the current third version from 2018, the definition of cluster headache has changed slightly [2, 5–7]. Firstly, pain-free intervals in episodic cluster headache have been expanded from at least 14 days to at least 3 months [2, 7]. Secondly, chronic cluster headache unremitting from onset and chronic cluster headache evolved from episodic cluster headache are merged to chronic cluster headache [2, 5]. Thirdly, remission periods in chronic cluster headache have been expanded from <14 days to <3 month per year [2, 7]. Fourthly, restlessness and agitations were added to the associated symptom list of cluster headache [5]. The latter is important, since some people with otherwise typical cluster headache lack autonomic symptoms [11–13].

Today the diagnosis cluster headache is universally accepted, and the clinical spectrum has been expanded with other subtypes that mimic cluster headache, i.e., paroxysmal hemicranias, short-lasting unilateral neuralgiform headache with conjunctival injection and tearing (SUNCT), and hemicranias continua [14–16].

2.2 Epidemiology

Epidemiological surveys of cluster headache are hampered by the fact that cluster headache is rare as compared to migraine and tension-type headache. Thus, many thousands of people need to be included in epidemiological surveys of cluster headache and even thought the prevalence figures have wide confidence intervals.

An epidemiological survey of 9803 18-year-old men from east central Sweden found 9 males with cluster headache, corresponding to a prevalence of 92 per 100,000 people (95% confidence interval 42–174) [17]. Thus, the “real prevalence” of cluster headache in the general population is considerably higher, since onset of cluster headache is generally after age 20 years and men have cluster headache twice as often as women.

Two epidemiological surveys were done in the Republic of San Marino 15 years apart including 21,792 and 26,628 people, respectively [18, 19]. An extensive data search was done in both San Marino surveys, i.e., the medical records of neurological, ophthalmological, and otorhinolaryngological services from the past 15 years were reviewed, family practitioners were contacted, and a letter was posted to all households of San Marino. Both surveys identified 15 people with cluster headache. The prevalence in the later survey was 64 per 100,000 people (95% confidence interval 36–106) (calculation by authors since two persons who no longer had attacks of cluster headache were not included in the papers’ calculations).

An American survey of Olmsted County, Minnesota, is imprecise, because people were included with only one attack and with attacks not clearly of 15–180 min duration [20]. Thus, patients did not necessarily fulfill the International Classification of Headache Disorders criteria for cluster headache. Furthermore, the diagnosis was based on case records and was not confirmed by a clinical interview. The suspected diagnosis of cluster headache was confirmed in only 13 of 30 neurological case records in the second San Marino survey [19].

A Norwegian epidemiological survey of 1838 people in Vågå county was based on a neurologist interview and classified according to the criteria of the International Headache Society [2, 21]. The prevalence was 326 per 100,000 people (95% confidence interval 153–783), i.e., five times higher than the prevalence in San Marino. However, three of those classified as cluster headache did not fulfill the strict criteria for cluster headache [2], since one had cluster periods of less than 1 week duration and two had only had one cluster period. Whether these persons later developed cluster headache or not is unknown. Change of cluster headache by time can be exemplified by a pair of monozygotic twins who were initially described as discordant but later became concordant for cluster headache [22, 23]. The cluster periods were very short in the beginning, and one of the twins had attacks of very short duration. Later the characteristics changed to be typical episodic cluster headache.

An Italian epidemiological survey included 10,071 people age above 14 years old [24]. The participants were all interviewed by an experienced neurologist and classified according to the International Classification of Headache Disorders [2]. The prevalence was 279 per 100,000 people (95% confidence interval 171–427).

A Swedish epidemiological survey of twins from the general population included 31,750 people [25]. Structured lay interviews identified 250 people with possible cluster headache, but only 45 of the 218 persons who were interviewed by the neurologist had cluster headache. Two screening negative twins and one index twin had their cluster headache diagnosis confirmed. The prevalence was 151 per 100,000 people (95% confidence interval 108–194).

The epidemiological survey included only people of European descent, but cluster headache has also been described in Africans, African-Americans, Japanese, and Chinese [26–29].

2.3 Conclusion

The true prevalence of cluster headache is likely to be 1 per 500–1000 people.