The definition of chronic headache still remains controversial. There is no agreement on how many days per month the headache must be present, the obligatory time from initiation of headache and the type of headache. Although a lot of studies have been done, only two of them used the same criteria.

Global prevalence of CDH was found in 3 %. CDH is less prevalent among children and adolescents. It is more common in Central/South America (5 %) than Africa (1.7 %). Medication-overuse headache (MOH), a potentially treatable and preventable headache type, is common among those with CDH. Possible MOH was found to occur in about 1 % [15].

In the Turkish study, chronic daily headache was found to be 3.3 %, of which 1.8 % had a diagnosis of chronic migraine (0.4 % for those without medication overuse and 1.3 % for those with medication overuse) and 0.2 % for chronic TTH.

Trigeminal autonomic cephalalgias consists of cluster headache (CH), paroxysmal hemicrania (PH), short-lasting unilateral neuralgiform headache attacks with conjunctival injection and tearing (SUNCT)/short lasting neuralgiform headache attacks with cranial autonomic features (SUNA), and hemicrania continua (HC).

Because of the low frequency of TACs, there are few available data on epidemiology.

In the Vågå study, 1838 people between 18 and 65 years were studied; two were diagnosed as having SUNCT and in 18 individuals (11 females and 7 males), the symptoms were consistent with hemicrania continua. For paroxysmal hemicrania, the one-year prevalence rate was estimated to be 0.5 per 1000 [46].

Epidemiological studies with CH are more common than the other TACs. Vågå study is the most comprehensive study estimating the prevalence of CH [47].

The study was conducted in Norwegian rural community among 1838 participants by face-to-face interviews. Prevalence of CH was found to be 326 per 100,000 in the total population (106 per 100,000 for females, and 558 per 100,000 for males).

In an Italian study similar to the Norwegian study, the prevalence was found to be 279/100,000 among >10,000 patients [48]. D’Alessandro et al. found the prevalence of CH 69/100,000 in Republic of San Marino [49]. Rasmussen et al. and Monteiro et al. showed similar prevalences (100/100,000) [50, 51]. In Germany, the one-year prevalence of CH was estimated to be 119/100,000 [52]. Ekbom et al. reported the lifetime prevalence and concordance risk of cluster headache in the Swedish twin population in 2006. They found the prevalence as 1 per 500 of the general population [53].

However, when considering the clinical practice, the number of cluster patients seems to be lower than these prevalence studies (personal comment: Rigmor Jensen)

Global Burden of Disease (GBD) Study 2010 revealed that TTH and migraine are, respectively, the second and third most common prevalent diseases after dental caries and migraine is ranked as the seventh highest cause of disability in the world. The main disadvantage of GBD 2010 is the lack of data regarding the interictal impairment in migraine and medication overuse headache [54].

The societal impact of headache consists of direct and indirect costs. Direct costs correspond to the sum of diagnostic investigations and treatment costs. Indirect costs, which include loss of productivity due to absenteeism and reduced performance, are the major leading causes of cost when compared with direct costs of headache. The cost of migraine in Europe is estimated at € 27 billion annually. Although there are many epidemiological studies with the prevalence of headache, migraine, TTH, the data on impact of headache is rare. The most comprehensive study evaluating the impact of headache was conducted by Eurolight project. In this study, personal impact of headache was assessed by seven questions to show headache-attributed lost work, housework, and social days in preceding three months. Eurolight project emphasized that the common headache disorders have very high personal impact [55]. However, much more studies with applicable questionnaires have to be done to indicate the impact of headache, which will provide awareness among physicians.