Abstract
Headache disorders constitute a public-health problem of enormous proportions, with an impact on both the individual sufferer and society. Epidemiological knowledge is required to quantitate the significance of these disorders. The effects on individuals can be assessed by examining prevalence, distribution, attack frequency and duration, and headache-related disability. The socio-economic burden includes both direct costs associated with health care utilization and costs associated with missed work due to sickness absence or reduced efficiency. The individual and socio-economic burden of headaches is substantial. Headache disorders deserve more attention, especially concerning strategies leading to adequate primary prevention, diagnosis and treatment.
Introduction
The public health significance of headaches is often overlooked, probably because of their episodic nature and lack of mortality, but headache disorders are often incapacitating, with considerable impact on social activities and work, and may lead to a significant consumption of drugs. Epidemiological knowledge is required to clarify the natural history and to quantify the significance of the disorders in the community, based on which decisions about research and medical services for these conditions can be made. The effects on individuals can be assessed by examining prevalence, distribution, attack frequency and duration, and headache-related disability. Previous classification systems of headache disorders lacked precision, included ambiguous expressions and were non-operational, which resulted in vague criteria. The introduction of explicit diagnostic criteria for all headache disorders introduced by the International Headache Society (IHS) under the chairmanship of Professor Jes Olesen in 1988 (1) provided new opportunities for valid epidemiological headache research. Recent epidemiological studies have clarified our understanding of the scope and the distribution of the public health problem posed by the two main types of primary headaches: migraine and tension-type headache.
Prevalence and frequency
Migraine
Previous estimates of the prevalence of migraine range from 3% to 35%, a variation due largely to differences in definitions and methodologies. It is remarkable that the most recent population-based studies in adults, all using the diagnostic criteria of the HIS, have reached very similar prevalence rates. Several European (2 –8) and American studies (6 –13) have reported somewhat congruent prevalence figures for migraine in adults (Table 1). The 1-year-period prevalence of migraine in adults was about 10–12%, 6% among men and 15–18% among women. Few studies have dealt with the frequency of migraine, i.e. the number of attacks within a defined period. Studies in general populations agree that it is most common for migraineurs to have less than one attack a month. In clinic samples the frequency is higher, since high frequency may be a compelling reason for referral. However, even in the general population about 20% of the migraineurs have frequent attacks (more than once a month) (2, 6, 13, 14). The 1-year-period prevalence of migraine without aura is about 6% and that of migraine with aura about 4% (3, 7, 9, 15 –17).
Tension-type headache
Tension-type headache varies widely, in frequency, duration and severity, from rare short-lasting episodes of discomfort to frequent, long-lasting, or even continuous, disabling headaches. Pooling these extremes in an overall prevalence may be somewhat misleading. When interpreting prevalence data, it is therefore important also to consider severity, frequency, duration and disability of the disorder. However, few studies have dealt with these aspects. In a Danish population-based study, it was found that 59% of persons experiencing tension-type headache had it 1 day a month or less and 37% had it several times a month. In the total population, 3% had chronic tension-type headache, i.e. headache ≥ 180 days a year (2). This is in agreement with other reports (8, 18 –20). Other population-based studies have agreed with the Danish study in finding a rather large proportion of persons with mild and infrequent (once a month or less) tension-type headache and frequent episodic tension-type headache (more than once a month) seems to occur among 20–30% of people from the general population (2, 8, 9, 18, 20).
Sex and age distribution
Migraine
The female preponderance in migraine is more consistent across studies than the overall prevalence figures of migraine. All studies show that migraine is more common in females than in males, with a male to female ratio c. 1:2–3 (Table 1). The over-representation of women may reflect an influence of female hormones. The most common onset of migraine is in the second and third decennium, and the onset of migraine is infrequent after middle age. The prevalence of migraine increases steadily from infancy to the fourth decade and decreases with increasing age (2, 4, 6, 8, 10, 13, 19).
Some prevalence studies of migraine in industrialized countries
Tension-type headache
Tension-type headache is also more prevalent in females than in males (male: female ratio c. 1:1.5) and in both sexes the prevalence seems to peak between the ages 30 and 39 and then decline with age (2, 8, 18, 20, 21). The most common onset of tension-type headache is in the second decennium.
Sociodemographic factors
A number of demographic factors besides sex and age have been explored in relation to the headaches. Epidemiological studies of general populations have shown a fairly uniform prevalence of the disorders in various social groups (6, 13, 18, 19, 21, 22). However, some recent studies from the United States of America have reported increased risk for migraine and chronic tension-type headache in less-educated or lower-income groups (9, 10, 20).
Socio-economic impact of headache
The socio-economic burden includes costs associated with health care utilization and costs associated with missed work due to sickness absence or reduced efficiency.
Utilization of medical services
Headaches strike individuals early in life and usually continue to affect them through the majority of their productive years, not infrequently continuing even after retirement. A large proportion of headache sufferers are never diagnosed or regularly treated (23, 24). In a Danish epidemiological study of headache disorders, nearly all persons with migraine stated that the pain impaired or abolished working capacity as well as social activities. This was also described by 60% of persons with tension-type headache (23). Nevertheless, nearly half of the persons with migraine and more than four-fifths of persons with tension-type headache had never contacted their general practitioner because of the headache (13, 14, 24). About half of migraineurs and one-seventh of persons with tension-type headache managed without medication in the current year. Among those who used medicine, over-the-counter drugs were the most frequently used (14). The relatively low consultation-rate and low consumption of medicine can be interpreted to mean that medical service is unnecessary for a large proportion of the sufferers. However, the majority of headache sufferers stated that their daily activities were inhibited due to the headaches. Therefore, it is plausible that many headache sufferers are unaware that effective treatment exists and therefore do not consult a physician. Thus, measures of medical care expenditure due to the disorders do not reflect the true burden of the diseases. Estimates of socio-economic loss should include direct costs associated with medical care as well as indirect costs associated with lost workplace productivity.
Sickness absence
In the general Danish population, 43% of employed migraineurs and 12% of employed persons with tension-type headache had missed 1 or more days of work in the preceding year because of their headache (14). Among all employed adults, 5% had been absent from work in the previous year at least once because of migraine and, correspondingly, 9% of the employed population had been absent due to tension-type headache. The majority of people lost 1–7 workdays per year. Regarding both migraine and tension-type headache, women had higher absence rates than men. The absence rates presented here are probably minimum estimates because of recall bias. Prospective studies are not available. Absence from work due to migraine found in the Danish study is higher than reported from Northern Finland (25), but similar to results from Great Britain (26) and Norway (27). The absence rate and the number of days off work due to tension-type headache are quite similar to results from Finland (25). Some recent American migraine studies reach higher estimates as regards impact of migraine on work absenteeism and reduced work effectiveness (28 –30). In a population-based sample of persons with headache it was estimated that individuals lose the equivalent of 4.2 days per year and that 70% of all work loss takes the form of reduced effectiveness at work (29). Subjects with migraine headache are much more likely to report actual lost workdays because of headache, whereas tension-type and other headache types account for a large proportion of decreased work effectiveness because of headache (29). Most other studies dealing with sickness absence have not analysed migraine and tension-type headache separately. In the Danish epidemiological study the total number of workdays lost per year due to migraine in the general employed population was estimated at 270 days per 1000 persons. For tension-type headache the corresponding figure was 820 workdays lost a year per 1000 employed (14). Thus, migraine and tension-type headache account for at least 1 day of absence from work per year per gainfully employed. This figure is very high considering that in Denmark the average number of absence days per year due to any disease is 6 days per gainfully employed. Thus, these primary headache disorders cause about 20% of all absenteeism due to sickness.
Conclusion
The burden associated with headaches has previously been underestimated and has never been the subject of any national public health project in any country. Considerable benefits can be gained by strategies leading to reductions in the amount of work absence caused by headache. The impact of headache disorders on the sufferer's quality of life may, however, be the most substantial burden. In addition to the disability related to attacks, many headache sufferers live with a fear of the next attack, which restricts their lifestyles, and may disrupt their ability to meet social obligations. In summary, the individual and societal burden of headache is substantial. This dictates that more attention should be given towards headache disorders, especially concerning strategies leading to adequate primary prevention, diagnosis and treatment.