Chapter 17. Epidemiology of Headaches

Headache is a common pain symptom that inflicts a substantial burden on individual sufferers and on society. Headache has many causes; a range of headache diagnoses was defined by the International Headache Society (IHS) in 1988.1 The IHS distinguishes two broad groups of headache disorders: primary headache disorders and secondary headache disorders. Secondary headache disorders result from an underlying condition, such as a sinus infection or brain tumor. In primary headache disorders, the headache disorder is the fundamental problem. The two most common types of primary headache disorders are episodic tension-type headache (ETTH) and migraine.

The epidemiology of headache varies by headache type and demographics. ETTH, the most common headache type, affects slightly more women than men.2–7 Between the ages of 18 and 65, about 36% of men and 42% of women suffer from ETTH.8 In contrast, migraine occurs approximately three times more often in women than in men: approximately 18% of women and 6% of men between 12 and 80 years of age suffer from migraine.9,10

ETTH exerts a modest impact on the individual; however, the aggregate societal impact is high because the disorder is so prevalent. Although migraine is less common, individual attacks are considerably more painful and disabling and often result in lost work time. Because the societal impact of both ETTH and migraine is significant, this chapter focuses on the epidemiology of the two disorders. This chapter does not cover secondary headaches because the epidemiology of the underlying condition is an important determinant of the epidemiology of the related headaches. This chapter begins with a review of the diagnostic criteria for migraine and tension-type headache, followed by a review of migraine epidemiology, including incidence, prevalence, and public health impact. We close with a review of the epidemiology of tension-type headache.

Migraine is characterized by various combinations of neurologic, gastrointestinal, and autonomic changes that occur during different phases of the migraine attack. Although the IHS defines seven subtypes of migraine (IHS, 1.0), by far, the two most important are migraine without aura (IHS, 1.1) and migraine with aura (IHS, 1.2). The IHS definitions for migraine with and without aura are found in Tables 17-1 and 17-2. Migraine is both a diagnosis of inclusion, because specific diagnostic features are required, and a diagnosis of exclusion, because secondary headache disorders have to be eliminated based on the history, physical examination, or laboratory studies.

The features of the migraine attack can be divided into four phases: premonitory phase, aura, headache phase, and resolution phase. None of these phases is obligatory for diagnosis, and most people with migraine do not have all four phases. The premonitory phase (prodrome) occurs in approximately 60% of migraine sufferers, with equal frequency in both migraine with and without aura.11 It usually begins hours to days before headache onset. Symptoms that may occur during the prodrome include depression, hyperactivity, yawning, food cravings, and thirst.11

In migraine with aura, various focal neurologic symptoms precede or accompany the attack. The aura usually develops over a period of 5 to 20 minutes, and typically lasts less than 1 hour. It is most often characterized by visual phenomena but may also involve other secondary features, motor, language, or brainstem disturbances. The aura is usually, but not always, followed by a headache. Approximately 20% to 25% of individuals have migraine with aura, but even in these individuals, most attacks are migraine without aura.12

The third phase is the headache phase. The headache of migraine is typically unilateral, pulsating, of moderate to severe intensity, and aggravated by routine physical activity. Other features, including photophobia, phonophobia, nausea, and vomiting, also occur during the headache phase.

During the final resolution phase the pain and accompanying symptoms subside. Although some migraine sufferers feel euphoric and energetic, others feel lethargic and tired.

The clinical diagnosis of tension-type headache (TTH) is also based on a characteristic symptom profile and the exclusion of secondary headache. Unlike migraine, there is no prodrome or aura in TTH. The headache is usually bilateral, dull, nonpulsating, and of mild to moderate intensity. Although associated features are not required for diagnosis, various symptoms are compatible with the diagnosis.

The diagnostic category TTH (IHS, 2.0) includes two main subtypes, ETTH (2.1) and chronic tension-type headache (CTTH) (2.2). The primary difference between these disorders is the frequency with which the headaches occur. Tension-type headaches that occur less than 15 days per month are classified as ETTH, while those that occur more than 15 days per month are classified as CTTH. In addition, for ETTH nausea is not permitted but either photophobia or phonophobia is permitted. For CTTH, any one of nausea, photophobia, or phonophobia is permitted. The diagnostic criteria for ETTH and CTTH are presented in Tables 17-3 and 17-4.

Incidence

Incidence is defined as the rate of onset of new cases of disease in a defined population. Estimating the incidence of migraine is challenging because the disease affects individuals of all ages, and the incidence rate varies substantially by age. To accurately describe incidence requires a large cohort study of individuals ranging in age from 4 to 60 years. There have been few studies of migraine incidence.

Breslau and colleagues13 conducted a prospective study to estimate the incidence of migraine. The investigators interviewed 1,007 members of a health maintenance organization who were between the ages of 21 and 30 years. Approximately 98% (972/1,007) of the participants completed a follow-up interview 3.5 and 5.5 years later. The at-risk population was composed of 848 participants who did not meet the criteria for migraine at baseline. The 5.5-year cumulative incidence was 8.4% (71/848; female, 60; male, 11), or a rate of 17.0 per 1,000 person years (24.0, female, 6.0, male).

Two population-based studies estimated the incidence of migraine using the reported age of migraine onset. Stewart and colleagues14 conducted telephone interviews among 10,169 residents of Washington County, Maryland, who were between the ages of 12 and 29 years. In this study, 392 male and 1,018 female interviewees were identified as migraine sufferers. As shown in Figure 17-1, in both males and females, the incidence rate of migraine with aura peaks 3 to 5 years earlier than migraine without aura. In addition, the onset of migraine in females occurs at a later age than in males. An important strength of this study is that it adjusted for telescoping. Telescoping is the tendency to report the occurrence of past events at times closer to the present.15 Thus, studies estimating the age-specific incidence of migraine based on recall would likely be biased toward older ages of headache onset.15,16 To minimize the effect of telescoping, Stewart and colleagues14 estimated the age-specific incidence rates, adjusted for the time interval between the reported age of onset and the age at interview. However, because of the limited age range of the participants, the generalizability of the results is limited.

The second population-based study, conducted by Rasmussen,17 reported that the age-adjusted annual incidence of migraine was 3.7 per 1,000 person years (females, 5.8; males, 1.6). Neither age-specific incidence nor incidence by migraine subtypes was reported.

One study, conducted in Olmstead County, Minnesota, used linked medical records to estimate the incidence of migraine.18 Of the 6,400 patient records reviewed, 629 fulfilled the criteria for migraine. The overall age-adjusted incidence rate was 1.37 per 1,000 person years for males and 2.94 per 1,000 person years for females. The incidence rates in this study are lower than those reported in the aforementioned studies, probably because only individuals who consulted a health care provider for headache were included.

Prevalence

In contrast to the limited number of studies of migraine incidence, there have been many studies of the prevalence of migraine. Prevalence is defined as the proportion of a given population that has migraine over a defined period of time. Lifetime prevalence refers to the proportion of individuals who have ever had the disease. Period prevalence refers to the proportion of individuals who have experienced at least one attack within a defined interval, usually within 1 year of the survey. Prevalence is a function of both the incidence and duration of a disease.

Migraine prevalence estimates have varied widely, largely because of differences in case definitions. Because migraine prevalence varies by age, gender, race, geography, and socioeconomic status, prevalence estimates from different studies also vary because of demographic differences among the populations studied.19,20Table 17-5 presents the age- and gender-specific migraine prevalence estimates from population-based studies that used the IHS criteria.19 Two meta-analyses of published population-based studies that help explain the variation in prevalence will be reviewed in this section.19,20

The meta-analysis by Stewart and associates20 included 24 studies published prior to 1994, only 5 of which used the IHS diagnostic criteria. Over 65% of the variation in prevalence estimates among studies was explained by only a few factors. Case definition accounted for the largest portion of variation in prevalence (36.1%) between studies. Gender (14.5%) and age (age 2.9%; age2 13.5%) also accounted for a substantial part of the variation. Migraine was more prevalent among females than males, peaking between 35 and 55 years of age in both genders. Factors such as the source of the population, the response rate, and whether diagnoses were confirmed by a clinical examination did not account for a significant portion of the variation among studies.

A second, more recent, meta-analysis included 18 population-based studies all using the IHS criteria.19 Because this study required the use of the IHS criteria for inclusion and stratified by gender, two major explanations from the first study were eliminated. In this second meta-analysis, migraine was again more prevalent among females than males, and peaked during the third and fourth decade of life in both genders. Among females, age and geographic location of the study population accounted for 74% of the variation in migraine prevalence. These two variables accounted for a slightly lower proportion, 58%, of the variation among males. Specifically, prevalence was highest in North America, South America, and Western Europe, intermediate in Africa, and lowest in Asia, as discussed later. Once again, variation in migraine prevalence was not explained by methodologic factors such as sampling method, response method, response rate, and recall period.

After standardizing the definition of migraine across studies, a substantial proportion of the variation in migraine prevalence is explained by remarkably few factors. However, some of the variation remains unexplained. Socioeconomic status, cultural differences in symptom reporting, or other unmeasured factors may account for part of the residual variation in migraine prevalence.19,20

Prevalence by Age and Gender

Migraine prevalence varies by age and gender. Most studies find that migraine prevalence is higher among females than males. As depicted in Figure 17-2, the prevalence of migraine is higher among females than males and varies considerably with age, with a peak between 35 to 45 years.

Prevalence by Race and Geographic Region

Migraine prevalence appears to vary by race and geographic region, two factors that are associated with each other. Stewart and colleagues32 conducted a population-based study to compare the prevalence of migraine among whites, African Americans, and Asian Americans in the United States. Both before and after adjusting for sociodemographic covariates, other than race, they found the lowest prevalence in Asian Americans (female, 9.2%; male, 4.2%), and the highest prevalence among whites (female, 20.4%; male, 8.6%). Similarly, the meta-analysis19 found the lowest prevalence in Asia and Africa, and considerably higher prevalence in Europe, Central and South America, and North America.

The international variation in migraine prevalence may be explained in several ways. The variation may be caused by regional differences in genetic susceptibility. This hypothesis is supported by the evidence of lower migraine prevalence in Africa and Asia, as well as among African Americans and Asian Americans in the United States.32 However, because the prevalence of migraine in Asia is considerably lower than the prevalence among Asian Americans in the United States, it suggests that a role for environmental or cultural factors is likely. A limitation in evaluating the role of genetic and environmental factors in race differences is that the exact same methods have not been used in studies conducted in different countries and race groups.

Prevalence by Socioeconomic Status

In the past, migraine was thought to be a disease of the affluent. However, population-based studies conducted in North America indicate that, in the community, migraine prevalence is inversely related to household income or education.9,10,32,35 That is, as income or education increases, migraine prevalence declines. Prior beliefs regarding the apparent direct relationship between migraine prevalence and income may be explained by patterns of health care utilization. Migraine may appear to be a disease of high income in the physician’s office, because medical diagnosis of migraine is more common among high-income groups.36 The contrasting finding of the National Health Interview Survey (NHIS)37 supports the inverse association and the role of access to care shaping physicians’ perceptions about this condition. In the NHIS study, prevalence was lower in the low-income compared with the middle-income group, and was highest in the high-income group. Because the NHIS relied on self-reported medical diagnosis, the higher prevalence among the high-income group is likely to be the result of an increased likelihood of diagnosis of migraine as income rises. Interestingly, studies outside the United States have generally not reported the inverse relationship between migraine prevalence and income.21,30,38,39 It is unclear what accounts for the international variation in these findings.

Public Health Significance

Migraine is a disabling disorder that exerts an impact on both the individual and society. The individual impact of migraine is measured by quantifying the frequency and severity of attacks, as well as through quality-of-life studies. The societal impact is measured in economic terms, which include direct costs, such as health care utilization, as well as indirect costs resulting from lost productivity at work.

Individual Impact: Frequency and Severity of Attacks

The individual impact of migraine is measured by the level of pain intensity, the presence and severity of associated symptoms (nausea, vomiting, photophobia, phonophobia), and the frequency and duration of attacks. The individual impact of migraine has been evaluated in several studies.9,10,32,40 Most recently, Stewart and colleagues32 reported the migraine characteristics of 1,748 migraine sufferers residing in Baltimore County, Maryland. In this study, approximately 40% of subjects (female, 38.9%; male, 46.0%) reported severe pain, while an additional 40% reported very severe pain (female, 43.3%; male, 31.8%). The remainder of subjects reported pain that ranged from mild to moderate.

The overwhelming majority of migraine sufferers reported photophobia (overall, 81.9%; female, 83.5%; male, 76.5%) and phonophobia (overall, 77.9%; female, 79.3%; male, 72.9%). More than half (58.8%) of migraine participants reported nausea that accompanies their migraine headache more than half the time (female, 62.1%; male, 47.3%), while only a small proportion reported vomiting.

The majority of subjects reported one to two migraine attacks per month. The duration of an untreated attack varies considerably by gender. Among females, approximately 71% of attacks last longer than 24 hours. In contrast, 48% of males reported attacks that last longer than 24 hours.

The results reported by Stewart and colleagues32 were comparable to those reported in previous studies.9,10,40

Quality‐of-Life Studies

Measures of quality of life provide a broad-based assessment of the specific and global impact of a condition such as migraine. However, most studies of the impact of migraine have been conducted in clinic-based samples. As such, the quality-of-life impact is likely to be overestimated by these studies. Nonetheless, using the generic health-related quality-of-life (HRQoL) instrument from the Medical Outcomes Study,41 these studies demonstrate that migraine sufferers have lower quality of life than control subjects in the United States population.42,43 One study43 found that individuals with migraine have quality-of-life scores similar to those of patients with osteoarthritis, hypertension, or diabetes. However, quality-of-life as measured in clinic-based samples may differ from that of migraine sufferers in the community. In addition, many studies conducted using clinic-based samples lack a contemporaneous control group. The lack of a control group can lead to an overestimation of treatment benefits because of regression toward the mean.

At least three population-based studies of quality of life in migraine cases versus controls have been conducted.44–46 In these studies, migraine sufferers had substantially lower HRQoL relative to population controls. In addition, quality of life and frequency of attacks were inversely correlated; as the frequency of attacks increased, quality of life decreased.44 Lower quality-of-life scores were associated with higher levels of headache-related disability.46

A recent study published by Lofland and colleagues47 assessed the outcomes of migraine sufferers enrolled in a mixed-model staff/independent practice association–managed care organization. The study enrolled patients with migraine when they received their first prescription for sumatriptan. After 6 months of sumatriptan therapy, four of the HRQoL dimensions and the physical component summary score of the Short-Form Health Survey (SF-36) showed significant improvement.41,48 Furthermore, the Migraine-Specific Quality of Life Questionnaire (MSQ),49 a disease-specific quality-of-life instrument, showed significant improvement at both 3 and 6 months after initiation of sumatriptan therapy. Despite these striking findings, results are limited by the lack of a control group, leading to a possible overestimation of treatment benefits resulting from apparent improvements that can be explained by regression towards the mean.

Societal Impact

Direct Costs

Direct costs related to health care include outpatient visits, hospitalization, the use of emergency department services, the cost of prescriptions, and other treatments. For migraine, most of the direct costs were the result of outpatient visits and the cost of prescription medications. Hospitalization37,50 and the use of emergency department services50–52 are uncommon among migraine sufferers.

According to the American Migraine Study,53 68% of females and 57% of males have ever consulted a physician for migraine. The study also found that certain demographic and headache characteristics are associated with consultation. For example, females and older age groups are more likely to consult a physician for migraine. In addition, higher pain intensity, the number of migraine symptoms, attack duration, and disability were all associated with an increased likelihood of physician consultation.

Migraine sufferers are more likely to consult their physicians for medical care than individuals in the general population who do not have migraine. Clouse and colleagues50 compared the health care utilization patterns of migraine sufferers and nonmigraine sufferers enrolled in a United Healthcare Corporation–affiliated plan. A total of 1,336 migraine sufferers were compared with a matched sample (i.e., matched on the basis of age, sex, duration of enrollment, and subscriber/dependent status) of individuals without migraine. During the 18-month study period, migraine sufferers made 22,587 physician visits, of which, 2,616 were for migraine. In contrast, individuals without migraine made a total of 13,072 visits during the same period. Interestingly, a small proportion of the migraine sufferers accounts for the majority of physician visits among migraine sufferers. Although most migraine sufferers reported only one to four visits per year, 7.6% reported more than 12 visits.

Few studies have translated use of medical care into direct monetary estimates. Hu and colleagues54 estimated the annual treatment costs of migraine in a population-based sample. They found that the annual treatment costs for migraine were over $1 billion, about $100 per migraine sufferer per year. These are likely to be underestimates because the figures were derived from 1994 data. Consultation and medication use have increased substantially over the past 5 years.55 Clouse and colleagues50 reported similar, though slightly higher, average claim costs per member per month ($145). The costs reported by Clouse and colleagues differ from those of Hu because they were not separated by diagnosis. Migraine sufferers are likely to have comorbid conditions; thus, the increased costs may reflect the costs resulting from the treatment of migraine as well as other medical conditions.

Indirect Costs

The indirect economic impact of migraine is primarily a consequence of lost productivity at work. Because many migraine sufferers continue to work, even while suffering from a migraine attack, it is important to account not only for lost workdays caused by migraine, but also reduced productivity while at work, when estimating indirect costs.

The early studies of migraine-associated disability had limitations. For example, some studies examined selected populations such as clinical trial participants or individuals with a self-reported diagnosis of migraine.37,56 Other studies employed designs that were limited by uncertainties regarding the accuracy of recall over a period of up to 1 year.57–59

Von Korff and colleagues60 conducted a population-based diary study as one approach to addressing the limitations of previous studies. They estimated the amount of lost work time among migraine sufferers who completed a daily diary for 3 months. During this 3-month period, migraine sufferers missed an average of 1.1 days per month because of headache. Subjects who worked during a migraine attack reported work effectiveness that was reduced by an average of 41%.

Some studies combine lost workdays and reduced effectiveness days into an overall measure, termed lost workday equivalent (LWDE). The LWDE equals actual days of missed work plus days at work with headache, times one, minus percent effectiveness while at work with headache.57 In the study reported by Stewart and colleagues,57 subjects experienced an average of three LWDEs during the study period. The majority of the lost workdays and LWDEs were the result of migraine rather than any other headache type.

The missed workday and LWDE estimates reported by Von Korff and colleagues60 are higher than those reported in other population-based studies.37,56,57,61 The use of a daily diary may have improved the accuracy of reporting that is inherently limited by asking people to recall events over the past 3 to 12 months.

In the study reported by Von Korff and colleagues,60 a relatively small percentage of migraine sufferers accounts for the majority of lost work time. In this study, the most disabled 20% of the participants accounted for 77% of the missed workdays; and 40% of subjects accounted for 75% of the total LWDEs. Similarly, Stewart and colleagues57 found that 51% of females and 38% of males reported six or more LWDEs per year, which accounted for more than 90% of all lost workdays.

The early studies that attempted to quantify the monetary cost of the lost work also had significant limitations. One study was based on self-identified migraine sufferers37; thus, it probably underestimated the cost because it excluded those who did not know they suffered from migraine. Another study56 probably overestimated the cost because it included a population of clinical trial participants who were likely to be more disabled than migraine sufferers in the community.

Using the human capital method, Hu and colleagues54 estimated the indirect costs of migraine. Human capital studies assume that the economic value of a missed day of work is equivalent to the wages for that day. According to the results of this population-based study, Hu estimated that migraine costs American employers $13 billion per year as a result of absenteeism and reduced effectiveness at work. Approximately 62% of the cost was a result of absenteeism. Importantly, the greatest indirect costs were found among middle-aged individuals (30–49 years). Although this study provided important information, the human capital method values the indirect costs of the disease according to the sufferers’ salary. A lost workday is valued by the individual’s wages for that day. This is a reasonable approach, but it may be an overestimate if the worker makes up for lost time or coworkers effectively cover for his or her absence. However, if a migraine sufferer makes an error while at work, it may be an underestimate. In addition, the impact of the disease in a laborer or homemaker would be valued at a lower rate than that of a business executive with a high salary. A limitation of all studies reported to date is that reduced productivity while at work when suffering from a migraine is based on self-report. No studies to date have actually quantified reductions in productivity.

Prevalence

The prevalence of TTH has not been widely studied. Among published studies of TTH that use the 1988 IHS criteria, the 1-year period prevalence ranges between 14.35 and 93.02 for ETTH, and 0.05,33 and 8.133 for CTTH (Table 17-6). Variation in prevalence estimates may be due, in part, to differences in study methodology. Lifetime prevalences are higher than 1-year period prevalences. The age distribution of the population studied influences prevalence estimates. Case definition also plays a role. Studies of the epidemiology of TTH use various levels of diagnostic specificity. Although some studies group all TTH (IHS 2.0) subjects together, other studies distinguish between subjects with ETTH, (IHS 2.1), CTTH (IHS 2.2), and headache of the tension-type fulfilling all criteria except one (IHS 2.3). Prevalence estimates may also vary because of differences in the diagnostic sensitivity and specificity of the methods used to collect symptom data. Methods of data collection (e.g., self-administered questionnaires, telephone interviews, and clinical examinations) as well as the quality of data collection may influence levels of diagnostic accuracy. Finally, the source of the study population, community based or clinic based, is likely to cause varying prevalence estimates. A meta-analysis might help explain how much each of these factors contributes to the variation in prevalence among studies. Following is a summary of several TTH studies that used the IHS criteria.

Schwartz and colleagues8 conducted the only large-scale population survey in the United States describing the epidemiology of ETTH and CTTH, as defined by the IHS criteria. These investigators used data from a telephone survey of 13,345 residents of the Baltimore County, Maryland, area32 to estimate the 1-year period prevalence of ETTH and CTTH by sex, age, education, and race. They found that the overall prevalence of ETTH in the past year was 38.3%.

Lavados and colleagues5 interviewed a representative sample of 1,385 adults (<14 years old) in Santiago, Chile, using an in-person interview. Subjects reported details about the type of headache from which they suffered most often. The 1-year prevalence of ETTH was 24.3%. The lower prevalence in this study compared with that of Schwartz and colleagues8 may be explained by the case definition used by the Lavados group. Cases were only identified if TTH was the most common headache. When Schwartz and colleagues8 used similar criteria, they found an ETTH prevalence of 25.3% compared with 38.3% after the second headache type was classified.

Rasmussen and colleagues,2 on the other hand, reported ETTH prevalence estimates that are higher than those of most other studies (see Table 17-5). In this study, potential participants were identified from the Danish National Central Person Registry and invited to a general health examination, with an emphasis on headache. Of the 1,000 potential study subjects, 740 males and females participated in the study. The 1-year period prevalence of ETTH was estimated to be 74.0%. The prevalence of ETTH may be higher in this study because of the way in which the invitation was worded. Because potential subjects were invited to a health examination with an emphasis on headache, individuals who had headaches may have been more likely to participate. Thus, there may have been an over-representation of headache sufferers among the participants.

The prevalence of CTTH is markedly lower than that of ETTH. Schwartz and colleagues8 found that the overall prevalence of CTTH was 2.2%. Tekle-Haimanot,33 Lavados,5 and Castillo and their colleagues62 reported similar estimates of 1.7%, 2.6%, and 2.2%, respectively, among the subjects they studied.

Prevalence by Demographic Features

The prevalences of both ETTH and CTTH vary by age, gender, race, and educational level. TTH is slightly more common among females than males. Schwartz and colleagues8 found that women had a higher prevalence of ETTH than men (men, 36.3; women, 42.0), with an overall prevalence ratio of 1.16 to 1.0. The female preponderance occurred at all age, race, and educational levels. Several other studies also report a higher prevalence of TTH among women,2–7 with female-to-male gender ratios ranging from 1.252 to 1.9.5

Similarly, the prevalence of CTTH is also higher among females than males. In the study published by Schwartz and colleagues,8 the prevalence was 2.8 in women and 1.4 in men, with an overall prevalence ratio of 2.0. Studies reported by Tekle-Haimanot,33 Lavados,5 and Castillo and their colleagues62 also reported higher prevalence among women than men.

Prevalence by Age

The prevalence of TTH varies by age. Prevalence peaks in the thirties and forties, with a decline thereafter.3,5,8 Pryse-Philips and colleagues6 reported a similar, although slightly earlier, peak prevalence in the 25- to 34-year-old age group. Rasmussen and colleagues,2 on the other hand, found that the prevalence of TTH decreased with increasing age, and Gobel and colleagues38 found no difference in prevalence by age. The lack of association reported by Gobel and colleagues may be attributed to the use of very wide age intervals; whereas in most studies age is categorized into 10-year intervals,2,3,5,6,8 Gobel and colleagues used 20-year age groupings.

Tekle-Haimanot,33 Lavados,5 and Schwartz and colleagues8 all reported an increase in the prevalence of CTTH with increasing age. This may be explained by the hypothesis that in some individuals, ETTH develops into a chronic form over a prolonged period of time.17,69 Gobel and colleagues38 did not find any difference in the prevalence of CTTH by age, but again this may be attributed to the use of very wide age intervals.

Prevalence by Geographic Region and Race

Some of the observed variation in the prevalence of TTH among studies may be the result of racial or ethnic differences. Each of the reported studies of the epidemiology of TTH using the IHS criteria was conducted in a different country, and based on these results, prevalence appears to be highest in the western hemisphere5,6,8 and Denmark,2 and lowest in the Asian countries.3

The only published study of TTH using the IHS criteria that reported the prevalence of TTH by race was that of Schwartz and colleagues.8 In this study, the prevalence of ETTH was significantly higher in whites than in African Americans in both men (40.1% versus 22.8%) and in women (46.8% versus 30.9%). The prevalence of CTTH by race paralleled that observed for ETTH: prevalence was higher in whites than in African Americans in both men (1.6% versus 1.0%) and women (3.0% versus 2.2%).

Prevalence by Socioeconomic Status

The evidence regarding the relationship between socioeconomic status and the prevalence of ETTH is mixed. Schwartz and colleagues8 reported that the prevalence of ETTH increases with increasing educational level, a measure of socioeconomic status. Prevalence peaked among those with a graduate-level education (men, 48.5%; women, 48.9%). Lavados and colleagues5 found a similar direct correlation between ETTH prevalence and socioeconomic status. Other studies have not found this direct association.6,38 Gobel and colleagues38 reported no significant differences by education; however, their study used only two educational categories (i.e., basic and secondary) and, as such, may simply lack the sensitivity to detect patterns observed in other studies that used a greater number of educational or income categories. It is also possible that the influence of socioeconomic status varies by country.

The relationship between socioeconomic status and migraine may differ for CTTH. Schwartz and colleagues8 and Lavados and colleagues5 reported that the prevalence of CTTH declines with increasing educational level, especially among women. Gobel and colleagues38 found no association between CTTH prevalence and educational level. Schwartz and colleagues8 suggested that the epidemiology of CTTH, with its higher risk in women and strong relationship to socioeconomic status, is intermediate between that of ETTH and migraine and may reflect the progression of both headache types to a chronic form.

Headache Characteristics

Over 90% of subjects with ETTH report mild to moderate headache pain intensity; attacks typically occur three times per month.2,8,38 Lavados and colleagues5 found that more than 86% of TTH sufferers report mild to moderate pain that occurs three to four times per month. The headache frequency reported by Lavados and colleagues is slightly higher than those reported by Schwartz,8 Gobel,38 and Rasmussen and their colleagues2 because the Lavados sample included both ETTH and CTTH sufferers.

CTTH, on the other hand, is typically associated with higher pain intensity and more frequent attacks. In one study, 86% of subjects reported moderate or severe pain (moderate, 44%; severe, 42%).38 Using a 10-point scale, Schwartz and colleagues8 found significantly higher pain intensity scores among CTTH subjects than ETTH subjects (CTTH, 5.55; ETTH, 4.98; P <.001). For CTTH, headache frequency ranges from 15 to 30 headaches per month.8,38

Certain clinical characteristics occur more frequently among TTH sufferers, and these characteristics often differ by gender. Lavados and colleagues5 reported that bilateral pain occurs in the majority of TTH sufferers but occurs with even greater frequency among women than men (men, 87.9%; women, 61.1%; P <.01). Throbbing pain occurs approximately equally in 66.4% of men and 56.8% of women; this feature, often viewed as a hallmark of migraine, does not discriminate the two disorders. These investigators also found that many TTH subjects report pain that is exacerbated with movement (men, 69.8%, women, 75.5%; P = .4), which is surprising because pain that is exacerbated by movement is normally associated with migraine rather than TTH. Pressing pain, photophobia, and phonophobia were also frequently reported: each of these characteristics occurred significantly more often among women than men. Nausea was not commonly reported by any of the study participants.

Disability

Rasmussen58 reported the first population-based study to examine work loss data in ETTH. Of the employed participants with ETTH, 12% reported being absent from work at least once during the previous year because of ETTH. Among those who reported lost workdays, the majority (68%) were absent from 1 to 7 days during the previous year, 25% were absent between 8 and 14 days during the year, and only 16% were absent more than 14 days during the previous year.

In their survey of Baltimore County residents,8 Schwartz and colleagues also measured the impact of headache in the workplace.59 In this study, reduced ability to function and an inability to function (actual missed work) were measured separately. Of the lost work time associated with headache, 19% of the missed workdays and 22% of the reduced effectiveness days were specifically the result of ETTH.59

Schwartz and colleagues8 also reported that among subjects with ETTH, 8.3% reported missed workdays, while 43.6% reported reduced effectiveness days because of headache. Among those with missed workdays, an average of 8.9 missed workdays were reported, whereas subjects with reduced effectiveness days reported approximately 5.0 reduced effectiveness days per person.

Lavados and colleagues5 found higher levels of missed work among their sample of TTH sufferers: 25% of males and 38.9% of females reported missed work because of their headaches. They also found that TTH sufferers were likely to miss family and social activities as a result of their headaches. Approximately 27.6% of males and 25.3% of females missed family or social activities because of their headaches.

In the Schwartz8 study, the proportions of CTTH subjects reporting lost and reduced effectiveness days were similar to those reported by ETTH subjects: 11.8% of CTTH sufferers reported lost workdays, and 46.5% reported reduced effectiveness days. However, in contrast with the ETTH sufferers, CTTH sufferers reported more frequent lost workdays and reduced effectiveness days. Subjects with lost workdays reported an average of 27.4 lost workdays per person, whereas subjects with reduced effectiveness days reported approximately 20.4 reduced effectiveness days per person.

Headache is a very common condition: approximately 36% of men and 42% of women suffer from ETTH,8 while 18% of women and 6% of men suffer from migraine.9,10 These disorders impose a burden on the individual, his or her family, and society.

Migraine sufferers report frequent and painful attacks that inhibit their ability to carry out their usual activities. In several population-based studies, migraine sufferers reported notable impairments in quality of life.44–46 In addition, Hu and colleagues54 estimated that the annual treatment costs for migraine were more than $1 billion, about $100 per migraine sufferer per year. Furthermore, Hu and colleagues reported that migraine costs American employers $13 billion per year because of absenteeism and reduced effectiveness at work.

When compared with migraine, the impact of ETTH is more modest. ETTH is generally less painful and has a smaller impact on daily functioning.8,59 However, the aggregate societal impact is large because the disorder is highly prevalent. The individual impact of CTTH is greater than that of ETTH, but the disorder is relatively rare, thus the societal impact is relatively small.

There have been many studies of the epidemiology of migraine; however, TTH has been studied less often. Future research should seek to expand understanding of the variation in TTH prevalence among certain subgroups, as well as heighten awareness of the impact of TTH on the individual and society. This information will enhance understanding about the relative impact of both migraine and TTH.