According to the 2005 population and housing census conducted by the National Statistical Office, the estimated population of individuals aged 19-69 years in Korea in 2009 was 37782000.14 The present study included all Korean territories (with the exception of Jeju-do) and classified residential areas into large city, medium-to-small city, and rural area according to the degree of urbanization (Table 1). Our sample targeted 1500 individuals and the selection process was based on the Korean population structure. We adopted a two-stage systematic clustered random sampling method. The 15 administrative divisions were designated as the primary sampling units. In each of the 15 administrative divisions, 4 representative basic units were randomly selected as secondary sampling units. The survey was therefore applied in 60 representative basic units where appropriate assessments of residential status, population structure, household income, and occupational structure were available. In each sampling unit, the target sample number was determined based on the distributions of sociodemographic parameters such as age, gender, educational level, and monthly household income. Interviewers recruited participants who met the assigned sociodemographic characteristics by door-to-door visits. The estimated sampling error of the KHS was ±2.5% with a 95% confidence interval (CI).15 Weighted values were assigned to each subject according to the distribution of the Korean population in order to estimate the adjusted prevalence. The representativeness of our sample was assessed by comparing sociodemographic distributions between our samples and the total Korean population by using data from the Korean National Statistical Office.14

The questionnaire was designed to assess demographic and socioeconomic characteristics (9 questions), headache profiles according to the second edition of the International Classification of Headache Disorders (ICHD, ICHD-2; 21 questions), and headache management plans (8 questions). We included the six-question Headache Impact Test (HIT-6) questionnaire to evaluate the impact of headache on quality of life.16 The questionnaire was validated for migraine (75.0% sensitivity and 88.2% specificity) and tension-type headache (86.2% sensitivity and 75.5% specificity) diagnoses by comparing the doctors' diagnoses in an additional telephone interview and the diagnoses in the survey. The additional telephone interview was applied to 135 subjects who consented to it.11

Subjects were stratified according to age, gender, and occupation. Prior to meeting the subjects, the interviewers were provided with the following information: 1) the aims of the present study, 2) the meaning of each question, 3) instructions for interpreting the subjects' responses, and 4) other details that were relevant to conducting a proper interview. All interviewers were employed by Gallup Korea and had previous social-survey interviewing experience. The interviewers were not medical personnel. The survey was conducted by door-to-door visits and face-to-face interviews.

If a subject responded positively to the statement "In the past year, you had at least one headache lasting more than 1 min," and the subject's headache presented ≥15 days per month for more than 3 months, he/she was classified as having CDH.

In the present study, we used the modified CM criteria outlined in the new appendix criteria for a broader concept of CM, which provides more inclusive criteria for the diagnosis of CM.9 A diagnosis of CM was assigned if a CDH subject's headaches met the ICHD-2 criteria for migraine or probable migraine (PM). A diagnosis of PM was based on the assigned A-D criteria for migraine without aura (code 1.1) in the third edition of the ICHD (beta version). If a participant's response met all criteria except one, he/she was identified as having PM.10

A diagnosis of medication overuse (MO) was based on the new appendix criteria for a broader concept of CM.9 A participant with CDH was diagnosed with MO if he/she reported regularly overusing acute/symptomatic treatment drugs that were defined in either criterion 1 or 2 for more than 3 months (criterion 1: ergotamine, triptans, opioids, or a combination of analgesics, triptans, or analgesic opioids ≥10 days/month for >3 months; criterion 2: simple analgesics or any combination of ergotamine, triptans, or analgesics opioids ≥15 days/month without the overuse of any single class alone).

We included the Korean version of the HIT-6 questionnaire in order to assess the impact of headaches on quality of life.17 The HIT-6 score was used to assign the subjects to an impact grade as follows: 36-49, little-to-no impact; 50-55, some impact; 56-59, substantial impact; or 60-78, severe impact.16

We examined the medical consultations and treatment that the subjects received. To assess medical consultations, we used the question "Have you ever visited a medical doctor for your headaches?" If a subject replied "yes", he/she was classified as having sought a medical consultation for his/her headaches.

The treatment of CDH was assessed by asking the question, "How did you treat your headaches over the past year?" This allowed the interviewer to distinguish between different treatment strategies such as 1) medical treatment, 2) over-the-counter (OTC) medications, 3) oriental medicine, 4) alternative methods other than oriental medicine, and 5) no treatment. If a subject treated his/her headaches with a medicine, an additional question was asked to evaluate the medical treatment regimen.

The 1-year prevalence with a 95% CI was calculated for each diagnosis, after adjustment of weighted values. Age- and gender-specific prevalence rates of CDH were also calculated using a 95% CI. The results were analyzed with statistical software for R (version 2.14.1) and R commander (version 1.7-3) (The R Foundation, GNU general public license). The Kolmogorov-Smirnov test was applied to test whether the continuous variables were normally distributed. After a normal distribution was confirmed, Student's t-tests, Mann-Whitney U tests, and chi-square tests were used for comparisons, as appropriate. Except where stated otherwise, the data are presented as mean±SD values, and the cutoff for statistical significance was set at p<0.05.

Of the 4054 individuals approached by our 76 interviewers, 1699 agreed to participate in the survey. Of these, 192 individuals suspended the interview, so that ultimately 1507 subjects completed the survey (cooperation rate of 37.2%) (Fig. 1). The distributions of age, gender, size of residential area, and educational level across our samples did not differ significantly from those for the total Korean population (Table 1).

Of the 1507 subjects interviewed, 27 patients were classified as having CDH (1.8% of total cases; 95% CI, 1.1-2.5%). The prevalence of CDH did not differ significantly between men and women (1.4% vs. 2.3%, respectively; p=0.28) (Table 1). The prevalence of CDH was higher among subjects with up to a middle-school level of education than among those with at least a high-school level of education. Of the 27 subjects with CDH, 6 (22.2%) reported that the headache had little or no impact, 8 (29.6%) reported some impact, 5 (14.8%) reported a substantial impact, and 8 (33.3%) reported a severe impact. The clinical characteristics of the subjects with CDH are summarized in Table 2. Seven (25.9%) of the subjects with CDH met the criteria for MO, of which three overused symptomatic prescription drugs, two overused acetaminophen, and one overused aspirin; the remaining subject did not know the name of the overused drug. Eighteen (66.7%) subjects with CDH were diagnosed with CM.

Of the subjects with CDH, 11 (40.7%) had previously consulted a doctor for their headaches. The reported HIT-6 score was higher in subjects with CDH who had participated in a medical consultation than in those who had not (61.7±9.3 vs. 53.7±8.7, p=0.04). CDH subjects with MO were more likely to have consulted a doctor than were CDH subjects without MO (87.5% vs. 25.0%, p<0.01). Headache frequency per month (28.3±3.7 vs. 27.9±5.2, p=0.39) and the score on a visual analogue scale for headache intensity (6.2±1.1 vs. 5.1±2.0, p=0.06) did not differ significantly between individuals who had and who had not participated in a medical consultation.

Of the subjects with CDH during the previous year, 11 subjects (40.7%) had not treated their headaches, 9 (33.3%) subjects had treated their headaches with prescription drugs after visiting a doctor, and 9 subjects (33.3%) had treated them with OTC medications. Among the nine CDH subjects who treated their headaches with OTC medications, two (7.4%) had used both OTC medications and prescription drugs, and seven (25.9%) had only used OTC medications.