Prevalence of Secondary Chronic Headaches in a Population-Based Sample of 30-44-Year-Old Persons. The Akershus Study of Chronic Headache

Abstract

We studied secondary chronic headaches (≥ 15 days/month for at least 3 months) in a random sample of 30 000 persons aged 30-44 years. They received a mailed questionnaire. Those with self-reported chronic headache within the last month and/or year were invited to an interview and examination by a neurological resident. The criteria of the International Classification of Headache Disorders (ICHD-II) were applied. The questionnaire response rate was 71%, and the participation rate of the interview was 74%. Of the 633 participants, 298 had a secondary chronic headache. The 1-year prevalence of secondary chronic headache was 2.14%, i.e. chronic posttraumatic headache 0.21%, chronic headache attributed to whiplash injury 0.17%, post-craniotomy headache 0.02%, medication-overuse headache (MOH) 1.72%, cervicogenic headache 0.17%, headache attributed to chronic rhinosinusitis 0.33% and miscellaneous headaches 0.04%. The majority of those with ICHD-II-defined secondary chronic headache had MOH, while about one-third had other secondary headaches often in combination with MOH.

Keywords

Epidemiology general population secondary chronic headaches

Introduction

The International Classification of Headache Disorders (ICHD) divides headaches into primary and secondary forms (1). The most common primary headaches are tension-type headache and migraine, whereas cluster headache and other primary headaches are relatively rare (2–12). The most common secondary headaches are induced by alcohol, fever, hunger and rhinosinusitis (13). Primary and secondary headaches are usually paroxysmal, but sometimes they appear in a chronic form. Among the secondary chronic headaches (≥ 15 days/month for at least 3 months), focus has been on medication-overuse headache (MOH), a world-wide problem and probably the most common cause of secondary chronic headaches (14–20). Studies on chronic headache based on the general population, have shown that medication overuse occurs in 17–62% of those with chronic headache (16–19, 21, 22). Otherwise, epidemiological data on secondary headaches are sparse. A few epidemiological surveys have reported a prevalence of chronic secondary headaches from 0.14% to 0.36% (16, 21, 22). However, these headache forms, including their subtypes, consist of extremely few cases and are not the main target of these surveys. Another epidemiological study has found that the prevalence of cervicogenic headache is 3% (23).

This study reports prevalence on secondary chronic headaches based on a survey of 30 000 people from the general population. To our knowledge, this is the first physician-conducted epidemiological survey providing meticulous data on secondary chronic headaches.

Materials and methods

Sampling

A random age-stratified sample of 15 000 men and 15 000 women, 30–44 years old and residing in the 20 eastern municipalities in Akershus County, was drawn from the National Personal Registry by Statistics Norway. Akershus County has both rural and urban areas and is situated in close proximity to Oslo. Data from Statistics Norway shows that the sampling area was representative of the total Norwegian population regarding age, sex and marital status. Regarding employment, trade, hotel/restaurant and transport were over-represented, whereas industry, oil and gas and financial services were under-represented in the sampling area compared with the total Norwegian population (24). The age range 30–44 years was chosen because the prevalence is relatively higher than in a younger age range, whereas comorbidity of other diseases is less than in an older age range. We have also planned a long-term follow-up of our population, and made the judgement that a cross-sectional sample of people aged 30–44 years would be suitable. Figure 1 shows a flow chart of the study.

Figure 1

Flow chart of study.

Questionnaire

All persons received a mailed questionnaire with a standard letter containing information about the project. Apart from ensuring confidentiality and emphasizing the importance of participation, it was stated that the object was to study headache. The questions ‘How many days during the last month have you had headache?’ and ‘How many days during the last year have you had headache?’ were used to screen for chronic headache. If the questionnaire evoked no response, a second and subsequently a third mail were issued.

Clinical interview, physical and neurological examination

The study took place at the Akershus University Hospital between May and December 2005. Persons with self-reported chronic headache who also consented by adding their telephone number on the questionnaire were invited to a clinical examination. Self-reported chronic headache was defined to be headache occurring ≥ 15 days within the last month and/or headache occurring ≥ 180 days within the last year. Inclusion required Norwegian languages skills. Two neurological residents (R.B.G. and K.A.) experienced in headache diagnostics conducted all interviews and the physical and neurological examinations. Prior to the study both were intensely trained at Copenhagen Headache Clinic as well as at Akershus University Hospital. Furthermore, clinical interviews were video filmed and the headache diagnoses were discussed among the authors. Those unable to meet at the clinic were interviewed by telephone. All headaches were classified according to explicit diagnostic criteria of the ICHD-II, 2004, the revised criteria for MOH, and new appendix criteria diagnosing MOH before ceasing medication overuse (1, 25–27). The diagnosis MOH was substantiated by detailed information on the different headache characteristics given by the ICHD-II as well as detailed information on medication consumption (Table 1). In order to secure precise data on the latter, we enclosed a scheme along with the invitation where the participant filled in details about their medicine consumption. The participants were also asked to bring their medication to the clinic. The ICHD-II criteria were formerly said to be operational, but today the term explicit diagnostic criteria is used synonymously (1).

Table 1

Revised criteria for medication-overuse headache (27)

A Headache present on ≥ 15 days/month

B Regular overuse for > 3 months of one or more acute/symptomatic treatment drugs as defined under sub forms of 8.2 (1)

1 Ergotamine, triptans, opioids or combination analgesic medications on ≥ 10 days/month on a regular basis for > 3 months

2 Simple analgesics or any combination of ergotamine, triptans, analgesics opioids on ≥ 15 days/month on a regular basis for > 3 months without overuse of any single class alone

C Headache has developed or markedly worsened during medication overuse

We defined secondary chronic headache as secondary headache ≥ 15 days/month for at least 3 months, as the ICHD-II do not provide an explicit definition of chronicity. Headache attributed to chronic rhinosinusitis was defined according to the criteria established by the American Academy of Otolaryngology—Head and Neck Surgery (Table 2), adding that the symptom had persisted 12 weeks or more (28). Cervicogenic headache was classified according to the revised criteria of Sjaastad (Table 3), requiring at least three criteria to be fulfilled, not including a diagnostic blockade in the neck (29). History of head trauma and whiplash was assessed by asking questions emphasizing the importance of the close relation in time between the trauma/whiplash and the headache according to the ICHD-II. The participants were further asked about severity of the trauma/whiplash, neck pain, amnesia and loss of consciousness.

Table 2

Definition of rhinosinusitis by the American Academy of Otolaryngology—Head and Neck Surgery (28)

Major factors

Facial pain/pressure

Nasal obstruction/blockage

Nasal discharge/purulence/discoloured postnasal drainage

Hyposmia/anosmia

Purulence in nasal cavity on examination

Fever (acute rhinosinusitis)

Minor factors

Headache

Fever (all non-acute)

Halitosis

Fatigue

Dental pain

Cough

Ear pain/pressure/fullness

Two major factors or one major and two minor factors are required for the diagnosis. Of note, facial pain requires another major factor associated with it for diagnosis, as facial pain plus two minor factors is not deemed sufficient for diagnoses of rhinosinusitis.

Table 3

Definition of cervicogenic headache (29)

Major criteria	I. Symptoms and signs of neck involvement
	Ia. Precipitation of head pain, similar to the usually occurring one: Ia1) by neck movement and/or sustained, awkward head positioning, and/or Ia2) by external pressure over the upper cervical or occipital region on the symptomatic side
	Ib. Restriction of the range of motion in the neck
	Ic. Ipsilateral neck, shoulder or arm pain of a rather vague, non-radicular nature, or—occasionally—arm pain of a radicular nature
	II. Confirmatory evidence by diagnostic anaesthetic blockades
III. Unilaterality of the head pain, without sideshift
Head pain characteristics	IV. Moderate–severe, non-throbbing pain, usually starting in the neck. Episodes of varying duration, or fluctuating, continuous pain
Other characteristics of some importance	V. Only marginal effect or lack of effect of indomethacin. Only marginal effect or lack of effect of ergotamine and sumatriptan. Female sex. Not infrequent occurrence of head or indirect neck trauma by history, usually of more than only medium severity
Other features of lesser importance	VI. Various attack-related phenomena, only occasionally present, and/or moderately expressed when present: (a) nausea, (b) phono- and photophobia, (c) dizziness, (d) ipsilateral ‘blurred vision’, (e) difficulties swallowing, (f) ipsilateral oedema, mostly in the periocular area

It is obligatory that one or more of the phenomena Ia–Ic are present.

Data processing

All questionnaires were scanned using TeleForm v9. Interviews were recorded electronically using spss Data Entry 4.0 (SPSS Inc., Chicago, IL, USA). The statistical analyses were performed using spss Base System for Windows 14.0. Adjusted prevalences were calculated with 95% confidence intervals using the Vassar Collage statistics web page (30). We used χ² test with 5% level of significance.

Ethical issues

The Regional Committees for Medical Research Ethics and the Norwegian Social Science Data Services approved the project.

Results

Questionnaire

The sample size was reduced to 28 871 because of error in the address list (n = 1065), emigration (n = 32), multi-handicap (n = 28), insufficient Norwegian language skills (n = 2) and death (n = 2). The overall response rate of the questionnaire was 71% (20 598/28 871). Women had a significantly higher response rate than men (women 77% vs. men 66%; P < 0.001). The questionnaire response rate among the three different age groups 30–34, 35–39 and 40–44 years increased significantly with age in both sexes (men 60%, 67% and 70%, and women 73%, 77% and 81%, respectively; P < 0.001 for both sexes). Among responders the first, second and third issued questionnaire were replied to by 64%, 23% and 13%, respectively. There was no significant difference between self-reported chronic headache within the last month per year and response to first, second and third issued questionnaire analysed separately by sex.

Clinical interview, physical and neurological examination

Of the 935 with self-reported chronic headache within the last month and/or year, 53 persons did not consent to further contact, and 30 persons did not speak Norwegian. Among the 852 eligible, 139 declined participation and 80 could not be reached by telephone. The resulting participation rate of the interview was 74% (633/852). The age and headache frequency was not significantly different in participants and non-participants. Women had a significant higher participation rate than men (79% vs. 63%; P < 0.001). Among the participants, 77% had an interview and a physical and neurological examination at the clinic, whereas 23% had an interview by telephone. The headache diagnoses were not significantly different in the two groups of participants. Two physicians each conducted half of the interviews and the headache diagnoses were not significantly different in the two groups of participants.

Prevalence

Two hundred and ninety-eight subjects had a secondary chronic headache. Table 4 shows the frequencies of the different subtypes of secondary chronic headaches. The majority had MOH, followed by chronic post-traumatic headache attributed to head injury, chronic headache attributed to whiplash injury, cervicogenic headache and headache attributed to chronic rhinosinusitis. Chronic post-craniotomy headache and miscellaneous secondary headaches constituted 3% of the secondary chronic headaches. Among those with chronic secondary headache, MOH was equally frequent in men and women (85.3% vs. 79.1%, P = 0.34).

Table 4

Frequencies of secondary chronic headaches

	Men N = 68 % (n)	Women N = 230 % (n)	All N = 298 % (n)
Chronic post-traumatic headache attributed to moderate/severe head injury	2.9 (2)	1.3 (3)	1.7 (5)
Chronic post-traumatic headache attributed to mild head injury	11.6 (8)	6.9 (16)	8.0 (24)
Chronic headache attributed to whiplash injury	2.9 (2)	9.5 (22)	8.0 (24)
Chronic post-craniotomy headache	0.0 (0)	1.3 (3)	1.0 (3)
Medication-overuse headache	85.3 (58)	79.1 (182)	80.5 (240)
Cervicogenic headache	10.1 (7)	7.3 (17)	8.0 (24)
Headache attributed to chronic rhinosinusitis	10.1 (7)	16.8 (39)	15.3 (46)
Miscellaneous secondary headaches	1.4 (1)	2.2 (5)	2.0 (6)

The diagnoses are not mutually exclusive.

Table 5 shows the co-occurrence of medication overuse in secondary chronic headaches. About half had co-occurrence of medication overuse. Medication overuse was nearly equally distributed by sex and in the different subtypes of secondary chronic headaches.

Table 5

Co-occurrence of medication overuse in secondary chronic headaches

	Men N = 15 x/n	Women N = 40 x/n	All N = 55 x/n
Chronic post-traumatic headache attributed to moderate/severe head injury	2/2	1/3	3/5
Chronic post-traumatic headache attributed to mild head injury	4/8	6/16	10/24
Chronic headache attributed to whiplash injury	2/2	10/22	12/24
Chronic post-craniotomy headache	—	1/3	1/3
Cervicogenic headache	4/7	8/17	12/24
Headache attributed to chronic rhinosinusitis	5/7	19/39	24/46
Miscellaneous secondary headaches	0/1	0/5	0/6

The diagnoses are not mutually exclusive. x, number with medication overuse; n, number with specific subtype of secondary chronic headache.

Table 6 shows the adjusted 1-year prevalence of secondary chronic headaches. The overall last year prevalence was 2.14%, a prevalence which is more than twice as high in women than in men (2.84% vs. 1.30%; P < 0.001). The prevalences of all the different subtypes of secondary chronic headaches were all higher in women than men.

Table 6

Adjusted last year prevalence of secondary chronic headaches∗

	Men N = 9475 % (95% CI) n	Women N = 11 123 % (95% CI) n	All N = 20 598 % (95% CI) n
Chronic post-traumatic headache attributed to moderate/severe head injury	0.04 (0.007, 0.15)	0.04 (0.01, 0.12)	0.04 (0.01, 0.09)
2	3	5
Chronic post-traumatic headache attributed to mild head injury	0.15 (0.07, 0.30)	0.20 (0.12, 0.32)	0.17 (0.11, 0.26)
8	16	24
Chronic headache attributed to whiplash injury	0.04 (0.007, 0.15)	0.27 (0.18, 0.41)	0.17 (0.11, 0.26)
2	22	24
Chronic post-craniotomy headache	0 (0)	0.4 (0.01, 0.12)	0.02 (0.005, 0.07)
0	3	3
Medication-overuse headache	1.11 (0.89, 1.35)	2.25 (1.99, 2.52)	1.72 (1.55, 1.90)
58	182	240
Cervicogenic headache	0.13 (0.06, 0.28)	0.21 (0.13, 0.34)	0.17 (0.11, 0.26)
7	17	24
Headache attributed to chronic rhinosinusitis	0.13 (0.06, 0.28)	0.48 (0.35, 0.65)	0.33 (0.25, 0.44)
7	39	46
Miscellaneous secondary chronic headaches	0.02 (0.0001, 0.12)	0.06 (0.02, 0.15)	0.04 (0.02, 0.10)
1	5	6
Overall secondary chronic headaches	1.30 (1.07, 1.54)	2.84 (2.57, 3.12)	2.14 (1.96, 2.32)
68	230	298

∗

The prevalence was calculated according to the following formulas: Men: n/N × 267/147 × 100. Women: n/N × 668/486 × 100. All: n/N × 935/633 × 100.

Table 7 shows MOH by age and gender. The prevalence in men was equal in the different age groups, whereas it increased with age in women.

Table 7

Last year prevalence of medication-overuse headache by age groups∗

	Men % (95% CI) n	Women % (95% CI) n	All % (95% CI) n
30–34	1.00 (0.60, 1.60)	1.56 (1.14, 2.12)	1.29 (0.99, 1.66)
16	40	56
35–39	1.25 (0.80, 1.89)	2.41 (2.00, 3.02)	1.81 (1.48, 2.21)
20	65	85
40–44	1.19 (0.78, 1.74)	2.72 (2.20, 3.34)	2.01 (1.67, 2.41)
22	77	99
All	1.11 (0.89, 1.35)	2.25 (1.99, 2.52)	1.72 (1.55, 1.90)
58	182	240

∗

The prevalence was calculated according to the following formulas: Men: n/N × 267/147 × 100. Women: n/N × 668/486 × 100. All: n/N × 935/633 × 100.

Discussion

Methodological considerations

The sample size was chosen to ensure adequate numbers of people with chronic headache for accurate descriptive statistics. As this was an epidemiological survey on headache, among responders to the questionnaire those with headache may be over-represented. However, replies to the first, second and third questionnaires did not imply that this was important in relation to self-reported chronic headache. Furthermore, a previous Danish epidemiological survey based on a short migraine screening questionnaire has found no significant difference in the frequency of migraine among responders and non-responders (4).

Questionnaires are generally not valid for diagnosing headaches (31). However, simple questions such as: ‘Have you ever had migraine?’, ‘Have you ever had tension-type headache?’ and ‘How many days did you have tension-type headache within the last year?’ are more valid, as comparison with physician-conducted interviews showed κ values of 0.77–0.87, 0.74 and 0.77, respectively (4, 10, 31). Thus, our question about unspecified headache frequency is likely to be valid.

Making specific headache diagnoses is not an easy task, especially among those with chronic headache and those with co-occurrence of several headaches. The gold standard is an interview and a physical and neurological examination by a physician experienced in headache diagnostics. For that reason, neurological residents with experience in headache diagnostics conducted our investigation. Since two physicians conducted the investigations, interobserver variation is a possibility. However, the headache diagnoses were equally frequent by both physicians, suggesting that interobserver variation was low. The majority were interviewed at the clinic. The neurological examination did not reveal abnormalities that caused a change in the headache diagnosis. Furthermore, the headache diagnoses were equally frequent in participants interviewed at the clinic and in those interviewed by telephone. This is in accordance with a previous Danish epidemiological survey (4). We preferred to apply Sjaastad criteria rather than the very strict ICHD-II criteria for cervicogenic headache in order to detect the syndrome as broadly as possible. Thus, even with the many methodological challenges, our study population is likely to be representative of the general population.

Medication-overuse headache

We found MOH to be the most prevalent ICHD-II-defined secondary chronic headache, as about four out of five people with secondary chronic headache had it. About half of those with other secondary headache had co-occurrence of MOH. Results of other epidemiological surveys of the general population are shown in Table 8 (16–19, 22, 32–34). The prevalence of MOH is generally two to five times more frequent in women than in men, and four to six times more frequent in adults than in adolescents, whereas it seems that the prevalence stabilizes in the elderly. Although ICHD-II defines MOH as a secondary chronic headache, we do not know whether the chronic headache is secondary to medication overuse or medication overuse is secondary to chronic headache.

Table 8

The prevalence of medication-overuse headache in population-based epidemiological surveys

	Study methods	Participant	Age (year) of participants	Men (%)	Women (%)	Overall (%)
Present study	Questionnaire	20 598	30–44	1.1	2.2	1.7
Interview by physician
Italy (22)	Lay interview	837	≥ 65	–	–	1.7
Interview by physician
The Netherlands (19)	Questionnaire	16 232	25–55	–	–	2.6
Norway (32)	Questionnaire	49 064	≥ 20	0.6	1.2	0.9
Norway (33)	Questionnaire	5 471	13–18	0.2	0.8	0.5
Spain (21)	Questionnaire	4 855	> 14	0.2	2.6	1.4
Interview by physician
Spain (16)	Questionnaire	1 881	> 14	–	–	1.2
Interview by physician
Taiwan (18)	Questionnaire	7 900	12–14	0.1	0.5	0.3
Interview by physician
Taiwan (34)	Questionnaire	1 533	≥ 65	–	–	1.1
Interview by physician
Taiwan (17)	Questionnaire	3 377	≥ 15	0.9	1.3	1.1
Interview by physician

Some of the prevalences were calculated by the authors of this paper.

Head trauma

The annual incidence of head injuries requiring hospital admission is 200/100 000 in Western countries (35, 36). However, many patients with head trauma are not hospitalized. We found that the prevalence of chronic headache attributed to head trauma was 0.21%. The prevalence was higher in women than in men, although the difference was not significant. The traumas were usually mild. A Danish epidemiological survey found that the lifetime prevalence of headache associated with head trauma is 4.65% in men and 2.27% in women (13). Another Danish epidemiological survey found that the lifetime prevalence of migraine associated with a head trauma is 0.77% in men and 1.81% in women (37). Even though men experience headache associated with head trauma more often than women, migraine and chronic headache are experienced more often in women than in men. This is in accordance with earlier observations (38, 39).

Whiplash

Whiplash is caused by sudden acceleration and/or deceleration of the neck (1). Rear-end collisions involve men 3.5 times more frequently than women, and whiplash is experienced by 35% (40). We found that the prevalence of chronic headache attributed to whiplash was 0.03% in men and 0.30% in women. This is an extremely high gender difference considering the preponderance of men involved in accidents. Thus, the whiplash trauma itself is not necessarily related to chronic headache. Earlier studies have concluded that expectation of disability, stress of the situation and pre-existing headache prior to the trauma are more important than the trauma (40, 41).

Headache attributed to chronic rhinosinusitis

Chronic rhinosinusitis occurs in 3.1–3.4% of men and 4.6–5.7% in women (42, 43). A single epidemiological survey has reported chronic rhinosinusitis in two persons, i.e. a prevalence of 0.04% (21). We found that the prevalence of headache attributed to chronic rhinosinusitis was 0.13% in men and 0.48% in women. Thus, many with chronic sinusitis in the general population may not experience chronic headache. However, in clinic populations, where patients undergo functional endoscopic sinus surgery for chronic rhinosinusitis, headache is a common symptom experienced by 73.6% (44).

Cervicogenic headache

Cervicogenic headache occurred in 0.13% of men and 0.21% of women. This is a much lower prevalence than the 3% prevalence found in a Danish epidemiological survey (23). The prevalence is affected by the classification applied. A Portuguese survey found a prevalence of 0.4% applying the criteria of the International Headache Society, 1.0% and 4.6% applying all six or less than six criteria, respectively, from Table 2 (29, 45–47). Our prevalence of cervicogenic headache is low compared with others, even though we required only three of six Sjaastad criteria to be fulfilled in order to acquire the diagnosis. We have no explanation for the low prevalence of cervicogenic headache in our survey.

Conclusion

Secondary chronic headache occurred in 2.1% of people from the general population. The majority had MOH, whereas the prevalence of chronic post-traumatic headache attributed to head and/or whiplash injury, chronic rhinosinusitis and cervicogenic headache was up to 0.5%. A minority had miscellaneous secondary headaches. Overall, women had secondary chronic headache two to three times more frequently than men. Our study emphasizes the importance of focus on MOH, since it is the most prevalent subtype of ICHD-II-defined secondary chronic headaches.

Footnotes

Acknowledgements

This study was supported by grants from East Norway Regional Health Authority and Faculty Division Akershus University Hospital. Senior researcher C.L. was supported by Akershus University Hospital. The authors thank Elisabeth Hossmann for technical assistance, Andre Øien and Øyvind Grimstad for IT support and Akershus University Hospital for providing research facilities.

References

1 Headache Classification Subcommittee of the International Headache Society. The International Classification of Headache Disorders: 2nd edition. Cephalalgia 2004;24 (Suppl. 1):1–160.

Rasmussen

Jensen

Schroll

Olesen

Epidemiology of headache in a general population—a prevalence study. J Clin Epidemiol 1991; 44:1147–57.

Rasmussen

Olesen

Migraine with aura and migraine without aura: an epidemiological study. Cephalalgia 1992; 12:221–8.

Russell

Rasmussen

Thorvaldsen

Olesen

Prevalence and sex-ratio of the subtypes of migraine. Int J Epidemiol 1995; 24:612–18.

Schwartz

Stewart

Simon

Lipton

RB.

Epidemiology of tension-type headache. JAMA 1998; 279:381–3.

Hagen

Zwart

Vatten

Stovner

Bovim

Prevalence of migraine and non-migrainous headache—head-HUNT, a large population-based study. Cephalalgia 2000; 20:900–6.

Lipton

Stewart

Diamond

Reed

Prevalence and burden of migraine in the United States: data from the American Migraine Study II. Headache 2001; 41:646–57.

Sjaastad

Bakketeig

LS.

Cluster headache prevalence. Vaga study of headache epidemiology. Cephalalgia 2003; 23:528–33.

Torelli

Beghi

Manzoni

GC.

Cluster headache prevalence in the Italian general population. Neurology 2005; 64:469–74.

10.

Russell

Levi

Saltyte-Benth

Fenger

Tension-type headache in adolescents and adults: a population based study of 33,764 twins. Eur J Epidemiol 2006; 21:153–60.

11.

Ekbom

Svensson

Pedersen

Waldenlind

Lifetime prevalence and concordance risk of cluster headache in the Swedish twin population. Neurology 2006; 67:798–803.

12.

Scher

Stewart

Liberman

Lipton

RB.

Prevalence of frequent headache in a population sample. Headache 1998; 38:497–506.

13.

Rasmussen

Olesen

Symptomatic and nonsymptomatic headaches in a general population. Neurology 1992; 42:1225–31.

14.

Bigal

Rapoport

Sheftell

Tepper

Lipton

RB.

Transformed migraine and medication overuse in a tertiary headache centre—clinical characteristics and treatment outcomes. Cephalalgia 2004; 24:483–90.

15.

Zeeberg

Olesen

Jensen

Probable medication-overuse headache: the effect of a 2-month drug-free period. Neurology 2006; 66:1894–8.

16.

Castillo

Munoz

Guitera

Pascual

Epidemiology of chronic daily headache in the general population. Headache: J Head Face Pain 1999; 39:190–6.

17.

Fuh

Chen

Juang

Wang

SJ.

Chronic daily headache in Taipei, Taiwan: prevalence, follow-up and outcome predictors. Cephalalgia 2001; 21:980–6.

18.

Wang

Fuh

Juang

KD.

Chronic daily headache in adolescents: prevalence, impact, and medication overuse. Neurology 2006; 66:193–7.

19.

Wiendels

Knuistingh

Rosendaal

Spinhoven

Zitman

Assendelft

Chronic frequent headache in the general population: prevalence and associated factors. Cephalalgia 2006; 26:1434–42.

20.

Diener

Limmroth

Medication-overuse headache: a worldwide problem. Lancet Neurol 2004; 3:475–83.

21.

Colas

Munoz

Temprano

Gomez

Pascual

Chronic daily headache with analgesic overuse: epidemiology and impact on quality of life. Neurology 2004; 62:1338–42.

22.

Prencipe

Casini

Ferretti

Santini

Pezzella

Scaldaferri

Prevalence of headache in an elderly population: attack frequency, disability, and use of medication. J Neurol Neurosurg Psychiatry 2001; 70:377–81.

23.

Nilsson

The prevalence of cervicogenic headache in a random population sample of 20–59 year olds. Spine 1995; 20:1884–8.

24.

25.

Silberstein

Olesen

Bousser

Diener

Dodick

First

The International Classification of Headache Disorders, 2nd edition (ICHD-II)—revision of criteria for 8.2 Medication-overuse headache. Cephalalgia 2005;25:460–5.

26.

26 ERRATUM. Cephalalgia 2006; 26:360.

27.

Olesen

Bousser

Diener

Dodick

First

Goadsby

New appendix criteria open for a broader concept of chronic migraine. Cephalalgia 2006; 26:742–6.

28.

Benninger

Ferguson

Hadley

Hamilos

Jacobs

Kennedy

Adult chronic rhinosinusitis: definitions, diagnosis, epidemiology, and pathophysiology. Otolaryngol Head Neck Surg 2003; 129:1–32.

29.

Sjaastad

Fredriksen

Pfaffenrath

Cervicogenic headache: diagnostic criteria. The Cervicogenic Headache International Study Group. Headache 1998; 38:442–5.

30.

31.

Rasmussen

Jensen

Olesen

Questionnaire versus clinical interview in the diagnosis of headache. Headache 1991; 31:290–5.

32.

Zwart

Dyb

Hagen

Svebak

Stovner

Holmen

Analgesic overuse among subjects with headache, neck, and low-back pain. Neurology 2004; 62:1540–4.

33.

Dyb

Holmen

Zwart

JA.

Analgesic overuse among adolescents with headache: the Head-HUNT-Youth Study. Neurology 2006; 66:198–201.

34.

Wang

Fuh

Liu

Hsu

Wang

Chronic daily headache in Chinese elderly: prevalence, risk factors, and biannual follow-up. Neurology 2000; 54:314–19.

35.

Annegers

Grabow

Kurland

Laws

Jr. The incidence, causes, and secular trends of head trauma in Olmsted County, Minnesota, 1935–1974. Neurology 1980; 30:912–19.

36.

Nestvold

Lundar

Blikra

Lonnum

Head injuries during one year in a central hospital in Norway: a prospective study. Epidemiologic features. Neuroepidemiology 1988; 7:134–44.

37.

Russell

Olesen

Migraine associated with head trauma. Eur J Neurol 1996; 3:424–8.

38.

Nestvold

Lundar

Mowinckel

Stavem

Predictors of headache 22 years after hospitalization for head injury. Acta Neurol Scand 2005; 112:13–18.

39.

Jensen

Nielsen

FF.

The influence of sex and pre-traumatic headache on the incidence and severity of headache after head injury. Cephalalgia 1990; 10:285–93.

40.

Schrader

Stovner

Obelieniene

Surkiene

Mickeviciene

Bovim

Examination of the diagnostic validity of ‘headache attributed to whiplash injury’: a controlled, prospective study. Eur J Neurol 2006; 13:1226–32.

41.

Schrader

Obelieniene

Bovim

Surkiene

Mickeviciene

Miseviciene

Natural evolution of late whiplash syndrome outside the medicolegal context. Lancet 1996; 347:1207–11.

42.

Kvaerner

Tambs

Harris

Mair

Magnus

Otitis media: relationship to tonsillitis, sinusitis and atopic diseases. Int J Pediatr Otorhinolaryngol 1996; 35:127–41.

43.

Chen

Dales

Lin

The epidemiology of chronic rhinosinusitis in Canadians. Laryngoscope 2003; 113:1199–205.

44.

Moretz

Kountakis

SE.

Subjective headache before and after endoscopic sinus surgery. Am J Rhinol 2006; 20:305–7.

45.

Pereira Monteiro

Cefaleia. Estudo epidemiologico e clinico de uma populacão urbana. Thesis, University of Porto 1995.

46.

46. Headache Classification Committee of the International Headache Society. Classification and diagnostic criteria for headache disorders, cranial neuralgias and facial pain, 1st edn. Cephalalgia 1988; 8 (Suppl. 7):1–96.

47.

Sjaastad

Fredriksen

TA.

Cervicogenic headache: criteria, classification and epidemiology. Clin Exp Rheumatol 2000; 18 (Suppl. 19):3–6.