The classification of chronic headache: Room for further improvement?

Introduction

The International Headache Society's (IHS) classification of and diagnostic criteria for headache disorders [ICHD-I; ICHD-II] has been adopted widely, fulfilling one of the main goals of the Classification Committee (1, 2). The purpose of this paper is to discuss the terminology and classification of chronic headache and propose suggestions for improvement.

Chronic headache or chronic daily headache?

Mathew et al. first coined the term ‘chronic daily headache’ (CDH) to describe the occurrence of headaches that occurred almost daily (3). The term CDH is now well entrenched in the paediatric and adult headache literature. Recent examples of usage include the papers by Midgette and Scher (4) and Sun-Edelstein et al. (5). More importantly, CDH is an apt description of what patients and families present with (‘I have headaches every day’; ‘she has headaches every day’). Patients and care-givers can relate easily to the diagnosis. As far as possible, medical terms must reflect the common experience. Thus, there are several reasons why the IHS should formally accept the simple but explicit term ‘chronic daily headache’, as Silberstein et al. initially (6), and Solomon more recently (7), suggested. Both ‘chronic headache’ and ‘chronic daily headache’ are syndromic terms, with the latter offering more clarity.

Definition of chronic in CDH

Silberstein et al. defined CDH as the occurrence of headache for at least 4 h/day, at least 15 days/month, for a month or more (6). CDH was not discussed in ICHD-II, but ‘chronic’ was defined as persistence (of headache) for > 3 months (2). ‘Chronic’ is used inconsistently within ICHD-II (2) and ICHD-IIR (8). Examples include: (ia) for chronic migraine (code 1.5.1): ‘headache occurring on 15 or more days per month for more than 3 months . . .’; (ib) revised chronic migraine (appendix 1.5.1): ‘headache . . . on ≥ 15 days per month for at least 3 months’; (ii) for chronic tension-type headache (CTTH; code 2.3): ‘headache occurring on ≥ 15 days per month on average for > 3 months (≥ 180 days per year)’; (iii) for trigeminal autonomic cephalalgias (code 3) the disorder has to be ‘unremitting’ for more than a year to be designated ‘chronic’; and (iv) for new daily-persistent headache (NDPH; code 4.8): A. ‘Headache > 3 months’ and B. ‘Headache is daily and unremitting from onset or from < 3 days from onset’. Would it not be more consistent to define any chronic (daily) headache type, including NDPH, unambiguously as: headache occurring ≥ 15 days/month for ≥ 3 months? Those with such headaches for 1–3 months could be labelled as acute or sub-acute as consensus dictates.

Silberstein et al. acknowledged that the 4 h/day requirement was arbitrary (6). Hence, consideration should also be given to subclassifying the duration as follows: daily ultra-short paroxysms (lasting seconds to < 10 min), daily short paroxysms (lasting 10–30 min) and daily longer paroxysms (> 30 min).

New appendix criteria for chronic migraine (8): a violation of fundamental principles?

ICHD-I and -II remind us that patients often have more than one type of headache (1, 2). Therefore, appendix criterion A for definition of chronic migraine (8) is a violation of the fundamental principle that each type of headache be classified separately. Criterion A reads: ‘Headache (tension-type and/or migraine) on ≥ 15 days per month for at least 3 months’. Dr Solomon questioned the validity of the definition and asked ‘Why call it “chronic migraine” when the majority of headaches are usually tension-type?’ (7).

The assimilation of TTH within the concept of migraine is not evidenced-based (9). The committee (8) used information from one study (10) to suggest that ‘a number of headaches in migraineurs fulfilling tension-type criteria may in fact be mild migraine attacks’. Only 215 of 311 completed the study; the confidence intervals for 4 h postdose response to sumatriptan (10) were rather large [examples: migrainous 3.81 (CI 1.43, 10.12); episodic TTH, 3.62 (CI 2.13, 6.13)], suggesting that the sample size was relatively small. Hence, any opinion that TTH in migraineurs is actually mild migraine is speculative. Manack et al. have recently drawn attention to the continuing lack of globally accepted criteria for chronic migraine (11).

Until there are biological markers to support the committee's opinion, chronic migraine should be restricted to headache that fulfils the accepted criteria for migraine. Those who have both migraine and TTH should be classified into a mixed category distinct from those who are subject to only migraine or TTH attacks. The mixed category could be merged with chronic migraine once more definitive proof for commonality becomes available

CDH deserves to be a major group (and have its own code) in its own right

Under ICHD-II, the subtypes of chronic headache appear in an unsystematic manner under categories such as migraine, TTH, cluster headache, other primary headaches, etc. CDH is now a recognized and accepted clinical headache syndrome in adults and children. It is to headache what status epilepticus is to epilepsy. Status epilepticus, a syndrome, is classified by cause and seizure type. CDH can also be subclassified by cause (primary/secondary) and headache type. Such a classification within the existing ICHD framework would reflect the clinical setting and serve research and clinical practice, an important objective of the IHS.

Time to embrace a multi-axial classification approach for headache disorders including CDH?

Headache disorders, including CDH, are frequently comorbid with anxiety and mood disorders; there are also many factors, such as adverse life events/stressors and medication overuse, that either precipitate or contribute to CDH. (9,12,13). Sun-Edelstein et al. suggest that ‘the designation of medication overuse headache as a separate entity in the ICHD-II is controversial’ (5). The use of axes as in psychiatry (14) will ensure that all important clinical data are captured. A multi-axial approach ‘facilitates comprehensive and systematic evaluation with attention to the various (mental) disorders and general medical conditions, psychosocial and environmental problems, level of functioning that might be overlooked if the focus were on assessing a single presenting problem . . .’. Such an approach also ‘promotes application of the biopsychosocial model in clinical, educational and research settings’ (14). A tentative axes classification (9) is shown in Table 1.

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Table 1.

Proposed axes for classification of CDH*

Axis I: Main headache category (CDH)

Axis II: Subtype/s (examples: chronic migraine, CTTH, etc.)

Axis III: Associated conditions (examples: anxiety or mood disorders, substance abuse, obesity, etc.)

Axis IV: contributory factors [examples: adverse life events/stressors—may need to be specified; medication overuse (specify); noise; light; caffeine, etc.]

Axis V: Functional impairment (specify sphere of activity and use standardized tool/s)

Axis VI: Pain severity (use standardized tools/measure; note that there may be dissociation between pain severity and functional impairment and hence these have to be listed separately)

Conclusion

We urge the IHS to include a distinct category labelled ‘chronic daily headache’, within the classification of headache disorders and also adopt a multi-axial classification approach to headache. In 1999, Nappi et al. opined that the existence of CDH and its importance in clinical practice should not continue to be ignored (15). They suggested that the IHS set up a subcommittee ‘entirely devoted to the classification of CDH’, a recommendation we wholeheartedly endorse. An evaluation of the axes classification scheme would be part of such a committee's mandate. Although our commentary reflects the opinions of paediatric headache specialists, the general principles underlying our discussion should apply also to adults. Our communication is in keeping with the philosophy of Professor Olesen, who has always encouraged vigorous debate to improve classification.