The ICHD definition of ‘facial pain’ should be revised

Abbreviations

AML: acanthiomeatal line

ICHD: International Classification of Headache Disorders

ICOP: International Classification of Orofacial Pain

IOML: infra-orbitomeatal line

OML: orbitomeatal line

V1: First branch of the trigeminal nerve, Nervus ophthalmicus

V2: Second branch of the trigeminal nerve, Nervus maxillaris

V3: Third branch of the trigeminal branch, Nervus mandibularis

Viewpoint

Over the course of the last 30 years, the International Classification of Headache Disorders (ICHD) (1) has proven to be extremely valuable for the scientific and clinical progress in the field of headache medicine. The ever more refined criteria for different headache disorders allowed for their systematic investigation and ultimately played a role in the development of disease-specific treatment regimens.

Compared to the headache field, the investigation and treatment options for facial pain disorders are lagging behind . This is owed to a set of different reasons, of which an important factor was certainly the lack of a comprehensive facial pain classification. With the recent implementation of the first International Classification of Orofacial Pain (ICOP) (2), this field hopefully will see new developments. When harmonizing ICHD and ICOP, we found some inconsistencies when reconsidering the definition of “facial pain” itself.

The ICHD currently defines facial pain as “pain below the orbitomeatal line, anterior to the pinnae and above the neck” (1) and this has been unchanged since its implementation in 1988 (3). By this definition the orbita is included in the face, rendering diseases affecting the orbital region such as glaucoma or even cluster headache as facial pain syndromes.

There is no question that the eyes do belong to the face and its definition. Nevertheless, when discussing pain syndromes and defining and distinguishing overlaps, similarities and differences between primary headaches and primary facial pain syndromes, the ICHD’s current definition of facial pain (1), with the inclusion of the eyes, renders this definition unprecise. The issue becomes important or indeed decisive when facial pain has to be distinguished from headache syndromes (4). Migraine will not be misdiagnosed only because the pain also spreads into the face. But is a facial pain that spreads to the head also a headache or indeed a syndrome in its own right? And how to interpret a facial pain without spread and no headache history, currently defined as type 3 facial pain in the current international classification of facial pain (2) and concomitant literature (4). This question can only be solved when we investigate and compare such diseases (here: Migraine following ICHD-3 and facial pain resembling migraine following ICOP). In order to investigate, we need to distinguish (5). If we do not want to rely on pure perception and verbal estimation of the patient (i.e. “the pain I am feeling is a facial pain, an eye pain or a headache”) we need anatomical landmarks. And for this reason, the orbita should be excluded in our definition of facial pain.

The orbitomeatal line (OML) currently used to define the upper boundary, connects the Porus acusticus externus and the mid-lateral orbital margin (outer canthus) (6.) Possible new upper boundaries could be the infra-orbitomeatal line (IOML) connecting the Porus acusticus externus and the Margo infraorbitalis or the acanthiomeatal line (AML) connecting the Porus acusticus externus and the acanthion (6) (see Figure 1).

Figure 1.

Human skull (OML: orbitomeatal line, IOML: infra-orbitomeatal line, AML: acanthiomeatal line).

One could argue that a definition that only follows anatomical landmarks is unpractical and that actual clinical syndromes are mainly defined by their innervation. This would mean that such a definition would a priori need to differentiate between V1 and V2/V3. If this is strictly followed, a herpes zoster of the tip of the nose (V1) would be defined as a ‘headache’ and a migraine of the temporal area only (V2/V3) would be considered to be ‘facial pain’. An arbitrary line, as suggested in the old classification, does make sense and for the above-mentioned reasons we need to redefine this line.

To include most parts of the maxilla, we suggest that the IOML is the more appropriate upper boundary and we therefore propose the following definition of facial pain: “Pain below the infra-orbitomeatal line, anterior to the pinnae and above the neck”.

Definitions are per se agreements (5,7) and we appreciate that such anatomical landmarks are highly artificial. But we need to have that agreement in order to study these diseases (4).