This chapter sets out a classification system for painful lesions of the cranial nerves and other facial pains based on a consensus between the International Headache Society (IHS) and the International Association for the Study of Pain (IASP).
The existing nosology of cranial-nerve pains does not fully portray the subtle differences between various conditions. However, rather than abandoning many long-established diagnostic terms, this classification retains them, providing detailed definitions for differential diagnoses and their types, subtypes and subforms.
Afferent fibres in the trigeminal, intermedius, glossopharyngeal and vagus nerves, in addition to the upper cervical roots via the occipital nerves, convey nociceptive input to central pathways in the brainstem and to the brain areas that process nociception and pain in the head and neck. The brain perceives pain in the innervated area.
The pain may manifest in any of many distinct forms that are believed to reflect differences in neural pathophysiologies, even if the details are not well known. What is known is that neuropathic facial pains can be classified on the basis of their distinct clinical characteristics and aetiology. Central to this concept is initial determination clinically of the main diagnostic group into which the patient’s pain best fits, to be followed by aetiological investigations for diagnostic types and subtypes and therapeutic decision-making.
There are several axes of classification.
a) Syndromology: neuralgia or neuropathy
The division between, for example, trigeminal neuralgia and trigeminal neuropathy should be viewed as a pragmatic way of distinguishing conditions in which clinical presentations and treatment approaches differ while the two conditions cannot be classified on the basis of currently known pathology or pathophysiology. The same applies to painful conditions associated with the glossopharyngeal and intermedius nerves.
An important cause of cranial-nerve pain is herpes zoster. Despite the fact that trigeminal pain following herpes zoster probably leads to different types of pathological change in trigeminal pathways (ie, “irritable nociceptor” versus “deafferentation” type), available data are too limited to classify them as neuralgia versus neuropathy. Therefore the well-established term post-herpetic neuralgia is maintained.
b) Location: central or peripheral neuropathic pain
A lesion or undue activation of these nerves (peripheral neuropathic pain), or of their central pathways (central neuropathic pain), causes neuropathic pain in the face.
c) Aetiology: classical, idiopathic or secondary
The cause of a neuropathic pain may be clear, such as infection by Varicella zoster virus or a structural abnormality (eg, multiple sclerosis plaque) demonstrated by imaging: such pain is termed secondary, and attributed to the cause. In other cases no cause is apparent (termed idiopathic).
For the trigeminal, glossopharyngeal and intermedius neuralgias, the term classical is reserved for cases where imaging or surgery has revealed vascular compression of the respective nerve. Strictly speaking, classical neuralgias are secondary (to the neurovascular compression), but it is beneficial to separate them from other causes on the basis of the wider therapeutic options and potentially different nerve pathophysiology.