Facial pain: from animal models to functional neuroimaging studies

This paper summarizes some recent findings on the physiopathology of facial pain. Over the past decade, a number of animal models of facial pain have been developed. Two of these models are discussed in more detail. The model developed by Strassman and Burstein has provided a useful tool for the study of the mechanisms underlying migraine. Single unit recordings revealed that medullary dorsal horn neurons show a long-lasting increased response to dural and cutaneous periorbital mechanical and thermal stimulation after application of inflammatory agents to the dura. In addition, dural and cutaneous receptive fields largely expanded and spontaneous ongoing activity developed. These findings suggest that the extracranial hypersensitivity that is often observed in headache may have an intracranial origin. The second model that is discussed is the model of facial neuropathic pain after infraorbital nerve ligation developed by Vos and colleagues. In contrast to the previous model which is an acute electrophysiological preparation, the infraorbital nerve model is a behavioural model. It can therefore be used both for electrophysiological and behavioural studies. In recent years, a number of functional neuroimaging studies of facial pain appeared. Studies performed in cluster headache patients seem to point to a crucial role of the hypothalamus in this syndrome. However, since hypothalamic activation has also been reported in some other types of (facial) pain, the specificity of the role of the hypothalamus in cluster headache remains to be proven.