IntroductionChronic daily headache (CDH) occurs in almost 2% of general population. Although CDH is the most frequent headache disorder observed in headache centers [1], it is not clearly classified by International Headache Society (IHS) diagnostic criteria [2]. The clinical variability characterizing CDH suggests that the underlying causes are heterogeneous.The pathophysiology of chronic daily headache is still unclear. Clinical research of basic mechanisms is often inconclusive for the variability of the involved factors. Furthermore, there is a recent trend to use neurophysiological tests in the investigation of functional disorders in brain structures. These techniques permit the selective investigation of well-known circuits in the peripheral nervous system, spinal cord and brainstem, although other than expected neurophysiological responses may occur as a consequence of modulation in the complex investigated pathways. In order to evaluate the pathophysiology of chronic daily headache, several methodologies have been employed.
ReflexologyNociceptive flexion reflex An objective method for measuring pain in humans is represented by the nociceptive flexion reflex (NFR). NFR has two successive components (RII and RIII). The RIII response is associated with pain sensation localized to the stimulation site. Nociceptive flexion reflexes are related to A-delta fiber activation. In normal subjects the value of the reflex threshold is strictly related to that of subjective pain perception.Sandrini and co-workers [3] observed a reduction of RIII reflex threshold, strictly related to the severity of the Abstract The pathophysiology of chronic daily headache is unclear. Clinical research aiming to elucidate causative mechanisms is often confounded by the variability of factors involved. Furthermore, there is a recent trend to use neurophysiological tests in the investigation of functional disorders in brain structures. In order to evaluate the pathophysiology of chronic daily headache, several methodologies have been employed. The aim of the present review is to relate the obtained results in the most frequently used neurophysiological tests to possible pathophysiological mechanisms of chronic daily headache. The neurophysiological findings suggest that a common neuronal dysfunction predisposes to primary headaches subtypes, whose chronicization may depend upon many factors, such as the influence of peripheral muscular factors and the dysfunction in central nociceptive modulation. These factors may explain the large variability observed in clinical features.