Present diagnostic criteria are too restrictive. We suggest adding the site of pain in the criteria, as described for trigeminal autonomic cephalalgias. Cranial autonomic features, a sense of restlessness during exacerbations, and marked to complete response to indomethacin are three characteristics features of HC. We suggest that the presence of any two may be sufficient to diagnose HC.

The neurobiology of chronic pain, including chronic daily headache (CDH) is not completely understood. “Pain memory” hypothesis is one of the mechanisms for phantom limb pain. We reviewed the literature to delineate a relation of “pain memory” for the development of CDH. There is a direct relation of pain to memory. Patients with poor memory have less chance to develop “pain memory”, hence less possibility to develop chronic pain. Progressive memory impairment may lead to decline in headache prevalence. A similar relation of pain is also noted with emotional or psychiatric symptoms. Literature review suggests that there is marked overlap in the neural network of pain to that of memory and emotions. We speculate that pain, memory, and emotions are interrelated in triangular pattern, and each of these three is related to other two in bidirectional pattern, i.e., stimulation of one of these will stimulate other symptoms/networks and vice versa (triangular theory for chronic pain). Longstanding or recurrent noxious stimuli will strengthen this interrelation, and this may be responsible for chronicity of pain. Reduction of both chronic pain and psychological symptoms by cognitive behavioral therapy or psychological interventions further suggests a bidirectional interrelation between pain and emotion. Longitudinal studies are warranted on the prevalence of headache and other painful conditions in patients with progressive memory impairment to delineate the relation of pain to memory. Interrelation of headache to emotional symptoms should also be explored.

Post-infectious disease syndrome includes both neurological and non-neurological disorders. However, headache as an isolated or a presenting complaint of post-infectious illness has not been well acknowledged in the literature. In this retrospective observation, patients having daily headache of more than 1 week and <4 weeks duration were included. We divided this group into patients having headache with preceding history of febrile illness in the recent past and patients without such history of febrile illness. We compared clinical features and therapeutic responses of various drugs between the groups. There were no significant differences in demographic features in these groups. However, associated neck pain, nausea, photophobia and meningeal signs were more prevalent in patients having history of preceding infection. A relatively lower proportion of subjects showed complete response to drugs at 3 months in post-infectious group. Good responses were noted to steroids in post-infectious group. In conclusion, a subset of patients with daily headache may be because of post-infectious pathology and treatment in the early stage may prevent it from becoming chronic. Large prospective studies are required to confirm these observations.