Post-traumatic headache: epidemiology and pathophysiological insights

Traumatic brain injury (TBI) is increasingly appreciated to be highly prevalent and deleterious to neurological function 1, 2 . At present no effective treatment options are available, and little is known about the complex cellular response to TBI during its acute phase. To gain novel insights into TBI pathogenesis, we developed a novel closed-skull brain injury model that mirrors some pathological features associated with mild TBI in humans and used long-term intravital microscopy to study the dynamics of the injury response from its inception. Here we demonstrate that acute brain injury induces vascular damage, meningeal cell death, and the generation of reactive oxygen species (ROS) that ultimately breach the glial limitans and promote spread of the injury into the parenchyma. In response, the brain elicits a neuroprotective, purinergic receptor dependent inflammatory response characterized by meningeal neutrophil swarming and microglial reconstitution of the damaged glial limitans. We additionally show that the skull bone is permeable to small molecular weight compounds and use this delivery route to modulate inflammation and therapeutically ameliorate brain injury through transcranial administration of the ROS scavenger, glutathione. Our results provide novel insights into the acute cellular response to TBI and a means to locally deliver therapeutic compounds to the site of injury.

Although the trigeminal nerve innervates the meninges and participates in the genesis of migraine headaches, triggering mechanisms remain controversial and poorly understood. Here we establish a link between migraine aura and headache by demonstrating that cortical spreading depression, implicated in migraine visual aura, activates trigeminovascular afferents and evokes a series of cortical meningeal and brainstem events consistent with the development of headache. Cortical spreading depression caused long-lasting blood-flow enhancement selectively within the middle meningeal artery dependent upon trigeminal and parasympathetic activation, and plasma protein leakage within the dura mater in part by a neurokinin-1-receptor mechanism. Our findings provide a neural mechanism by which extracerebral cephalic blood flow couples to brain events; this mechanism explains vasodilation during headache and links intense neurometabolic brain activity with the transmission of headache pain by the trigeminal nerve.

Recent animal studies on the mechanism of migraine show that intracranial pain is accompanied by increased periorbital skin sensitivity. These findings suggest that the pathophysiology of migraine involves not only irritation of meningeal perivascular pain fibers but also a transient increase in the responsiveness (ie, sensitization) of central pain neurons that process information arising from intracranial structures and skin. The purpose of this study was to determine whether the increased skin sensitivity observed in animal also develops in humans during migraine attacks. Repeated measurements of mechanical and thermal pain thresholds of periorbital and forearm skin areas in the absence of, and during, migraine attacks enabled us to determine the occurrence of cutaneous allodynia during migraine. Cutaneous allodynia is pain resulting from a nonnoxious stimulus to normal skin. In 79% of the patients, migraine was associated with cutaneous allodynia as defined, and in 21% of the patients it was not. The cutaneous allodynia occurred either solely within the referred pain area on the ipsilateral head, or within and outside the ipsilateral head. Cutaneous allodynia in certain well-defined regions of the skin during migraine is an as yet unreported neurological finding that points to hyperexcitability of a specific central pain pathway that subserves intracranial sensation.