Functional and structural neuroimaging in trigeminal autonomic cephalalgias

The trigeminal autonomic cephalalgias (TACs) are a group of primary headache disorders characterized by unilateral trigeminal distribution pain that occurs in association with ipsilateral cranial autonomic features. They include cluster headache, paroxysmal hemicrania, and short-lasting unilateral neuralgiform headache with conjunctival injection and tearing. Until recently, primary headache disorders, including the TACs, were widely considered to be caused by peripheral mechanisms such as vascular changes or neurogenic inflammation. Developments in neuroimaging are revolutionizing our understanding of the pathophysiology of primary headache syndromes. Functional imaging studies have demonstrated hypothalamic activation in all the TACs. Furthermore, neuroimaging studies using voxel-based morphometry and magnetic resonance spectroscopy techniques have demonstrated structural and biochemical alterations, respectively, in the hypothalamus of patients with cluster headache. These studies suggest that the hypothalamus plays a crucial role in the pathophysiology of TACs, thereby supporting the notion that these disorders are primarily due to central rather than peripheral mechanisms.