Rafael Benoliel: The trigeminal nerve supplies all sensory innervation to the head and associated structures. This is Âour nerve, and it is inevitably involved in the multitude of painful disorders that we manage on a daily basis. Despite distinct embryogenic origins and processing capabilities, trigeminal neurons and cell bodies should not be significantly different from their spinal counterparts. The anatomical set-up is similar, with afferent neuronal cell bodies, both spinal and trigeminal, located in the dorsal root (DRG) and the trigeminal (TG) ganglia, respectively. The DRG and TG structures are similar, and they are essentially homologs of each other. The clearest anatomical differences are the proximity of the trigeminal system to the CNS and the relatively long trajectory of afferent axons in the spinal system. Additionally, the TG is the only sensory ganglion of the body that resides within the CNS. Clinical and laboratory observations, however, suggest differences between the trigeminal and spinal systems that may underlie dissimilar functional properties and response to injury or disease. There may be subtle differences between these two systems, the basis and consequences of which are still unclear. One interesting difference: The trigeminal system is the only nerve involved in spontaneous denervation during shedding of deciduous teeth. Is that indicative of any differences that may be of consequence? Evidence in animal models suggests that the trigeminal nerve is more resistant than the spinal system in developing neuropathic pain (NP) following insult or disease. Moreover, sprouting of sympathetic nerves around large ganglionic neurons following injury is not observed in the TG, but is present in the DRG, and cervical sympathectomy does not alter trigeminal NP behavior, but does so in the spinal system. Together with a rich vascular supply to trigeminally innervated regions (ie, the trigeminovascular system), these may explain some differences in the clinical phenotypes of trigeminal vs spinal pain syndromes. For example, clinically diabetic neuropathy is a common source of NP in the limbs and trunk involving classical signs of paresthesia and allodynia. In the trigeminal system, these effects are not pronounced, and there is a sparsity of reports of typical painful diabetic neuropathic pain or indeed neuropathy at all. Insults to trigeminal nerve branches such as microinjuries (eg, tooth extraction, root canal treatment) or macroinjuries rarely result in chronic NP and are consistently reported at a lower prevalence than following injury to the spinal system. Complex regional pain syndrome, a disabling painful disorder resulting from mild to severe nerve injury that commonly presents in the extremities and is accompanied by sensory, vascular, and muscular problems, has no true equivalent with all of these diagnostic features in the trigeminal system. Conversely, pain syndromes such as migraine, cluster headache, and trigeminal neuralgia exist only in the trigeminal system. Taken together, these findings suggest that the spinal and trigeminal systems may differ, particularly in the way they respond to injury. How and why remain a mystery. Gene expression data may offer some insights. Studies in naïve animals reveal differences in expressed genes between the DRG and the TG, indicating that different molecular mechanisms are involved in the baseline functionality of the two systems. The animal data seem to align well with human data on the DRG and TG. It has recently been shown that spinal and trigeminal neuropathies caused by trauma are accompanied by differentially regulated genes within the DRG and TG. The genes involved suggest that neuroinflammatory signaling and other related pathways are involved. These studies are limited due to the fact that they are Âwhole-ganglion analyses and do not indicate from which type of neuron or nonneuronal cell present in the ganglia the changes originate. Yet, overall, the message is that the trigeminal system is different. As molecular techniques improve, we may be able to use these data to identify protective genes and pathways that may be novel targets for pain intervention and to elucidate some of the fascinating differences in diseases present exclusively in the trigeminal system.<sup>1Â3</sup> Rafael Benoliel Editor-in-Chief
Read full abstract