Itch and analgesia resulting from intrathecal application of morphine: contrasting effects on different populations of trigeminothalamic tract neurons.

Intrathecal application of morphine is among the most powerful methods used to treat severe chronic pain. However, this approach commonly produces itch sufficiently severe that patients are forced to choose between relief of pain or itch. The neuronal populations responsible for processing and transmitting information underlying itch caused by intrathecal application of morphine have not been identified and characterized. We describe two populations of antidromically identified trigeminothalamic tract (VTT) neurons in anesthetized rats that are differentially affected by morphine and explain several aspects of opioid-induced itch and analgesia. We found that intrathecal application of morphine increased ongoing activity of itch-responsive VTT neurons. In addition, intrathecal application of morphine increased responses to pruritogens injected into the skin and greatly heightened responses to innocuous mechanical stimuli. In contrast, the ongoing activity and responses to noxious pinches in nociceptive VTT neurons were frequently inhibited by the same dose of morphine. These results reveal that i.t. application of morphine affects specific subpopulations of VTT neurons in ways that may produce itch, hyperknesis, alloknesis, and analgesia.

The trigeminal trophic syndrome: an unusual cause of face pain, dysaesthesias, anaesthesia and skin/soft tissue lesions.

The trigeminal trophic syndrome is an unusual consequence of trigeminal nerve injury that results in facial anaesthesia, dysaesthesia and skin ulceration. Limited knowledge is available. The aim of this study was to increase the knowledge of this syndrome by performing a retrospective medical record review and case series report. Fourteen cases were identified. The female:male ratio was 6:1. Mean age of onset was 45 years (range 6-82). The cause was iatrogenic in most. Latent period to onset ranged from days to almost one decade. The majority (n = 12) had bothersome dysaesthesias. Most (n = 9) self-manipulated the face; a third (n = 5) did not. Most ulcers affected the second trigeminal division, mainly in the infraorbital nerve distribution. Neuropathic and/or neuralgic facial pain occurred in 50% (n = 7). Pain intensity was severe in most (n = 6). Gabapentin gave relief in two. To conclude, trigeminal trophic syndrome follows injury to the trigeminal nerve or its nuclei. For unclear reasons, most ulcerations follow infraorbital nerve distribution. Self-manipulation may contribute to ulcer development rather than being required. Gabapentin may help pain.