Disease mechanisms in neuropathic itch

Itch, also known as pruritus, is an unpleasant cutaneous sensation that provokes the desire to scratch. Itch is a common symptom of inflammatory skin disorders, but it can also occur in neurological diseases associated with injury to nervous tissue, in the absence of any skin disease and without any notable physiological stimuli in the periphery. This 'neuropathic' type of itch occurs either in combination with neuropathic pain or independently and is thought to be underdiagnosed. In this Review, we describe the physiological characteristics of specific neuronal systems in the PNS and CNS that transmit and process pruriceptive information, and we consider pathological changes that occur in these systems after nerve lesions. We then introduce a classification system for itch and highlight the similarities and differences between neuropathic itch and neuropathic pain. A summary of neuropathic syndromes in the PNS and CNS that are associated with itch is presented. Finally, we propose appropriate treatment strategies for neuropathic itch, in view of the fact that this condition has different mechanisms of itch generation to other types of itch and consequently requires different therapies.