Neuropathic Itch, its neurophysiology and putative treatment
Neuropathic Itch is a result of a damage of the nerves
At the plenary session at the NeuPSIG 2017 at Gothenburg, Sweden a topic discussed was Neuropathic Itch. A state of the art lecture was given by Sarah Ross, USA
In PubMed only few entries can be found on neuropathic itch, and no clear therapies are known.
Many neuropathic pain conditions have an itch component, such as post herpetic neuralgia and various neuropathic pain conditions. Itch and Pain are both sides of the same coin. Itch is always related to the body surface. Many aversive chemicals are not specific, and can result either pain or itch. So the question would be, how can itch and pain be differentially coded?
Perhaps lateral interaction at the spinal cord level could differentiate between pain and itch, a simple circuit model. Several neurobiological experiments indeed could support the existence of such inhibition neurons (B5-I neurons). Kappa antagonists indeed can enhance itch, supporting the dynorphin hypothesis. Separate studies, transplanting certain cells, also supported the idea of inhibitory neurons.  Nalfurafine, a kappa opioid is on the market for itch in Japan.
Neuropathic Itch could be related to dynorphin at a interneuron in the spinal cord
The treatment of neuropathic itch theoretically would be quite served based on more attention for phenytoin cream. Phenytoin, as a broad acting sodium channel blocker, has already been described as a efficient and safe topical treatment in various neuropathic pain conditions.