Neuropathic Pruritus - Our Itchy and Scratchy Show

    19/07/2014

    As the summer heats up all sorts of bugs and critters emerge from the bushes. When I am out walking or hiking around with others I am more susceptible to being bitten by mosquitoes than anyone else. I often wake up the next day with multiple itchy lumps around my body, which I fight the urge to scratch constantly. My itchiness, or pruritus, is induced by histamine release at the site of the inflamed skin, due to the insect bite and usually subsides in a few days. I am very lucky, because there are many forms of pruritus which are far more severe and they are classified into dermatological, systemic, neurological and psychogenic.

    One of the most baffling is Neurological Pruritus caused by neuronal or glial damage or by endogenous opioids. While some neuropathies manifest as pain, others manifest as itch sensation but no one knows why one sensation is activated over the other. Just as in pain, where responses can lead to peripheral and central sensitization, allodynia (innocuous stimulus causing pain) and hyperalgesia (small stimulus causing heightened pain), the itch response can lead to alloknesia (innocuous touch stimulating itch) and hyperkinesis (light touch or heat causing increased itchiness). In fact the signals for pain and itch are both carried up the nociceptive unmyelinated C-fibers and thinly myelinated A-delta fibers from the skin to the central nervous system. Receptors for pain (nociceptors) and receptors for itch (pruriceptors) are often localized to the same nerve endings, which explains why neuropathic itch is often accompanied by pain. Furthermore, neuropathic pruritus can be activated by at least two distinct pathways, one mediated by histamine release, activated by capsaicin and one which is PAR-mediated, activated by cowhage.

    Itch is caused by stimulation of nerve endings in the Dermo-epidermal junction

    In the brain and spinal cord, if a lesion occurs somewhere along primary afferent sensory neurons, either due to injury, tumour growth or aneurysms, this can trigger neuropathic pruritus and manifestation of chronic scratching in patients. In certain neurological disorders like multiple sclerosis, where demyelination occurs, pruritus can also appear in patients. In the peripheral nervous system, ganglionopathies such as postherpetic itch, caused by herpes zoster (shingles) is most commonly known to induce neuropathic pruritus and pain. Small-fibre neuropathies, caused by a variety of other aetiologies is also a common cause of itch.

    TTITypical presentation of Trigeminal Trophic Itch syndrome caused by postherpetic itch. The patient scratches himself due to damage in the trigeminal nerve.

    This latest review in Nature Reviews Neurology nicely summarizes the list of known causes for neuropathic itch and some current treatments:

    Clinicians often have a hard time distinguishing between neuropathic pruritus and dermatological disorders. According to the review, proper diagnosis should include skin biopsies to measure IENF density, electromyography and tests for sural nerve conduction. Only when the itch is diagnosed as localized or generalized can a set of opioid, acupuncture or neurostimulation treatments be applied.

    It is interesting that so much research has been done on neuropathic pain in the fields of spinal cord injury and certain neurological diseases but so little attention has been paid to itch. There may be room for development here.