Neuropathic itch.

Chronic itch can be caused by dysfunctions of itch-sensing neurons that produce sensory hallucinations of pruritogenic stimuli. The cellular and molecular mechanisms are still unknown. All neurological disease categories have been implicated, and neurological causes should be considered for patients with otherwise-unexplained itch. The same neurological illnesses that cause neuropathic pain can also or instead cause itch. These include shingles (particularly of the head or neck), small-fiber polyneuropathies, radiculopathies (eg, notalgia paresthetica and brachioradial pruritis), and diverse lesions of the trigeminal nerve, root, and central tracts. Central nervous system lesions affecting sensory pathways, including strokes, multiple sclerosis, and cavernous hemangiomas, can cause central itch. Neuropathic itch is a potent trigger of reflex and volitional scratching although this provides only fleeting relief. Rare patients whose lesion causes sensory loss as well as neuropathic itch can scratch deeply enough to cause painless self-injury. The most common location is on the face (trigeminal trophic syndrome). Treating neuropathic itch is difficult; antihistamines, corticosteroids, and most pain medications are largely ineffective. Current treatment recommendations include local or systemic administration of inhibitors of neuronal excitability (especially local anesthetics) and barriers to reduce scratching.