Neuropathic itch

Abstract

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.

Figure 1. A patient with chronic itch and scratching-induced lesions on her distal legs

negligible sweat production from the forearm, proximal leg, distal leg and foot sites after iontophoresis of acetylcholine, reflecting dysfunction of her cholinergic sudomotor sweat fibers, consistent with small-fiber polyneuropathy.

Figure 2. A man with trigeminal trophic syndrome

This 63-year-old developed left trigeminal itch and pain after left Wallenberg’s syndrome (dorsolateral medullary infarction). His trigeminal trophic syndrome (TTS) was maintained by his scratching. Reproduced with permission from Elsevier.