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The Neurology of Itch

The neurology of itch.
Dhand A1, Aminoff MJ. Brain. 2014 Feb;137(Pt 2):313-22
Abstract: Research over the past 15 years has helped to clarify the anatomy and physiology of itch, the clinical features of neuropathic itch syndromes and the scientific underpinning of effective treatments. Two itch-sensitive pathways exist: a histamine-stimulated pathway that uses mechanically insensitive C-fibres, and a cowhage-stimulated pathway primarily involving polymodal C-fibres. Interactions with pain continue to be central to explaining various aspects of itch. Certain spinal interneurons (Bhlhb5) inhibit itch pathways within the dorsal horn; they may represent mediators between noxious and pruritic pathways, and allow scratch to inhibit itch. In the brain, functional imaging studies reveal diffuse activation maps for itch that overlap, but not identically, with pain maps. Neuropathic itch syndromes are chronic itch states due to dysfunction of peripheral or central nervous system structures. The most recognized are postherpetic itch, brachioradial pruritus, trigeminal trophic syndrome, and ischaemic stroke-related itch. These disorders affect a patient's quality of life to a similar extent as neuropathic pain. Treatment of neuropathic itch focuses on behavioural interventions (e.g., skin protection) followed by stepwise trials of topical agents (e.g., capsaicin), antiepileptic drugs (e.g., gabapentin), injection of other agents (e.g., botulinum A toxin), and neurostimulation techniques (e.g., cutaneous field stimulation). The involved mechanisms of action include desensitization of nerve fibres (in the case of capsaicin) and postsynaptic blockade of calcium channels (for gabapentin). In the future, particular histamine receptors, protease pathway molecules, and vanilloids may serve as targets for novel antipruritic agents. Free Full Text.

Neurogenic Pruritus: An Unrecognised Problem?

Full Article at British Medical Journal

Summary
Intractable localised segmental pruritus without a rash has been reported over the years under various titles depending on the area of the body affected. Notalgia paraesthetica and brachioradial pruritus are the two terms used for what is believed to be a form of neuropathy. The clinical observations reported here suggest that other localised cases of pruritus exist that share common clinical features, and the term neurogenic pruritus is suggested to encompass these under one clinical condition. Acupuncture has been used to treat skin conditions, of which pruritus is one symptom. This retrospective study looked at the symptomatic relief of neurogenic pruritus in 16 patients using acupuncture. In 12 cases the affected dermatomes of the body were innervated by cervical spinal nerves, seven innervated by dorsal spinal nerves and four innervated by the lumbar spinal nerves. Seven patients had areas affected by two different regions of the spine. Restricted neck or back movements were noted in patients as were areas of paravertebral spasm or tenderness of the muscles. Total resolution of symptoms as judged by VAS occurred in 75% of patients. Relapse occurred in 37% of patients within 1-12 months following treatment. Acupuncture appeared to be effective in alleviating the distressing symptom of itching in patients presenting with neurogenic pruritus

Notalgia paresthetica

By Dr. Carolyn Ellis, Largo Medical Center, Largo, Florida

The 57 year-old woman pictured below has had episodic pruritus of the mid-back for six years.  She denied axny apparent cause for the pruritus.  At times there was an accompanying sensation of 'pins and needles'.   She also complained of heightened sensitivity in the area, which she noticed when putting on clothes or rubbing her back against the bed.  She located the affected area medial to the left scapula.  She could not identify anything that improved or worsened the symptoms.

Notalgia Paresthetica (NP) is an ecxeeding common disorder.  Dr. Carolyn Ellis has written a fine introduction to NP. See: Download NP edit

Typical Distribution of NP

Subtle Hyperpigmentation and Lichenification

 

Dr. Ellis' Teaching Points

 

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