Itch has been described for many years as an unpleasant sensation that evokes the urgent desire to scratch. Studies of the neurobiology, neurophysiology, and cellular biology of itch have gradually been clarifying the mechanism of itch both peripherally and centrally. The discussion has been focused on which nerves and neuroreceptors play major roles in itch induction. The “intensity theory” hypothesizes that signal transduction on the same nerves leads to either pain (high intensity) or itch (low intensity), depending on the signal intensity. The “labeled-line coding theory” hypothesizes the complete separation of pain and itch pathways. Itch sensitization must also be considered in discussions of itch. This review highlights anatomical and functional properties of itch pathways and their relation to understanding itch perception and pruritic diseases. Semin Cutan Med Surg 30:64-70 © 2011 Elsevier Inc. All rights reserved.
Atopic dermatitis is a common, pruritic, inflammatory skin disorder. Chronic, localized, or
even generalized pruritus is the diagnostic hallmark of atopic dermatitis, and its management
remains a challenge for physicians. The threshold for itch and alloknesis is markedly
reduced in these patients, and infections can promote exacerbation and thereby increase
the itch. Modern management consists of anti-inflammatory, occasionally antiseptic, as
well as antipruritic therapies to address the epidermal barrier as well as immunomodulation
or infection. Mild forms of atopic dermatitis may be controlled with topical therapies, but
moderate-to-severe forms often require a combination of systemic treatments consisting of
antipruritic and immunosuppressive drugs, phototherapy, and topical compounds. In addition,
patient education and a therapeutic regimen to help the patient cope with the itch and
eczema are important adjuvant strategies for optimized long-term management. This review
highlights various topical, systemic, and complementary and alternative therapies, as well
as provide a therapeutic ladder for optimized long-term control of itch in atopic dermatitis.
Semin Cutan Med Surg 30:71-86 © 2011 Elsevier Inc. All rights reserved.
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.
Semin Cutan Med Surg 30:87-92 © 2011 Elsevier Inc. All rights reserved.
Itch is a complication of liver disease. It is hypothesized that this type of itch is mediated,
at least in part, by increased central opioidergic tone; a peripheral component may coexist.
The role of serotonin, bile acids, substance P, and lipophosphatidic acid and the activity of
the enzyme that generates it, autotoxin, has been proposed in the pathogenesis of itch.
Scratching activity was significantly suppressed in association with the placebo tablet in a
controlled, double-blind study; this finding supports the exploration of the placebo effect on
the itch sensation and the inclusion of behavioral methodology in clinical trials in patients
with this complication of liver disease.
Semin Cutan Med Surg 30:93-98 © 2011 Elsevier Inc. All rights reserved.