Why We Itch
Consider the famous case study which formed the basis for a New Yorker magazine piece. One year after developing ophthalmic herpes zoster (shingles), affecting the skin above the right eye, a woman developed an uncontrollable itch which resisted all treatment. Her hands and scalp were bandaged. Despite all this, she was able to scratch through the scalp and, incredibly, the skull to the point that her brain protruded through a large skull defect.
Itch can be dermatological, such as atopic dermatitis; systemic, such as in renal failure; psychogenic, such as in schizophrenia with delusions of insect infestation; and neuropathic such as in shingles, spinal nerve root compression, or stroke. Itch can be a phantom limb syndrome after an amputation. It may be the presenting symptom of malignancies, including lymphoma. Opioids commonly cause itch.
Unlike pain, itch can be felt only in the skin or mucosa lining the body’s entrances. Repeated scratching from neuropathic itch can discolor the skin and lead erroneously to a search for a dermatological cause.
An interesting neuropathic itch is “notalgia paresthetica,” affecting a large area in the back around the lower shoulder blade. This is a difficult spot to reach by hand and sufferers tend to rub their backs against door frames (think bears).
Unsurprisingly, treatment for neuropathic itch is difficult. Scratching temporarily relieves the itch by augmenting sensory input and becomes self-reinforcing. Breaking this cycle is critical. Cognitive behavioral therapy or medication can help. Medications that treat epilepsy and neuropathic pain can be tried. Antihistamines can help allergic but not neuropathic itch. Occlusive patches can help.
None of this, however, helped the woman with the protruding brain.
See Lancet Neurology, Vol 17, Aug 2018. Lead author is Steinhoff.
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