Neurological Itches and Why Scratching Doesn't Help

Causes and Burden of Itching from Your Nervous System

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A young woman scratches her neck. Getty Images
“Fill what’s empty, empty what’s full and scratch where it itches.”
- Wallis Simpson, Duchess of Windsor

In a famous case in 2008, a woman with itching caused by shingles on her head scratched so furiously that she went through her skull and into her right forebrain.  Although aside from the itch she was neurologically and psychiatrically well, she needed to be physically restrained to prevent injuring herself further.

In a seeming paradox, a skin biopsy of the itching area showed almost no nerve fibers in the affected region.  Where did this terrible itch come from?

What Exactly is an "Itch"?

Despite its universality, itch has been relatively poorly investigated compared to pain or other sensations.  One of the best definitions of itch is an annoying sensation that provokes a scratch response, which can sometimes provide temporary relief.  This definition is not completely satisfactory, however, since there are itches for which scratching provides no benefit.

The original purpose of itch may have been to protect the skin from harmful agents by causing us to scratch the culprit away. Such itch-inducing substances are called pruritogens.  Like all sensations, though, itch can also result from misfiring in the nervous system.  In such cases, the skin needn't be directly involved, and scratching may do little to relieve the chronic itch.

The Neurology of Itch and Its Relationship to Pain

It used to be believed that itch was just low intensity pain.  We now know this view to be wrong. It’s true that a non-specific pathway is shared by signals for both pain and itch. This pathway triggers itching when provoked by pruritogens such as cowhage, the plant from which pranksters derived “itching powder.”  However, like pain or light touch, the sensation of itch also travels through its own unique dedicated pathways from the skin to the brain.

  This specific pathway is triggered by histamine release.  

That all being said, the lines of information for pain and itch, while separate, do interconnect and communicate in the spinal cord.  They do this via interneurons.  This may explain the inexorable desire to scratch an itch.  Also, if pain is firing, it may inhibit or mask the activity of the itch pathway. This intriguing relationship between itch and pain can be seen in the use of some opioids, which may cause itching when inhibiting pain.

Chronic Itch That Does Not Respond to Scratching

In some cases of chronic itch, the normal cross-talk between pain and itch pathways in the spine does not exist.  To explain this, it's possible that the peripheral nerves, or the nerves that travel from the spinal cord to the skin and other parts if the body, become more sensitive.  Or perhaps the brain changes in such a way that small irritants that would normally not be considered itchy are misperceived.  There is some evidence for each of these arguments, and the true nature of chronic itch that does not respond to scratching is likely some combination of those mechanisms.

Neurological Causes of Itching

While itching is classically thought of as a problem with the skin, some problems of the nervous system have also been described as causing itch.  In some cases, this may result from peripheral nerve damage leading to spontaneous signaling from the nerve or spinal cord.  Examples include brachioradial pruritis and post-herpetic neuralgia.  In other cases, small sensations may lead to a pattern of signals which is incorrectly decoded by the brain as being an itch.

Here are a few examples of neurological conditions that cause itching:

  • Trigeminal trophic syndrome- this is a condition caused by interrupted sensory pathway s of the trigeminal nerve, which usually transmits sensation from the face to the brain.  A slowly enlarged ulcer spreads on the cartilage on the outer edge of a nostril.  Treatment is usually unhelpful, consisting mainly of using protective gloves or other physical barriers to prevent the patient from scratching.
  • Brachioradial pruritis- this itch usually develops on an arm, and may be related to exposure to sunlight.  Though an exam and electrodiagnostic studies will be normal, people may scratch their arm to the point of bleeding.  The condition may be helped somewhat with ice and some antiseizure medications.
  • Notalgia paresthetica—this disorder usually occurs on the left hand side just below the shoulder blade, where most of us can’t quite reach.  It may result from spinal nerve impingement.  Capsaicin, some antiseizure drugs, nerve blocks, and even botulinum toxin injections may be helpful. 
  • Spinal cord lesions- Several kinds of spinal lesions, including neurofibroma, a tumor of the nerve fiber and cavernous hemangiomas, when blood vessels for a tumor, have been associated with itching.  This is often unrelated to the cite of the lesion.
  • Brain lesions-  Stroke and other problems may cause itching that is usually on the side opposite of the lesion.

How to Treat a Neurological Itch

Chronic neurological itches are difficult to treat.  Typical avenues like oral antihistamines (e.g. Benadryl) or topical steroids are usually no help.  A stepwise approach can explore capsaicin, antiseizure medication like gabapentin, and even agents like thalidomide or botulinum toxin may be considered. Neurostimulation techniques are also being explored.

Sources:

MJ Aminoff, the neurology of itch, Recent Advances in Neurology Conference, San Francisco Ritz-Carlton, February 13, 2014

A Dhand and MJ Aminoff: The neurology of itch, Brain 2013119128

DISCLAIMER: The information in this site is for educational purposes only. It should not be used as a substitute for personal care by a licensed physician. Please see your doctor for diagnosis and treatment of any concerning symptoms or medical condition.

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