It begins with an itch. That familiar irritating feeling, swiftly followed by the inevitable scratch. For most of us it ends here, in a fleeting moment of bliss. But then there are those tortured few for whom scratching provides little relief.
In 1660, the German physician Samuel Hafenreffer defined an itch as “an unpleasant sensation associated with the desire to scratch.” As an operational definition, it does the job. As far as we know, every animal with a backbone has a scratching reflex. It’s a useful instinct to rid yourself of fleas, mites, mosquitoes and other small insects that might carry infection. But this protective mechanism can also go awry.
In a masterful essay entitled The Itch, the surgeon Atul Gawande recounts the case of an HIV patient suffering from a severe chronic itch. The patient had recently been diagnosed with shingles, a disease whose symptoms often include extreme itchiness. After many sleepless nights of relentless scratching, she woke up one morning with a greenish fluid trickling down her face. Hours later, in the emergency room, her doctors informed her that she had managed to scratch through her skull, all the way to her brain.
Chronic itching is triggered by various diseases, such as eczema, shingles, HIV, chronic kidney problems, or even as a side effect from some medications. In most cases, it adversely affects quality of life, as patients are constantly tortured by their incessant need to scratch themselves. Standard medications often have no effect. These are people who are suffering from an itch that they can’t get rid of.
The story of itch is inextricably woven with the story of pain. Starting from the discovery of morphine in the early 1800s, there has been steady progress in the medical understanding of pain. Researchers have mapped the circuitry that transmits pain, and have developed increasingly effective painkillers and anaesthetics. In contrast, an itch was not considered life threatening, and relatively little effort was spent trying to understand it. For a long time, it was simply thought to be a dull form of pain.
But this picture is changing fast. In the last decade, researchers have learned about receptors in the nerves under our skin that react specifically to itchy substances. When these receptors fire, they send a signal racing up our spinal cord, headed to our brain where it creates an urge to scratch. Scientists now have a basic map of the roads that an itch takes on its way to our brain. And they have even been able to block some of these roads in mice, essentially preventing them from feeling an itch.