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Understanding the science of psoriasis itch

Burning, biting, stinging, tingling: No matter what form it takes, the itch caused by psoriasis can be maddening.

Itch affects up to 90 percent of people with psoriasis, and is the most troublesome symptom, according to a study by the National Psoriasis Foundation. Yet only in the past decade has itch been acknowledged as a common symptom of the disease, said itch expert Dr. Gil Yosipovitch, professor and chair of dermatology at Temple University School of Medicine in Philadelphia.

Why? Itch has taken a backseat to pain among researchers, who have struggled to understand the relationship between the two sensations. Scientists have recently discovered molecules, receptors and nerve pathways that are specific to itch, leading scientists to recognize itch as a distinct phenomenon from pain. These findings have helped trigger a surge of research into the science of itch and how to treat it.

On the leading edge are research centers such as the Temple Itch Center and the Center for the Study of Itch at Washington University in St. Louis—the first of its kind when it was established in 2011. Their research promises to one day give new relief to people experiencing chronic itch.

"I predict in the next five years there will be drugs on the market that address this problem," Yosipovitch said.

Itching Explained

Itch typically occurs when something external touches the skin, irritating itch receptors. Those receptors transmit a signal to the brain via the spinal cord, which triggers the desire to scratch. Even just thinking about itch or seeing others scratch can induce itchiness, Yosipovitch said.

Scratching provides temporary relief by interrupting the itch signal. But for people with psoriasis, scratching tends to only make the problem worse in the long run.

"Any damage to the skin barrier triggers itch more," Yosipovitch said. Scratching can over-activate nerve fibers, amplifying itch and intensifying the urge to scratch. Scratching can trigger the Koebner phenomenon, in which new plaques form on healthy skin as a response to skin trauma.

Among the numerous itch receptors identified in the past decade, those connected to psoriatic itch include the nerve growth factor (NGF), tyrosine protein kinase (TrkA), and transient receptor potential vanilloid 1 (TRPV1) receptors. Medicines targeting some of these receptors are now in development, and preliminary data have shown an encouraging reduction in itch among people with psoriasis, Yosipovitch said.

Identifying Itch

Not all itch is created equal.

Take, for instance, the itch caused by an insect bite. In response, your immune system releases the chemical histamine, which causes itch. But histamine is not a primary factor in psoriatic itch.

Psoriasis itch has unique qualities, Yosipovitch said. It tends to affect certain parts of the body—such as the scalp, groin, palms and soles of feet—more so than other types of chronic itch, he said. Psoriasis itch is bothersome at night, making sleep difficult.

Scientists don't know exactly why psoriasis itch behaves the way it does, but new insights are bringing this aspect of the disease into clearer focus. One revelation: Psoriasis itch is more than skin deep. Data suggests that itch has little to do with psoriasis severity, and itch occurs on skin that does not yet have a plaque.

As the science of itch continues to evolve, Yosipovitch has one central message for people dealing with the frustrating itch of psoriasis: With new treatments in the pipeline, the future is bright.

Learn more about the science of itch »

—Sarah L. Stewart

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