Contagious itch in humans: a study of visual ‘transmission’ of itch in atopic dermatitis and healthy subjects

Abstract

Anecdotal evidence suggests “contagious” itch occurs in daily life when we see other people itch and scratch. This phenomenon has not been systematically studied previously, and factors which can amplify itch perception were unknown. We investigated whether exposure to visual cues of itch can induce or intensify itch in healthy and atopic dermatitis subjects. Participants received histamine or a saline control delivered to the forearm and were asked to watch short video clips of people scratching. Spontaneous scratching induced by visual cues was monitored and analyzed. Atopic dermatitis patients reported a higher itch intensity and scratched more frequently while watching itch videos, even in the presence of mock itch stimuli. Human susceptibility to develop itch when exposed to visual cues is confirmed and it appears amplified in atopic dermatitis sufferers. These findings suggest that interpersonal social cues can dramatically alter the subjective sensory experience of itch.

Neural basis of contagious itch and why some people are more prone to it

“…Although no previous study has examined the neural correlates of visually induced itch, several researchers have suggested that the "mirror neuron system" may be essential for contagious itching (2,4). Mirror neurons, first reported in the macaque brain, respond to both a self-executed action and the sight of an action performed by another person (15).…”

“…Subjective feelings of itchiness and observable increases in scratching can be evoked by watching someone scratch himself or by listening to a lecture on dermatologic conditions (1,2). Although many aspects of the neurobiology of itch are now appreciated (3,4), the standard definition of itch ("an unpleasant sensation associated with an urge to scratch") and its description as a symptom within clinical disorders remain essentially subjective and based on self-report.…”

Are sex and history of pruritic skin conditions factors which affect the phenomenon of visually evoked itch? An exploratory study

“…Although recent studies have shown sexspecific differences in the quality, localization and triggering of chronic pruritus, and in the underlying disease and scratching behavior [6] , studies from the experimental literature have typically found no sex differences in itch contagion [3] , itch ratings or scratch behavior [1] . However, this may be due to limited sample sizes (eg, only 3 males and 8 females with AD were tested in Papoiu et al [1] ) or biased samples that recruited twice as many females to males [3] . To address this bias, similar numbers of males and females were recruited in the current study.…”

“…One factor that could influence susceptibility to VEI is having a history of pruritic skin conditions (PSCs). Previous studies have demonstrated that a combination of watching videos of people scratching while participants received either histamine or saline administration caused self-reported itch intensity to increase in all participants, but resulted in a doubling of spontaneous scratching episodes in those with AD who also appeared to scratch a more widespread area for longer [1] . Lloyd et al [5] have shown that images containing itch-related stimuli in contact with the body (such as ants crawling on the skin) produce the highest ratings of itch intensity.…”

“…This cycle can be triggered by "contagious itch," that is an induced feeling of itch typically followed by a scratch response, resulting from viewing another person scratching [1] . While several studies have explored this effect using different methods for triggering the phenomenon [1][2][3][4] , only 1 study has thus far compared the effectiveness of different visual stimuli to evoke itch and scratch [5] . However, it did not assess the impact of individual differences such as sex or history of pruritic dermatological conditions on visually evoked itch (VEI).…”