Articles > Giving In To The Urge To Scratch: Researchers Find Not All Itches Created Equal

Giving In To The Urge To Scratch: Researchers Find Not All Itches Created Equal


Different parts of brain activated by pollen and dust vs. histamine

(BETHESDA, MD) – Intense itching and the urge to
scratch are symptoms of many chronic skin ailments. A new study conducted by
Oxford University researchers has found that different reactions in the
brain to two common allergy triggers — allergens (pollen and dust) and
histamine (allergy cells within the body caused by foods, drugs or
infection) — may shed some light on the itch-scratch cycle.  

The Study

The study is entitled Itch and Motivation to
Scratch: An Investigation of the Central and
Peripheral Correlates of
Allergen- and Histamine-Induced Itch in Humans.
The research team was
comprised ofSiri G. Leknes, Susanna Bantick, Richard G. Wise and
Irene Tracey, all of the Department of Physiology, Anatomy and Genetics,
Oxford University, Oxford, UK and Centre for Functional Magnetic Resonance
Imaging of the Brain (FMRIB), Clinical Neurology, Oxford University, Oxford,
UK; and Carolyn M. Willis and John D. Wilkinson, both of the Department of
Dermatology, Amersham Hospital, Amersham, UK. 

The results of the study are published in the online
edition of the Journal of Neurophysiology (http://jn.physiology.org/).
The journal is one of the 14 scientific publications published by the
American Physiological Society (APS) (www.The-APS.org)
every month. 

Methodology Overview

Laser Doppler Study

Twenty eight female volunteers were recruited for the
study, of which 14 tested positive (atopic cohort) for type I allergens
(grass pollen and/or house dust mite) and 14 did not (non-atopic cohort).
Over three consecutive days the atopic cohort was challenged with either
their specific allergen or histamine, along with the saline control group,
by applying a skin prick to the forearm. Non-atopic subjects were challenged
with histamine and saline on two consecutive days.

The subjects rated itch intensity continuously on a
scale of 0 (no itch) to 10 (worst itch imaginable). Itch related skin blood
flow changes for both groups were measured by laser Doppler.  Data points
from the laser Doppler were summated and mean values and standard deviations
were calculated for each cohort.

fMRI Imaging Study 

Sixteen males and females comprised a second group of
atopic and non-atopic individuals. As part of the study participants laid
down in an fMRI (functional magnetic resonance imaging) scanner. A
standard whole-brain gradient echno-planar imaging sequence was used to
obtain the functional scans. Sixty seconds after participants reclined they
received a skin challenge to the toe area.

The volunteers were presented with a screen showing a
simplified itch rating scale of 0 (no itch) to 5 (worst itch imaginable).
Subjects pressed a button to rate the itch they felt when the scale was
presented. Following the test each subject’s mean rated itch intensity was
computed and the differences between types of itch were explored using a
t-test. Correlations between mean itch ratings in the two groups were
investigated using SPSS. 

Results

After examining the data obtained at the different itch
sites, the different itch scales, and the gender differences between the
study populations, the researchers determined that extensive commonalities
existed between allergen- and histamine-induced itch. Among them was the
extensive involvement of the brain’s motivation circuitry in response to
both types of itches.

Researchers also observed differences, including: 

  • allergen-induced itch intensity ratings were higher
    compared to histamine;

  • perception of itch and changes in blood flow were
    significantly greater when allergen induced;   

  • itch intensity perception and the changes in blood flow
    occurred significantly later in response to allergen, and while the
    sensations took longer to appear, they were perceived to exist for
    significantly longer periods;

  • itch elicited by allergens activated different parts of
    the brain, specifically the supplementary motor and posterior parietal
    areas; and

  • the differences found in the orbifrontal regions of the
    brain imply a compulsion to so something (i.e., scratch) that is very
    strong in the allergen group. This is presumably due to the heightened
    intensity of the itch. There are similarities to the activity in this
    area of the brain and other disorders that display compulsive behavior.
    This might help to explain why eczema sufferers scratch to the point of
    harm because they are compelled to do so and cannot help themselves.

Conclusions

While there are extensive commonalties between
allergen- and histamine-induced itch, perceptions about the intensity and
the parts of the brain that are activated by allergens differ when compared
to histamine. As a result, mothers who admonish their children to stop
itching may now be rightly told “I can’t.” For mold and grass-related
itches, it appears that science is on their side. 

American Physiological Society. December 2006.


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