Central sensitization of chronic pain and itch: generalized and symptom-specific mechanisms.

Physicians of all disciplines are frequently confronted with patients with high
symptom reports that do not correspond to biomedical findings and that
frequently result in enormous costs for diagnostics and treatment without
satisfying results. Central sensitization of physical symptoms (i.e.
central-controlled sensory sensitivity) is assumed to be primarily responsible
for these high symptom reports of patients, independently of a known
pathophysiological etiology. Particularly in patients suffering from chronic
physical symptoms, such as chronic pain and itch, central sensitization is
assumed to be involved in the maintenance and increase of physical symptoms in
the long term.
In line with basic psychophysiological theories, two processes can be
distinguished with respect to the symptom report of sensations of e.g. pain and
itch: the tendency to react with various degrees of intensity to the sensation
(quantity / non-specificity) and to ascribe a qualitative attribute to the
sensation (quality / specificity of e.g., pain and itch). Recent findings of
our and other research suggest that both processes might be involved in central
sensitization and disregulated in patients with chronic physical symptoms,
resulting in 1. generalized sensitization: the tendency to report an overall
lowered threshold to somatosensory stimuli and 2. symptom-specific
sensitization: the tendency to interpret an ambiguous sensory stimulus in
correspondence to the patient*s main physical symptom (e.g. pain in chronic
pain patients). That is to say, patients with chronic physical symptoms may be
more sensitive to somatosensory stimuli in terms of lower sensory thresholds
(generalized sensitization) and simultaneously interpret / identify an
ambiguous stimulus as a symptom of their main physical symptom, e.g. pain in
chronic pain (symptom-specific sensitization).
In line with preliminary international findings, we recently conducted a pilot
study to validate a method of assessing these phenomena for the first time in
both patients with chronic pain and itch, by making use of a validated method
to assess central sensitization in chronic pain: Quantitative Sensory Testing
(QST). Results showed that QST enables both central sensitization mechanisms to
be studied with the same experimental approach in patients with chronic pain
and itch, and indicated that generalized and symptom-specific sensitization
plays a role in patients with and without a known pathophysiology of their
symptoms. In the present study the specific sensory, affective, cognitive and
central processes, affecting these phenomena, are investigated.

The present research project is directed at delivering further evidence for the
mechanisms underlying generalized and symptom-specific sensitization in
patients with chronic pain and itching. For this purpose, 4 experiments are set
up to provide insight into specific sensory, affective, cognitive and central
characteristics of these sensitization phenomena in patients with chronic pain
and itch.

In this study, subjects receive different short-term stimuli (e.g., electrical,
mechanical, thermal) of low intensity. In addition, a low level of short-term
psychological stress and cognitions towards the stimuli are induced during one
of the experiments (see protocol).

Subjects are asked to come twice for two hours to the UMC St. Radboud. The
applied sensory stimuli are standardized, of short duration and of low
intensity. After the experiments, it is expected that subjects will experience
a low intensity of itch and/or pain for maximal one hour. The validated
questionnaires have frequently been used. Unless a time investment, no risks
and disadvantages are expected of this study.