Quantitative Sensory Testing (QST) - Does assessing ... - TI Pharma

6. General discussion and Summary
General discussion
The assessment of sensory signs in chronic pain is of major importance in clinical
practice, in part to help to identify neuropathic pain. Standardized Quantitative Sensory
Testing (QST) such as by the DNFS (German Research Network on Neuropathic Pain)
protocol is not only advantageous in the clinic for diagnosis, where sensory signs in
patients can now be described quantitatively and qualitatively, but QST also facilitates
new opportunities for research. QST can be categorized as psychometric testing using
both thermal and mechanical nociceptive as well as non-nociceptive stimuli. It allows
the evaluation of different sub-modalities of nerve fibres involved in the transduction of
sensory information from the periphery to the spinal cord such as Aβ-fibres, Aδ-fibres
and C-fibres.
Originally developed to explore the prospects of a mechanism-based classification
of neuropathic pain, in this thesis additional potentials of QST are investigated. As a
research tool it provides valuable information of differences in somatosensory function
between healthy subjects and chronic pain patients. We have shown that in a substantial
number of patients with unilateral neuropathic pain bilateral somatosensory changes
are found. This challenges the application of sensory testing as a clinical evaluation tool
when the contralateral side is used as reference side. Therefore, we suggest that only
reference values obtained from healthy controls can be used for the correct interpretation
of sensory signs in neuropathic pain patients.
The results of our QST studies are also of relevance for the development of novel
therapeutics in chronic pain, in terms of improved assay sensitivity. Examples are the
selection of homogenous subpopulations based on specific QST parameters for the
clinical evaluation of novel drugs, and the identification of distinct QST responders
(e.g. Mechanical Pain Sensitivity) for mechanistic studies.
Implications of QST for clinical neuropathic pain practice
We demonstrated that by using QST values from healthy controls the interpretation of
sensory function and its clinical manifestation for a patient with neuropathic pain may
be different compared to the outcome based on bed side testing (chapter 2). In clinical
practice, most often the contralateral, non-affected side of a patient is used as reference
for the identification of sensory signs, such as hyperalgesia and allodynia, at the
affected side. We have shown that in unilateral neuropathic pain bilateral somatosensory
changes i.e. changes at the affected as well as the contralateral side occur frequently.
Contralateral changes are in fact more rule than exception in unilateral neuropathic
pain and could be attributed to plastic changes of the nervous system. These plastic
changes might be “triggered” by pain since the presence of contralateral abnormalities
was correlated with the ongoing pain intensity reported by patients. The observation of
bilateral sensory abnormalities in unilateral neuropathic pain is very valuable clinically
since it indicates that the contralateral side should be used cautiously as a control side
in sensory examinations such as bedside tests. We have shown that the interpretation of