What is Quantitative Sensory Testing?
Quantitative Sensory Testing (QST) is the systematic examination of sensations arising from the skin and underlying tissues (i.e. somatosensory sensations). In its simplest form, a single type of sensation (or 'sensory modality') is tested to determine wither the threshold at which it is felt, or how intensely it is perceived. For example, a heat stimulus, applied to the skin, might be gradually increased until the participant reports the first sensation of warmth, or a fixed temperature might be applied and the participant asked how painful it is.
The power of QST rests on stimuli being applied in a carefully controlled and standardised manner, usually with an identical set of instructions for experimenters to read to every participant that ensures that participants' subjective responses to stimulation are as reproducible and comparable as possible.
Usually, the term QST implies multiple sensory modalities are being tested. A comprehensive, standardised QST battery has been devised by the German Neuropathic Society (DFNS) for this very purpose (Rolke et al 2006 A). As such, within clinical and academic circles, particularly in the field of pain, the label 'QST' is often used as shorthand for the full multi-modality DFNS QST protocol.
Phenotyping with DFNS Protocol
One of the strengths of the DFNS QST protocol is in somatosensory phenotyping, useful for defining where in the nervous system a problem is occurring, like a long-lasting (chronic) pain. This is a powerful tool in both the clinical and research world, and the DFNS reference dataset allows for meaningful interpretation of a participant or patient's DFNS QST phenotype compared to normative values (Rolke et al 2006 B).
Because many of the nerve fibres assessed by QST (see subsections below) are inaccessible to standard electrophysiological nerve conduction studies, phenotyping with QST is often the only practicable quantitative measure of small-fibre function. This is highly relevant in pain clinics and pain research where small fibres are the primary fibres subserving peripheral pain transduction.
As above, QST phenotyping relies on multiple modalities being tested in a standardised manner. In the following subsections, the commonly tested modalities are outlined, along with details about the tools used to test them and an outline of how they are assessed as per the QST DFNS standardised protocol.
It is important to note, however, that QST is not limited to phenotyping, and assessing a subset (sometimes just one) of these various modalities can also be extremely useful in the clinic and in the lab. It should also be noted that the methods used by the DFNS QST protocol are intentionally simple in order for a high-number of modalities to be assessed in a clinically practicable timeframe. When testing fewer modalities, and/or especially within research environments, more sophisticated psychophysical testing might be preferential.
Types of Stimuli
Superficial Blunt Stimuli ('Mechanical')
Superficial mechanical stimuli are conveyed by small and large diameter rapidly conducting low-threshold (i.e. easily activated) fibres. These fibres are important for discriminative touch, i.e. being able to precisely locate on your body where something or someone has touched you.
They are commonly assessed with Von Frey filaments: round-ended nylon filaments that bend at a set force and subsequently maintain the set force (whilst bent). The contact area is typically 0.5mm in diameter and rounded to prevent sharp percepts. QST DFNS recommends using a descending and ascending force series to determine the 'mechanical detection threshold', or MDT.
A number of manufacturers produce calibrated Von Frey filaments.
Superficial Sharp Stimuli / Pinprick ('Mechanical Pain')
Superficial sharp (i.e. painful) stimuli are conveyed, primarily, by narrow diameter moderately rapid high-threshold (i.e. resistant to activation) fibres that respond to sheer forces created by sharp edged stimuli, such as pins.
Sharp stimuli are commonly assessed by weighted pinprick devices that exert a specific force when deployed vertically, perpendicular to the skin. The contact area is necessarily small and sharp-edged, usually in the region of 0.2mm diameter.
As per DFNS QST protocol, an ascending series of pinprick force is applied to a participant's skin until they report a sharp percept. Specialised pinprick stimulators are available, including MR-compatible devices.
Deep Blunt Stimuli ('Pressure Pain')
Deep blunt stimuli are thought to activate sensory fibres within muscle tissue. They also have a range of small and large low threshold fibres subserving non-painful deformation of muscle as well as high-threshold small diameter slow and fast fibres that convey pain (Graven-Nielsen et al 2004).
Pressure pain is tested at the bedside, in both the clinic and the lab, using relatively large contact area (~1cm2), rubber-tipped, probes. The probes are pressed against a person's skin at a site overlying a muscle, and the force increased at a steady pace until the person reports a change from pressure percept to pain: the 'pressure pain threshold', or PPT.
Manual application of force has been the mainstay of PPT assessment, however an automated device is currently being developed.
Thermal
Warm and cold sensations are conveyed by small diameter fibres, the former slowly conducting and the latter more rapidly conducting.
Testing of thermal sensation requires a specialist device called a thermode. These have contact areas (or various sizes) that can cool or warm a patch of skin.
The DFNS QST protocol recommends gradual heating or cooling from the skin baseline temperature until the participant reports the pre-specified percept, e.g. until they detect warmth, cooling, heat-pain or cold-pain.
Vibration
Vibration sensation is transmitted via large-diameter, rapidly-conducting nerve fibres. And whilst these can be tested neurophysiologically, vibration sense testing is a convenient bedside measure for the function of such fibres.
DFNS QST protocol advises use of a 64Hz Rydel–Seiffer tuning fork. Once struck, the base of the tuning fork is placed over a bony prominence and the lowest detectable vibration amplitude recorded.
Automated vibrotactile stimulators are also available for more subtle psychophysical assessments.
Soft Tactile Stimuli and Dynamic Mechanical Allodynia
Soft touch is a complex phenomena that is primarily conveyed via large diameter rapidly conducting fibres. However, slow stroking, especially the type you might observe during affiliative touch, is also conveyed via slowly conducting narrow nerve fibres. The former is most important within a DFNS QST assessment and here soft touch is used as a means to assess 'dynamic mechanical allodynia'.
Allodynia is the sensation of pain for a stimulus that would ordinarily be perceived as non-painful, for example a stroke on the forearm with a soft brush evoking pain. Standardised brushes and soft stimuli are available.
Temporal Summation or Wind-up
'Wind-up' has historically been used to refer to the increased firing of central nerve cells in the spinal cord in response to repeated painful stimuli. Temporal summation (TS), often used interchangeably with wind-up, is the report of increased pain experience in response to repeated pain stimuli: i.e. it is the perceptual correlate of wind-up. TS can be demonstrated within any pain modality. The DFNS QST assesses 'wind-up ratio' (TS) via means of pinpricks of the same force applied repeatedly (10 times) to the skin.
QST and Functional MRI
The majority of QST stimuli can be delivered by MRI-compatible devices, with varying degrees of automation. Functional MRI (fMRI) in combination with QST stimuli can provide a powerful read-out of central nervous system responses to different types of pain and pain paradigms.
Is causing pain ethical?
Pain, by definition, is unpleasant and commonly leads people to question whether inducing pain in people is ethical. First and foremost, if used within the specified operational parameters, none of the above tools will cause damage to people.
Secondly, pain research is essential to better understand the ongoing suffering that millions of people experience worldwide.
Thirdly, the paradigms used for pain research, such as those described here, usually require mild to moderate amounts of pain. Very few experiments (if any) expect participants to suffer severe pain for the name of science.
By way of example, the pain induced by the DFNS QST protocol is limited to threshold detection: participants are instructed to alert the experimenter as soon as sensations become painful, that is, at the lowest level of perceivable pain. Some custom QST experimental set-ups require moderate pain stimuli to be applied, but as with all ethically conducted research, the tolerance of these stimuli is tested at an individual level, and participants are always free to withdraw at any time.
- Quantitative sensory testing: a comprehensive protocol for clinical trials.. Rolke, R., Magerl, W., Campbell, K.A., Schalber, C., Caspari, S., Birklein, F., Treede, R.-D. European Journal of Pain. January 2006
- Quantitative sensory testing in the German Research Network on Neuropathic Pain (DFNS): Standardized protocol and reference values. Rolke, R., Baron, R., Maier, C., Tölle, T. R., Treede, - D. R., Beyer, A., Binder, A., Birbaumer, N., Birklein, F., Bötefür, I. C., Braune, S., Flor, H., Huge, V., Klug, R., Landwehrmeyer, G. B., Magerl, W., Maihöfner, C., et al. Pain. August 2006
- Painful and non-painful pressure sensations from human skeletal muscle. Graven-Nielsen, T., Mense, S., Arendt-Nielsen, L. Experimental Brain Research. October 2004
Associated Products
The following products from our catalogue are associated with this technique. To find out more about these supported devices, follow the links below or get in touch via email or phone.
QST
