Quantitative Sensory Testing (QST)

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Original Editor - Melissa Coetsee

Top Contributors - Melissa Coetsee and Carina Therese Magtibay  

Introduction

Quantitative sensory testing (QST) is a systematic psychophysical test method used to measure sensory thresholds for pain, touch, vibration, and temperature sensations.It quantifies individual sensory perceptions using direct patient feedback. It tests for sensory loss (hypoesthesia, hypoalgesia) and sensory gain (hyperesthesia, hyperalgesia, allodynia). The procedure tests nociceptive and non-nociceptive properties of different afferent nerve fibres and central pathways.

Clinical Application

QST can be used to evaluate any condition that affects sensory function - it may help with diagnosis and disease monitoring. QST can be very useful to identify underlying contributing pain mechanisms and pathophysiology, which can assist with targeted intervention strategies. It is best to compare results with normative values, and it is best conducted early after onset of a condition - the reason for this is that people with central sensitisation will present with widespread QST abnormalities, rendering the contralateral side an inaccurate comparison site. In early onset cases, the contralateral side is more likely to still reflect the patient's norm, but if predisposing existing sensory abnormalities are present it could interfere with interpretation.

Conditions for which QST can be useful include:

• Neuropathic pain
• Polyneuropathy (diabetic, HIV-related, chemotherapy-related)
• Postherpetic neuralgia
• Complex regional pain syndrome (CRPS)
• Chronic low back pain
• Knee osteoarthritis

Components

To perform QST, a patient is stimulated with quantified sensory stimuli and based on patient feedback, perception thresholds are identified for the following sensory functions: light touch, pressure, vibration, thermal sensations, heat and cold pain. This way A-delta fibres (small diameter myelinated) and C-fibres (unmyelinated) are assessed. The categories of the full test include:

• Thermal detection thresholds for the perception of cold, warm and paradoxical heat sensations
• Thermal pain thresholds for cold and hot stimuli
• Mechanical detection thresholds for touch and vibration
• Mechanical pain sensitivity including:
  • Pin-prick thresholds
  • Blunt pressure thresholds
  • Stimulus response for pin-prick sensitivity and mechanical allodynia
  • Pain summation to repetitive pin prick stimuli

Procedure

Sophisticated computerised equipment is often used in clinical trials, but hand-held tools are also available for clinical use. The following table presents the standardised procedure as recommended by the German Research Network on Neuropathic Pain. For fully detailed description of this procedure see the Resources section. Simplified versions will be discussed later:

QST Standardised Procedure

Test	Equipment	Nerves tested	Instructions	Measurement
Thermal Testing	Thermode with element and cooling water system	A-delta fibres and C-fibres	Thermode is placed on the skin and measure heat and pain/ cold and pain detection between 0 and 50 degrees Celsius	Average of three consecutive measures
Mechanical Detection	Von Frey Filaments	A-beta fibres	Contact with skin for 2 seconds determine at what point the patient detects tactile input	Average of five above and below-threshold stimulus intensities
Mechanical pain threshold	Needle stimulators	A-delta fibre subtypes	Needle applied perpendicular to the skin in ascending and descending stimulus intensity with skin contact time of 1-2s	Average of five above and below-threshold stimulus intensities
Mechanical pain sensitivity	Needle stimulators, Q-tip, soft brush, cotton pad	Allodynia and pin-prick hyperalgesia (central pathways)	Gentle stimuli applied to the 2cm of skin (stroking) with the various instruments	Subjects are asked to rate the perception of the stimulus using a pain scale of 0-100
Vibration detection threshold	Tuning fork	A-beta fibre	Place fork over bony prominence until subject can not feel vibration any more	Disappearance threshold taken on 3 repetitions
Pressure pain threshold	Pressure gauge device (Wagner instruments)	Deep somatosensory function	Apply gradually increasing pressure over muscle area	3 series of ascending stimulus intensities
Temporal Summation	Needle stimulator (pin prick)	Wind-up phenomenon (spinal cord neurons)	A series of 10 repetitive pinprick stimuli of the same intensity	Subjects are asked to rate the perception of the stimulus using a pain scale of 0-100

Normal values

Factors that affect normal values:

• Sex: men and women differ in their sensitivity to pain; women are significantly more sensitive to pain
• Age: aging is correlated with an increase in perception and pain threshold
• Body site: sensitivity will vary according to body sites

Bed-side testing

The full protocol of QST is not practical for regular use in clinical practise. Modifications of these tests may however provide useful insights. The following bedside methods have been shown to be valid:

Loss of cold perception to 22 °C metal, hypersensitivity towards 45 °C metal, loss of tactile perception to Q-tip and loss of pain perception to 0.7mm CMS hair as well as DMA by Q-tip, brush and cotton wisp (ie, allodynia sum score). However, the sensitivity and specificity of parameters assessing gain of function towards cold pain (ie, cold hyperalgesia) has to be interpreted with caution due to the fact that only 3 patients suffered from cold hyperalgesia

performed in significantly shorter time (ie, <5 minutes for the 5 selected test parameters per test area), whereas a full lab-QST assessment is much more time-consuming (ie, 30 minutes per test area). Another advantage of the bedside-QST is that expensive devices such as a thermotest device, a pinprick set, as well as von Frey hairs are not required. Therefore, the proposed bedside-QST battery could be implemented in clinical settings as well as clinical trials, which so far have not used QST for feasibility reasons.

Simplified QST

Test	Equipment	Instructions
Thermal tests	Metal iron of 3cm; for cold detection at 22C (metal at room temp) and for cold pain detection at 8C (cooled in fridge); for warm detection at37C and warm pain detection 45C (warmed using milk heater)	Apply stimulus for 3 sec. Subjects rate the stimulus as painful or not, and if painful - the pain intensity from 0-10; they also report if the stimulus was felt or not
Dynamic mechanical detection
Static mechanical detection
Mechanical pain sensitivity
Temporal summation
Mechanical allodynia
Pressure pain sensitivity
Vibration detection

Complementary Tests

• Temporal summation (TS): Apply rapidly repeated mechanical stimuli (for 5-10s) to a painful region - if TS is present it will result in an increased pain perception indicating central sensitisation. Although part of the QST protocol it can be used in isolation to detect increased firing of secondary spinal cord neurons. Pressure or pin-prick can be sued as a stimulus.
• 2-point discrimination: Especially useful in upper limb to test tactile perception. Can be impaired in both PNS and CNS conditions. In the absence of peripheral pathology, it could indicate cortical changes in the homunculus.
• Conditional pain modulation test (CPM): CPM tests the function of central pain inhibition/facilitation, and is based on the phenomenon that "pain inhibits pain". It involves applying an initial noxious stimulus (thermal or mechanical), which is measured before and after applying a second stimulus (cold or hot water bath applied to a distal extremity) - if descending inhibitory control is functioning properly, the initial source of pain will be less intense during the application of the remote stimulus.

Interpretation

The table below summarises the link between underlying pain mechanisms / pathophysiology and specific QST findings:

Interpretation of QST results: Mechanisms

Underlying mechanism/ Sign	QST finding
Hypoesthesia (sensory loss)	Elevated sensory thresholds, i.e. decreased sensitivity to non-noxious stimuli
Hypoalgesia	Elevated sensory thresholds, i.e. decreased sensitivity to noxious stimuli
Hyperalgesia	Increased sensitivity (lowered thresholds) to heat, cold, pinprick or deep pressure
Allodynia	Pain in response to non-noxious stimuli on skin
Peripheral sensitisation	Heat hyperalgesia (lowered hot pain threshold) Lowered pain thresholds for pressure/pin prick proximal to painful area or along nerve distribution / segmental region
Central sensitisation	Increased sensitivity to cold/pin prick/pressure/cold hyperalgesia that is widespread and contralateral; Mechanical allodynia; Temporal summation (repeated mechanical and thermal stimuli causes increased pain)
Polyneuropathy	All thresholds elevated
Small fibre neuropathy	Vibration thresholds are normal, but others elevated
Nerve deafferation	Reduced sensitivity with all tests

The table below summarises QST findings for specific conditions, as identified in research studies:

Common QST results: Conditions

Condition	QST findings	Implication
Chronic low back pain	(1) Increased sensitivity (decreased threshold) to heat and pressure (2) Temporal summation	(1) Indicates peripheral sensitiasation (2) Central sensitisation components present
Lower limb fractures	Decreased sensitivity (elevated threshold) to touch and increased sensitivity (decreased threshold) to warmth	Identified underlying peripheral nerve injury which can affect limb function and balance
Knee OA	(1) Increased sensitivity (lowered thresholds) to pressure around the knee (2) Increased sensitivity to pressure and cold at distal sites; Temporal summation (TS with pinprick	(1) Indicates peripheral sensitisation (2) Indicates central sensitisation - predicts poor surgery outcomes if not addressed; will likely not respond fully to NSAIDs
CRPS

Limitations

• QST abnormalities are present in non neuropathic pain, making it difficult to use QST as a definitive tool for identifying neuropathic pain
• Abnormal findings are not specific for peripheral nerve dysfunction, as central nervous system disorders will also alter sensory thresholds
• It is a subjective psychophysical test, entirely dependent on patient alertness, motivation and willingness to supply accurate feedback. There is large intra- and interindividual variation

Summary

QST can be a very valuable clinical tool in patients with chronic pain by assisting to characterise pain experiences. It provides information about potential underlying mechanisms contributing to pain, therefore incorporating QST may improve the ability to implement individualised treatment plans based on underlying mechanisms.

Resources

• Quantitative Sensory Testing - standardised full procedure

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