Management of painBMJ 2003; 326 doi: https://doi.org/10.1136/bmj.326.7390.635 (Published 22 March 2003) Cite this as: BMJ 2003;326:635
- Quantitative sensory testing
Sensory disturbances are common in patients with pain and are not confined only to neuropathic pain states. Examinations using cotton wool for light touch and a pin for nociceptive sensation are inaccurate. For quantitative sensory testing the stimuli are standardised and reproducible thus providing precise and accurate testing. Sensations such as light touch, temperature, pressure, and vibration are tested depending on their modality—for example, unimodal for vibration or, in contrast, in varying perceptions for temperature (see table A).
Methods are standardised. Firstly, attention is encouraged by, for example, reducing fatigue. Threshold is measured either by the method of limits, where stimuli are increased or decreased stepwise in intensity, or by the fixed choice method where a choice is made between two stimuli. The fixed choice method is relatively slower. The sites of stimuli include the segmental area of pain, the contralateral side for comparison, and other segments relevant to referred pain.w1
Quantitative sensory testing can be applied to both diagnosis and treatment. The modality specific techniques can provide a clear profile of the status of the different somatosensory channels. For example, if it is unimodal and limited to the site of pain then the diagnosis is unlikely to be a neuropathy. Therapies may be evaluated by serial assessment and by a return to normal sensation.
Table A Methods used to detect abnormalities in perception of sensation
Modality Instrument Characteristic of nerve fibres Measurement Light touch Von Frey filaments Low threshold, mechanosensitive Threshold for perception of light touch (mg) Thermal: Peltier element based thermode Heat <2° C/sec temperature change Innocuous or non-painful Threshold (° C) for perception of heat or warmth Nociceptive, C fibres Pain threshold (° C) for heat Cold <2° C/sec temperature change Non-painful Threshold (° C) for perception of cold Nociceptive, A delta fibres Pain threshold (° C) for cold Pressure Pressure algometer (area 1 cm2) C fibre, mechanosensitive Pain threshold for pressure (kPa) Vibration Tuning forks, for example, 30-256 Hz Low threshold, mechanosensitive Threshold (Hz) for perception of vibration
Table B Approach to management of neuropathic pain based on diagnostic tests and mechanism based classification, Italics represent neurotransmitter at receptor
Mechanism Diagnostic features Molecular target Drugs General sodium channels: redistribution or altered expression Spontaneous pain, paraesthesia Sodium channels sensitive to tetrodotoxin Local anaesthetics, antiepileptics, antiarrhythmics, tricyclic antidepressantsw2-w4 Specific sodium channels Spontaneous pain Sodium channels resistant to tetrodotoxin Selective blockersw4 Central sensitisation Hyperalgesia (in response to tests using touch, cold, pin prick) N-methyl d-aspartate receptor (glutamate, glycine); neurokinin 1 receptor (bradykinin); neuronal nitric oxide synthase; protein kinase N-methyl d-aspartate antagonist (for example, ketamine, dextromethorphan, memantine), glycine site antagonists; neurokinin 1 receptor antagonist; neuronal nitric oxide synthase inhibitors; protein kinase inhibitorsw2 w5-w9 Peripheral sensitisation Hyperalgesia in response to pressure Vanilloid receptor Capsaicin, cannabinoidsw10 w11 Hyperalgesia in response to thermal stimuli Neurokinin 1 receptor Neurokinin 1 receptor antagonistw7 Neurogenic inflammation Nerve growth factor Nerve growth factor antagonistsw12 Sympathetic activity Spontaneous pain Adrenergic receptors (α adrenergic), nerve growth factor or trKA Phentolamine, guanethidine, clonidine, nerve growth factor antagonistsw2 w12 w13 Reduced inhibition Hyperalgesia Opioid receptors, γ -aminobutyric acid transaminase, neurokinin 1, adenosine, purine, kainite, cholecystokinin, acetyl choline (nicotinic) Morphine, gabapentinw3 w13
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