The power function of the ten test for measuring neural sensitivity in clinical pain or sensory abnormalities

Abstract

This review highlights a simple psychophysical quantitative sensory testing (QST) method (the ten test) for research and clinical practice as it relates to sensitivity change and symptom improvement in pain populations. This cost-effective QST has a three-fold benefit of being diagnostic, prognostic and providing outcome evaluation. The power function of the ten test is discussed with the theoretical foundation of levels of measurement and psychophysical method that can approach ratio scaling in mind. The ratio level measurement might be useful for the researcher as the normative values of different QSTs are not well established. As a reliable and valid testing method, it provides an option for clinicians in busy clinical settings, and/or where QST equipment is unavailable.

Introduction

The need for assessment of clinical phenomena is a central issue in any scientific or clinical process since it is difficult to make valid conclusions about a disease mechanism, epidemiology, natural history or therapy response without assessing the relevant parameters. A good clinical assessment tool has a three-fold benefit of being diagnostic, prognostic and providing outcome evaluation. ¹ Pain is a complex phenomenon and difficult to measure in the clinical context. However, there is increasing awareness about neural sensitivity measures to detect abnormal responses,^2,25 as the disturbance of sensory function is a feature of neurological illness or clinical pain (Figure 1).

Figure 1.

Deviation of sensitivity in compared with basal/normal pain sensitivity phenomena and its clinical representation. The ten test can help to detect these abnormal pain responses (hyper/hypoesthesia) by comparing normal sensitivity.

The majority of the clinical-pain or sensation-measuring tools suffer from poor level scaling (nominal or ordinal). ² The data from these scales are arbitrary, ³ as they are derived from the difference between two numbers or the choice of two descriptors (e.g. verbal rating scale). In fact, the precision of verbal and numeric scales is limited by the number of descriptive words or numbers. In contrast, a ratio scale is considered a higher level of measurement in view of logical/mathematical operation (Table 1), and in that respect differs from nominal/ordinal scales. ⁴ The term “ratio” refers to the measurement properties based on the evaluative calculation of the ratio between a magnitude of a continuous quantity and a unit of the same kind of magnitude.^5,6 Data from a ratio scale are meaningful and non-arbitrary. ⁴ Ratio scaling is considered to be superior to nominal or ordinal scales for evaluation of pain progression or its treatment, especially in intervention evaluation. ²

Table 1.

Levels of measurements and permissible mathematical operations.

		Nominal	Ordinal	Interval	Ratio
Logical (math) operations	× ÷	×	×	×	✓
	+ −	×	×	✓	✓
	< >	×	✓	✓	✓
	= ≠	✓	✓	✓	✓

Note: Scale types with their properties according to Stevens. ³

The clinical measures of therapeutic efficacy are determined from clinical trials by comparing active and control therapy in view of pain relief as judged by patients (reporting the extent/degree of pain relief). However, patients’ pain reports are subjective and dependent on an unmeasurable stimulus. Therefore, a pain sensitivity measure based on a psychophysical paradigm is a comparatively better approach, as altered sensitivity is related to clinical phenomena (Figure 1). Although psychophysical measures are semi-subjective given that the controlled stimulus intensity (objective) is evaluated on perception magnitude (subjective), these measures are capable of targeting the whole neural axis, from the peripheral receptors to the brain. ⁷ The objective of this paper is to highlight a simple psychophysical quantitative sensory testing (QST) method for research and clinical practice as it relates to somatosensory changes and symptom improvement in pain populations, by comparing costs versus benefits of the application.

The ten test and its reliability and validity

The ten test is a psychophysical QST test requiring no test equipment (a cost-efficient and simple approach) that was first described by Strauch et al. in 1997. ⁸ The test is based on the stimulus rating (or pain magnitude rating) on a 10-point numeric scale in relation to normal sensitivity (Figures 2 and 3). This test is capable of providing a quantitative score to the ratings obtained while the examiner administers light moving touch stimuli to a test area and simultaneously comparing that with a reference area of “normal” sensation. The procedure of the ten test (to quantify neural sensitivity in pain) is available at http://www.youtube.com/watch?v=ktvjsqbIfUM and an overview of the test is provided in Table 2.

Figure 2.

The examiner (right) stroking a normal part (left) to elicit patient’s normal sensibility at palmer aspect of the tip of a digit, which is assigned a score of 10 on a 1–10 scale. Reproduced by permission from Uddin et al. ¹³

Figure 3.

The normal and abnormal area being simultaneously stroked, with equal pressure maintained by examiner’s both fingers (right). The procedure produces an analog ratio of the abnormal body area compared with the normal area. Reproduced by permission from Uddin et al. ¹³

Table 2.

Overview of the ten test. Modified with permission from Uddin et al. ¹³

Test name	Application method	Advantage	Disadvantage	Cost	Neural target
The ten test 7	Test is based on comparing a light moving touch stimuli (provided by examiner) to a test area and comparing that to an area of “normal” sensation. The rating is verbally rated from 10–1 (for hypoesthesia) and 1–10 (for hyperesthesia) with anchors 2 moving touch (figure 3).	Rapid and simple sensibility test without any equipment requirements. Ratio between normal light moving touch and diminished moving touch compared with a known scale of sensibility (reliable and valid).	Only comparable to contra-lateral innervated body part within same dermatome.	Evaluation requires no instrumentation (virtually no cost).	Mainly adaptive large (A-β) fibers, also small A-δ fibers in glabrous skin.8,18,19

The robustness of the ten test in terms of its reliability and repeatability has been verified in several studies. Inter-observer reliability was reported to be excellent (ICC = 0.91) and very strong agreement (κ =1.00, p<0.003) was found between examiners.^8,9 Good to excellent intra-observer reliability (ICC = 0.62 to 0.90, p<0.05) was also reported ⁸ when equal delivery of the stimulus pressure to the test and normal areas was evaluated. Multiple studies have demonstrated the utility of the test for clinical outcome measurement.^10–15 Comparison of the test with other sensory testing techniques (e.g. Weinstein enhanced sensory test with calibrated monofilaments, static two-point discrimination, moving two-point discrimination, and pressure specified sensory device) supported the applicability of the ten test in hand sensitivity measure for both clinical and research purposes. ¹⁶ The test has thus been recommended for clinical use in patients age >5 years, ⁹ different conditions of upper and lower extremities^11,12,16,17 and pre/post-operative sensory evaluation.^8,10,18

The ten test is a cost-effective QST tool that is now used in research projects at McMaster and Western Universities. The testing manual/protocol is available from McMaster University. ¹⁹ Studies that compare the ten test with allodynography and the Rainbow Pain Scale have been proposed. ²⁰

The power function of the ten test

In psychophysics, a ratio scale is composed of perceptual intervals adjusted for stimuli and associated with the power function of Fechner’s Law. ²¹ The evaluation of stimuli can be adjusted according to the perception magnitude in a ratio scale; therefore, a comparison between two stimuli is valid on a ratio scale. An illustration of this fact is the two classical psychophysical methods that can approach ratio scaling:^22,23 (1) magnitude matching (e.g. cross-modality matching, magnitude estimation or production), where subjects evaluate intensities of sensations from two or more modalities on a common/single scale, and (2) ratio matching (e.g. cross-modality ratio pairings, ratio estimation or production), where subjects match numbers to sensations in a way of ratio scale formation. Using this paradigm, the ten test can be categorized as a ratio-matching approach, where the ratio scale is constructed through intermodal pairing (e.g. touch or force applied with the hand). In fact, Merkel formulated this in 1888, ²⁴ which required producing or finding the stimulus that best relates to a given standard stimulus in the subject’s body (the earliest form of ratio matching). Later, Stevens included Merkel’s task into the ratio-matching category of psychophysical methods approaching ratio scaling.^4,21 According the Stevens theory, ⁴ the ten test should be considered a ratio level measurement.

Interpretation and limitation of the ten test

The response from the patient concerning the abnormal area is recorded as a fraction of 10 between 1/10 to 10/10 (diminished to normal sensory perception). The test may be repeated (according to the need and time) to produce an average score. If any test area is found to be more sensitive than normal, then the interpretation would be different. In case of hyperesthesia (sensory hyperfunction that is common in abnormal pain response like hyperalgesia and allodynia), the normal area sensation score is assigned 1 (instead of 10) in a 1–10 scale, then the test area compared by a fraction from 1/10 to 10/10 (normal to increased sensory perception). Usually, the affected body area is compared with the contra-lateral normal body area. In the case of bilateral symptoms, another body area may be chosen by taking into consideration similar nerve innervation density. The test requires patient cooperation and might have some cognitive bias. Maintaining equal pressure and a precise test area for simultaneous stimulation of both the normal and abnormal part may be initially challenging; this limitation can be overcome with repeated trials/practices.

Conclusion

The ten test is quick to administer, requires no equipment and can be used where self-report measures or other forms of QST ²⁵ are not feasible or possible. It provides a reliable option for clinicians in busy clinical settings, and/or where QST equipment is unavailable. Despite some limitations, the test is a good option as it can minimize individual variability in pain or sensory physiology by comparison to an individual’s normal/basal neural sensitivity. Simplicity, capacity to detect subtle sensory changes, and ease of test administration are the most significant practical appeals of the test. Since the normative value of other QSTs are not well established, it can be used as a powerful measurement tool for outcome evaluation of interventions.