Chronic Pain Institute - Response Consistency Testing

Response Consistency

Response Consistency Testing
-the significance of variance in consistency of skin pressure readings as a tool for studying the psychophysiology of human pain.*

August L. Stemmer, M.D., D.M.D., F.A.C.S. a**, Peter F. Chase, DDS, MA b

Facial Pain Clinic of the Department of Basic Sciences

School of Dentistry University of the Pacific

San Francisco, California 94115 USA

** Correspondence to: August L. Stemmer, M.D.1280 Lynwood Dr. Novato, CA 94947. email: stemmer@home.com Tel: 415-897-9659,
Fax 415-897-9659

a) August L. Stemmer, M.D., D.M.D., F.A.C.S. is Clinical Associate Professor and Member of the Oro-Facial Pain Clinic at the School of Dentistry University of the Pacific 2155 Webster Street Suite 615, San Francisco, CA 94115

b) Peter F. Chase, DDS, MA is Clinical Associate Professor and Director of the Oro-Facial Pain Clinic at the School of Dentistry University of the Pacific 2155 Webster Street Suite 615, San Francisco, CA 94115.

Publishers Note
This original work was first submitted to a physiology journal on 6/8/2000 in it's present form, but not accepted for publication due to a lack of broad interest in the subject. It was submitted to The Chronic Pain Institute on 2/18/2001 and accepted here for publication on 03/20/2001.

Page 1

Abstract

Response Consistency testing

-the significance of variance in consistency of skin pressure readings as a tool for studying the psychophysiology of human pain.

In recognizing the need for a baseline work in the application of the algometer to measure skin pressure responses for the study of such psychophysiology as that of human pain the importance of even minor variations from the norm has become more evident than may have been the case in the past. In order to be able to place greater emphasis on minor degrees of variation in response, a method is here set forth to better allow the future development of algometer testing in the field of psychophysiology to progress more rapidly with greater validity than has previously been available.

This first phase study, of a relatively homogeneous group of 113 dental students as controls, has shown enough group variation statistically to reveal the advantages of the described method over that which has been the case in the past from the researched papers of previous authors. If the method is universally accepted and applied, it may well be of advantage in preliminarily screening larger groups of subjects in order to be able to use more definitive, although more costly, methods such as cranial imaging economically for research purposes.

Page 2

Response Consistency Testing
-the significance of variance in consistency of skin pressure readings as a tool for studying the psychophysiology of human pain.*

August L. Stemmer, M.D., D.M.D., F.A.C.S. a**, Peter F. Chase, DDS, MA b

Facial Pain Clinic of the Department of Basic Sciences

School of Dentistry University of the Pacific

San Francisco, California 94115 USA

Although cranial imaging, in the forms of such procedures as PET and SPECT scans and more recently Functional MRI, is rapidly allowing identification and association of brain structure with function, the cost of such testing generally makes it difficult to do large groups of patients in order to easily study such a human condition as pain --particularly from the standpoint of understanding its chronic nature. It was with the long range view of establishing a firm footing for a simple device to help with the problem of objectively identifying individuals who might benefit from the more expensive investigative procedures which stimulated one of the authors1 to undertake this baseline study of the less glamorous aspect of the physiology of skin pressure measurement in the human as a first step in the long process of ultimately definitively controlling the entity called chronic pain. The pursuit of such an instrument for the ease of studying the more psychological illnesses associated with pain complaints almost certainly antedates such work as that of Merskey et al2 who studied the reaction to pain in a population made up of chronic schizophrenic patients in 1962 by using a skin pressure algometer along with soft pin-pricking to specific anatomic locations. Although that study led to the conclusion that the method was valid and their results were determined to be statistically significant, it never the less suffers from the same shortcomings of some of the others referenced below. This led to the present effort in order to establish a better method which would allow for the differentiation of more subtle psychological factors of the sort involved in chronic pain to a reliable degree in order to reduce the costs of studying such problems. Since the costs, taking a SPECT study as an example, can well be on the order of $1000.00 to $1800.00 one can see that it will not be easy to establish programs which might require large numbers of tests in order to clearly define questionable areas of brain function. An inexpensive test such as could easily be performed with the algometer would be worthwhile in sorting out certain categories of psychological factors which would be more likely to show positive imaging results.

There are a large number of scientific papers that cover the study of various forms of stimulation of skin surface ranging from light skin stroking to noxious temperatures such as, for example, is the case of Ferracuti et al3 who in 1994 related certain EEG changes to pain intensity using an analogue scale by cold water pressor testing. However, in that large group of studies there is a smaller subsection which seeks to demonstrate the significance of skin pressure in the study of pain. In this subsection this author1 has reviewed the experimental design and found that the differences between subjects are based upon averages and group means of readings with little attention to the degree of scatter relative to inter subject analysis,2,4,5,6,7,8,9,10 and none to intra-subject analysis. This includes one paper by Merskey and Spear4 where coefficient of variation was mistakenly referenced by one of the other authors who stated that Merskey and Spear had studied variance as well as averages; however Merskey and Spear did infer that they were thereby depicting scatter. We refer to our greater emphasis on scatter in terms of 'degree of Variance' (dVAR) as the mean of the Variances (VAR) which we have identified. Since we can not be sure that some of the skin pressure readings of many of the papers in the group were not taken under varying circumstances and probably at various times of the day we do not know that one can simply presume there is no significant intra-subject variance in these papers. Finally, further confounding factors in the form of a multiplicity of difficult to control variables, such as those involving several operators, and even some situations which are open to the presumption that important variables have been overlooked --resulted in our present effort to come closer to a more accurate prospective experimental design than the previous more correlative studies.

Left completely unanswered, in this author's1 survey of the literature, has been the question of whether one has the right to presume that an individual subject will not vary in his or her response to skin pressure testing from one reading to the next --to at least a small measurable degree, and if this can be measured then can we not more accurately arrive at the true Variance between subjects under various conditions? An answer pro or con would seem to be highly significant when one wishes to use any particular device or method for the study of human responses since it is well known that we all have some variation between our answers even to very finite questions often simply when the same question is separated by different intervals of time. Consequently, if one wishes to use such a subjective tool, commonly referred to as an analogue scale, from even one short period to the next, in order to have an individual subject rate almost any sensation, especially pain, on a scale of 1 to 5 or greater it is best to have an understanding of the degree of any natural Variance between readings within the same individual (intra-subject) such as Variance resulting from circadian rhythm or proximity to meals. Actually even before Pressure-Pain thresholds (PPT) are determined, as used in the work of Fredriksson et al10 for example, it may well be better to arrive at the Response Consistency (RC) of the group to be studied rather than simply making a statement such as "The increase in absolute PPTs could be due to individual variation."

Therefore in order to arrive at a baseline in terms of whether such a Variance in Response Consistency (RC) is or is not significant, a group of individuals was selected that would likely be somewhat homogeneous as to their constancy relative to this form of testing in order to ascertain whether there is any variation in the group that can be found to be significant. It was thought that once this is or is not established then the method can be used with other more heterogeneous groups and finally between various well defined groups in the study of similar types of stimulation's such as pain to verify the inter-subject work that has been done in the past by many researchers who probably overlooked or at least simply presumed that there was no need for concern about that variability in their tests of skin pressure.

In seeking support for an argument that there is an underlying consistency, to the Average individuals response in his or her judgement of skin pressure, which remains constant unless influenced either by past experiences or by extraneous external circumstances, it was not difficult to recognize that it would first be necessary to break this larger question down to one of identification of this consistency which up to the present has mainly been presumed to exist. Then, once clear as to which plays a greater part in influencing this consistency, either internal or external factors, it may more easily be possible to arrive at not only why, but even where and when the factors are operational in the human.

Page 3

Method

One hundred and thirteen dental students participated in this phase of testing over a period of approximately one year as they rotated in small groups almost weekly through the Facial Pain Clinic of the University of the Pacific Dental School in San Francisco. After filling out a questionnaire containing 29 questions with 77 possible answers covering questions that included demographic information as well as questions covering past and present pain experiences, they acted as their own test subjects by placing the 1 cm rubber tip of the same type of PDT Algometer 10 as that used in three of the above cited papers 7,8,9 at four different anatomic locations under only the direction, but not the actual skin stimulation, of one technician that was used for all testing. The technician then recorded the algometer readings which could not be observed by the subject and after the subject was satisfied with the degree of pressure that he or she applied to themselves the result was recorded by the technician. The tests were done in succession by rotation to both right and left nasal labial folds as well as the anatomic snuff box area of each wrist at the distal end of the radius. Prior to this the students had the benefit of reading a detailed description of the test and the reasoning behind wanting to perform it as well as informing them as to the very minimal limited risk involved with the students acting as their own subjects in order to control the point and degree of pressure applied to themselves to where they perceived that the stimulus of the pressure was an identical amount at the same area tested during the previous cycle. This method was chosen to avoid the common confounding error in experiments which is due to differences between testers who must assess the degree of pressure which they apply to subjects. Incidentally this older instrument was still available to us despite our being unable to reach the company any longer. It was chosen for this study in an effort to corroborate the previous studies which were cited. For future studies we will be using the newer digital readout Commander algometer from JTech Instruments for greater accuracy in readings. Since the degree of Variance between readings is a constant, in that it is not dependent on any particular type of algometer, with a given series of studies it would seem acceptable that test results of various examiners throughout the country could be compared to each other regardless of instruments used in the future in order to corroborate outcomes --provided they do not change instruments throughout a particular series of tests. In reviewing a paper by McMillan and Walshaw11 it is even possible that various different methods of testing on different areas of the anatomy will eventually have their own degree of Variation (dVAR) in Response Consistency (RC) in order to arrive at better ways of comparing groups more universally.

Upon completion of the entire series, SAS software was used to analyze the results both in terms of the means figuring from the standpoint of Averages as well as from the Variances of each set of 5 readings per each of the four test sites per individual by the well known formula:

nS x2-(S x)2

n(n-1)

Then not only the individuals average for the four sites was applied to the analysis of the SAS software separately to give the mean of the Average (AV), as commonly used by previous examiners, but also the mean of the variances was determined to give the Variance (VA). The Averages as well as the Variances of the readings along with the gender determinations then provided the dependent variables against which the independent variables in terms of the differences of the means, of the two groups making up the answers to the two questions, were further analyzed and reported below in the Results and Conclusions section.

Of the 113 dental students who participated in the study each was given the series of 24 questions with the possibility of 77 separate answers --the two which are significant for this analysis are as follows:

Have you had any pain in the past which can be described as being unusual beyond that normally expected with childbirth or minor trauma lasting longer than expected or of different character? (Past Unus. Pn)
('Yes' was ranked as 1 and 'No' as 2).

Do you have any pain or even physical discomfort of any type today? (Pres. Pn)
('Yes' was ranked as 1 and 'No' as 2).

As already clarified, the students were first instructed in groups of 4 to 6 that the series of tests was to determine their consistency in being able to judge skin pressure repetitively. At this time it was emphasized that the test was not to determine pain. Then after a short discussion as to risk and rewards where this innocuous study was concerned, the questionnaire was explained by clarifying that it was to help identify the results of the testing in order to see whether there were any variations by groups based upon their answers.

Upon completion of the entire questionnaire each student reported to the technician who identified the sites upon which the student placed the PDT algometer described above in rotation starting with the left nasal labial fold and rotating to the right nasal labial fold then to the left wrist and finally to the right wrist. Five tests were performed alternately at each site under the supervision of the technician, who merely observed to make certain that each student applied the pressure by the hand held algometer in the correct location and so that he or she could not see the meter. Prior to first undertaking the study the importance of the rotation of test sites was emphasized to the technician since it was wished to require a consistent length of memory disassociation prior to placing the algometer on the same location five different times in rotation rather than in testing each site by having each reading in immediate succession.

Page 4

Results

Table 1

All 113 Dental Students were analyzed both from the stand point of the degree of Variance in their series of five tests on the four sites mentioned and then the 20 separate results were subject to a Variance analysis which is termed (VAR) for ease of presentation. The overall results of the statistics are presented in Table 1 above.

Also the identical procedure was used, but the individual results were Averaged and the overall Average was referenced as (AV). These differences can be recognized by their separate histograms as seen in Fig. 1 below.

Averages of Responses

Figure 1

Since the averages of the reponses does not demonstrate what we are attempting to study, each response individually was calculated as to its variance from the mean and then plotted separately to give us the histogram shown below in Fig. 2. This better demonstrates how little variance there actually was in this group of dental students in their response to skin pressure testing when the problem of tester evaluation is taken out of the equation by allowing them to judge their own impressions.

Variances of Responses

Figure 2

The One Sample t-test below demonstrates that by comparison to their hypothesized means each set of results is clearly significant with a P-Value of <.0001 as demonstrated by the box plot shown in Fig. 3 below.

Figure 3

As recognized from this box plot there is a significant difference of 2.064 kilograms per square centimeter (kg/cm 2 ) between the means of the two methods of arriving at an understanding of the overall distribution of the results when calculated from these two different standpoints which also provides a significant P-Value of <.0001 when the Paired t-test is used to compare them to each other rather than their hypothesized mean.

Looking back to Figures 1 and 2, the importance of this difference in terms of a conclusion as to the distribution of the results of testing makes itself evident when one realizes that the Kurtosis of 36.859 with a 5.447 Skewness for the Variances (VAR) of the distribution compares to a Kurtosis of 2.059 and Skewness of 1.245 for the Averages (AV). Thus in comparing the two different methods of data analysis by groups --figuring them each from the standpoint of Averages as well as Variances, the distributions of the their Histograms clarify that one reason for the different appearance is that the outliers, as noted in the longer tail of the (VAR) histogram, are thereby given greater weight than when one simply calculates the results in the form of Averages (AV).

Page 5

Conclusions

From at least two standpoints the general conclusion would seem justified that Response Consistency, at least where skin pressure is concerned, is more finite than pain interpretation at the present time and thus closer to a true physiologic principal akin to temperature or blood pressure and may well be of help in the future where psychophysiologic studies are concerned.

Reasoning for the conclusion from a lack of gender differences.

The reason for emphasizing the importance of this difference in classification of distributions, and ultimately drawing conclusions from them, begins to make itself evident when one studies the results as based upon gender since it has long been considered that females have a different pain threshold than males. This is supported in the conclusions of the work of A. Fisher 5 through algometry in 1987 at least where skin pressure over certain muscle groups was concerned. Their results were based upon Averages and on cursory examination our results, where responses to skin pressure are concerned at first glance, one might think we support their findings of a gender difference. Here, with skin pressure as reflected in a Mean Difference of the AV as 1.082 kg/cm2 higher pressure in males than females and a significant P-Value of .0016 seen graphically in the box plot of Fig. 4 below.

Males Females Males Females
Averages Variances

Figure 4

However, when the VAR is then analyzed by the Unpaired t-test one finds only a P-Value of .0689 for the Mean Difference of 1.011 kg/cm2 as well as the same results for Fisher's PLSD test strongly suggesting that given larger population samples, perhaps at a later date, we may be able to further substantiate that there is no real difference in response consistency (RC) where individual judgement of the degree of skin pressure is involved between the sexes even though there may well be a difference between them in the degree of pressure used in order to interprete a judgement of the term 'pain'. This then would support a contention that RC, in being based upon differences in Variances, does not simply show differences in skin pressure alone -as is the case when only Averages are used to differentiate between groups, because it serves better to demonstrate the degree of scatter per individual in a given population.

Conclusion based upon Past verses Present Pain

Rather fortuitously two of the 77 questions in this early portion of the study already showed a reportable difference in sufficiently large enough numbers to further illustrate the use of the method. These results are relative to the two questions stated under Method above.

Reasoning for the conclusion from an unexpected finding.

To the first question concerning the presence of a significant degree of past pain (Past Unus. Pn) there were 22 'Yes' answers as signified by a number 1 in the analysis and 91 'No' signified by 2s. The Unpaired t-test demonstrates a P-Value of .0011 for the difference of 2.112 kg/cm 2 between the two means of the group .This is clearly significant as seen in graphically in the box plot Fig. 5 below and is supported by a Fisher's PLSD of .0010.

Yes No

Past Pain

Figure 5

There were 28 '1's and 85 '2's to the second question stated above for which we use the symbol (Pres.Pn). This means that 28 of the 113 Dental Students actually had some degree of discomfort of one sort or the other ranging from a sinus headache to the gout at the time of testing. While one might think that having some degree of discomfort during the actual testing would be more likely to be demonstrated by larger Variances in consistency of judgements during the test procedure this is not borne out by these results. Here the Unpaired t-test gives only a P-Value of .2596 as seen with its accompanying box plot in Fig. 6 below.

Yes No
Present Pain

Figure 6.

The Fisher's PLSD here is .3219. The Averages (Av) of .P-Values, being .0325 for the Past Unusual Pn group and .1114 for the Pres. Pn group are not as clear in their definition of a difference in outcome also splitting by gender adds nothing to either significance here as the groups consequently become too small.

Even a quick glance at the two box plots shown for the Past Unus. Pn and the Pres.Pn groups reveals that there were two outliers identified in the Past Unus. Pn group who clearly were appreciably above the mean of the group. Undoubtedly it will effect the overall results when these are removed from the whole study because they automatically shift to the opposite side when we change the analysis from Past to Present pain since they , after all, are still a part of the total group. This changes the results to where the Unpaired t-test

P-Value for the Past Unus. Pn group is now only .0801 and that for the Pres. Pn group is .3846. But this result, in still being close to a 95% significance level, gives us a greater appreciation for this method of analysis by Variance rather than by Averages as was seen earlier in the two markedly different shapes of the distributions shown in the histograms of Fig.1.

The question is do we have a right to exclude those two outliers? Perhaps with a clearer understanding of the importance of this analysis, in being based upon Variance rather than simply Averages, we can now come a little closer to the answer by excluding only the one most drastic outlier since it is more than double that of its closest neighbor. When we drop only the one outlier we now once again clearly reach significance with a

P-Value of .008 for the Past Pn group. Yet for the Pres.Pn group the P-Value of .2925 still does not nearly approach significance. Upon some reflection, we might say that --even if this one subject was perversely trying to defeat the test, his presence alone serves to prove the test --in that one might wonder about the type of underlying psychology that accounts for such motivation.

Finally, in looking back to Table 1 we can say that, since it is the smaller mean relative to the standard deviation of the Averages (AV), if we can take the mean of the Variances (VAR) in our rather homogenous group of dental students, in being reflective of a natural, if not even normal, Response Consistency (RC) then roughly .5 dVAR may well be used to correct for intra-subject variations. It can also be used to figure 'normal' intra-subject variation when future Response Consistency (RC) studies on other groups are performed which are identified as 'abnormal'.

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Summary

Despite the fact that at this early stage it is not yet possible to be certain that an important vital sign has here been identified to add to the understanding of the psychology of humans, it never the less does seem that we are on the right track to developing a tool that is more useful than, the less discriminating, simple skin pressure algometry has proven to be in the past. Skin Response Consistency (RC) testing also seems to be of consequence from the standpoint of its cost and ease of use thereby giving it a niche in the rapidly developing area of functional cranial imaging. It does not seem unreasonable to expect that, following further validation, this tool may play a large part in helping to arrive at a better understanding of lower level memory, aptly termed the attentional system as elucidated by Gazzaniga11, which some now consider a factor in the development of chronic pain.

Page 7

References

1 Stemmer, A. L., (1998) Verbal communication, http://www.ChronicPainInstitute.org

2 Merskey, H., Giles, A. and Marsalek, K. S., (1962 Clinical Investigation of Reactions to Pain. Journal of Mental Science, 108: 347-355

3 Ferracuti, S., Seri, S., Mattia, D., Crucca, G. (1994) Quantitative EEG modifications during the Cold Water Pressor Test: hemispheric and hand differences. International Journal of Psychophysiology, 17: 261-268

4 Merskey, H., and Spear, F. G. (1964) The Reliability of the Pressure Algometer, British Journal of Social Clinical Psychology, 3: 130-136.

5 Keele, K.D., Pain-Sensitivity Tests, (1954) Lancet, 1:636-639.

6 Moldofsky, H. Chester, W. J. (1970 Pain and Mood Patterns in Patients with Rheumatoid Arthritis: A Prospective Study, Psychosomatic Medicine, 32, 3 309-318

7 Reeves, J.L., Jaeger, B. and Graff-Radford, S. B., (1986) Reliability of the Pressure Algometer as a measure of Myofascial Trigger Point Sensitivity. Pain 24: 313-321.

8 Fischer, A. A. (1987) Pressure algometry over normal muscles. Standard values, validity and reproducibility of pressure threshold. Pain, 30: 115-126

9 Fisher, A. A., (1986) Pressure Threshold Meter: Its Use for Quantification of Tender Spots. Archives of Physical Medicine Rehabilitation 67: 836-838

10 Fredriksson, L., Alstergren, P., Kopp, S., (2000) Absolute and Relative Facial Pressure-Pain Thresholds in Healthy Individuals. Journal of Orofacial Pain 14:2 98-104

11 McMillan, A. S., Walshaw, D., (2000) Pressure-Pain Threshold in the Human Tongue. Journal of Orofacial Pain 14:2 93-97

12 Pain Diagnostic and Thermography Algometer Company of 17 Wooley Lane East, Great Neck NY 11021. .

13 Gazzaniga, M. S., (1998)The Split Brain Revisited. Scientific American. July 1998 51-55

*With some support from The Chronic Pain Institute, http://www.ChronicPainInstitute.org

** Correspondence to: August L. Stemmer, M.D.118 Post Street, Petaluma, CA 94952. email: stemmer@home.com Tel: 707-776-0737, Fax 707-776-0747

a) August L. Stemmer, M. D., D.M.D., F.A.C.S. is Clinical Associate Professor and Member of the Oro-Facial Pain Clinic at the School of Dentistry University of the Pacific 2155 Webster Street Suite 615, San Francisco, CA 94115

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