STUDIES ON PAIN. A NEW METHOD FOR MEASURING PAIN THRESHOLD: OBSERVATIONS ON SPATIAL SUMMATION OF PAIN J. D. Hardy, … , H. G. Wolff, H. Goodell J Clin Invest. 1940;19(4):649-657. https://doi.org/10.1172/JCI101168. Research Article Find the latest version: https://jci.me/101168/pdf STUDIES ON PAIN. A NEW METHOD FOR MEASURING PAIN THRESHOLD: OBSERVATIONS ON SPATIAL SUMMATION OF PAIN By J. D. HARDY, H. G. WOLFF, AND H. GOODELL (From the Russell Sage Institute of Pathology, in affiliation with the New York Hospital and Departments of Medicine and Psychiatry, Cornell University Medical College, Newv York) (Received for publication April 18, 1940) The purpose of these studies has been to pre- has many advantages, the most important of sent a new method for measuring pain thresholds, which are: together with experimental observations on the physiology of pain and on the effect of various 1. The necessary apparatus is simple and easily chemical agents upon pain thresholds. constructed. Methods for estimating the intensity of the 2. The intensity of the stimulating agent can be stimulus required to evoke a painful sensation in precisely measured. the skin may be classified under the headings: 3. The sensory threshold to pain as a result of mechanical, chemical, electrical, and thermal. The this stimulus is a sharply defined experience so mechanical devices as used by von Frey (1) and that thresholds may be determined with accuracy by Eddy (2), involving production of pain by higher than that of other methods. pressure, empirically correlated the amount of 4. The method is flexible so that the time of pressure and the pain threshold. The deforma- exposure to the stimulus, the state of the skin, tion and internal stresses in the tissues responsible etc., can be varied at will. for the sensation were not investigated, and it may 5. The stimulus can be used for large and small not be assumed that there is a simple proportion- areas of skin even though the surface be irregular. ality between the pressure and the intensity of the 6. The stimulus can be repeated in rapid suc- sensory stimulus. The chemical methods of irri- cession without injury to the skin surface tested. tating the skin have not been thoroughly explored. The thermal methods of producing pain are Radiation was first used to stimulate skin sen- among the oldest and were introduced as a method sation by Alrutz (8) in 1897. In 1921 Sonne of study by Goldscheider (3) in 1884. These (9) focussed the rays of the sun onto the skin to methods, involving application of hot objects to produce pain. Neither of these authors was the skin or immersion of a member in hot water, studying pain, although. Sonne showed that the have added little of a quantitative nature to the white human could stand more penetrating radi- study of pain sense. The only method of study ation than non-penetrating radiation. In 1934 for which the relation of the stimulus to its physi- Dallenbach (10) used radiation to produce pain ologic and sensory effects has been investigated, is in an investigation of adaptation. the electrical method of stimulating with Faradic The experiments of Oppel and Hardy (11) current, introduced by Helmholtz (4) and studied demonstrated that the radiation technique could be carefully by Martin (5). This method has been applied quantitatively to the study of temperature widely used, largely in the study of the action of sense, and the methods devised by these authors analgesics by Martin and his co-workers and by have been adapted to the measurement of pain Macht and his associates (6). Recently, Hauck thresholds. The pain is produced by concentrat- and Neuert (7) have used high frequency currents ing the radiant energy from a powerful source in the study of pain thresholds. onto the skin. The sensation produced is sharp, Valuable work has been done with the above a " bright pain " (12), and is to be distinguished methods but none of them combines the flexibility from an ache or deep pain. Whereas our obser- and precision necessary for some investigations. vations apply specifically to pain in the skin, it is The use of thermal radiation for the study of pain probable that they have broader implications. 649 650 J. D. HARDY, H. G. WOLFF, AND H. GOODELL METHOD the end of the exposure. This threshold pain was easily recognizable even by untrained subjects. The sensation The apparatus for measuring the pain threshold is was that of heat finally "swelling" to a distinct, sharp shown schematically in Figure 1. stab of pain at the end. When this condition had been The light from a 1000 watt lamp, L, was focussed by a reached the radiometer, R, was placed in the aperture in condensing lens, C, through a fixed aperture onto the place of the forehead of the subject and the intensity of blackened forehead of the subject, H. The surface of the radiation measured in gm. cal./sec./cm.2 This value the forehead to be tested was thoroughly blackened with was considered to be the minimum stimulus for pain and India ink. This measure was taken to insure total ab- was shown to be proportional to any thermal changes tak- sorption of the radiation, regardless of pigmentation of ing place in the skin, whether the total change or rate of the skin, and to eliminate possible effects due to the pene- change of skin temperature or both be considered. The tration of the rays below the skin surface. The stimulus radiometer was calibrated by means of a radiation could thus be considered as purely thermal. standard of the U. S. Bureau of Standards and also with The intensity of the radiation was controlled by means an experimental black body. of a rheostat, RH. Immediately in front of the lamp The time required to make a single observation was was mounted an automatic shutter, P, which was arranged usually less than 2 minutes. The maximum variation to allow the radiation to pass through to the subject for from the mean was in all cases less than 12 per cent, and exactly 3 seconds. This time interval was so short that several measurements agreeing within 2 per cent were the heating of the skin was local and effects due to con- made to establish a threshold. Among the physical fac- duction at the edges of the aperture could be neglected. found to influence the threshold were Thus, the temperature changes in the exposed area were tors which were that time of those which tended to increase the rate of heat loss from assured to be uniform It was necessary the a great decrease stimulation be fixed precisely as the pain threshold de- the skin. Sweating was found to cause pended upon this factor. In the present apparatus P was in the effectiveness of this type of stimulus. Also strong fixed to a heavy pendulum. The shutter, S, was operated drafts and especially cold rooms had to be avoided. Cir- manually, and allowed stimulation of the subjects when cumstances which affected the condition of the skin, such desired. as sunburn, calluses, or skin lesions, changed the level of The method of making the measurement of pain the threshold. As the skin temperature had some in- threshold was as follows: The subject seated himself and fluence on the threshold, most tests were made on the placed his forehead in position. The aperture was ar- forehead. The forehead temperature is constant (34.0 ± ranged so that 3.5 cm.2 of blackened skin could be ex- 0.5° C.) over a wide range of external temperatures posed. After a minute or so the shutter, S, was lowered (20 28° C.), barring sweating from any cause. and the radiation allowed to fall on the skin for 3 sec- onds. The subject reported on his sensation. If no pain RESULTS was experienced, the intensity of the light was increased and after 30 to 60 seconds the test was repeated. This The measurements of pain threshold on the 3 procedure was followed until the subject just felt pain at subjects over a period of a year are shown in IR_ yTO GAL IR FIG. 1. ARANGEMENT OF APPARATUS FOR MEASURING PAIN THREsHoLDs STlUDIES ON PAIN 651 40 - RG. J.H. H.W. 30 - qfl z 0 I- a:> 20 - 0 0 Ua. 10 - z ox .4 .20 .24 CAL /CM"/SEC FIG. 2. FREQUENCY CHARTS OF PAIN THRESHOLD MEASUREMENTS FOR 3 SUBJECTS Figure 2. Half the measurements on any subject 9 a.m. and 7 p.m., showed no significant changes were within + 2 per cent. Further, all 3 sub- with time of day. jects had nearly the same thresholds, i.e., 0.229, Except for procedures directly affecting the 0.231, 0.233 gm. cal./sec./cm.2 for H. G. W., skin and chemical agents having analgesic action, J. D. H., and H. G., respectively. Any single we have demonstrated few factors affecting the measurement could be expected to be within + 5 pain threshold. Binding the head firmly with a per cent of this average value. Daily measure- bandage through which a window was cut for the ments of pain thresholds were made over a period threshold measurement was found to increase the of about a year. Before measuring pain thresh- average threshold by about 4 to 6 per cent. This olds, estimates were made by each subject con- rise, though definite, shows that an intense sen- cerning his or her general effectiveness as regards sation due to a stimulus which is not painful daily tasks and emotional state.