increasing pressure registered in kilograms was applied until the subject began to feel a sensation of pain. With an intelligent person the responses

Transcription

1 VARIATIONS IN THE SENSIBILITY TO PRESSURE PAIN CAUSED BY NERVE STIMULATION IN MAN. BY R. C. SHAWE. (From the Royal Infirmary, Manchester.) IN a previous paper (Brit. Journ. Surgery, Jan. 1922) I have given an account of a case of tabes in which the posterior roots of the 5th-9th thoracic nerves were cut to relieve the pain of the gastric crises. In this case the pressure on the abdominal muscles in the area of cutaneous anaesthesia requisite to elicit pain was markedly decreased during a crisis. The association of vascular phenomena such as widespread cutaneous pallor and rapid rise of blood-pressure with the above change in pressure sensitivity suggested making observations on the changes in the tint of the skin concurrently with those on the threshold of pressure required to produce pain. Method. The median nerve was stimulated, one electrode being placed about four inches above the wrist and the other at the wrist. The stimulation was by faradic currents of three degrees of intensity which may be roughly described as weak, moderate and strong. The weak currents caused a tingling sensation only; they were applied for half a minute or more. The moderately strong currents produced active contraction and caused discomfort but not actual pain. The strong currents could only be borne for a moment or two in some cases, in others for several seconds, and evoked vigorous muscular contraction. The pressure pain threshold on the thenar eminence was determined by vesthesiometer; increasing pressure registered in kilograms was applied until the subject began to feel a sensation of pain. With an intelligent person the responses given differ very little for the stimulation of the same spot, showing generally a slight diminution of the requisite stimulus after continued pressure. The pallor or flushing of the skin was noted in the region of distribution of the median nerve in the hand. Weak stimulation. Weak stimulation caused in all cases a marked lowering of the threshold. The weight required to produce pain decreased by 1*2 to 3*25 kgm., cf. Exps. 1 and 2. A summary of further experiments is given in the appendix. Since these observations were made through

2 PRESSURE PAIN. 289 the normal skin both superficial and deep receptors participated in the response to the pressure stimulation; it was therefore necessary to eliminate cutaneous receptors to decide whether there was an exaltation of deep sensation. This was carried out by freezing the skin with ethyl chloride. Owing to the pain produced by this procedure and its unpleasant after effects in some cases the freezing was complete in five only of the experiments. Exp. 2 may be taken as typical of those subjects in which cutaneous sensation was lost. It will be seen that after freezing the normal response rose by 1-2 kgm. The nerve was then stimulated with the weak current and on re-testing there was still a diminution of the threshold stimulus by 3-66 kgm. for deep sensation. Thus weak nerve stimulation increased the sensitivity to cutaneous and deep pressure sensation. In certain nerve lesions in which there is a loss of cutaneous sensation and retention of deep sensation it has been found that the deep sensibility is greater on the side of the lesion than on the opposite side. The result must be due to the lesion and not to the loss of cutaneous sensation, for, as we have seen, loss of cutaneous sensation in a normal region increases the threshold for deep sensibility. Weak stimulation generally produced flushing in the area of the palm supplied by the median nerve; it was especially marked in the index finger and the thenar eminence; in some cases the flushing was slight. The increased sensitivity might be due to the increased blood supply of the receptor structures. In order to decide this point, the circulation was excluded from the arm by adjusting a sphygmomahometer over the brachial artery. Observations were commenced after the circulation had ceased for two or three minutes, the final readings being taken from five to eight minutes after the stoppage of the blood flow. It was found that weak nerve stimulation during cessation of the circulation still caused increased sensitivity both to combined cutaneous and deep pressure and to deep pressure alone. The cessation of the circulation by itself generally caused a slight increase of threshold indicating a slight lowering of excitability of the receptors; there was however sometimes a slight decrease of threshold probably due to the accumulation of metabolites during the anaemia. Since cessation of the blood supply causes at the most but a slight increase in threshold of pressure pain it may be concluded that increase of blood supply can only cause a slight decrease in the threshold and that the decrease of the threshold caused by weak nerve stimulation is in the main produced in some other way than by vaso-dilation.

3 290 R. C. SHA WE. SERIES A. Exp. 1. Pressures in kilos. The successive numbers give the results of successive stimulations. With circulation Without circulation Skin and deep Deep tissues Skin and Deep tissues tissues only (skin deep tissues only (skin Median nerve frozen) frozen) Normal... 8, 8, 8, 8 10, 10, 10 10,10, 10 10, 10, 10 Weak stim.... 6, 6, 5*, 6* 5, 5, 5 6*, 6i, 61, 6* 7, 8, 8 7, 6, 6 7, 7, 7, 6, 6 10, 10, 9 Strong stim... 8*, 10, 9i, 8j, 10, 10 9, 9, 10 9, 11, 9, 10 12, 13, 12 Exp. 2. Normal... 8, 9, 10, 9, 8 10, 10, 10 7, 8, 7 9, 9, 9 Weak stim... 6j, 7, 6, 7j 6*, 6i, 6*, 7 4*, 5*, 4*, 4* 6*, 6, 6j 4, 5, 4, 6, 5 9, 9, 9 6*, 61, 6*, 61 10, 14, 11, 12 Very strong stimulation. Very strong nerve stimulation following a weak stimulation, increases the threshold for pressure pain and in some cases raises it above that present before the weak stimulus was applied. Thus in Exp. 1 pain was felt normally with a pressure of 8 kgm., weak stimulation lowered it so that 6 kgm. was sufficient to produce pain, whilst immediately after very strong nerve stimulation 9-3 kgm. was required. Similar results were obtained when the skin was anaesthetic and deep sensibility only left. The stimulation causes also very marked blanching of the skin and, as with the weak stimulation, the question arises whether the changed sensibility is caused by the changed blood flow. In order to decide this the same method as with the weak stimulation was employed. The circulation was stopped by an arm bandage and during the cessation of the circulation the median nerve was stimulated with strong induction shocks, it caused a marked increase in the threshold for pressure pain. I conclude therefore that the decreased sensibility is not in the main due to vaso-constriction. Moderately strong nerve stimulation (i.e. currents, which whilst causing vigorous contraction, were comfortably borne). Faradic stimulation of this degree produced marked pallor and lowered the threshold, but it differed in its effects from weak stimulation: (a) In causing in most cases a greater decrease of the threshold for pressure pain. This was especially as in Exp. 2. With weak stimulation a pressure of 6*5 to 7 kgm. was required whereas with moderately strong stimulation the pressure required was only 4 to 6 kgm. (b) In causing during cutaneous anaesthesia a less decrease of the threshold than is caused by weak stimulation. Thus in Exp. 3 during cutaneous anaesthesia weak stimulation reduced the pressure required

4 PRESSURE PAIN. from 10 to 61 kgm., whilst moderately strong stimulation reduced it to 9 kgm. only. It appears then that moderately strong stimulation increases skin sensibility much more than it increases deep sensibility. Since vascular changes do not account for the changes in pressure pain sensibility caused by nerve stimulation some other explanation must be sought for. The explanation might conceivably lie in a change of conductivity of the nerve fibres in the stimulated region; whilst it is possible that some decrease of conductivity is caused by strong currents there does not seem to be any evidence that it can be increased by weak currents and it will be seen from what follows that there is evidence that the chief cause of the change in the threshold of pressure pain is a change in the central nervous system. The nerve stimulation necessarily sets up impulses which affect the central nervous system. Since afferent impulses set up on one side of the body readily affect the nerve centres of the opposite side, either exciting or inhibiting them, the state of the palm of the hand was noted on the opposite side to that on which the median nerve was stimulated. It was found that there were in general changes in the pressure pain sensibility on the opposite palm of the same nature as those produced on the stimulated side. With weak stimulation there was increased sensibility on the opposite side in five out of six cases, and on several occasions flushing was observed in the thenar region on the opposite hand to that stimulated, although this was never very marked. With strong stimulation on one side, the sensibility on the opposite side diminished in five cases. SERIES B. Pressure in kilos. The successive numbers give the results of successive stimulations. Exp. 1. Right Right Left Left Stimulus thenar hypo-thenar thenar hypo-thenar Normal... 8, 8i, 8, 8 9, 9* 8, 81, 8 81 Weak stim j, 51, 5j 6j, 7j Strong stim... 10, 9 10, 9, 9 10, 9, 8 8, 8, 9 Exp. 2. Normal... 7, 7, 61 8, 71, 7 8, 8, 71 8, 8, 7, 71 Weak stim... 5, 5, 5 6, 51, 51, 5 6, 51, 5, 5 6, 51, 51 6, 6, 51 6, 61, 6 6, 6, 6 6i, 61 Strong stim.... 8j, 8 81, 8, 8 7j, 7 8, 71, 7 In some instances the strong stimulation caused pallor in both arms and widespread sweating; and in one subject there was the interesting result that the sweating both on the stimulated and on the opposite side PH. LVIII

5 292 R. C. SHA WE. were confined to the median nerve areas of both palms. The widespread pallor and sweating might be the result of pain, but it is most unlikely that the local effects found would be so produced. Moreover by adjusting the strength of the stimulus a decrease of sensitivity can be obtained in the opposite side before the sensation in the stimulated area amounts to pain, and weak stimulation does not cause pain. It appears then clear that the variations in the threshold of pressure pain caused by nerve stimulation are in the main due to changes in the central nerve cells with which the afferent fibres are connected: weak nerve stimulation causing an increase in excitability of the nerve cells and strong nerve stimulation causing a decrease of their excitability. The alterations in sensory responses described above are, I think, of utility in the several grades of mental excitement. This question I shall deal with in a later paper. SUMMARY. 1. Weak faradic stimulation of the median nerve causes a marked lowering of the threshold in both cutaneous and deep receptors. This is accompanied by cutaneous flushing in the area of the hand supplied by the median being especially noticeable in the index finger and the thenar eminence. 2. Very strong faradic stimulation raises the threshold in both cutaneous and deep receptors. It is accompanied by marked vasoconstriction in the palmar distribution of the median. 3. Faradic stimulation of the median nerve of a degree intermediate to the above causes a lowering of the threshold in the cutaneous receptors only. 4. The same changes were found to occur even if the circulation had been excluded for 5 to 8 minutes, and therefore vascular change cannot be held to explain the sensory variation. 5. Change of conductivity in the nerve fibres cannot explain these results because although strong currents may decrease conductivity, weak currents are not known to increase it. 6. Observations on the contra-lateral side to that stimulated showed that during weak nerve stimulation there was a corresponding lowering of the threshold in the median and ulnar areas which in some cases was associated with a flushing in the thenar region of the side stimulated and the opposite side. During very strong faradic stimulation the threshold -was raised in both ulnar and median areas of the opposite side to that stimulated. This was associated in one case with localised sweating in

6 PRESSURE PAIN. 293 the median area of the palm on the contra-lateral and homo-lateral sides. 7. I conclude that the variations in the threshold of pressure pain caused by nerve stimulation are due principally to changes in the nerve cells with which the afferent fibres are connected. Finally, I wish to express my thanks to Prof. Langley for invaluable criticism and assistance with the paper, to Mr J. Morley, F.R.C.S., for reading the MS. and to Miss Lee for assistance with the electrical work. SERIES A. Pressure in kilos. The mean of the results of the successive stimulations are here given. With circulation Without circulation Skin and Deep tissues Skin and Deep tissues deep tissues only (skin deep tissues only (skin Median nerve frozen) frozen) Exp. 3. Normal , 7 Exp. 4. Normal * Exp. 5*. Normal * Exp. 6. Normal * * Exp. 7. Normal Exp. 8*. Normal * *Freezing incomplete. Stimulus Exp. 3. Normal... Exp. 4. Normal... SERIEES B. Pressure in kilos. Means Right Right thenar hypo-thenar of results. Left thenar Left hypo-thenar