Myasthenia gravis is a chronic disease characterized by excessive fatigue and

Transcription

1 252 J. Physiol. (1953) I22, NEUROMUSCULAR TRANSMISSION IN MYASTHENIA GRAVIS By H. C. CHURCHILL-DAVIDSON AND A. T. RICHARDSON From the Departments of Anaesthetics and Physical Medicine, St Thomas's Hospital, London (Received 26 February 1953) Myasthenia gravis is a chronic disease characterized by excessive fatigue and weakness of the voluntary muscles. It is generally accepted that the functional abnormality in this disease is a failure of neuromuscular transmission, and the previous theories of causation have been derived from studies of the action of acetylcholine in normal neuromuscular transmission. The introduction of decamethonium salts (C. 1), which act in a manner similar to acetylcholine (Zaimis, 1951; Burns & Paton, 1951), has allowed a re-investigation of the mechanism of not only normal but also myasthenic neuromuscular transmission, and, in particular, has indicated the response of the motor end-plates in these states to chemical excitation. In this study the reaction of myasthenic muscles to decamethonium is contrasted with that of normal muscles under identical conditions and the mechanism of these different reactions investigated. Based on this investigation a new theory of the failure of neuromuscular transmission in myasthenia gravis is presented. METHODS Electromyographic and ergographic 8tudies of muscle function in normal and myasthenic patients before and after decamethonium All the ergographic and electromyographic recordings were made before and after 2-5 mg of decamethonium iodide was given intravenously into an arm vein in divided doses of 1-5, 5 and 5 mg over a period of 8 min to twelve myasthenics and to six normal control subjects. The effect of neostigmine and 'Tensilon' (Ro , 3-hydroxyphenyldimaethylethylammonium chloride) upon changes in muscle function was also measured. Throughout all the experiments, routine medication with neostigmine was stopped for a minimum period of 18 hr before the investigations. Measurement of ptosis. The subjects were placed in a supported position on a couch with their heads held at rest. The width of the palpebral fissure was recorded photographically under conditions of constant lighting and positioning, and finally expressed as a percentage change of the mean control value. Measurement of grip strength. The height to which a column of mercury could be raised by squeezing a hand-bulb was measured and expressed as a percentage change of the mean control value.

2 NEUROMUSCULAR TRANSMISSION IN MYASTHENIA 253 Straight leg raising. The height to which the extended leg could be raised above the horizontal level of the couch was determined and alterations in this height expressed as a percentage of the mean control value. Electromyographic measurements. To measure the effect of decamethonium iodide on muscle function, the ulnar nerve was stimulated percutaneously at a rate of 1/sec and in selected cases at 4/sec and 2/sec. Action potentials derived from the hypothenar muscles of both hands were recorded by skin electrodes. The -3 msec stimulus was delivered from an earth-free source and the skin electrodes were placed so as to obtain integrated action potentials, peak-to-peak measurements of which were used as an index of the number of muscle fibres responding. To detect errors due to any change in position of the skin electrodes, the action potentials were observed both during the paralysing and recovery phase; the final action potentials after recovery were matched with the initial ones. The potentials were amplified by a conventional 2-channel differential amplifier (time constant, 3 msec; high-frequency response, 6 db down at 5 keyc). The potentials were photographed from a cathode-ray oscillograph, the time-base of which was triggered by the stimulator. The excretion of decamethonium in normal and myasthenic subjects The activity of decamethonium iodide excreted in the urine was investigated in six normal and five myasthenic subjects. After withholding all fluids for 2 hr, 2-5 mg of decamethonium iodide was given intravenously in a single dose, and all the urine excreted by the subject during the next 3 hr collected. This urine was found to produce a contracture when diluted samples were injected into the neck veins of chicks and the duration of this ensuing contracture was noted (Zaimis, personal communication). Similarly, known quantities of decamethonium in urine were injected into chicks and the log-dose curve of the duration of contractures plotted. From these results the concentration and activity of decamethonium in the samples of urine from both myasthenics and normal subjects were calculated. RESULTS Effect of 2-5 mg of decamethonium iodide on muscle function in normal and myasthenic subjects In this series, all the six control subjects developed widespread paralysis of the limb musculature, together with a marked fall of the action potentials of the hypothenar muscles after the administration of decamethonium iodide. The twelve myasthenic patients showed varying and generally only slight degrees of paralysis to volitional effort and the action potentials of the hypothenar muscles were sustained (Fig. 1). In fact, the action potentials of eight of the myasthenic patients showed an initial increase after the injection of decamethonium. No such increase of the potentials was recorded in any of the control subjects. Further, whereas in normal subjects a common feature was twitching of the back and limb muscles with a feeling of tightness in the jaw and calf muscles, in patients with myasthenia this was consistently absent. Effect of different rates of motor nerve stimulation on muscle action potentials after decamethonium and D-tubocurarine Measurements of the action potentials produced by motor nerve stimulation at 2/sec (tetanus) and at 4/sec (twitch) were made in three normal subjects after the administration of D-tubocurarine chloride and of decamethonium

3 254 H. C. CHURCHILL-DAVIDSON AND A. T. RICHARDSON iodide; similar observations were made on two myasthenic subjects before and after decamethonium iodide. With D-tubocurarine, tetanic stimulation in normal subjects rapidly failed, whereas twitch remained unaffected until a dosage sufficient to produce a severe neuromuscular block was used. With decamethonium, however, 2-5 mg decamethonium iodide Fig * * C *... * '-1- U 2 E ~ 3- C -4-5 C U v r -9. * U -1 Myasthenic patients Normal subjects The effect of an intravenous injection of 2-5 mg of C. 1 on the action potentials of the hypothenar muscles in fifteen control and sixteen myasthenic subjects. The mean response is shown as a latticed line. tetanus and twitch were affected equally. In the clinically weak muscles of myasthenic subjects, tetanus was not sustained for so long as twitch; and furthermore, the rate of failure of tetanic stimulation was accelerated by the administration of decamethonium (Fig. 2). The effect of neostigmine and 'Tensilon' (Ro ) on the skeletal muscle paralysis induced in normal and myasthenic subjects by decamethonium We have previously shown that two types of response may be anticipated in patients with myasthenia gravis after an injection of decamethonium iodide (Churchill-Davidson & Richardson, 1952a). First, the muscles may exhibit a diminished sensitivity to the drug so that large doses are tolerated without effect. Secondly, when increasing doses of decamethonium are given, the tolerance of the muscles of myasthenic subjects to this drug is first overcome in those muscles which showed clinical evidence of fatigue and weakness. Once these susceptible muscles begin to be paralysed by decamethonium, they remain in this state for a prolonged period as compared with the muscles of normal subjects.

4 NEUROMUSCULAR TRANSMISSION IN MYASTHENIA 255 The effect of neostigmine upon this persisting paralysis after decamethonium in these weak muscles of myasthenics was in contrast to its effect on the muscles of the controls. Thus, one myasthenic who remained in a very weak state for over 3 hr after decamethonium with no sign of recovery, improved dramatically 1 min after the intramuscular injection of 2-5 mg of neostigmine. In a further three cases the effect of neostigmine, given soon after the onset of decamethonium paralysis, was again a rapid improvement in muscle function. When neostigmine was given before the injection of decamethonium, the tolerance of the weak myasthenic muscles to decamethonium was markedly increased, whereas the muscles of normal subjects became slightly more sensitive. C Before C.1 -v 5- E 6- E 6 ~~After C.1\\ bo 7 - C U Time (sec) Fig. 2. The effect of tetanic stimulation (2/see) before and after 2-5 mg C. 1 (intravenously) in a case of myasthenia gravis. Neostigmine, in doses of mg even when combined with large doses of atropine sulphate, produced marked and unpleasant side-effects in normal subjects. Further, the slow onset of action made it difficult to obtain clear-cut end-points of its effect on the paralysis produced by decamethonium. For these reasons, it was more convenient to use a neostigmine-analogue, Tensilon; this compound, like neostigmine, acts mainly by the inhibition ofcholinesterases and to a small extent by a direct acetylcholine-like action on the muscle fibres, but it has only weak muscarinic effects (Hobbiger, 1952). It can therefore be given intravenously with minimal discomfort. The action of 2 mg of Tensilon given 12 min after the onset of paralysis from 2*5 mg of decamethonium was studied in six normal control subjects (Table 1A) and twelve myasthenic patients (Table 1 B). All the normal control subjects showed an increase of the decamethonium-induced paralysis

5 256 H. C. CHURCHILL-DAVIDSON AND A. T. RICHARDSON 1-: oo ~~~~~4) C 4) r C) -@ C) 4)4) oto ;L -a 4. bio 1:1 P4 *E E<- W b ~~~~ P.- >-3n la ;4 C) bo 1. t C) ^ 1 b ^. "^ E-) ^ C) 4.4 4)> 4)W Hb 4).o C) 514 EH w M.~ -4 4) Ca -4) H 4) ~ r. o- W PA ma 1ttio -4) 14) 6GF -4) rm _1 g -4) z eooox U: 1 _ CO _ o -O Ci 114 O O o ci _ ei 1C)1C 1-1- co I C) cm _ CO I C) C;l t-1o CO O 4 ** *C* O CO C4-1ec ~ ca es ce cs _4) coo azt-col t--t - COb C) 1 1 CO oco wo o b- 1 1CO I 4) *$ 4 o + ka 4)- W ) C~~~~~~~~ o** **o**o* * * -CCWI=M b (m-4o COCO~~~~~~~~~~~C * * 4) & ocd XO CO CO1CO o cq coo8s CO P4 xo Xb Io sr rr O4t-OC'N - C1 44X 11. W 4.1 4) o C4 ~ Ca E4) ob- 4) _w_)".,g- V.n - E 1S C). 1- gsq fjiq ~II e 4)4 W ) t- 1 Co Co - C;.~ o E-11.C 11cCO C co Co Co t-a CO E CO1 CO CO CO CO C mc COC 4 =C 1 CO - CO "- 1 CO 1 M.;4 s *

6 NEUROMUSCULAR TRANSMISSION IN MYASTHENIA 257 starting 3-4 sec after the injection of Tensilon and lasting for 2-3 mi (Fig. 3). These findings, however, are in contrast to the reaction in cases of myasthenia gravis, where-as previously stated-the mode of response depended largely on the extent of the myasthenic weakness. For example, those cases in which the disease was localized to a single group of muscles, such as the ocular or bulbar musculature, showed a high degree of tolerance, so that there was no significant alteration of either volitional or electromyographic response following both decamethonium and Tensilon. Three of our cases were in this 'resistant phase' of myasthenia (Fig. 4). On the other hand, eight cases showed widespread clinical weakness and all exhibited a remarkable reversal of the decamethonium-induced paralysis within 45 sec ight ~~~~~~~~~~~~Right -8 I~~~~~~plpba palpebral.o2v.. +2 fissure - -4 t 2 ft-arm ~~~~~~~~~~Right-arm - W. e.m. e.m.g. +2 Right-hand 2-6 ~~~~~~~~~~~~-2- iu - X Xl,/R 4 Right-hand C w 2.ergometer ~~~~~~~~Right-hand (xmn Tieergometer.. - ~~~~~~~~~~~~~~~~~4-Left-arm Fig. C1 41 aising 82 F. 4 1 e.m.g. 41 right leg -2 m -6: ~~~~~~~~~~~-4-1-9o an5-5 -5amg C.1 Tensilon 2g mg1e mg C ensilon 2mg 2 9 T lb '6 Time (min) Time (min) Fig. 3. Fig. 4. Fig. 3. The effect of the intravenous injection of Tensilon (2 mg) on ptosis, voluntary muscle power and the action potentials of the hypothenar muscles after 2-5 mg. 1 in a control subject. Fig. 4. The effect of the intravenous injection of Tensilon (2 mg) on ptosis, voluntary muscle power, and the action potentials of the hypothenar muscles in a myasthenic patient with localized weakness. of the intravenous injection of 2 mg Tensilon, resulting in complete restoration of muscle power (Fig. 5). After 5-1 min, this muscle power began to fail again, but in some instances the recovery was maintained for up to 3 min. It was noteworthy that in three of these eight cases the injection of decamethonium produced paralysis of muscles that had not previously shown any clinical weakness or fatigue characteristic of myasthenia, and that Tensilon completely reversed this paralysis also. In the remaining one myasthenic PH. CXXII. 17

7 258 H. C. CHURCHILL-DAVIDSON AND A. T. RICHARDSON patient, a fall in the power of straight-leg raising occurred after Tensilon as in normal subjects. Fig. 6 shows a comparative example of the effect of Tensilon on the degree of ptosis induced by decamethonium in both normal and myasthenic subjects. It illustrates the fact that Tensilon potentiates the decamethonium-block in normal subjects, whilst antagonizing it in myasthenic patients \ ~~~~~~~fissure -1* 5 >> I >. Right-hand e.m.g. +5 +t26-1 Left-hand ergometer +5 5 Raising left leg -7 '1 2125mg Tensllon 2 mg Time (min) Fig. 5. The effect of the intravenous injection of Tensilon (2 mg) on ptosis, voluntary muscle power and the action potentials of the hypothenar muscles after 2-5 mg C. 1 (intravenously) in a myasthenic patient with widespread clinical weakness. In two cases the return of muscle power produced by Tensilon after decamethonium was almost matched by that produced with the same dose of Tensilon alone (Fig. 7). The excretion of decamethonium in normal and myasthenic subjects Determinations of the rate of excretion of decamethonium in the urine of both myasthenic and normal subjects failed to show any significant differences between the two groups. Over the 3 hr period, a mean of 948 mg was excreted by the six normal subjects with a range of ,g, whereas 955,g was excreted by the five myasthenic patients with a range of jg-the amount recovered representing approximately 4 % of the total given. It does not, therefore, appear that the elimination of decamethonium was significantly different in the two groups.

8 NEUROMUSCULAR TRANSMISSION IN MYASTHENIA 259 C.n.- _ 7- -_ C. IU _ 1 min TEN min 11 min Fig. 6. Normal I1 Myasthenic 115 min 17min The effect of the intravenous injection of Tensilon (2 mg) on the degree of ptosis after 2-5 mg C. 1 (intravenously) in a normal and a myasthenic subject. 5- -~4- C 44i3 c2 C JC 1 I 2m2mg g 12mg Tensilon ~C.1 Tensilon Time (min) Fig. 7. The effect of the intravenous injection of Tensilon (2mg) on the voluntary muscle power before and after 2-5 mg C. 1 (intravenously) in a myasthenic patient with marked clinical weakness. 17-2

9 26 H. C. CHURCHILL-DAVIDSON AND A. T. RICHARDSON DISCUSSION Our results indicate that the neuromuscular block induced by decamethonium in the clinically-weak muscles of myasthenic patients differs profoundly from that in the corresponding muscles of normal subjects under the same conditions. Thus, the block occurring in normal subjects has the characteristics of one due to pure depolarization, in that it is constant for varying rates of motor nerve stimulation, potentiated by neostigmine and Tensilon, and muscle twitches and fasciculations are a common feature. In contrast, the neuromuscular block produced by decamethonium in myasthenic subjects is greater for tetanus than twitch, reversed by neostigmine and Tensilon, and unaccompanied by the muscle tightness and fasciculations seen in normal subjects, although preceded by a brief improvement in muscle strength. In fact this block, when developed, exhibits the characteristics of one due to competitive inhibition. Although Tensilon, like neostigmine, will temporarily reverse the inherent neuromuscular block of myasthenia gravis, which has long been known to be of the competitive inhibition type, our results show that if a further block is produced by decamethonium, Tensilon will still completely restore muscle function. This leaves no doubt that Tensilon in our cases did more than reverse any underlying myasthenic block, and, in fact, also reversed the additional decamethonium block. It is noteworthy that although the response of various muscles to decamethonium followed by Tensilon in normal subjects (Table 1 A) and in patients suffering from myasthenia gravis (Table 1 B) showed a consistent difference with volitional measurements, such a consistency was not always obtained with electromyographic measurements. The reason for this appears to be twofold. First, in normal subjects, decamethonium in doses of 2-5 mg produces a severe fall in the action potential of the hypothenar muscles (average change = %), so great, in fact, that any further potentiation by Tensilon is difficult to detect. Secondly, in the majority of cases of myasthenia the same dose failed to produce in the hypothenar muscles a sufficient drop in the action potential (average change= %) to make the reversing action of Tensilon significant. The fact that muscle weakness is more difficult to demonstrate at low rates of motor unit stimulation as used in electromyography (1/sec) than with the higher rates involved in a volitional effort (3-4/sec) is additional evidence for the production of a competitive inhibition block in myasthenia from decamethonium. Differences between the drop in amplitude of the action potentials of the right and left hypothenar muscles were also observed. These differences, related to the exercise involved in ergographic measurements used on one limb, have previously been discussed in relation to the action of decamethonium on normal muscles (Churchill-Davidson & Richardson, 1952b).

10 NEUROMUSCULAR TRANSMISSION IN MYASTHENIA 261 The finding that the majority of cases of myasthenia showed a brief improvement in muscle function (recorded electromyographically) after decamethonium but before the onset of any neuromuscular block would seem to indicate an initial anti-competitive inhibition function of the decamethonium, as this was not seen in normal subjects. This function must presumably be due to the direct depolarizing activity which it possesses. It appears, therefore, that the weak myasthenic muscles exhibit a dual mode of reaction to decamethonium. Thus, the initial effect is one of depolarization and this is immediately followed by competitive inhibition. The series described in this paper also confirms our original finding that whereas normal subjects undergo a marked paralysis with decamethonium, the muscles of myasthenic patients appear to be relatively resistant to this drug except where those muscles exhibit excessive fatigue and weakness. Such a state of resistance can also be explained by a dual mode of reaction to decamethonium, for in the transition of response of the myasthenic motor end-plate from that of depolarization (normal muscle) to that of competitive inhibition (fatiguable myasthenic muscle), a stage will be reached when the two types of block are antagonistic, so that the end-plate displays a decreasing sensitivity to the action of depolarizing drugs. A dual mode of response to decamethonium, identical to that we have observed in myasthenia, has been described as occurring physiologically in various species of animals (Zaimis, 1952). Thus, while cat, avian and frog muscle responds to decamethonium by pure depolarization, the muscles of monkeys, dogs and hares can exhibit features of both a depolarization and a competitive inhibition block with this substance. Further, a transition of response from depolarization to competitive inhibition produced by decamethonium can be demonstrated in the white muscles of the latter species, including a stage when high doses are needed to produce any paralysis at all. Zaimis has also shown that succinylcholine and tridecamethonium may lead to similar results. Such a dual mode of reaction produced by decamethonium in myasthenia could be accounted for in two ways; first, by changes in the drug itself (Randall, 1952); and secondly, by an alteration in the mode of response of the motor end-plate. The fact that decamethonium is excreted by myasthenics unchanged indicates that the ultimate production of competitive inhibition in myasthenia by decamethonium is in fact due to changes in the motor end-plate. Although there is no evidence, as yet, that the motor end-plates of any animal respond to acetylcholine-or one of its breakdown products-in this dual manner, such a conception in man would account for all the clinical features of myasthenia gravis, notably the usual limitation of weakness to isolated muscles. In the circumstances obtaining in myasthenia gravis it is

11 262 H. C. CHURCHILL-DAVIDSON AND A. T. RICHARDSON reasonable to suppose that the molecules of acetylcholine liberated at the motor end-plate would, as is probably the case with decamethonium, first bring about a momentary depolarization of the end-plate and then these molecules, or those produced by the breakdown of acetylcholine, would persist in an inactive form, so combining with the receptor protein that the excitation threshold is raised and a block with all the characteristics of competitive inhibition results. Certainly the muscle dysfunction of myasthenia gravis cannot be explained in terms of abnormalities of the production or the destruction of acetylcholine, because the inherent neuromuscular block in this disease has all the characteristics of one due to competitive inhibition. Further, the presence of a circulating curare-like substance affecting the muscles-a hypothesis which lacks the support of consistent evidence-would not explain our findings with decamethonium. In the event of such a substance being responsible for the myasthenic syndrome, the weakest muscles should be the most resistant to decamethonium, because neuromuscular blocks of the competitive inhibition and depolarizing type are antagonistic. In fact, the reverse is the case. Finally, the conception of myasthenia gravis as due to a dual mode of response of the motor end-plate to acetylcholine is directly supported by two clinical findings. First, some of the muscles of myasthenics have been shown to be resistant to acetylcholine (Acheson, 1944; Buchthal & Engbaek, 1948; Wilson & Stoner, 1947) as they are to decamethonium. Secondly, both neostigmine and Tensilon fail to produce muscle fasciculations in this disease, and the former, if used repeatedly, can on occasions not only fail to relieve the neuromuscular block of myasthenia but actually lead to its potentiation (Wilson, personal communication). SUMMARY 1. If sufficiently high doses of decamethonium are given to patients suffering from myasthenia gravis, the muscles which show the greatest degree of clinical weakness are the first to undergo a failure of neuromuscular transmission. This neuromuscular block, when produced, has the characteristics of one due to competitive inhibition. 2. The clinically normal muscles of myasthenics possess a marked tolerance to the depolarizing action of decamethonium iodide. 3. A comparison of the urinary excretion of decamethonium in myasthenic patients and normal subjects indicates that there is no significant difference in the two groups. 4. The importance of these findings in relation to the action of acetylcholine at the motor end-plate in cases of myasthenia gravis is discussed, and it is suggested that the failure of neuromuscular transmission in myasthenia is derived from abnormalities in the response of the motor end-plate to acetylcholine.

12 NEUROMUSCULAR TRANSMISSION IN MYASTHENIA 263 We would like to thank Dr E. J. Zaimis for her co-operation in the analysis of decamethonium in the urine of our subjects and her helpful advice throughout this work; Dr J. M. Tanner for statistical advice; Roche Products Ltd. for generous supplies of 'Tensilon'; and the members of the Anaesthetic and Physical Medicine Departments at this Hospital who acted as control subjects. This work was carried out with the aid of a grant from the Endowment Fund of St Thomas's Hospital. REFERENCES ACHESON, G. H. (1944). Physiological and pharmacological aspects of neuromuscular diseases. J. nerv. ment. Di8. 1, BUCHTHAL, F. & ENGBAEK, L. (1948). On neuromuscular transmission in normal and myasthenic subjects. Acta p8ychiat., Kbh., 23, 3-7. BURNS, B. D. & PATON, W. D. M. (1951). Depolarization of the motor end-plate by decamethonium and acetylcholine. J. Phy8iol. 115, CHURCHILL-DAVIDSON, H. C. & RICHARDSON, A. T. (1952a). The action of decamethonium iodide (C. 1) in myasthenia gravis. J. Neurol. 15, CHuracILL-DAvIDsoN, H. C. & RICEaIDsoN, A. T. (1952b). Decamethonium iodide (C1): some observations on its action using electromyography. Proc. R. Soc. Med. 45, HOBBIGER, F. (1952). The mechanism of anticurare action of certain neostigmine analogues. Brit. J. Pharmacol. 7, RANDALL, L. J. (1952). Synthetic curare-like agents which are reversible by Tensilon. J. Pharmacol. 15, WILSON, A. & STONER, H. B. (1947). Effect of injection of acetylcholine into brachial artery of normal subjects and patients with myasthenia gravis. Quart. J. Med., N.S., 16, ZMMIs, E. J. (1951). The action of decamethonium on normal and denervated mammalian muscle. J. Phy8iol. 112, ZMS, E. J. (1952). Motor end-plate differences as a determining factor in the mode of action of neuro-muscular blocking slibstances. Nature, Lond., 17, 617.