Pathogenesis of myogenic trigger points

Prof. Ivanichev G.A Kazan, Russia Any kind of stimulation which directly or through a reflex is able to initiate muscle contractions resulting in a motor act. The completion of the accomodative process of muscle contraction implies muscle relaxation producing this effect. This occurs under conditions similar to the ideal ones. More often the muscle activity cannot be characterized as accomodative. A prolonged muscular tension in chronic pain is a convincing example to that. The signal essence of pain in the following stages of the disease is characterized by pathologic features while a prolonged contraction turns out to be a dialectical converse. Let us consider some steps of the formation of the local muscular tension. The initial stage is a residual muscular tension. The trigger moment of a Myogenic trigger point (M.T.P.) is a static (isometric) work with minimal intensity during a long period of time. This kind of work is due to a reflex muscular tension in inner organ pathology (defans), spine disturbance (immobilization of the destructed segment), cold effect on the skin (reflex tension), defective motor stereotype (overload of some separate muscular groups) etc. In contrast to the dynamic work, the static work is known to have its own characteristics. First and foremost it is a very small range of physiological functions. From the point of view of phylogenetics the dynamic work may be more perfect: physiologic adaptation measures are more dynamic, there is a profitable energy supply. The lability of the neuromotor system, proprioceptive afferentation, coordinating relations — are prerequisites for any muscle contraction. A simple analysis indicates that all these indices have been historically evolved to become more and more perfect. Otherwise the static muscle work would perform posture activity in posture support, while the dynamic one has a precise, prompt, transitory activity, connected with the reaction of choice. The ways how these moments are organized are principally different. Tough, determining (circular type of organization according to N.A.Bernstein, (1947) is primarily responsible for the static kind of activity. It is due to functional organization of the spinal-segmental apparatus. Less tough, more changeable (a programmed N.A.Bernstein's type) provides a supraspinal control of predominantly dynamic activity of the neuromotor system. The latter type is more liable in motor control. Hence, there occurs in long term static work with minimal intensity a complicated change in the functional activity of the corresponding neuromotor system. Mostly it is a space deformation of the working muscle. The most thick and strong portion of the muscle stretches the most thin and weak one, which is a well known physiological phenomenon (I.S.Beritov, 1947). On release from tension this deformity disappears due to a natural muscle elasticity; muscle relaxation is known to be a passive act due to its physico-chemical properties and the antagonist state. The period of relaxation is used for muscle rest (restoration of energy reserve, lability, inhibition systems etc.). This is just a physiologic measure of adoptation of the motor apparatus under natural activity conditions. During a prolonged work though of minimum intensity, the reserved potentialities, especially during a transient interval have not enough time to supply the initial physiologic parameters of the motor substratum. The residual tension — a formed space deformation of the muscle portion in its weak part — is preserved. While the static work is going on under the above regimen this deformation is increasing as a result of accumulation of all changes. Indeed, this process may be local only for a short period of time. Conditions for neuromotor system activity naturally involve the segmental-spinal mechanisms. Probably, they are aimed at repairing of physiologic and morphologic parameters of this muscle. Depletion of the restoration resources of the initial state implies inclusion of mechanisms whose effect should be regarded as being pathological. This process is thought to begin as an impaired proprioceptive impulsation on the border of a muscle region with a residual deformation and normal structure. It is natural to admit that this region is the most tense from the point of view of morphologic orientation of the muscle and afferent innervation under all other conditions of the functional state. The receptor apparatus in this zone may turn out to be under complicated conditions of functioning — overstretching of the portion of the neuromuscular spindle (as a linear substratum) during some relative compression of another portion. This induces diversely directed deformation of the receptors which belong to one and the same afferent fibre. Therefore sensor impulsation would occur under conditions different from normal. The main tendency of afferent imbalance implies an imbalance of dynamic and static modality impulsation. This suggestion is confirmed by studying the relationship between the bioelectrical activity of the muscle while evoking the tonic vibration reflex (static activity imitation of the proprioreceptors) and in muscle stretching (dynamic activity imitation). If 2 min. vibration of a muscle with M.T.P. does not substantially change the level of the muscle bioelectrical activity than its even transient stretching sharply intensifies it — much more than the stretching of a normal «healthy» muscle. Inadequate impulsation overcoming the control at the input into segmentary apparatus is able to bomb the anterior horn motoneurons as a result of its continuation, producing the appropriate changes in them. Efferent impulsation supported by a prolonged afferent stimulation through group Ia afferents is able in its turn to aggravate the local space changes of the muscle architectonics. Probably, the segment interneuron activity, reverse inhibition included is characterized by the reduction of effectiveness of the inhibitory processes. It is through this the way is «ushered» of the proprioceptive constant reflex, which has unequivocal pathologic meaning in the muscle activity. Impairment of the architectonics terminals of the motor units in the hypertonicity zone (space aberration) is the result of this pathologic reflex and the reason why the subsequent space changes of the muscular band — fascicule occur. To regard the spinal-segmental and local mechanisms as just being pathological ones means a mere simplification of the above mentioned complicated processes. The problem is complicated not only because the involved structures are multicomponent and because the relationship of the phenomena should be determined, but because all these processes are greatly influenced by the supra-segmental structures. The study of the functional state of these bands under clinical conditions is known to be rather difficult from the methodical point of view. All kinds of assessment of monosynaptic reflex (H-reflex), plantar response of lumbar motoneurons (F-wave), M-response, so far employed fail to bring solution to the problem under study. What seems to be promising in this respect is the method of investigation into the evoked responses manifested by long reflex relations, e.g. the evoked sensor potentials of different brain structures (L.P.Zenkov, M.A. Ronkin, 1982; L.P.Zenkov, 1984). Though this technique seems to be very attractive still the evoked cerebral cortex potentials cannot provide an exhaustive study of M.T.P. pathogenesis. For our studies we needed a technique of registration of evoked responses of muscular fibers themselves. This approach could be made by development of a new technique of evoking a spinal-truncal polysynaptic reflex (STPR). The mechanism of reflex implies motoneuron discharge production in the spinal cord during irritation of the mixed nerve of the upper extremity (median or ulnar). The response is registered as a polysynaptic discharge 40 — 60 ms long with the latent period of 60 — 150 ms. Such information about the reflex nature was given in our publications (G.I.Ivanichev, 1983, 1985). Let us notify the characteristic manifestations of this reflex which are necessary for understanding of the nature of M.T.P. pathogenesis. One of the features of STPR is that the STPR latent period has a lot of fluctuations due to a characteristic manifestation of the pain syndrome in the registration region. In pronounced tenderness of the soft tissues the STPR latent period is reduced, and the duration of the discharge is quite varied. As tenderness diminishes the STPR latent time increases due to compact response. Another peculiarity is that STPR has a refractory period due to delay which is the result of activation of the truncal structures. In mild voluntary contractions of muscles where STPR is studied the above reflex is detected quite often, i.e. some relief is achieved. In strong muscular contractions STPR is naturally produced too but only when there is a significant bioelectrical activity which makes it hard to identify it. The so called period of inhibition of the bioelectrical activity makes the task easier. It follows the produced STPR. This period is clearly seen on the electromyogram, it corresponds to the end of the reflex discharge front. The earlier assumptions about the segmental origin of the inhibitory period due to the activity of the segmental apparatus of the spinal cord (P.Shahani, R.Yung, 1973; I.A.Zavalishin, V.P.Novikova, 1979; R.S.Person, 1985) were not confirmed. This information is due to findings obtained from STPR which made it possible to observe at least one way of regulation of the activity of the spinal segmental apparatus. Probably it has a physiological nature in producing posture motor reactions. This point of view is confirmed by clinical examples observed- models of structural impairment of the nervous system when STPR is distorted or disappears altogether. Using this method of STPR study it was possible to make an impression on the disturbance of its manifestation in myofascicular hypertonisity — M.T.P. Thus, dispersion of the latent time (period) and the response duration in M.T.P. is rather significant as compared to that in the region of a normal muscle or on the border between them. One of the most obvious features of pathological manifestation of STPR, one should consider the STPR inhibitory failure in M.T.P center. This disorder in inhibition may be complete and may be not, i.e. some responses cannot be inhibited. These data serve as an electrophysiologic evidence of participation of suprasegmental structures in M.T.P. pathogenesis. The assumption about it becomes rather convincing from the point of view of pains which become more intense in neurotic states and psychovegetative syndroms. Functional changes in the limbicoreticular complex are known to influence significantly on the tonic muscular activity (A.M.Vein, V.S.Maltsina, 1974). Disfunction of these sections accompanied by the concurrent changes in regulation of the neuromotor activity of the subordinate systems is a component of M.T.P. pathogenesis. Obviously this system exerts the most significant effect on the subjective assessment of M.T.P. tenderness. Very often this part of clinical manifestations is from the patient's point of view most essential: M.T.P. appearance is connected with an episode of psychoemotional stress, long-term stress situations. In this respect M.T.P. manifestations may be evidenced psychoemotionally. Obviously, all systems of motor regulation are manifested in M.T.P. pathogenesis. A variety of the functional levels evidenced in M.T.P. pathogenesis makes it possible to consider the local elevation of muscular tonisity as a sign of the posture basic activity under pathologic conditions. In a strict sense of hierarchic subordination M.T.P. pathogenesis must have occurred with the participation of cortex prefrontal region. However, the clinical and electrophysiological techniques of examination offer no help to prove that. Thus, the reflex routes may be traced which include a deformed proprioceptive apparatus, ways of deep sensibility, spinal segmental apparatus and structures of truncus cerebri, efferent descending tracts (reticulospinal, rubrospinal, pyramidal) anterior horn of the spinal cord, motoneuron with deformed territory of the motor unit. The stable pathologic state (N.P.Bekhterev et al., 1978) of this system signifies a completion of peripheric determinant structure development which is generated by M.T.P. Activation of this system is possible due to extended zone of its activity with formation of new generators and stability of pathologic processes whereas the effectiveness of the inhibitory functions is decreased. A special place in M.T.P. pathogenesis is occupied by tenderness of an indispensable element of the above phenomena. The development of local pain and muscular tension one should regard as an interrelated process. Disturbance in the correlation of proprioceptive and extraceptive impulsation (absence of impulse inhibition of nociceptive modality due to decreased afferentation of the proprioceptive nature) in the postesior horn region is known to be responsible for feeling of pain (R.Melzack at. al., 1965). Imbalance in proprioceptive impulsation while the extraceptive impulsation level is constant (afferentation of the nociceptive nature may be partly increased) makes conditions for generation of pain potentials. From the point of view of determinant structure functioning the evidance of pain syndrome may be explained by the development of a hyperactive structure in the posterior horn region where the decrease of presynaptic impulse inhibition coming from M.T.P. is surmised. The assumption about the biologically active substances produced in the M.T.P. region which are responsible for generation of local tenderness proves to be not very convincing from the point of view of prompt disapearance of tenderness during manual therapy (see further). The analgesic effect thus achieved within some seconds with simultaneous myorelaxation can not be due to a normalization of the biochemical content in the M.T.P zone. Naturally, microcirculation and metabolism disorders as component of any long muscular tension may occur but only as secondary and not obligatory phenomena. Possible distrophic changes with sclerosis (fibrosis) as we see it, in the muscular activity under the above conditions are extremely rare, which is confirmed by clinical observations. The more so for these changes are detected morphologically more seldom than it is diagnosed by palpatory assessment. In this diagram the mimic muscle contracture occupies a separate place. Though the clinical M.T.P. manifestation in the sceletal musculature and mimic muscles seem to be unlike the pathogenesis of their development has much in common. The characteristic features of these mimic muscles have a significant influence on the development and clinical M.T.P. manifestations. They are as follows: 1) the mimic muscles are hypodermic and compose an anatomical structure as a symplast, having no fascial intermuscular septa (resembling somehow of the myocardium); 2) they have no statistic functions; 3) the ratio of the muscle efferent innervation is high, these muscles are exact and quick; 4) the mimic muscles have a high sensibility to acetylcholine. To fully understand the mechanism of the initial stages of contracture one should emphasize two more things necessary to make the likelihood of secondary contracture very high. This is first of all a mild (of median degree) affection of the nerve — efferent tract preservation, though a defective one; second, it is an evidence of pain phenomena accompanying facial nerve neuropathy over the whole period of time including the antecedent period (precursors). Taking into account all this and the above considerations concerning M.T.P pathogenesis of the skeletal muscles, the development of the secondary contracture of the mimic muscles may be seen in the following way. It is stated that at the earlier stages of impairment of facial nerve hypertonicity of the mimic muscles is detected in all cases and at any degree of mimic muscle lesion (G.A.Ivanichev, 1982, 1985). The formation of trigger points one should relate to a variety of contributing factors (increased sensibility of denervated muscles to acetylcholine, release of biologically active substances as a result of pain syndrome, more intense contractile activities of denervated muscles), which result in the development of palpable indurations in the mimic musculature. They may be revealed early — in 3-4 days. They are characterized by marked tenderness during muscle stretching, though they may not be revealed by palpation alone. When nerve affection is mild and the functions can be well restored, the hypertonicities thus developed can have involution and the mimic musculature has all functions restored without a defect. In severe nerve affection both the developed hypertonicity and the affected muscles rapidly undergo all degenerative changes resulting in the eventual death of the muscles. In moderately severe nerve affections there may be total complex of phenomena when nerve repair is rather late as compared to the processes occuring in muscles. During partial denervation when the muscle has a prolonged deficiency in stimulation, any exogenic (from the face skin) as well as endogenic effect exerted (the humoral medium etc) may be inadequate. Increased sensibility of denervated muscles to acetylcholine as evidenced by tonomotor effect is an indication of raised exitability in M.T.P. Electromyographically it is evident by a rhythmic activity of certain muscular portions, observed by many authers in needle electrode technique. This activity is manifested by potential discharges 3-4 ms long with intervals between them of 1,2 — 2 s. Irritation of the facial nerve trunk stimulates these potentials for a long period of time (5-6 s). They may be provoked by mechanical irritations of muscles also. In this connection it should be noted that there is so caled secondary contraction of a resting muscle overlaying a contracting one (I.S.Beritov, 1947). Transmission of stimulation from one muscle to another easily occurs when there is a slight insulator between them. A denervated mimic symplast may be an ideal medium in this respect. The area of muscle contraction thus appearing, which is active in relation to the neighbouring ones, causes currents of action in these neighbouring muscles, while relaxation of this contraction occurs slowly and is incomplete due to passive generators. This is contributed by relative autonomy of a contractile process due to deficiency of the efferent control under conditions of invariable proprioceptive impulsation. Generated by this process a continuous current called contractural (I.S.Beritov, 1947) can by itself support a contractile process and prolong the time of muscle relaxation. Often erroneously administered proserin or electrical procedures (taking into account an increased muscle excitability to acetylcholine and tendency to contractile reactions) are able to maintain this process. Thus, supported over a long period of time a contractile process in the isolated muscular groups signifies M.T.P. developments in the mimic musculature as a functional substrate of secondary contracture. The statement that a neural scar at the site of lession is able to «mix» impulses and not deliver them to a precise addressee's place, producing contracture (A.K.Popov, 1968) is not confirmed electromyographically: transmission of the nerve impulse in the formed scar is physiologically ridiculous. It is stated that mixing of impulses in the affected nerve is possible to occur only over a short period of time at the beginning of attack when there is an area of the demyelinized nerve (G.A.Ivanichev, 1980; Ja.B. Yudelson, 1982). Further re- and hypermyelinization of the zones of the affected nerve cancels this completely as a pathogenic factor (N.N.Bogolepov et al., 1974). Even if we assume that such probability exists then "at the moment when all fibres growing along the peripheral fascial tubes are growing into the peripheral organs and establish the organ synapses, immediately some relations are established between the organ and the central apparatus according to the type of functional loads" (P.K.Anokhin, 1975). Admitting the existance of such mechanism we must expect functional restoration of the mimic muscles with no complications at all. But in our case regenerating nerve is growing into a changed, qualitativly special work organ — hypertonicity. The qualitative characteristics of hypertonicity have been already mentioned by us: imbalanced proprioceptive afferentation, distortion of Motor Units terminals territories, reduced intensity of inhibitory processes, the high level of exitability. Then, how do synkineses arise? In none of the skeletal muscles which has M.T.P. or other pathological elements, synkinesis does not arise.Muscle contraction even under these conditions is an isolated one without participation of the neighbouring muscles. Muscle synkineses are not observed even in such severe lesions as polymyositis. The assumption that the neural scar is responsible for synkineses turned out not to be true. The participation of the facial nerve nucleus in synkineses generation due to a pathologic relationship between the neuron groups were not confirmed clinically or electrophysiologically. A variety of affected bodies of scull motoneurons and spinal neurons are known which are clinically and anatomically verified when no synkineses occurred. In this connection only considerations concerning peculiarities of the mimic musculature itself allows to clearly understand this phenomenon. As it was already reported the mimic muscles contain no fascial elements and intertwine with the facial skin. The coefficient of innervation is higher than in the skeletal muscles. It is due to these peculiarities that a quick and precise mimic muscles play may occur in displaying emotions. The same properties of the mimic muscles are underlying mechanisms of generating pathologic movements — synkineses. In the absence of a reliable insulator between the deformed fascicles and fibers, the abundance of intertissue liquid around the denervated tissue and dystrophic changes of sarcolemma, endo- and perimysium, create conditions for transmission of the nervous impulse from one muscular fiber to a neighbouring one. In other words "an artificial synapse" develops between the muscles and not at the site of nerve lesion. The intermuscular transmission of nerve impulses may be possible under normal conditions too (W.Trojaborg, 1977) While regenerating fibers are growing into the normal muscle and M.T.P. a muscular conglomerate is created with differently functioning zones approaching each other. Even the junctions of the contiguous parts of fibers may be possible. It is due to this defect that the normal relationship between the center and periphery according to the type of functional adjustment is impossible. Any impulse transmitted from the center will produce a stereotyped motor effect as more or less synchronous contractions of the mimic muscles (the impulse simultaneously involves different by their function and far from each other located muscles). Assuming the existance of such mechanism allows to understand the distribution of stereotyped synkineses by a clinical feature-palpebro-labial, oculo-zigomatic etc. As it was already mentioned, hypertonicity arises in the most strong and thick parts of the muscle. In the mimic muscles these portions occupy the sites of their fixation with bones along the basic muscles. Very often it is difficult to see what certain muscles these points of fixation belong to. The existence of intertwinings, zones of muscle overlapping at M.T.P. site creates conditions for cross transmission of a motor impulse to muscles far located from each other and different by their functions for example from a zygomatic to an orbicular muscle and vice versa. In primary (active) muscle excitation in the region of mouth circumference (grinning) impulses go upward to be transferred then in the region of maxillary puberosity to the muscles of the upper part of the face. On the contrary in screwing up one's eyes impulse transmission is possible in the zone where zygomatic muscles raising the mouth angle and cheek muscles are approaching each other. Otherwise a contractile process, isolated under normal conditions occurs almost synchronously in different by their functions muscles. This explains a stereotype way and invariability of synkineses not only in one particular patient but in all patients with contractures. M.T.P. is a generator of synkineses in any localization if the reflex ring is preserved with reduced threshold of excitability of the total neuromotor system. In these cases one may assume the appearance of a peculiar «pace maker» or an ectopic generator of excitability in the mimic musculature which reminds of heterotopic pace makers in the affected myocardium. The activity of the latter is known to be very high. The participation of limbico-reticulo complex in this scheme seems to be delayed (Ja.B.Yudelson, 1982). The participation of these systems at the stage of a formed determinant structure in the trigeminal facial nerve system is precisely demonstrated by relief from reflex reactions, i.e. a decreased supraspinal inhibitory control. The dysfunction of this section of the brain is largely responsible for an emotional colouring of the main clinical evidences of contracture — mimic musculature is a main manifestation of the functional state of the limbic brain.

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