SUMMARY. Burn victims present considerable biochemical and receptor alterations. Recent improvements in instrumental research techniques have clarified some of the mechanisms involved. One of the problems is resistance to curarizing drugs (which is probably not related only to receptor upregulation); also involved are metabolic alterations, immunodepression, hyperalgesia and pain “memory” phenomena, and, in particular, problems of skin tissue regeneration.
The damage caused by burns is a serious pathology that presents multiple aspects that have to be identified. In addition to the considerable and protracted simultaneous effect of various physiological alterations, the survivors of burn accidents also have to face complex psychosocial consequences that affect their quality of life.
The skin is the body’s most extensive organ. It makes up 15% of body weight and in an adult has an area of about 1.7 sq. m. It has multiple functions, including that of being an indispensable interface between the inner and the outer body. Any damage to this multifunctional barrier impairs the inner internal environment. Of its two constituent strata, only the epidermis possesses a real capacity for self-regeneration.
A burn causes damage and destruction to the skin, with its various components, and the underlying inner structures with a mechanism that may be thermal, chemical, electrical, or a combination of all these. Thermal damage is the most frequent type and is often associated with inhalation damage. Thermal damage is due to heating of the tissues above a critical level. The extent of the damage is a function of the thermal energy of the burn agent, the duration of exposure, and the thermal conductivity of the tissue affected. The skin overheats slowly, but also cools slowly, for which reason thermal damage continues even after the burn agent is removed.
The skin is a complex organ of vital importance because of its function as dynamic barrier: a lesion, especially if extensive, causes in the patient important systemic and sensorial biochemical alterations.
Recent improvements in instrumental research have made it possible to clarify some of the mechanisms involved.
One of the problems is resistance to curarizing drugs (which is probably not related only to receptor upregulation); also involved are metabolic alterations, immunodepression, hyperalgesia and pain “memory” phenomena, and, in particular, problems of skin tissue regeneration.
The extent of burn damage depends on various factors: the majority of tissue loss is caused by thermocoagulation of tissue proteins. The central area of irreversible tissue destruction is surrounded by a zone of coagulation, which is itself surrounded by a poorly perfused area known as the “stasis zone”, followed by a zone of hyperaemia: these last zones must be considered as being at risk, although they can be saved with appropriate therapy. The amount of tissue destruction is generally a function of temperature and exposure.
Various tissue mediators capable of causing ischaemia in acute burns have been identified, e.g. certain oedematogenic substances such as prostaglandin, histamine, bradykinin and xanthine-oxidase (this last substance plays an important role in the formation of oedema, and pre-treatment with appropriate inhibitors can consequently significantly reduce its extent). Substances like TNF-a, IL-2, and IL-8 promote the activation and migration of neutrophils to the burn area, where the neutrophils degranulate abundantly, liberating protease and free radicals, which cause tissue necrosis. Experimentally, the application of antibodies directed at membrane integrins or neutrophil surfaces prevented this migration, although it is not yet possible to use this procedure in clinical practice.
The physiopathology of the neuromuscular plate is a frequent problem in the intensive care of the burn patient.
The neuromuscular junction can be regarded as a specialized synapsis dedicated to the transmission of an action potential. The neurotransmitter is acetylcholine. The receptor is a nicotine-type cholinergic receptor - a polymer consisting of five units. For its activation, it requires the binding of two molecules of acetylcholine.
The neurotransmitter is liberated in packets, known as “quanta”, in the synaptic fissure. The excess neurotransmitter is split off by an enzyme known as acetylcholinesterase.
There are two types of curare: 1. depolarizing (succinylcholine); 2. competitive non-depolarizing.
Depolarizing curares bind weakly to the nicotinic receptor and initially produce fasciculations acting as agonists; also, by blocking the receptor, they prevent the bond with acetylcholine (Ach). The only depolarizing curare in use is succinylcholine, which is known for its rapid onset and the brief duration of its action.
Non-depolarizing curares compete with Ach for the binding to the nicotinic receptor: if the curare binds and blocks one of the two binding sites of the nicotinic receptor, this is sufficient for it to be inactivated. The duration of their action varies: it is either short to intermediate (e.g. rapacuronium, rococuronium, and atracurium) or protracted (e.g. pancuronium).
When depolarizing curares are used, the upregulation of nicotinic receptors (mature and non-mature) leads to the onset of hyperkalaemia. Hyperkalaemia can in these cases be so severe as to cause cardiac arrest.
This receptor upregulation is among the phenomena behind the well-known resistance to non-depolarizing curares in burn patients.
The resistance to d-tubocurarin derivatives is more marked in muscles near the burn due to the expression of mature and (in particular) immature receptors for acetylcholine. In the rat, this resistance is also observed in distant muscles; however, it is later, less intensive, and not related to the expression of immature receptors.1 In the present study, burns were inflicted on the surface of the paw (above the tibial muscle) or the side of anaesthetized rats. On days 1, 4, 7, and 14 the curarizing dose of t-tubocurarine, the acetylcholine receptors, and the RNA messenger (m-RNA) in the tibial muscle were observed. In the case of local burns (paw) there was an early (4th day) increase in resistance to curarization and an increase in both the mature and the immature receptors. Resistance after distant burns (burn in the side and control in tibial muscle) was observed later (day 14). Distance resistance would appear, on the basis of the doses, to be attributable to unexplained phenomena of muscular atrophy - the number of cholinergic receptors was found to be unaltered (as also the m-RNAs of the gamma subunit).
Other experimental results suggest that there are other mechanisms of resistance to non-depolarizing curares besides the above-mentioned receptor upregulation. Edwards et al.2 used the following experimental model in the rat: 30% burns to the animal’s body surface, and a study of slow fibres of the soleus and rapid fibres of the long extensor of the digits. The animals were examined 72 h post-burn. Compared with controls, there was a marked increase (50%) in the frequency of miniature elicited plate potentials (mEPP) both in the soleus and in the long extensor of the digits. In particular, there was a 10% increase in amplitude in the mEPP of the long extensor of the digits (the amplitude of the soleus mEPPs was unaltered). There was also an approximately 30% increase in the elicited plate potential in both muscles.
These data indicate beyond any doubt an increase in quantum release 72 h post-burn. The phenomenon is certainly an interesting mechanism of resistance to non-depolarizing curares.
The problems of burn patient management include that of metabolic alterations.
The hypocalcaemia and hypoparathyroidism that follow a severe burn would appear to be related to an upregulation of the calcium-sensitive receptor of the parathyroids. This mechanism may therefore be related to suppression by this receptor of the levels of parathyroid hormone in response to the circulating calcium. The “set-point” of calcium would thus be lowered: the receptors, increased in number, interpret what is a normal calcaemia as if it were in fact increased, and therefore reduce it, leading inevitably to hypocalcaemia. Some researchers3 have inflicted 40% burns in animal models (sheep), sacrificing them at 48 h. Compared with controls, these animals presented histologically hypocalcaemia and hypomagnesaemia and a 50% decrease to the Northern blot of the messenger RNA of the calcium receptors. These data confirm the hypothesis that hypocalcaemia in seriously burned patients is related to an upregulation of these receptors.
Lipolysis alterations are an important metabolic problem in the burn patient. It has been shown4 that there is a modification in the response of beta-3 adrenergic receptors to a specific agonist (BRL-37344) in isolated rat adipocytes after a burn. The maximum lipolytic response to stimulation with BRL-37344 of the beta-3 receptors was much reduced at 3 and 7 days. There was also a reduction in the expression of hormone-sensitive lipase and a reduced beta-3 correlated incretion of insulin.
Severe burns are followed by an increase in the serum levels of TNF-a and its soluble type I and II receptors. It is known that high levels of these soluble receptors are related to a high risk of shock and the multiple organ dysfunction syndrome (MODS).5 The levels of TNF-a and its soluble type I and II receptors (TNFRI and TNFRII) would appear to be an important prognostic factor. In a study conducted in 2000, the levels of TNFRI and TNFRII proved to be proportional to the extent of the burned area and were very elevated in patients who eventually died.6
The involvement of the immune response is another important factor in the physiopathology of burn patients.
One of the factors that expose burn patients to the development of infection is dysfunction of polymorphonuclear phagocytosis (PMN). The expression of receptors for opsonin and the complement by PMN has been quantified.7 The phagocytosis of Candida albicans mediated by the complement and immunoglobulins has also been assessed, and a decrease in immunoglobulin-mediated phagocytosis has been observed. A potentially dangerous systemic activation of the PMNs has been observed. Also, the increase in serum levels of ICAM (intercellular adhesion molecule, involved in PMN accumulation) has proved to be related to an increase in the damage caused by PMN activation following extravasation due to endothelial lesion of the skin and lung.
After extensive burns there are high serum concentrations of the alpha chain of the receptor of interleukin-2 (sIL-2R a). The consequences of this phenomenon have not been fully established. Jobin et al.8 attempted to clarify this phenomenon that is observed in burn patients. The sIL-2R a was elevated in all the burn patients examined, and the western blot analysis showed that this receptor represents the extracellular domain of lymphocytes CD2 and CD11b. The sIL-2R a, in the presence of IL-2, inhibited 50% of the activity of the natural killer cells and suppressed production of interferon-g mediated by IL-2. It was also found that a low-fat diet had beneficial effects on the concentrations of sIL-2R a. The activation of IL-2 mediated by TH1 and TH2 would appear to be inhibited by elevated serum levels of sIL-2R a.
A clear phenomenon that is peculiar to immune involvement in the burn patient is the alteration of monocytopoiesis and granulocytopoiesis at medullary level. In cases of burn sepsis it is possible to observe an increase in the progenitors of monocytes and mature macrophages, while there is a decrease in the precursors of the polymorphonuclears and in mature cells. In the light of recent observations9 it may be possible to correct these alterations by using a receptor antagonist of PGE-2 (SC-19220). This very interesting finding makes it possible to confirm the central role of PGE-2 in the pathogenesis of sepsis in burn patients, and at the same time provides some therapeutic options.
With regard to immunodepression, it is interesting to observe that mice subjected to severe burns and an elevated dose of type I herpex simplex exhibit an insufficient immune response. A protective therapeutic effect is related to an increase in IFNg produced by spleen LFN T after treatment with IL-12 and with the soluble receptor of IL-4.10
In view of the “hypermetabolic” state induced by serious burns, it is also important to consider the nutritional aspect during treatment. Nutrition should if possible be enteral, in order to prevent atrophy of the intestinal villi and the subsequent phenomenon of bacterial translocation. This consists of the colonization of atrophic intestinal mucosa by pathogenic bacteria, which is aided by the organism’s state of immune deficiency. This condition can trigger liver, lung, bladder, and blood infections as well as systemic sepsis. It is also related to the onset of multiple organ deficiency. This cascade of events can be relatively simply avoided by maintaining enteral nutrition, and in particular the administration of glutamine, which appears to have a trophic effect on enterocytes.
It is of prime importance to understand the hyperalgesia that afflicts burn patients.
From the clinical and metabolic point of view, burns can be subdivided into an acute phase and an evolutive phase.
Acute phase
The acute phase lasts from the moment of the burn until the patient is haemodynamically stable. This corresponds to the period of intensive care.
The intensity of the initial pain caused by burns depends on the degree of the burn. The scale of intensity ranges from slight to moderate pain, in first-degree burns, to the unbearable pain characteristic of second-degree burns when more than 30% TBSA is burned. The correlation of pain intensity and percentage TBSA burned would appear to be confirmed, a correlation that is proportional and positive. Contrary to widely held opinion, it has recently been observed that third-degree burns, generally believed to be characterized by little or no pain, are all the more painful the greater the extent of the area of full-thickness burns. The intensity of the pain, whatever the initial degree of the burn, may increase after the onset of infective processes, when the degree of the burn becomes more serious, or when the surface is removed, suffers a trauma, or becomes exsiccated.
a) Immediate effects
Harmful thermal action sets off a regular sequence of events:
This activation, due to the burning of the peripheral sensory fibres, causes the release of neuropeptide vasoactive tachykinins. Among the tachykinins the most studied are substance P and neurokinin A. These neuropeptides are involved in local inflammatory reactions and in nociceptive transmission. The action of these molecules is closely related to two main types of receptor: receptors for neurokinin 1 and 2 (NK1 and NK2). After a burn there is an upregulation of NK1 and NK2, a phenomenon that promotes the formation of oedema and phenomena of hyperalgesia. Lofgren studied the experimental use of antagonists of the receptors NK1 and NK2 for the limitation of oedema and nociceptive transmission, obtaining excellent results in the rat model. This confirms the potential prospects of therapies capable of blocking these receptors.
b) Secondary effects
A burn is followed within a few minutes by the development of an area of hyperalgesia (and/or allodynia) around the lesion, known as that of “primary hyperalgesia”, around which, in the undamaged skin, there appears a zone of “secondary hyperalgesia” (and/or allodynia), which gradually increases in diameter with the passing of time. Hyperalgesia means a greater sensitivity to pain caused by a lowering of the pain threshold and an increase in intensity of the response to supraliminal noxious stimuli, while allodynia is the painful sensation induced by normally non-painful supraliminal noxious stimuli. It is important to distinguish between the definitions of “hyperalgesia and allodynia” and those of “primary and secondary hyperalgesia”: the former are expressions of algological semeiotics, while the latter, in the physiopathology of thermal lesions, define topographical areas where various events may occur.
The areas of primary and secondary hyperalgesia differ:
Sensory features
In the burn area (zone of primary hyperalgesia), the stimulus/response function due to a thermal and/or mechanical stimulus is shifted to the left, indicating a clinical situation of hyperalgesia and/or allodynia towards mechanical stimuli and heat. The application of cold to the lesion site reduces spontaneous pain and the dimension of the area of secondary hyperalgesia.
In the area of secondary algesia, there is hyperalgesia and/or allodynia only for mechanical stimuli, and not for heat: hyperalgesia due to noxious stimuli is therefore present only in the zone of primary hyperalgesia but not in that of secondary hyperalgesia. This may present hyperalgesia due to cold, but not systematically or in a clinically remarkable manner.
This particular sensorial dissociation in the two areas of hyperalgesia suggests different pathogenic mechanisms.
Initial acute pain
An intense, prolonged, and/or repeated harmful thermal stimulus in an extensive skin area produces a rapid and acute sensation of pain, owing to the activation:
In both cases, mechanisms develop of temporal and spatial summation with the result that, for example, the receptive field of AMHs and CPNS expands until it includes surrounding undamaged tissue, so that a stimulus in the lesion area activates several nociceptors with an increase in the intensity of the response.
The activation of nociceptor activity can occur by direct activation, due to the stimulus of the lesion, and/or by possible transduction mechanisms involving algogenic chemical substances that have been liberated from cells damaged by the burn stimulus or have been synthesized locally from substrates liberated by the lesion, or conveyed by exudation or migration of lymphocytes, or produced by the activity of the nociceptor.
Secondary acute pain - hyperalgesia
The persistence and successive evolution of pain depend on the presence, evolution, and variable interaction of several factors that are capable of continually affecting the symptoms, such as for example the progress of tissue damage, the successive role of proalgogenic or simply algogenic substances, and the effect of peripheral and central neural mechanisms, such as those involved in the pathogenesis of areas of primary and secondary hyperalgesia.
a) Zone of primary hyperalgesia - peripheral mechanisms
The sensitization of the peripheral nociceptors is the neurophysiological substrate of the hyperalgesia to thermal stimuli that occurs in the lesion site. The specific role played by myelinic and amyelinic nociceptive afferences depends, in the case of hyperalgesia and/or allodynia to heat, on the type of skin: in hairy skin, both the AMH and the CPN nociceptors can be sensitized to thermal stimuli, even if the action of CPNs seems to be predominant; in hairless skin, sensitization appears to affect exclusively Ad afferences, while CPN nociceptors manifest an increase in threshold and a reduction in intensity of the response to heat stimuli.
Contrary to what one might expect, the mechanism of sensitization to heat stimuli is not accompanied by an analogous mechanism after mechanical stimuli. For this reason the hyperalgesia and/or allodynia that occur in the lesion site owing to mechanical stimuli cannot be explained as a result of the sensitization of the primary nociceptive afferences. It may be useful to consider the hypothesis that - as in the case of HTMs due to heat - afferences that are initially mechanically insensitive stimuli (MIAs) develop mechanical sensitivity as a consequence of tissue lesion.
In both cases, i.e. hyperalgesia to heat and to mechanical stimuli, in addition to the mechanisms indicated there are other mechanisms that lead to an increased response of the central neurons: a spatial summation due to expansion of the nociceptorial receptive field in the adjacent undamaged area, and a reduction in the responsiveness of the low threshold mechanoceptors (LTMs) that leads to a reduction in the inhibitory input to the dorsal horns (gate control theory).
b) Central mechanisms
A phenomenon of sensitization to thermal stimuli has been observed not only in the peripheral afferences but also at spinal, thalamic, and cortical level, even if such observations may not imply an autonomous central process of sensitization but only the effect of a potentiated peripheral input due to sensitization of the nociceptors.
Mechanical primary hyperalgesia is a different matter. Several studies have indicated a specific sensitization of the central neurons, to which is added a central spatial summation due to expansion of the mechanized CR of the central neurons (especially WDR) and a process of disinhibition of the central neurons due to suppression of the stimuli from the a‚ fibres. The role of the latter in primary analgesia is not clear, as also the changes that some researchers have indicated in descending inhibition.
c) Zone of secondary hyperalgesia - peripheral mechanisms
Hyperalgesia and/or allodynia in the undamaged area adjacent to the burn are, as we have seen, elicited only by mechanical stimuli and not by thermal stimuli. Why this should be so, i.e. by what mechanism a state of altered sensitivity spreads to the surrounding undamaged territory and even to territory at a distance, is a question that divides the scientific community interested in pain neurobiology.
Lewis, on the basis of a series of experiments conducted using nervous blocks, suggested a peripheral mechanism (“axon reflex”), involving the following phases:
d) Central mechanisms
More recent data and evidence suggest a central model of secondary hyperalgesia. First proposed by Hardy, this hypothesis - according to which there is a central mechanism of sensitization that is all the more specific the more evident is the absence of the phenomenon in the peripheral nociceptors - is more substantially supported than the peripheral hypothesis.
Rat paw burn induces allodynia distal to the lesion. This phenomenon would appear to be related to Ca++-permeable receptors of type AMPA of glutamate; the NMDA receptors of glutamate would appear to be uninvolved, or only involved to a minor extent. Joro spider (Nephila clavata) toxin (JSTX) is an antagonist of the calcium-permeable subtype of the receptors of non-NMDA glutamate. In fact, the intratecal use of JSTX in burned rats reduces the phenomenon of allodynia (but does not alter the threshold of thermal sensation).12
It is useful to bear in mind that hyperalgesia in burn patients is related to phenomena of wind-up and central hyperexcitability of the neurons of the dorsal horn. A recent study13 evaluated the soothing action of dextromethorphan on this form of hyperalgesia. Dextromethorphan is a non-competitive antagonist of receptors of type NMDA of glutamate and is capable of interfering with processes of wind-up and sensitization of the sensitive neurons of the dorsal horns. In this study, healthy adult volunteers subjected to burns were treated with dextromethorphan with good analgesic effect and limited side-effects.
Ketamine (an antagonist of glutamergic receptors of type NMDA) is also effective against hyperalgesia in burn patients. Morphine is effective in primary hyperalgesia (lesion zone) but ineffective, unlike ketamine, in secondary hyperalgesia (around the burn area). Morphine is also related to wind-up owing to phenomena of spatial and temporal summation.14 It was thus shown that the phenomena of spatial and temporal summation underlying secondary hyperalgesia are mediated by glutamergic receptors of type NMDA. It should be noted that neither ketamine nor morphine altered the threshold for thermal stimuli.
An interesting study on morphine15 was conducted on healthy volunteers who were subjected to first-degree burns. This study showed that morphine, whether systemic or topical, reduced pain during the burn but not post-burn primary and secondary thermal or mechanical hyperalgesia; the question of the receptors involved in nociceptive transmission is still a matter of discussion.
The present study is of particular interest because it reopens the debate on the controversial role of the receptors of opioids inducible on peripheral nerves (the existence of such receptors has in fact sometimes been doubted).
In the central model, besides the sensitization of the central neurons of the marrow or of the superior centres, there is also a description of an expansion of the mechanical CR in the WDR neurons (spatial summation) and the development of a low-threshold CR in the NS neurons after activation by the C fibres, which transforms them into WDR neurons, accentuating the importance of the LTM mechanoceptors in the development of secondary hyperalgesia.
Evolutive phase
After the acute phase, manifestations of pain are even more variable than before, with different trends and intensities from patient to patient and, in time, in the same patient. The various therapies for dealing with the lesions, the procedures of hydrotherapy and debriding, and the possible use of transplants and rehabilitation exercises all contribute to the variable profile of pain. Various attempts to have been made to define this profile, using different pain scales. These include the Present Pain Integrity Scale and the Number of Words Chosen Scale, according to which the intensity of pain in burn patients is comparable in most cases with that of patients with post-herpes neuralgia, while on the Pain Rating Index the pain was analogous to that of arthritic patients.
It must also be added that this is a phase in which the psychological reactions of burn patients play a decisive role, especially as regards pain.
Clinically speaking, when a lesion heals - if the repair is performed by first intention - chronic painful sequelae are rarely encountered except in patients with extensive burns subjected to primary excision and then grafting, who may suffer from dysaesthesia in donor sites used more than once.
Repair by second intention would appear to be more suitable in the event of causalgia, dysaesthesia, and ghost pain syndromes, considering that granulation tissue skin biopsies have indicated involvement of nerve tissue in the scar.
The importance of these complaints tends to be underestimated because - appearing as they do in the post-burn period of depression - they are sometimes taken as a sign of an altered psychological condition, thus delaying correct diagnosis and therapy.
Some words on the skin lesion itself may be useful. Burn tissue repair is certainly a strong expression of type 1 receptors of the fibroblast growth factor. It is now known for certain that the administration of fibroblast growth factor promotes better keratinogenesis and angiogenesis.16
It is also interesting to remark that after a burn the increase in endothelin in injured and adjacent areas rarely causes tissue thromboses or lesions. The use in the rat of TAK-044, a nonselective antagonist of endothelin A and B, makes it possible to considerably reduce oedema and tissue necrosis.17
In the last 25 years, the survival rate of burn patients, including those with serious burns, has increased by 1% a year, thanks to the great progress made in intensive care techniques, respiratory damage treatment, and all emergency medicine practices. Burns are still however an extremely complex type of lesion, and optimal treatment requires wide and deep knowledge of the metabolic interactions between all the major systems of the body. The physiopathology of these lesions evolves in time, and correct and timely treatment and careful monitoring of the pathogenetic mechanisms involved in lesions due to heat are the key to a successful clinical outcome.
RESUME. Les patients atteints de brûlures présentent des altérations biochimiques et réceptrices considérables. Les progrès récents des techniques instrumentales de recherche ont clarifié certains des mécanismes intéressés. Un des problèmes est la résistance aux médicaments curarisants (ce qui probablement n’est pas lié seulement à l’intensification des récepteurs); il faut aussi considérer les altérations métaboliques, l’immunosuppression, l’hyperalgésie et certains phénomènes de “mémoire” de la douleur et, en particulier, les problèmes de la régénération de la peau.