Ann. Medit. Burns Club - voL VI - n. I - March 1993

ACUTE AND CHRONIC PAIN RESULTING FROM BURN INJURIES

Anoesthesiology Department, General Hospital, Athens, Greece

SUMMARY. After a description of the physiology of burn pain, and the development of pain in the successive phases of the bum disease, suggestions are given for appropriate treatment, which may be local or systemic. Medical and nursing staff frequently underestimate pain and are unnecessarily worried about undesirable secondary effects, such as addiction, that it is feared may follow opioid administration. With individualized drug treatment, all bum patients can benefit from pain relief.

From the first moments of a burn injury, as well as throughout the entire period of treatment, pain affects the patient not only as a symptom but also as a difficult medical problem. Taking into account the average extended hospital stay and the necessity for the closest co-operation between the patient and the medical and nursing staff, especially during the various procedures, it is clear how essential it is to deal aggressively with the problem of pain in the burns patient (1,2).

For many decades the treatment of pain, not only in burns patient but also more generally, was based on two major hypotheses:
1. The degree of pain is proportional to the degree of tissue damage;
2. The degree of pain release obtained was directly proportional to the analgesic medication administered.

Modern research has now shown that:
1. A specific system of neurons of the central nervous system is responsible for the central control of pain;
2. Specially developed neural cells produce endogenic opioid-like compounds, generally known as endorphins, which have similar pharmacological properties to those used in medical treatment as potent analgesics. These two discoveries provided the basis for understanding what was known empirically, but little understood scientifically, i.e. the variability in pain intensity. observed in various cases (3).

Pain as a result of burn injuries starts starts immediately after the thermal, chemical or electrical injury to the tissues. This is due to the production of high frequency impulses by the neural end plates which come into contact with the surrounding air or have undergone partial destruction.
The neurons that have undergone complete destruction of their end plates have a period of respite following which, due to regeneration, they again come into contact with the air. During the immediate hours and days following an injury the pain produced is a result of the irritative chemical substances released close to the area of the burn. Such substances are histamine, serotonin, kinins'and prostaglandins.
Laboratory research has also indicated that the thermal injury not only causes the area of burn and the immediate surrounding tissues to be painful but also results in a phenomenon known as hyperalgesia. This phenomenon lowers the pain threshold, i.e. even a minor impulse will result in an increase of pain. Hyperalgesia can also cause automatic pain (pain without apparent cause) and can last from a few minutes up to several hours.

Emergency phase. This phase starts immediately after the accident and lasts a few hours or days. The main treatment consists of the replacement of fluids and electrolytes and the maintenance of satisfactory circulatory, pulmonary and renal function. The pain during the initial phase is due to two reasons (I):

1. The direct trauma to the burn area and surrounding tissue, which in most cases begins immediately. Cases have however been reported in which the burn victim has proceeded immediately after the injury to carry out incredible feats of strength and endurance without feeling pain (i.e. to save others, to put out flames or to find assistance). This has been chronicled as Wall's Theory (1979), according to which the sense of pain is suppresed to allow other more vital biological mechanisms to operate. Also, in full skin-thickness burns the nerve endings are completely destroyed, so that the pain felt is either mild or moderate in nature and intensity, and refers more generally to the surrounding area. This pain is described as continuous and irritant during voluntary and involuntary movements.

2. Pain as a result of medical procedures (treatment and dressing of the burn area, venous catheterization, intubation, bronchoscopy, etc.). The pain produced is described as mild, moderate or intense, the degree depend depending on the personality of the patient, and the degree of emotional, psychological and physical stress present.

Acute phase. This stage follows and may last days or months. The management consists of local treatment and dressing of the burned areas and includes many procedures such as surgical intervention, physiotherapy, hydrotherapy and dressing changes. Pain during this stage is described as relatively low in intensity, interspersed however with brief periods of acute to unbearable pain during treatment procedures. This treatment-related pain has been described as "hell" by burn patients even when pre-treatment analgesics have been administered. It is interesting to note that after auto-transplant of skin the burned area ceases to cause pain while the donor site causes discomfort (1,4).

Recovery phase. This commences as soon as the burned surfaces have been covered or have healed. The primary target is functional rehabilitation and the minimalization of scar formation. This phase is related to the rebirth of tissue and causes local discomfort and paraesthesia. These symptoms, though relatively mild, have at times been described as being as unbearable as the initial burn pain (5).

Medical factors (1,4,6)
Apart from the fact that patients with more extensive burns have to face a longer stay in hospital, it seems that no relation exists between the degree of pain and:
a) the extent, the depth or the site of the burn injury;
b) alcoholism;
c) narcotic drug addiction;
d) psychiatric disorders; and
e) length of treatment.

Klein and Charlton, who studied the effects of the duration of hospital confinement on the level of pain in adult burn patients, found no correlation between length of stay and the amount of pain experienced. Other researchers (Andreasen et al., 1972) report that many burn patients sense ever-increasing levels of pain from the same procedures, with the passage of time. This can be explained as a result of either lowering of the pain threshold or reduction of the patient's resistance to pain.
The latter is evidenced by the following: a) return of sensation to the burned area; b) increasing fear and anxiety about treatment; and c) progressive reduction of the patient's will-power and fatigue as a result of deep disturbances and repeated stress situations (1,7).

A number of researchers have indicated that in adult burn patients factors such as age, sex, nationality, education or socioeconomic class do not appear to influence the level of pain felt and its externalization in the behaviour of the patients (6,8).
In children pain levels from similar burns are greater in very young and low-weight children (1).

Psycli.ological.factors

It has been described that pain and anxiety are two indissociable aspects of the same phenomenon, triggered by severe tissue injury (9).
In burn patients, anxiety relates to the immediate past (for example, the circumstances leading to the injury and even feelings of guilt for injuries caused to others), to the present (worry about the progress of treatment), and to the future (survival, appearance, ability to work, etc.). Moreover, from the initial treatments, the experience of pain leads to worry about the next and so on, leading to a vicious circle of anxiety/ pain/anxiety. Feelings of animosity may develop, for this reason, between the patient and medical personnel. Many researchers have noted that the degree of pain expressed is greater when the patient is aware of the dressing and treatment routine (1,4).

1. The choice of method for local treatment: the closed method is considered the most satisfactory despite the painful dressing changes.
2. Choice of the painless antiseptic to be used.
3. Early debridement: reduces the need for painful dressings and the formation of scar tissue and keloids.

The most widely accepted method of treatment for severe pain is the administration of potent narcotic analgesics such as morphine, pethidine, fentanyl or nalbuphine, together with the simultaneous administration of tranquillizers or anxiolytic drugs (6,10).
The method of delivery, the choice of drug, the dosage and the frequency have to be individualized, according to factors such as the patient's stage of recovery and physical state. It should be noted that during the acute phase the blood supply to various organs and tissues is diminished. Analgesics which are administered either by subcutaneous (SC) or intramuscular (IM) injection have a delayed absorption rate and a reduced peak concentration. If this is countered by increased doses the result may be toxic sideeffects. It is suggested therefore that during the acute phase analgesics should be administered via the intravenous route (IV).
Forty-eight hours after a burn injury the hypermetabolic state usually occurs. This is characterized by increased blood llow to the tissues and organs and alteration of the drugs' pharmacokinetics.
The concentration of plasma proteins is altered during the acute and emergency phases. The level of albumin is reduced, causing a decrease in the binding fraction of drugs which bind primarily with albumin, such as the benzodiazepines and the antiepileptic drugs. This results in an increase of their free plasma fraction. On the other hand, the levels of 'certain glycoproteins are increased and there is an increase in the binding fraction of antidepressant and muscle relaxant drugs (6).

1. Anxiolytics, tranquillizers: these drugs should be delivered IV in small doses repeatedly. It should also be borne in mind that hepatic metabolism in burn patients is delayed.
2. Analgesics: as previously stated the simultaneous administration of opioid analgesics and anxiolytic drugs IM or IV is the most common analgesia regime (2). It is known that the requirements of opioid analgesics in burn patients are increased, but it is uncertain whether the reason for this is altered pharmacodynamics or pharmacokinetcs. Moreover, the functioning of the body's endogenous system of endorphins may alter the response to exogenous opioids. It is prudent to note at this stage that although the undesirable effects of opioids are well known there is a tendency to administer less than the required dose of these drugs because of fear of the effects. It has been proved that 30% of nursing and medical staff underestimate the degree of pain severity. The failure to provide adequate pain relief is due to the following reasons:
a. Fear of patient addiction: yet in a study of over 10,000 cases in 33 Burns Centres, not a single case of iatrogenic addiction to narcotics was noted.
b. It is wrongly believed that over frequent administration of analgesics will cause a decrease in the patient's response to the drugs.
c. It is also wrongly believed that the metabolism of these drugs is altered (this is true only during the acute phase and in very young and very old patients).

For the suppression of the very severe pain that occurs during procedures or dressing changes, it is suggested that adequate doses of opioid analgesics individualized to each patient's requirements be administered. The action of these drugs may be reinforced by simultaneous administration of nitrous oxide or ketamine.
Adequate pain relief must also be afforded during periods of rest. This can be in the form of repeated small doses of opioid and, successively, less potent analgesics. This also applies to patients on mechanical ventilation, who cannot express their needs.
Me most modern trend is analgesia with special self-administration devices. In conclusion, it must be emphasized that with individual treatment programmes analgesia can be administered safely and adequately to every patient.

RESUME. Après avoir décrit la physiologie de la douleur due aux brûlures et le développement de la douleur dans les phases successives, l'auteur présente des indications pour le traitement approprie, qui peut être local ou systémique. Uéquipe médicale et paramédicale fréquemment sous-estime la douleur et s'inquiète inutilement des effets secondaires indésirables, par exemple l'assuétude, que l'on craint puissent être causés par l'administration des opiacés. Avec un traitement pharmacologique individualisé, tous les patients brûlés peuvent tirer avantage du soulagement de la douleur.

BIBLIOGRAPHY

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