Key points

Introduction

From the moment of injury through rehabilitation and beyond, pain control is a major challenge in the management of patients with burn injuries. In fact, some argue that burn pain is the most difficult to treat among any etiology of acute pain. The therapies used to treat burn injuries may exacerbate the difficulty of pain control because most of these interventions are associated with pain, be it dressing changes, excision and grafting, or physical therapy. These therapies can cause pain that is equivalent to or worse than the pain of an initial burn injury. Therefore, pain management must be a foundation of burn care. Good pain control is linked to better wound healing, sleep, participation in activities of daily living, quality of life, and recovery.

Despite profound improvements in modern burn care, suboptimal and inconsistent pain management persists throughout all stages of burn treatment. Without aggressive pain control, patients are likely to suffer not only from the acute experience of pain in itself, but the secondary morbidities of higher pain levels, including long-term anxiety and posttraumatic stress, or even delayed wound healing. The unique challenge of burn pain is further complicated by a relative dearth of standardized approaches. Instead, tradition and personal/institutional biases often dictate pain management. The complex interaction of anatomic, physiologic, pharmacologic, psychosocial, and premorbid issues can make the treatment of burn pain particularly difficult. An overview of pain management strategies specific to the treatment of burn injuries is summarized here.

Introduction

From the moment of injury through rehabilitation and beyond, pain control is a major challenge in the management of patients with burn injuries. In fact, some argue that burn pain is the most difficult to treat among any etiology of acute pain. The therapies used to treat burn injuries may exacerbate the difficulty of pain control because most of these interventions are associated with pain, be it dressing changes, excision and grafting, or physical therapy. These therapies can cause pain that is equivalent to or worse than the pain of an initial burn injury. Therefore, pain management must be a foundation of burn care. Good pain control is linked to better wound healing, sleep, participation in activities of daily living, quality of life, and recovery.

Despite profound improvements in modern burn care, suboptimal and inconsistent pain management persists throughout all stages of burn treatment. Without aggressive pain control, patients are likely to suffer not only from the acute experience of pain in itself, but the secondary morbidities of higher pain levels, including long-term anxiety and posttraumatic stress, or even delayed wound healing. The unique challenge of burn pain is further complicated by a relative dearth of standardized approaches. Instead, tradition and personal/institutional biases often dictate pain management. The complex interaction of anatomic, physiologic, pharmacologic, psychosocial, and premorbid issues can make the treatment of burn pain particularly difficult. An overview of pain management strategies specific to the treatment of burn injuries is summarized here.

Mechanisms and types of pain in burns

Although burns are classified according to depth, area, and severity of injury, pain does not necessarily correlate with these measures. The individual experience of pain varies widely between patients and throughout the healing process in burn injuries. Because individuals have varying pain thresholds, coping abilities, and even physiologic responses to injury, patients may experience disparate levels of pain despite having similar injuries. The most immediate and acute form of burn pain is the inflammatory nociceptive pain attributed to burn injury and tissue trauma. Nociceptive pain is often followed by and potentially exacerbated by procedural pain related to the treatment of burn wounds, be it surgical debridement, grafting, staple application and removal, physical therapy, or dressing changes. As burn wounds begin to heal, neuropathic pain, characterized by a throbbing or constant burning sensation potentially adds an additional layer of discomfort.

Although all burns are painful, conventionally, deeper, full-thickness burns are thought to be somewhat less painful than superficial and partial thickness burns because of afferent nerve destruction. However, this does not always play out in clinical practice. Additionally, full-thickness burns eventually require debridement and grafting and subsequent dressing changes that all lead to substantial pain. At the time of burn injury, tissue damage is the primary mechanism of pain. Stimulation of local nociceptors transmits an impulse via Ad and C fibers to the dorsal horn of the spinal cord. Peripheral sensory nerves and descending influences from cortical areas can modulate the magnitude of the pain impulse. Ultimately, conscious perception of pain is regulated by areas of the brain, often named the “pain matrix,” which is thought to involve a network of higher cortical areas and the thalamus. The conscious perception of pain is affected not only by the burn wound itself but also by context, cognition, pharmacologics, mood, and other predisposing factors. Burn pain also may vary and fluctuate widely over the span of recovery. Therefore, the successful treatment of burn pain should involve a multimodal approach tailored to the patient and scenario.

Pain assessment

The first step in determining a pain treatment plan is assessing the degree of the patient’s pain, which, in the case of burn injuries, may be mild to excruciating. Reliable, valid pain assessment tools in form of verbal adjective, numeric, or visual analog scales (VASs) can be useful guides for pain management in burns. In adults, VAS and numeric rating scales (NRSs) are commonly used. Both NRS and VAS have undergone repeated validation and have performed well in different patient populations. Children, especially those who are preverbal, and noncommunicative adults present a more difficult challenge. Observational scales and physiologic indicators, such as heart rate and blood pressure, may be used to gauge pain in these populations.

Second, understanding the type and chronicity of a patient’s pain is useful for tailoring pain management strategies. The Patterson burn pain paradigm provides a roadmap for the management of burn pain through 5 different phases of injury, treatment, and recovery. (1) Background pain is pain that is present while the patient is at rest, results from the thermal tissue injury itself, and is typically of low to moderate intensity and long duration. (2) Procedural pain is brief but intense pain that is generated by wound debridement and dressing changes and/or rehabilitation activities. (3) Breakthrough pain describes unexpected spikes of pain that occur when background analgesic effects are exceeded, when at rest, during procedures, or with stress. (4) Procedural pain is an expected and temporary increase in pain that occurs after burn excision, donor skin harvesting, grafting, or interventions, such as the placement of central lines due to the creation of new and painful wounds in the process. (5) Chronic pain is pain that lasts longer than 6 months or remains after all burn wounds and skin graft donor sites have healed. The most common form of chronic pain is neuropathic pain, which is the result of damage sustained by the nerve endings in the skin. Each of these 5 phases presents unique challenges in the management of burn pain. Clinicians should be prepared to adjust treatment strategies using both pharmacologic and nonpharmacologic techniques, discussed in further detail in the next sections.

Pathophysiology and pharmacologic considerations

Major burns cause massive tissue destruction and activation of a cytokine-mediated inflammatory response leading to dramatic pathophysiologic effects. The inflammatory response is initiated within minutes of burn injury, which results in a cascade of irritants that sensitize and stimulate pain fibers. Burn wounds may become primarily hyperalgesic to mechanical and/or thermal stimuli. Two distinct phases, a burn shock phase followed by a hypermetabolic phase, were first described by Cuthbertson in 1942. Burns involving more than 20% of total body surface area cause generalized edema even in noninjured tissues. Continued loss of plasma into burned tissue can occur up to the first 48 hours or even longer in these larger burns. These profound physiologic changes contribute to altered pharmacokinetic and pharmacodynamic responses to many drugs. Plasma protein loss through burned skin and further dilution of plasma proteins by resuscitation fluids decrease the concentration of albumin. Increase in volume of distribution has been shown in almost every drug studied, including propofol, fentanyl, and muscle relaxants. Burned patients may demonstrate variable or unpredictable responses to drugs, thereby requiring adjustments to dosing or complete exclusion of certain drugs (eg, succinylcholine). Cardiac output goes down in the acute injury phase (0–48 hours) even with aggressive volume resuscitation. As a result, elimination of some drugs by the kidney and liver may be decreased. Next, the hyperdynamic phase leads to increased cardiac output and blood flow to the kidneys and liver, meaning increased clearance of drugs dependent on organ blood flow for elimination.

Pharmacologic management

Oral nonsteroidal anti-inflammatory drugs (NSAIDs) and acetaminophen are mild analgesics that exhibit a ceiling effect in their dose-response relationship. Such limitations render these agents unsuitable for the treatment of typical, severe burn pain. Oral NSAIDs and acetaminophen are of benefit in treating minor burns, usually in the outpatient setting. For hospitalized patients with burn injuries, opioids are the cornerstone of pharmacologic pain control. Opioids are inexpensive, widely available, and familiar to most clinicians. Opioid requirements are increased in patients with burn injuries and may far exceed standard dosing recommendations; therefore, tolerance is a challenge throughout burn care. Patient-controlled analgesia (PCA) with intravenous opioids is a safe and efficient method of achieving flexible analgesia in burn-injured patients. Studies comparing PCA with other routes of administration have shown mixed results as to benefit and patient satisfaction. Although opioids delivered via oral and intravenous routes are a mainstay of burn pain treatment, it is important to note that pharmacokinetic changes have been documented for morphine, fentanyl, and propofol throughout the hyperdynamic and hypermetabolic stages of burn recovery. Animal studies of burn injury have shown changes in spinal cord receptors, including downregulation of μ opioid receptors, and upregulation of protein kinase C (PKC)-γ and N-methyl- d -aspartate (NMDA) receptors. Therefore, ketamine requirements to anesthetize patients with burn injuries also may be increased. For patients with burn injuries who develop extreme tolerance to morphine, treatment with clonidine, dexmedetomidine, ketamine, and methadone has been found to be effective. Dexmedetomidine has been used to provide sedation–analgesia for burned patients and to decrease opioid requirements. Titration of dexmedetomidine may also allow weaning of benzodiazepine, as patients get close to extubation. Dexmedetomidine has been shown to result in less delirium than benzodiazepines in several critical care studies.

Ketamine has many potential advantages for induction and maintenance of anesthesia in patients with burn injuries. Ketamine is associated with hemodynamic stability, preserving airway patency as well as hypoxic and hypercapnic responses, and decreasing airway resistance; therefore, ketamine may be the agent of choice, particularly in scenarios in which airway manipulation is to be avoided. Intensive dressing changes at the bedside, removal of hundreds of staples, or other procedures requiring conscious sedation are examples in which ketamine might be the agent of choice. Administration of benzodiazepines along with ketamine can reduce dysphoria, and coadministered glycopyrrolate can reduce the severity of increased secretions associated with ketamine. Burned patients receiving ketamine must be closely monitored for myocardial depressant affects, however, because persistently high levels of catecholamines results in desensitization and downregulation of β-adrenoreceptors in these patients. Additionally, patients with prior drug use might experience distressing anxiety with ketamine use.

Anxiety is a common issue for burn-injured patients and may be closely linked to pain. Background pain and the anticipation of procedural pain exacerbates anxiety, which can in turn exacerbate the pain. Anxiolytic drugs have commonly been used in conjunction with opioids in the treatment of burn pain. When administered as an adjunct to opioids, benzodiazepines have been shown to decrease both background pain and pain in those patients with high levels of procedural pain. Furthermore, low-dose benzodiazepine administration may reduce burn wound care pain reports. Patients with high anticipatory procedural anxiety and high levels of pain are most likely to benefit from anxiolytic therapy. However, benzodiazepine use, as in other critical care populations, can lead to short-term and long-term delirium. Antipsychotic medications (eg, haloperidol and quetiapine) also are good options and are increasingly used for management of anxiety and agitation associated with burns. Antidepressants appear to enhance opiate-induced analgesia, especially in patients with chronic (neuropathic) pain.