Definition/Introduction
An individual’s capacity to feel pain is an essential component of the body’s ability to heal. Pain is the body’s way of telling us there is an injury, and we need to do something about it to ensure that healing occurs. An important consideration when discussing pain is that one patient’s pain is not the same as another’s, even if they have experienced similar injuries. Pain perception, indeed, is a subjective experience, influenced by complex interactions of biological, psychological, and social factors.[1]
Although inflammation and pain are essential parts of healing (acute pain), in some circumstances, they can have negative effects. Several mechanisms involving the activation of peripheral and central sensitization pathways through the sensitization of peripheral nociceptors, alterations in spinal dorsal horn neurons, and central nervous system (CNS) brain areas can trigger a pathogenetic cascade that ends with the development of chronic pain. Again, evidence suggests the paramount roles of the environment (ie, epigenetic) and genetics.[2][3] Thus, the chronicity of pain results from changes in pain processing through transcription and transduction. As a consequence, chronic pain is no longer simply a symptom but becomes a disease in itself. Currently, in the United States, estimates are that 100 million individuals suffer from the negative effects of protracted courses of pain and inflammation.[3] This epidemic is growing at an alarmingly fast rate and is taking a significant toll on the economy as well as patients and providers. In fact, there are reports that the cost for chronic pain patients is anywhere between 560 and 635 billion dollars annually.[3] Furthermore, increased opioid prescription and use, leading to opioid addiction in the United States and Canada, is a dramatic phenomenon that has been responsible for up to 70.000 drug overdose deaths in 2017. Although it appears that in other countries (eg, Western Europe), the data is not so alarming.[4]
The underlying foundations of pain perception (and its processing) have long interested researchers. Throughout the history of medicine, several theories have been proposed to explain why and how individuals experience pain. Although beliefs about where the pain originates have varied over the years, the desire to elucidate this phenomenon has led to the development of many different philosophies. The pain theories that this topic discusses below include the intensity theory, Cartesian dualism theory, specificity theory, pattern theory, gate control theory, neuromatrix model, and biopsychosocial model.[5] The topic has not only historical and didactic interests but also involves important practical and therapeutic implications.[5][6][7]
Issues of Concern
Intensity Theory
The theory goes back to the Athenian philosopher Plato (c. 428 to 347 B.C.), who, in his work Timaeus, defined pain not as a unique experience, but as an 'emotion' that occurs when the stimulus is intense and lasting. Centuries later, we are aware that chronic pain is a dynamic experience, profoundly changeable in spatial and temporal dimensions. A series of experiments, conducted during the nineteenth century, sought to establish the scientific basis of the theory. These investigations, using tactile and other natural stimuli such as electrical stimulation, provided important information on the threshold for tactile perception and the role of dorsal horn neurons in the transmission/processing of pain.[8]
Cartesian Dualistic Theory
The oldest explanation for why pain manifested in specific populations was rooted in religious beliefs. Throughout history, religious ideologies have had a substantial influence on people’s thoughts and actions. As a result, the majority of people believed that pain was the consequence of committing immoral acts. There was also a belief that the suffering they endured was the individual’s way to repent for these sins.[9] Although this belief remained popular up until the nineteenth century, this was not due to the lack of other available theories. One of the first alternative scientific theories of pain was bravely introduced in 1644 by the French philosopher René Descartes (1596-1650). This theory is known in the current literature as the Cartesian dualism theory of pain.[10] The dualism theory of pain hypothesized that pain was a mutually exclusive phenomenon. Pain can result from physical or psychological injury. However, the 2 types of injury did not influence each other, and at no point did they combine to create a synergistic effect on pain, thereby making pain a mutually exclusive entity.[11] In an attempt to placate the church, Descartes also included in his theory the idea that pain is connected to the soul. He claimed that his research uncovered that the soul of pain was in the pineal gland, consequently designating the brain as the moderator of painful sensations.[1] The dualistic approach to pain theory fails to account for many factors that are known to contribute to pain today. Furthermore, it lacks an explanation as to why no 2 chronic pain patients have the same experience with pain, even if they have had similar injuries. Despite these shortcomings, it provided future researchers with a solid foundation for further expanding the scientific understanding of the intricate phenomenon of pain.
Specificity Theory
Many scientists continued to do research long after Descartes proposed the dualistic theory of pain. However, it wasn’t until 1811 that another well-known theory of pain emerged. This theory, initially proposed by Charles Bell (1774–1842), is known as the specificity theory.[1][5] This theory is similar to Descartes' dualistic approach to pain in that it delineates different types of sensations along different pathways. In addition to identifying specific pathways for different sensory inputs, Bell also postulated that the brain was not the homogeneous object that Descartes believed it was, but rather a complex structure with various components.[1][5] Scientists and philosophers alike spent the next century and a half further developing the specificity theory. One of the many contributors to this theory was Johannes Müller. In the mid-1800s, Müller published in the Manual of Physiology that individual sensations were the result of specific energy experienced at certain receptors. Furthermore, Muller believed there were an infinite number of receptors in the skin, and this surplus accounted for an individual's ability to discriminate between different sensations.[6] In 1894, Maximilian von Frey made another critical addition to the specificity theory, advancing the concept. This contribution to the theory was the discovery of 4 distinct somatosensory modalities distributed throughout the body. These sensations include cold, pain, heat, and touch.[12] This concept aligns well with previous research on this pain theory, which reiterates the presence of distinct pathways for different sensations. Although this theory and the research surrounding it have significantly advanced the understanding of pain, it still fails to account for non-physical factors that give rise to the sensation of pain. Much like the dualistic approach to pain, this theory also lacks an explanation for why sometimes pain persists long after the healing of the initial injury. The incompleteness of the specificity theory of pain etiology necessitated additional theories and ongoing research.
Pattern Theory
Following the specificity theory, a handful of other philosophies were introduced regarding the sensation of pain. Of these philosophies, the pattern theory of pain has the greatest coverage in the scientific literature. The American psychologist John Paul Nafe (1888-1970) presented this theory in 1929. The ideas contained in the pattern theory were directly opposite to the ideas suggested in the Specificity theory regarding sensation. Nafe indicated that there are no separate receptors for each of the 4 sensory modalities. Instead, he suggested that each sensation relays a specific pattern or sequence of signals to the brain. The brain then decodes this pattern. Depending on which pattern the brain reads, it correlates with the sensation felt.[5][13] At the time of its introduction, the pattern theory gained significant popularity among many researchers. However, through further research and the discovery of unique receptors for each type of sensation, it can be stated with certainty that this theory is an inaccurate explanation for how we feel pain.
Gate Control Theory
In 1965, Patrick David Wall (1925–2001) and Ronald Melzack announced the first theory that viewed pain through a mind-body perspective. This theory became known as the gate control theory.[14] The new theory by Melzack and Wall partially supported both previous theories of pain and also provided additional knowledge to advance the understanding of pain. The gate control theory of pain states that when a stimulus gets sent to the brain, it must first travel to 3 locations within the spinal cord. These include cells within the substantia gelatinosa of the dorsal horn, fibers in the dorsal column, and transmission cells in the dorsal horn.[6][15]The substantia gelatinosa of the spinal cord's dorsal horn modulates signals passing through it, acting as a “gate” for information traveling to the brain.[13] The sensation of pain an individual feels results from the complex interaction among these 3 components of the spinal cord. Simply stated, when the “gate” closes, the brain does not receive the information that is coming from the periphery to the spinal cord. However, when the signal traveling to the spinal cord reaches a certain threshold, the “gate” opens. Once the gate is open, the signal can travel to the brain, where it is processed, and the individual feels pain. The information mentioned above accounts for the physical component of pain, but, as stated earlier, the Gate Control Theory was among the first to acknowledge that psychological factors also contribute to pain. In their original study, Melzack and Wall suggested that, in addition to the control provided by the substantia gelatinosa, there was another control mechanism in the brain's cortical regions.[15] In recent years, researchers have proposed that these cortical control centers mediate the effects of cognitive and emotional factors on pain. Current research has also suggested that a negative state of mind amplifies the intensity of signals sent to the brain.[16] For example, somebody who is depressed has a “gate” that is open more often, allowing more signals to get through, increasing the probability that an individual experiences pain from an otherwise normal stimulus. Also, there are reports that certain unhealthy lifestyle choices also result in an “open gate,” which in turn leads to pain that is disproportionate to the stimulus.[17][18] The gate control theory has proven to be one of the most significant contributions to the study of pain throughout history. The concepts that Melzack and Wall introduced to the study of pain are still utilized by researchers today. Even though this theory introduced the idea that pain wasn’t solely a result of physical injury but rather a complex experience influenced by cognitive and emotional factors, additional research was still necessary to fully comprehend the mechanisms and etiology of pain. This need precipitated the introduction of the following 2 philosophies regarding pain.
Neuromatrix Model
Almost thirty years after introducing the gate control theory of pain, Ronald Melzack proposed another model that helped explain how and why people feel pain. Until the mid-1900s, most theories of pain held that the experience was exclusively due to an injury somewhere in the body. The thinking was that if an individual suffered an injury, whether through trauma, infection, or disease, a signal would be transmitted to the brain, which would, in turn, result in the sensation of pain. Although Melzack had contributed to these previous theories, it was his exposure to amputees who were experiencing phantom limb pain in well-healed areas that prompted his inquiry into this more accurate philosophy of pain. The theory he proposed is known as the neuromatrix model of pain. This philosophy suggests that the central nervous system, rather than the periphery, is responsible for eliciting painful sensations.[19][20] The neuromatrix model posits that 4 components within the central nervous system are responsible for the perception of pain. The 4 components are the “body-self neuromatrix, the cyclic processing and synthesis of signals, the sentinel neural hub, and the activation of the neuromatrix.”[13] According to Melzack, the neuromatrix comprises multiple areas within the central nervous system that contribute to the neural signal that produces the feeling of pain. These areas include the spinal cord, brain stem and thalamus, limbic system, insular cortex, somatosensory cortex, motor cortex, and prefrontal cortex.[19] The signal that these areas of the central nervous system work together to create is responsible for allowing an individual to feel pain, and he referred to as the “neurosignature.” Furthermore, this theory states that input coming in from the periphery can initiate or influence the neurosignature, but these peripheral signals cannot create a neurosignature of their own.[13] The idea that peripheral signals can alter the neurosignature is an important concept when considering how nonphysical factors affect an individual’s experience of pain. The theory by Melzack claimed that not only are there specific neurosignatures that elicit certain sensations, but that when a signal is altered, this allows for the formation of memories of these particular experiences.[19] If the same circumstances recur, it is this memory that allows the same sensation to be felt. In addition to the hypothesis that pain was a product of different patterns of signals from the central nervous system, the neuromatrix model further elaborated on the idea initially proposed in the gate control theory: that pain can be affected not only by physical factors but also by cognitive and emotional factors. Melzack suggested that hyperactivity of the stress response has a direct effect on pain. Hyperactivity of the stress response occurs when an individual exposed to increased stress experiences heightened pain.[19] Taking all of these claims into consideration, it is evident that pain is a complex issue that cannot be accounted for by physical factors alone. Even though the neuromatrix model further established that pain is influenced by cognitive, emotional, and physical factors, it still fails to account for the social constructs of pain. Therefore, a new theory of pain must be utilized to appropriately explain the mechanism behind pain and why each individual’s experience with pain is unique.
Biopsychosocial Model
The biopsychosocial model provides the most comprehensive explanation of the etiology of pain. This specific theory of pain hypothesizes that pain results from complex interactions among biological, psychological, and sociological factors, and any theory that fails to include all 3 constructs of pain fails to provide an accurate explanation for why an individual is experiencing pain.[21][22] Although the term biopsychosocial was not introduced until 1954 by Roy Grinker (1900-1993), a neurologist and psychologist, many physicians had long considered the utility of using such a model to approach the management of a patient’s pain.[20][23] One of the most prominent physicians who utilized this more comprehensive approach to pain was John Joseph Bonica (1917-1994), a Sicilian American anesthesiologist at Madigan Army Hospital, who is widely regarded as the founding father of the discipline of pain medicine. In the 1940s, Bonica was caring for many patients who had returned home from World War II and were now experiencing debilitating pain due to injuries they had suffered in the war. He had recognized that the pain these wounded soldiers were experiencing was rather complex and not easily managed. This situation led him to propose that to adequately manage these patients, physicians needed to create interprofessional pain clinics comprising multiple disciplines.[24] At this moment in history, there was little support for the idea that pain was more than just the result of an injury, and Bonica was relatively unsuccessful in establishing these clinics. It wasn’t until 1977 that the biopsychosocial model was scientifically suggested as an explanation for the etiology of some medical conditions. George Engle claimed that to treat disease adequately, one must consider multidimensional concepts and manage the whole patient instead of focusing on a single issue. This methodology takes into account that the human body cannot be divided into separate categories when considering treatment options. Instead, it is beneficial to acknowledge that illness and disease result from complex interactions among biological, psychological, and sociological factors, which all affect an individual’s physical and mental well-being. Although Bonica had technically been the first physician to recognize the importance of a biopsychosocial approach to pain, John D. Loeser, another anesthesiologist, has been credited as the first to apply this model to pain.[25] Loeser suggested that 4 elements should be considered when evaluating a patient with pain. These elements include nociception, pain, suffering, and pain behaviors. Nociception is the signal sent from the periphery to the brain to alert the body to injury or tissue damage. Pain, on the other hand, is the subjective experience that occurs after the brain has processed the nociceptive input. The last 2 components of pain to consider are suffering and pain behaviors. The thinking is that suffering is an individual’s emotional response to the nociceptive signals and that pain behaviors are the actions that people carry out in response to the experience of pain. Both of these can be either conscious or subconscious.[25] The Loeser 4 elements of pain account for the biological, psychological, and sociological factors that can create or influence an individual’s experience with pain. Failing to consider any of these 4 elements when determining the cause or establishing a management plan could constitute inadequate assessment or care. With a better understanding of the cause of a patient's pain, the doctor has a more accurate foundation for formulating a treatment plan. The findings by Loeser prove that the biopsychosocial model of pain offers the most comprehensive philosophy and provides the framework that is needed to start appropriate therapy to manage patients with chronic pain adequately.
Clinical Significance
Pain results from complex interactions among biological, psychological, and sociological factors, and no 2 individuals’ experiences are the same. It makes sense that an individual’s treatment approach should align with what it is treating. An appropriate method for treating pain must be multidimensional and tailored to the individual's experience.[26][27] Utilizing the biopsychosocial model to treat chronic pain allows health practitioners to do just that. This model has been proven to deliver superior outcomes, including increased patient satisfaction and greater functional restoration. In addition, it has been shown to be a more cost-effective method for treating patients with chronic pain than other commonly used treatment plans. When taking all of this into consideration, it becomes quite evident that an interprofessional biopsychosocial model for the management of chronic pain should be the standard of care for all patients. Ethically speaking, when a means of reducing a patient's suffering exists, it should be used to its fullest extent.[28]
Addressing a patient who is suffering from chronic pain using a biopsychosocial approach provides the physician with the knowledge that is necessary to treat all factors contributing to pain instead of only treating the pain itself. There are multiple advantages to utilizing this type of model to address pain. One of the most critical benefits observed in clinics when this model is in use is a higher success rate, as measured by increased functional capacity and patient satisfaction. This type of treatment focuses not on curing the problem but on regaining maximal functional capacity. Certain specialties need to be included in a biopsychosocial treatment plan when managing patients with chronic pain to achieve this goal. These mandatory specialties include a primary care physician, a psychiatrist or psychologist, a physical therapist, an occupational therapist, and sometimes a disability case manager. Multiple research studies evaluating the effectiveness of this approach have continuously shown that, versus other more traditional methods of managing pain, the biopsychosocial approach to pain allows for greater restoration of functionality. This type of treatment provides the patients with the tools that they need to take control of their pain, as opposed to letting their pain control them.
In addition to better outcomes and increased patient satisfaction, a biopsychosocial approach to pain management would serve to reduce the financial costs associated with caring for chronic pain patients. Research has been conducted comparing an interprofessional biopsychosocial pain management protocol to alternative methods.[22][29] All the studies have reached the same conclusion: an interprofessional approach to pain management is 21 times more cost-effective than other methods.[5] This reduction in cost can be attributed to a decreased need for pain medication, fewer calls to healthcare and emergency rooms, and reduced disability payments. In a country where 17.5% of gross domestic product is spent on healthcare, it would be logical to implement methods that help reduce this financial burden, which is why applying an interprofessional approach to pain management is the obvious solution.[5]
The biopsychosocial model is the only theory of pain that provides the most comprehensive explanation as to why people have pain, as well as the unique nature of each patient’s experience. With chronic pain now considered a public health crisis, clinicians must alleviate suffering by trying all means possible instead of continuously using old methods that have been shown to be ineffective. In conducting future research on the concept of pain and its management, clinicians must maintain this biopsychosocial approach and continue to uphold the prescient observation by Aristotle that “pain is the quality of all senses.” Even with these newer comprehensive approaches available, pain management remains challenging and continues to be an important area for future research.
