Models of Pain

Gate-Control Theory of Pain

The first major modernized medical model theory of pain, the gate-control theory, emphasized the close interaction between psychosocial and physiologic processes. The gate-control theory of pain describes how thoughts, feelings, and behavior affect pain.^1,⁵ The hypothesis is that a “gate,” located within the human brain, determines the individual’s impression of pain. The gate may be opened or closed—this determines the amount of pain the individual experiences. The underlying assumption is that the pain message originates at the site of aggravation, the signal is transmitted to the brain, and the pain is then brought into the individual’s awareness.

There are many ways in which an individual may “open” or “close” the gate. Using coping strategies may close the gate (meaning that the brain will either not recognize or give credence to the pain signal), while allowing oneself to focus on thoughts of pain may open the gate (bringing the pain signal into the brain’s awareness). Negative thinking, nonconstructive, pessimistic thinking may also open the gate, as will stress, anxiety, tension, helplessness, anger, hopelessness, and despair.

The ultimate conclusion from this theory is that the process of pain can therefore be mediated by changing the way an individual cognitively processes the pain experience. This theory is often useful in clinical practice as a means of explaining pain to patients, and aids the clinician in treating pain via cognitive therapy; however, the scientific community has demanded a more comprehensive theory that accounts for the neurophysiology, neurotransmission, and opioid receptors that may all be involved in understanding and defining pain. This demand was the precursor to the neuromatrix model of pain.

Neuromatrix Model of Pain

The term neuromatrix refers to the neural network involved in the perception of pain. The neuromatrix theory integrates physiologic and psychological evidences, and assumes pain to be a multifaceted experience, with pain sensations produced by specific patterns of nerve impulses generated by a widely distributed neural network.

The neuromatrix model may be viewed as a diathesis-stress approach, meaning that predispositional factors interact with acute stressors to result in a pathologic state. The experience of pain might be thought of as such a stressor. Further explained, the theory proposes that when an organism is injured, there is an interruption of homeostatic regulation. This disruption is not only physically stressful, but it also creates psychological stress. This in turn initiates a complex response aimed at restoring homeostasis (homeostasis being the previously nonpainful state of the body). This process of homeostatic restoration can add further physical and psychological stress.

Physiologically, the body may experience deleterious effects, such as immune system suppression, hypertension, and physical discomfort such as stomach pains or heart burn. The psychological aspects of pain result in the body activating the limbic system. The limbic system plays an important role in experiencing and regulating emotions, motivation of actions, and contributes to thought patterns. In the case of pain, one’s subjective interpretation of the pain experience, fear, and anxiety all further remove the body from homeostasis. Thus, once pain is established and the body activates the necessary mechanisms to return to homeostasis, any future or additional experience of pain will be physiologically and psychologically viewed as a continual threat that creates harmful demands on the body. Thus, a cycle develops that contributes to and maintains the pain-stress process. The neuromatrix hypothesis suggests that an individual’s unique genetic makeup and his or her own subjective experience of pain are the chief components that determine the nature of the pain the organism will experience and is the basis for individual differences in the pain experience.

Both the gate-control theory and the neuromatrix model have attempted to integrate and define a great deal of psychological and physiologic scientific data, although it is thought neither of them provides a fully adequate theory to define the pain experience. They do, however, point to what is currently the most promising approach to understanding pain: the biopsychosocial approach. This approach views physical disorders, including pain, as the result of a dynamic interaction between physiologic, psychologic, and social factors that can heavily influence a subject’s clinical presentation.

Biopsychosocial Model of Pain

In an effort to explain why individual experiences of pain are unique, the biopsychosocial model examines how psychological, social, and economic factors can interact with physical pathology to modulate a patient’s report of symptoms and subsequent disability. This understanding has been the foundation for a major paradigm shift in the assessment and management of pain, moving away from a traditional biomedical reductionist approach to this more comprehensive biopsychosocial approach. In fact, this paradigm shift is so dramatic that it has resulted is a mandate from the Joint Commission on the Accreditation of Healthcare Organizations requiring physicians to consider pain as a fifth vital sign. The Pain Care Bill of Rights of the nonprofit American Pain Foundation calls for management of all types of pain, both malignant and nonmalignant.

In order to understand pain in view of the biopsychosocial model, it seems helpful to examine the distinction between disease and illness. The term disease is generally used to define “an objective biological event” that involves the disruption of specific body structures or organ systems, caused by anatomic, pathologic, or physiologic changes. Illness, in contrast, is generally defined as a subjective experience or self-attribution of disease being present. An illness will yield physical discomfort, behavioral limitations, and psychosocial distress. Therefore, illness references how a sick individual and members of his or her family live with and respond to symptoms and their resulting disabilities.

To illustrate this distinction between disease and illness is analogous to the distinction made between nociception and pain. Nociception involves the stimulation of nerves that convey information about tissue damage to the brain. Pain, however, is a more subjective perception that is the result of the transduction, transmission, and modulation of sensory input, and may be filtered through an individual’s genetic composition, prior learning history, current physiologic status, and sociocultural influences. The combination of the physiologic experience of pain and the debilitating behavior that can accompany it are the expressions of suffering and pain behavior.

Based on this notion, it is thought that pain cannot be comprehensively assessed without a full understanding of the person who is exposed to the nociception. The biopsychosocial model focuses on illness. With this perspective, diversity in pain behavior can be expected as illness experience varies from person to person. This may include severity, duration, and psychological consequences. The interrelationships among biologic changes, psychological status, and the social and cultural context all need to be taken into account in fully understanding the pain patient’s perception of and response to illness. A model or treatment approach that focuses on only one of these core set of factors will be insufficient to effectively assess and treat the patient. The biopsychosocial model has consistently demonstrated the heuristic technique in treatment.²

When interpreting pain using the biopsychosocial model, clinicians should be aware that each of the three constructs in the model are different in their composition. Therefore, their assessment will be accomplished through different means and processes. Pain likely should be viewed longitudinally as an ongoing, multifactorial process in which there is dynamic interplay between the biologic, psychological, and social cultural factors that shape the experience and responses of patients.^2,⁵

To comprehensively assess pain, it is important to account for potential interactions in the process of prescribing the best treatment regimen, individualized for a particular patient with pain. For example, a patient may present with pain resulting from an earlier accident that produced severe musculoskeletal injuries, such as bone fractures and ligament tears, that have not completely healed. In addition to these physical injuries and resultant pain, the accident may have led to the inability to return to work. The patient might also have self-esteem problems because he or she is viewed as being disabled and is stigmatized by this situation. This may have resulted in economic problems and stressors because of the sudden decrease in income. There are debts to be paid, causing family stress, turmoil, and guilt. If this patient comes from a culture in which work and activity are highly valued there may be even more psychosocial distress. Thus, there are potentially multiple levels of psychosocial stressors that all need to be assessed and considered before one can develop a comprehensive pain management program for a patient who may not be responding to conventional or conservative care as might be expected.

Another model outlined four dimensions associated with the concept of pain: (1) nociception (2) pain (3) suffering and (4) pain behavior.^3,⁵ Nociception refers to the actual physical units that might affect specialized nerve fibers and signal the central nervous system that an aversive event has occurred. This may include chemical irritant, physical/mechanical, or thermal pain. Pain is the sensation arising as the result of perceived nociception. However, this definition is overly simplistic because sometimes pain is perceived in the absence of nociception. An example of this would be phantom limb pain. On the contrary nociception has been recorded to occur without being perceived, such as an individual who is in shock after experiencing a very severe injury. Nociception and pain act as signals to the central nervous system. Suffering is a reaction to these signals that can be affected by past experiences as well as anticipation of future events, and refers to the emotional association with it, such as fear, threat, or loss.³ Because of a specific painful episode, anxiety and depression may develop as a consequence to the pain behavior. Pain behavior refers to things that individuals do when they are suffering or currently experiencing pain. For example, a person may avoid driving after experiencing an injury due to an accident. The implications for pain behavior can range from avoiding certain activities to more debilitating problems such as developing generalized anxiety surrounding any activity the person must participate in to have a functional life. As such, the interaction in the range of biopsychosocial factors can be quite broad. There are times when the nature of the patient’s response to treatment may have less to do with the objective physical condition than it does with their psychological receptivity to treatment as well as their expectations. This is the grist for the mill of the psychological evaluation and psychotherapy-related treatment process of the person with pain.

Psychological Evaluation

Because of the biopsychosocial complexity associated with pain, pain-related psychological evaluation can be helpful in cases in which symptoms are in excess of expectation or do not correlate with known physiologic processes. Psychological factors may be producing delayed recovery of function or preventing the individual from otherwise benefiting from appropriate medical treatment which, if identified, can improve the treatment process and the ultimate outcome of the case.

If psychological factors are identified as moderating or mediating the patient’s pain-related behavior, it can result in treatment recommendations that remove or ameliorate the barriers to improvement and recovery. Thus, it is thought to be helpful for treating physicians to have a basic understanding of the pain-related psychological evaluation and treatment process.

The psychological evaluation of patients with pain begins with the establishment of rapport with the individual to be assessed. In a psychology practice it is not unusual to initially encounter a patient with pain who enters the evaluation room defensively at best and offended, angry, and/or suspicious at worst. The patient with pain may interpret the pain-related psychological consultation to imply the referral source believes their problems are not real or that their complaints are psychogenic in origin. For this reason, in addition to addressing issues of informed consent and establishing rapport with the patient, there is usually a need to provide some education as to the purpose of the evaluation and how biopsychosocial variables fit into the clinical picture and subjective situation of the patient’s life.

It can be extremely helpful for the referring physician to approach the referral for a pain-related psychological evaluation in a sensitive, compassionate manner. We suggest initially explaining to patients that the dualistic view in which the mind and body are separate does not appear to hold true, and that there is a dynamic, interdependent relationship between the individual’s psyche and their physical condition. We have found it makes sense to our patients when we explain our goal to be to treat the whole person and that we want to ensure they are as psychologically and mentally fit as they can be while they are in the process of physically rehabilitating and becoming more physically fit following an injury or in treating their painful condition.

The basic purpose of the pain-related psychological evaluation is to answer the questions posed by the referral source as clearly as possible. Often, if no referral questions are forwarded with the referral, the consulting psychological examiner may need to call the referral source to clarify if there are any specific issues that need to be addressed in the evaluation. Another goal of the evaluation is to generate psychological and behavioral information that is helpful to the referral source in understanding the psychological issues in the case and promotes the care in a more efficient and effective manner. The psychological evaluation documents and preserves a record of the assessment for use in the future and may provide a baseline or outcome information regarding progress. Ultimately, appropriate diagnosis leading to specific, practical, and functional recommendations that advance the patient’s care in a meaningful way become the goal for appropriate evaluation.⁴

To achieve the purposes of the evaluation, sufficient records need to be gathered and reviewed to provide an understanding of the medical issues and physiologic underpinnings of the case. A comprehensive clinical interview is necessary to elicit historical information about the onset and history of the pain, injuries, and background that may be psychologically contributing to the onset, severity, exacerbation, or maintenance of the pain. Opportunities for behavioral observation when the patient may or may not be aware he or she is being observed provide excellent data regarding the consistency of subjective complaints. Psychological testing can provide data derived from standardized samples of behavior that are quantifiable and illustrate how the individual being evaluated deviates from a normative base related to the concepts that are being assessed.

The clinical interview in these cases tends to be comprehensive in nature and covers important factors that can serve as potential barriers to recovery. Important topics for the clinical interview should likely include an understanding of the person’s cultural and ethnic background, because various cultures deal with issues of pain differently. The individual’s own personal and familial history of mental health problems should be explored to include issues of depression, anxiety, problems dealing with reality, and substance use. How the patient may have previously dealt with illness and injury may shed light on their ability to cope or the models for coping they may have witnessed in the past.

The individual’s cognitive capacity, level of intellectual functioning, ability to understand the nature of their condition, treatment options, and likely outcomes are important features to understand because they have bearing on how compliant, anxious, depressed, and motivated the individual may be in completing their treatment regimen. Contemporaneous stressors that the person may be experiencing in addition to their injury, illness, or painful condition for which they are being assessed will be important to explore to evaluate how taxed their resources are and also may provide necessary information regarding potential sources of secondary gain that may be promoting abnormal illness behavior.

The exploration and history obtained during the psychological clinical interview should likely also provide information about spousal availability or family solicitousness that may be unnecessarily reinforcing pain behaviors. Work history, prior work-related injuries, job changes or losses, and job dissatisfaction are important variables to survey as such factors may be either pressuring and propelling the person toward or repulsing them from relinquishing the disabled role and maintaining symptoms.

An awareness of issues involving litigation, finances, and availability of disability compensation can be important to understanding prolonged disability. Other significant pieces of psychosocial history that should be explored include the individual’s educational achievement, military service record, marital or relationship background, legal history, substance use patterns and habits, history of abuse, and available support systems.

Screening versus Objective Personality Tests

One may also use a stepwise approach to psychologically pain-related evaluation that proceeds from global indices of emotional distress and disturbance to a more detailed evaluation of the most important interactive factors of the diagnosis that may include Axis I clinical disorders and Axis II personality disorders.⁵ There are two basic types of psychological instruments that can provide useful information when working with pain patients: screening tests and objective personality tests. Some screening tests can assist persons in describing, characterizing, and quantifying pain. Other screening tests can be used to identify conditions that may complicate the course of treatment and need further treatment or evaluation. However, screening tests are typically overly sensitive, are obvious in their intent, and lack validity measures. The advantages of screening tests include: they are inexpensive, quick, and patients typically understand their purpose. Objective personality tests can provide a broader, more detailed evaluation of a patient’s functioning, but they are lengthy and require specialized training to interpret. Objective tests have greater validity and reliability than screening tests.