What Is the Latest in Pain Mechanisms and Management?

During the past 4 decades, major advances have been made in pain science. Chronic pain is now understood to involve a neural response to tissue injury where peripheral and central events related to disease or injury can trigger long-lasting changes that result in sensitization.¹ In this way, the nerves become stuck in the on position, such that, after injury, the neurons are capable of firing spontaneously or in response to stimuli that normally would not cause pain. With this information, we now understand such clinical observations as allodynia (pain evoked by a stimulus that normally does not cause pain, such as light touch), hyperalgesia (increased pain response to stimuli that are painful), or phantom pain (pain referred to an area of the body that has been amputated or has lost its sensory innervation). Gone are the days when physicians would suggest that regional pain or sensory findings support a diagnosis of psychogenic pain.²

In this issue of The Canadian Journal of Psychiatry, Dr Katz et al³ delve into the controversial area of “medically unexplained pains.”^{p 160} While significant progress has been made in our understanding of underlying pathophysiology of persistent pain, little progress has been made as to how pain is treated in the Diagnostic and Statistical Manual of Mental Disorders (DSM), Fifth Edition. In this journal in 1989, Harold Merskey identified that most of the emotional change seen in people with chronic pain is a consequence of physical disorder, and that a common response to pain will be some degree of depression, irritability, and anxiety.⁴ Katz et al present recent work supporting the same conclusion. Merskey warned that psychiatrists should be wary of accepting the criteria of the DSM-III-Revised on somatoform pain disorder; today Katz and colleagues warn us about using the DSM-5 diagnostic category of somatic symptom disorder with predominant pain arguing it is overly inclusive and stigmatizing. A quarter of a century has led to no real progress in how pain is treated in the DSM. I would suggest that this is because pain is not a psychiatric disorder. However, it may be associated with psychiatric disorders, in which case specific psychiatric diagnoses may be applied. Merskey identified that it is best to make 2 diagnoses (encompassing physical and psychological aspects) and estimate their importance. If a patient exhibits the criteria for posttraumatic stress disorder, depression, or other psychiatric disorder, this diagnosis should be made. In situations where patients do not have a major psychiatric disorder but where the psychiatrist thinks the patient is overly concerned about the pain, Katz et al³ support that adjustment disorder is the most appropriate diagnosis and will minimize stigmatization of people suffering with chronic pain.

Major advances have also been made in our understanding of the descending pain modulatory system. There is a now a literature that we can cite as we explain to patients that their body is equipped with a sophisticated pain defence network that can be exploited in a way that will decrease pain. This is useful in assisting patients with understanding that there are multiple ways of activating the pain defence system using nonpharmacotherapeutic approaches as well as medications.⁵ Nondrug strategies include healthy lifestyle, therapeutic exercise, psychological strategies, and complementary approaches. In this In Review series, Morley and Williams⁶ provide an excellent and concise review of key developments in the psychological management of pain.

Despite compelling science this growing body of knowledge has not led to improvements in care for most people afflicted with pain conditions in Canada or in the rest of the world. In many countries timely access to pain care is a growing problem.⁷ Wait times for care are greater than 1 year at more than one-third of publicly funded pain clinics across Canada, and there are vast areas of the country with no access to appropriate care.⁸ This is despite the well-documented fact that patients experience a significant deterioration in health-related quality of life and psychological well-being while waiting for treatment for pain,⁷ and that more than 50% of people waiting for care at Canadian pain clinics have severe levels of depression, with 34.6% thinking about suicide and 72.9% reporting interference with their normal work.⁹ The magnitude of the problem is increasing, with 1 in 5 Canadians experiencing chronic pain, including children (prevalence is also 1 in 5). The prevalence increases with age and, ironically, with improvements in medical care. The reason for this is the growing cohort of people surviving diseases, such as cancer, human immunodeficiency virus, and cardiovascular disease, many of whom have been left with chronic pain caused by either the disease or the treatments used to cure it, such as chemotherapy, radiotherapy, or surgery.^10–14

A big part of the problem is that we are not very good at managing acute pain in Canada. For example, even in the best academic hospitals in the country, patients continue to receive inadequate pain control in emergency departments¹⁵ and after common surgeries. In a study of postoperative pain treatment after coronary artery bypass grafting, less than 30% of ordered medication was actually given, with about 50% of patients continuing to report moderate to severe pain 1 to 5 days after surgery. One-quarter of patients rated the pain as “extremely unpleasant,” with significant interference with ability to function even the day before discharge.¹⁶ In another study involving patients undergoing same-day shoulder surgery, patients continued to report severe levels of pain and poor sleep 7 days following the surgery. This has obvious implications for healing and function.¹⁷ This undertreatment of postoperative pain was identified over a decade ago, and a recent study reveals the situation has not improved.¹⁸ Given that the presence of significant pain immediately after surgery is a critical risk factor for persistent postoperative pain, which is severe in 2% to 10% of cases,¹⁹ it is imperative to address this problem, especially in light of the recent Canadian study that identified that 1 in 10 patients continue to report pain 2 years after cardiac surgery, 4% of whom describe the pain as moderate to severe.²⁰

The situation is most critical in children. As recently as 20 years ago, many believed that because children did not have a fully developed nervous system, they did not experience pain as much as adults. This belief led to the undertreatment of children’s pain. Excellent work, much of this by Canadian scientists, has demonstrated this is inaccurate, and, in fact, that the opposite is true. Children and infants experience pain, and premature neonates with developing nervous systems experience more severe pain. This is because the pain sensing systems develop before the pain inhibitory systems.²¹ Further, recent work has identified that pain experience in early life has significant impact on pain experience and general function later. In this issue, Beggs²² provides an excellent review of this recent literature.

In summary, this In Review series will provide the reader with an overview of the latest in pathophysiology and psychology of chronic pain as well as implications for management.