The medicolegal implications of the link between chronic pain and depression

Chronic pain affects around 20% of adults at any one time. It can have a significant effect on the patient’s relationships, social life and employment prospects. The isolation and reduced quality of life that often accompanies chronic pain is associated with symptoms of depression and anxiety. Depression and anxiety are also associated with disability and poor quality of life, and they are also important predictors of chronic pain. However, under-diagnosis of these co-occurring conditions, leading to inadequate treatment, remains an issue.

The stress resulting from chronic pain often leads to the development of psychological symptoms. Studies suggest that around 40% of patients with chronic pain also have symptoms of depression, although the figure varies according to the specific pain condition. The prevalence of depression is highest among patients with pain conditions associated with nociplastic alterations in the absence of tissue damage, including fibromyalgia, complex regional pain syndrome and temporal mandibular disorder. Patients with conditions such as arthritis, which typically have a greater nociceptive involvement, are less likely to experience depressive symptoms. These findings align with other evidence that psychological distress and adverse life experiences increase the risk of chronic nociplastic pain. Prevalence is also higher among younger patients and women, but surprisingly there does not appear to be a link with pain duration.

It is therefore clear that chronic pain and depression are closely correlated, and each can influence the progression and severity of the other. Patients with both chronic pain and depression have a poorer prognosis than those with either condition in isolation. Although the pathophysiological mechanisms of chronic pain and depression have not been identified, recent studies have demonstrated considerable overlap between the neuroplastic and neurobiological changes associated with both pain and depression. In particular, the injury sensory pathways for pain appear to share the same brain regions involved in mood management, thus providing a structural explanation for the co-existence of pain and depression. Furthermore, these changes may also occur in the sensory conduction pathways from the peripheral to the central nervous system, thus influencing the development and maintenance of chronic pain.

Most chronic pain treatments do not specifically address underlying depressive symptoms, but it is important that these conditions are treated, particularly as they can influence the outcome of pain treatment. As well as their analgesic properties, opioids have shown some effectiveness in treating depression. Certain opioids may enhance synaptic plasticity and promote antidepressant effects by influencing neurotransmitter signals. However, the use of opioids in the treatment of depression is controversial because of their known association with patient addiction and dependency. Furthermore, long-term use of opioids can increase the risk of depression and cause an increased sensitivity to pain, which may subsequently lead to depression. Co-existing depression can also prolong opioid use.

Antidepressant drugs may provide a better alternative for treating co-existing depression and chronic pain. As well as being a treatment for depression, tricyclic antidepressants can also provide meaningful pain relief for conditions such as postherpetic neuralgia. Similarly, monoamine reuptake inhibitors are effective against chronic neuropathic pain, with lower average pain scores and maximum pain intensity being recorded by patients receiving these drugs. Ketamine can achieve simultaneous analgesic and antidepressant effects through its influence on the glutamatergic NMDA receptor, which eventually leads to significant changes in synaptic plasticity and connectivity. However, NMDA receptor antagonists are also associated with significant side effects and their usefulness as a treatment for chronic pain-induced depression has yet to be clarified.

As psychosocial factors have a significant influence on the development of both chronic pain and depression, appropriate adjuvant psychotherapy also plays an important therapeutic role. With the advent of remote delivery options, short-term psychological treatments are increasingly available and provide a cost-effective option for patients with chronic pain. Patients with back pain who received cognitive behavioural therapy experienced a greater improvement in function than those receiving usual care. Psychological therapy also decreased pain levels in children and adolescents suffering from headaches. Thus, psychotherapy reduces clinical symptoms, shortens recovery times and improves prognosis. However, while the efficacy of these programs on pain-related outcomes is clear, significant improvements in psychological symptoms are not generally seen and there are few psychological treatments available that target both chronic pain and co-occurring depression. Treatment options that take a holistic view of the patient’s condition and incorporate interventions for both pain and psychological symptoms are more likely to result in a good outcome for the patient.

Due to the frequency of co-occurring psychological symptoms, changes in chronic pain care are needed to address the current challenges in diagnosis and treatment. Systematic screening of chronic pain patients for depression and anxiety should be introduced in primary care. Various screening instruments are available, with the Patient Health Questionnaire-9 being one of the most widely used. A network of mental health referral resources should be developed so that patients with a positive screening can receive adequate support and treatment. Identification of the neuroplastic changes shared by chronic pain and depression would enable the development of new therapeutic options, thus contributing to an improved prognosis and quality of life for these patients.