Chronic pain after surgery is common. The standard of research has improved markedly in recent years, but much work remains to be done, particularly in the fields of mechanisms and risk factors. Improving the management of acute postoperative pain is one strategy that may prevent chronic post surgical pain, but there are many technical and organizational barriers to be overcome in order to achieve that improvement.
There is no universally agreed definition of chronic post-surgical pain; however, the working definition proposed by Macrae and Davies is commonly used. Chronic post-surgical pain is associated with increased analgesic use, restriction of activities of daily living, significant effects on quality of life and increased health-care utilization. More than 4 million people undergo surgery every year in the UK, so chronic post-surgical pain poses a significant economic and health-care burden.
Nerve injury during surgery has been implicated in the development of chronic post-surgical pain; some (but not all) patients with chronic post-surgical pain have neuropathic pain. Inflammatory and immune reactions after damage to axons results in release of neurotransmitters that act locally and in the spinal cord to produce hypersensitivity and ectopic neural activity; this contributes to central sensitization. Central sensitization occurs when repetitive nociceptive stimuli result in altered dorsal horn activity and amplification of sensory flow; this can lead to persistent nervous system changes, for example, death of inhibitory neurones, their replacement with excitatory afferent neurones, and microglial activation. These changes lead to evoked and spontaneous symptoms associated with neuropathic pain, for example, allodynia and hyperalgesia. chronic post-surgical pain is reported in more than 50% of patients who have surgery associated with nerve and tissue damage, for example, mastectomy, thoracotomy, and amputation. Despite this, there is no simple relationship between nerve injury during surgery and the development of chronic post-surgical pain.
Any link between nerve damage during surgery and the development of chronic post-surgical pain is complicated. Not all patients with nerve damage develop chronic post-surgical pain, and those who do develop chronic post-surgical pain do not necessarily have neuropathic pain. Some operations not associated with nerve damage can result in chronic post-surgical pain. Although the mechanisms behind the development of chronic post-surgical pain have yet to be fully elucidated, a number of risk factors have been identified.
A number of pre-, intra-, and postoperative risk factors have been identified for the development of chronic post-surgical pain.
The existence and intensity of preoperative pain is a risk factor for the development of chronic post-surgical pain after hernia repair, thoracotomy, amputation, and mastectomy. Increasing age is inversely related to the development of chronic post-surgical pain; the probability of developing chronic post-surgical pain after breast cancer surgery decreases by 5% for each yearly increase in the patient’s age. Genetic susceptibility is likely to play a role in the development of chronic post-surgical pain. Genetic variability in the expression of enzymes responsible for neurotransmitter synthesis in the dorsal root ganglion is associated with persistent pain after lumbar discectomy. Psychosocial factors have an important effect on chronic pain. Preoperative psychological measures have been shown to predict pain severity a year after breast surgery, and fear of surgery is associated with worse pain and quality of life outcomes.
Longer and more complicated operations are associated with more chronic pain. Laparoscopic surgical approaches result in less chronic pain after hernia repair and cholecystectomy. Repeat surgery for hernia repair has a higher incidence of moderate to severe pain intensity at 12 months compared with primary repair.
The severity of postoperative pain significantly predicts the development of chronic post-surgical pain, supporting the hypothesis that repetitive nociceptive stimulation during the perioperative period results in nervous system changes such as central sensitization.
Preventative regional analgesia has demonstrated some promising results (although data are limited). Preventative analgesia is given in the perioperative period, but has an effect that extends beyond the duration of the drugs used. Epidural analgesia when commenced before surgery and continued into the postoperative period reduces the incidence of CPSP in patients undergoing thoracotomy and laparotomy. Similarly, paravertebral block initiated before incision and continued into the postoperative period reduces the incidence of CPSP in thoracic and breast cancer surgery patients. It may be that establishing sufficient afferent block before the surgical incision and continuing this well into the postoperative period reduces the nociceptive barrage that results in central sensitization.
TENS and acupuncture can be helpful in some cases of chronic post-surgical pain.
Anti neuropathic medication is the main stay of treatment in chronic post- surgical pain. These agents include:
Topical agents like lidocaine 5% patches and topical capsaicin cream can help in some cases.
Interventional treatments can be tried in patients who have not responded to conservative management and oral medications.