Chronic Postoperative Pain Needs Additional Clinical Study

A review in Anaesthesia provides a sobering picture of the missing clinical links in research regarding current pharmacological treatment for chronic postoperative pain. The review looked at the pathogenic mechanisms thought to contribute to persistent postoperative pain, peri-operative preventative strategies with the potential to reduce the risk of chronic pain, the impact of psychosocial factors and the potential value of genetic analysis to stratify risk and treatment for the individual.

The review is an excellent primer on the criteria for diagnosing chronic postoperative pain, which is characterized by pain of at least 2 months’ duration that developed after a surgical procedure and for which other causes of pain (such as malignancy or chronic infection) have been ruled out.

Among the more interesting findings of the review:

• Interventional trials to prevent chronic postoperative pain have been underpowered with inadequate patient follow-up.
• Ketamine may reduce chronic postoperative pain, although the optimum treatment duration and dose for different operations have yet to be identified.
• The evidence for gabapentin and pregabalin is “encouraging but weak; further work is needed before these drugs can be recommended for the prevention of chronic pain,” the reviewers noted.
• Nerve-sparing surgical techniques may be of benefit, although nerve injury is not necessary or sufficient for chronic pain to develop.
• The mechanisms that mediate the transition from acute, adaptive postoperative pain to chronic, maladaptive pain are not fully understood, although nerve injury and ongoing inflammation play important roles.
• Some studies have shown that peri-operative analgesics that have “antihyperalgesic” actions appear to reduce central sensitization and hence the progression to chronic pain.
• Among the most commonly used current treatments, there is some clinical evidence that ketamine reduces chronic postoperative pain, but further study is needed to confirm these findings and determine which subgroups benefit and the optimum dose and duration of therapy.
• Gabapentin and pregabalin reduce immediate postoperative pain and opioid consumption, but long-term effects are not well understood, and studies with longer-term follow-ups are split on their effectiveness for periods up to 6 months after surgery. “The sample sizes in the majority of trials of gabapentin, pregabalin and ketamine have been small, powered to detect differences in outcomes other than chronic pain: future trials need to recruit sufficient participants to detect statistically significant, clinically important effects of drugs on chronic pain,” the reviewers noted.

The review also makes important points about the psychology of pain sufferers and pain catastrophizing, as well as providing a brief overview of candidate genes that may play a role in who may develop chronic postoperative pain.

“A better understanding of the natural history and consequences of chronic postoperative pain would facilitate effective strategies for its prevention and treatment,” the reviewers conclude. “Patients should be made aware of the risk of chronic postsurgical pain, particularly where there is a high risk due to the type of surgery or known patient risk factors. This will allow patients to make better informed decisions about whether to proceed with surgery and to understand that surgery may alleviate an existing, intermittent pain but in exchange for a different lifelong pain.”