Predicting postoperative pain. Clinical and genetic studies of relationships between pain sensitivity and pain after surgery

Abstract: Many patients experience pain after surgery. Postoperative pain may lead to delayed mobilization, persisting pain, and psychosocial distress. Others are given excessive analgesic doses and experience side effects. More optimal use of pain relief has several advantages, such as faster postoperative mobilization and fewer incidents of venous thromboembolism or infection. To achieve this, we must identify individuals at increased risk of severe postoperative pain, but simple and reliable techniques for prediction of postoperative pain have yet to be discovered.We know that individual factors such as female gender, low age, considerable pre-operative pain, and expectations on a painless postoperative course all increase the risk of severe postoperative pain. Studies have also shown that the estimation of pain thresholds with various modalities, such as heat, cold, pressure, or electricity, can be linked to pain intensity after surgery, but those tools are rarely used in clinical routine practice. To instead use painful components of routine preparation for surgery in order to evaluate individual pain sensitivity and predict postoperative pain would be almost revolutionary. This thesis (I-IV) was based on cinical studies designed to investigate whether painful procedures during routine preparation for surgery can be used to predict postoperative pain (I, IV), to test whether electrical pain threshold levels can be used to predict postoperative pain (II), and to identify possible genetic differences accountable for individual pain sensitivity (III). Painful routine procedures can be used for prediction of acute postoperative pain. Patients reporting pain intensity at or above 2.0 VAS units to be associated with peripheral venous cannulation were found to have 3.4 times higher risk of severe acute pain after laparoscopic cholecystectomy (I), and 1.7 times higher risk of severe postoperative pain after various surgical procedures (IV).Electrical pain theshold levels are reproducible, and the technique is well tolerated by patients. However, the method was found to be useful for prediction of postoperative pain only in women and not in men (II). Weak correlation with postoperative pain intensity, found here as well as previously, and high gender-dependency, considerably limit the clinical value of this technique for routine use in peri-operative practice.In our analysis of possible genetic contributions to individual pain sensitivity we found minor-allele single nucleotide polymorphisms in the ABCB1 and COMT genes to be more common in patients with higher pain sensitivity (III). These findings suggest a possible genetic contribution of those single nucleotide polymorphisms to individual pain sensitivity, however without reaching statistical significance, probably due to insufficient numbers of study patients.