Introduction

Pain is defined as an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage. Pain in the post-operative period is one of the major factors that impede recovery from anaesthesia and surgery. Total abdominal hysterectomy is commonly performed via a Pfannenstiel incision and causes significant moderate to severe pain that requires multimodal post­operative pain management.

Pain after hysterectomy is often multifactorial and arises from different sources. The sources of pain may include a combination of incision pain, pain from deeper visceral struc­tures, and dynamic pain or pain on movement, such as during straining, coughing, or mobilizing that may be severe. The main aim of multimodal analgesia is to obtain synergistic or additive analgesia, allowing a smaller dose of each drug with improved safety profile and less side effects. This can be achieved by combining analgesics acting at different locations of the pain pathway.

Pain Assessment

The essential prerequisite to comprehensive pain management is reliable and accurate assessment of pain. This should include history of the site of pain, pain characteristics, pain intensity, associated symptoms such as nausea and effect of pain on activities, and sleep.

A number of scales are available to quantify and measure pain intensity, such as the verbal descriptor scale and numerical rating scale. These can be used together with self-reporting, as pain is itself a subjective experience. The verbal descriptor scale uses words to describe the magnitude of pain using terms such as none, mild, moderate, and severe pain. This is quick and simple but may be limited by personal, cultural and language differences in interpretation by patients and doctors. The numerical rating scale is used by asking patients to rate their pain intensity on a scale of 0 to 10, with 0 being no pain and 10 being the worst pain imaginable. This scale can be done both in a verbal and written manner.

Post-operative pain management should also include monitoring the adverse effects of therapy and haemodynamic monitoring such as blood pressure, pulse rate and respiratory rate. The availability of an acute pain service would also be beneficial to better manage pain by dedicated staff including anaesthetists and nurse clinicians.

Opioid Analgesia

Opioids remain the cornerstone for treatment of moderate to severe pain for post-operative pain management. However, they should not be the sole analgesic of choice. The use of opioids by patient-controlled analgesia is popular but limited by side effects, and certain types of pain such as neuro­pathic pain respond poorly to opioids.¹ Neuropathic pain is defined as pain arising as a direct conse­quence of a lesion or disease affecting the soma­tosensory system. The consumption of morphine is often high, particularly in the initial post-operative period, leading to marked opioid adverse effects.^2,3

Morphine, pethidine and tramadol are the most frequently used drugs for post-operative pain manage­ment.^2,4 In patients undergoing abdominal hysterectomy, the administration of intravenous morphine, pethidine and tramadol via patient-controlled analgesia resulted in equivalent pain scores and side effects.⁵ Intravenous opioid patient-controlled analgesia provides better anal­gesia than conventional intramuscular or subcutaneous opioid regimens, although the magnitude of the differ­ence in analgesia is small.⁶ Opioid consumption may be greater though. There are no differences in duration of hospital stay or opioid-related adverse effects other than pruritus. Intravenous patient-controlled analgesia is associated with increased pruritus but higher patient satisfaction compared with most conventional opioid regimens.

Morphine remains the most widely used opioid for the management of pain and is considered the gold standard against which other opioids are compared. Morphine-6-glucuronide and morphine-3-glucuronide, the main metabolites, are formed by morphine gluc­uronidation in the liver. Morphine-6-glucuronide is an active metabolite and may contribute to respiratory depression. Both are dependent on renal excretion. Impaired renal function, higher doses and increased age may lead to higher concentration and thus potential risk of sedation and respiratory depression.

Tramadol is commonly referred to as an atypical, centrally acting opioid often used as an adjuvant in pain management. Its efficacy is a result of two complemen­tary mechanisms of action that include stimulation of opioid µ receptors and inhibition of the noradrenaline and serotonin reuptake in the pain pathway.⁷ Although an effective analgesic, it may not provide adequate pain relief if used as the sole agent for management of moderate to severe pain.⁸ The adverse effect profile is different from those of other opioids. The risk of respiratory depression is significantly lower compared with other opioids at equianalgesic doses.^9–11 Nausea and vomiting are the most common adverse effects and occur at rates similar to other opioids.¹²

Pethidine is a synthetic opioid still widely used even though there are several disadvantages. Pethidine induced more nausea and vomiting than morphine in the first 2 hours after gynaecological surgery.¹³ Accumula­tion of its active metabolite, norpethidine, is associated with neuroexcitatory effects including nervousness, tremors, twitches, myoclonus, and seizures. This is especially evident with renal impairment. Intramuscu­lar pethidine is a popular choice even though there are drawbacks, such as pain on injection and unpredictable duration of analgesia especially with multiple dosing.¹⁴ Overall, the use of pethidine should be discouraged in favour of other opioids.¹⁵

Oral opioids are a viable option owing to the short time to feeding after abdominal hysterectomy. Common options include tramadol and oxycodone. Oxycodone is a potent opioid agonist derived from the opium alka­loid thebaine. It is metabolized in the liver primarily to noroxycodone and oxymorphone, but these metabolites have clinically negligible analgesic effects.¹⁶ Animal studies have suggested that oxycodone is thought to act as a . receptor agonist, and these receptors may play an important role in the treatment and mediation of visceral pain.^17,18

Dextropropoxyphene has low analgesic efficacy and should be discouraged in favour of other opioids. It is often combined with paracetamol, but this combi­nation does not lead to superior analgesia and there is an increased incidence of dizziness.¹⁹ Furthermore, there are other adverse effects of dextropropoxyphene, including risk of abuse, risk of accumulation in the elderly, and cardiac arrhythmia from prolonged QT inter­val. There is a current phase withdrawal of dextropro­poxyphene in the UK of dextropropoxyphene-containing medications.²⁰

Opioid administration is associated with adverse effects such as pruritus, urinary retention, sedation, constipation, paralytic ileus, and nausea and vomit-ing. Because of the multiple mechanisms involved in post-operative pain, a multimodal analgesic regimen, with a combination of opioid and non-opioid analgesic drugs, is often used to enhance analgesic efficacy, and reduce opioid requirements and opioid side effects. In many institutions, this would involve combining patient-controlled opioid analgesia with paracetamol and non-steroidal anti-inflammatory drugs (NSAIDs). There are several adjuvants that have been added to patient-controlled opiod analgesia. Dexmedetomidine, an α₂ agonist, has been shown to improve analgesia and be opioid-sparing, and may reduce opioid adverse effects.^21,22 Ketamine has not been useful to reduce opioid consumption after abdominal hysterectomy.²²