Manoj Narayan Ravindran, MD
Andreas, a 35 year old marine, was leading a night patrol in the battlefield. He stepped over a land mine and sustained a blast injury to his right leg. The blast shattered the bones of his leg and feet and produced extensive damage to soft tissues. After the initial resuscitation at the frontline he was airlifted to the regional command hospital. He is now awaiting urgent surgery and having a lot of pain. As an anesthesiologist, you are requested to see him to provide effective pain relief. He is otherwise a healthy man with no significant past medical history.
What is the mechanism of acute pain in Andreas?
Andreas's pain has resulted from traumatic injury to foot and is thus an example of nociceptive pain. This pain results from the release of inflammatory mediators at the site of trauma and their stimulation of the peripheral pain receptors called nociceptors. The pain sensation is then carried to central nervous system by AS and C fibers. These fibers first synapse in the thalamus and then the sensory cortex.
How would you deal with his acute pain?
It is important to first determine the full medical history, drug history and find out any drug allergies that may be present. In this situation it is difficult to follow the WHO pain ladder. Andreas needs strong opioid analgesics. Though we can supplement this with acetaminophen, using NSAIDS in hypovolemic patients with major trauma should be done with care as there is risk of renal toxicity and platelet dysfunction. Adding weak opioids is another option. Neuraxial block can provide good quality analgesia, though this could prove risky in the presence of hypovolemia and coagulopathy.
How do these drugs relieve acute pain?
NSAIDs are used to overcome mild to moderate pain. They act by preventing the production of prostaglandins and thromboxanes by inhibiting the enzyme cyclooxygenase.
This translates into reduction of inflammatory mediators such as prostaglandins.
The exact mechanism of action of acetaminophen is still not entirely understood. Its antipyretic action is thought to be due to inhibition of prostaglandin synthesis in the central nervous system.
Opioids are very effective analgesics because of their affinity for the opioid receptors.
The opioid receptors are divided into mu-1 (^1), mu-2, kappa (k) and delta (S) receptors. Mu-1 receptors are mainly involved in analgesia and euphoria, while mu-2 cause respiratory depression and inhibition of gut mobility. Kappa receptors are associated with spinal analgesia, meiosis and sedation, whereas delta receptors cause respiratory depression, physical dependence and analgesia. Opioid receptors activate G1 proteins and cause hyperpolarization of the cell membrane.
Could you suggest an intervention to block the transmission of nociceptive impulses in the above situation?
A combined femoral and sciatic nerve block using catheters could be used. This technique can provide adequate acute pain relief. It has the advantage of being useful in providing adequate surgical anesthesia even during a limb salvage operation which Andreas might be undergoing.
Andreas undergoes extensive limb salvage surgery and after evaluation of the clinical situation, you decide to err on the side of caution and administer morphine PCA along with acetaminophen for pain relief; you also prescribe tramadol as needed. Andrea's pain is now reasonably well-controlled. Unfortunately, over the following week the limb became unsalvageable due to infection. His surgeon decides that amputation is the best option and hence takes him to the operating room for a below-the-knee amputation. Andreas is worried about phantom limb pain, as he has heard dreadful stories about it.
What is phantom limb pain?
Phantom limb is a type of chronic pain. It results in a sensation that an amputated limb is still attached to the body. More than half of amputees experience some phantom sensation in their amputated limb, with pain being the most common sensation. It is most common if amputation is delayed after initial injury and it is more common in arm amputations. The perceived limb may be felt to be in an abnormal position.
Could you elaborate on the mechanism of phantom limb pain?
The exact mechanism is still unknown. Various theories that have been suggested to explain this, including abnormal re-growth of nerve endings in the stump of the amputated limb. These nerve endings then cause altered and painful discharges, leading to phantom limb pain. There is also possibility of altered nervous activity in the spinal cord and brain in these patients.
Is there any way of preventing this?
Effective control of pain before amputation can prevent dorsal root sensitization and help prevent or reduce severity of phantom limb pain. For this reason patients are routinely prescribed opioids, anti-depressants, and anti-convulsants. Ketamine, which is an NMDA receptor antagonist, also has been tried for this purpose. Use of epidural analgesia before the actual amputation has been claimed to prevent the development ofphantom limb pain.
List the various forms of treatments available for phantom limb pain? Traditional treatment options include:
• Simple analgesics
• Anti-convulsants and anti-depressants, e.g. phenytoin, carbamazapine and gabapentin.
• Dorsal column stimulation
• Injection around the stump neuroma with local anesthetic and depot steroid, if pain is thought to be due to neuroma in the stump.
• Prosthetic assessment: a correct fitting prosthesis may help phantom limb pain due to stump neuroma.
• Surgery to refashion the stump is advised if the pain is thought to be due to the presence of neuroma in the stump, close to weight-bearing area when a prosthesis is used.
Other options include:
• Mirror box: Ramchandran et al, found that stimulation of the motor cortex can help reduce phantom limb pain. In this study patients were asked to put their normal limb in a mirror box, so that they saw their normal limbs mirror-reversed to look like their amputated limb. When they moved their normal limb in the mirror box, their brains were fooled to believe that they were moving their amputated limb - this helped to reduce pain.
• Merely getting patients to imagine their paralyzed arms moving in relation to a moving arm on a screen in front of them can relieve phantom limb pain.
• Virtual reality: By attaching an interface to the patient's amputated limb, the amputee is able to see both of his limbs being moved in a computer generated simulation - this also has been shown to relieve phantom limb pain.
Cite the key differences between acute and chronic pain?
Acute pain occurs at the time of injury and disappears once the healing process is complete. It protects the body from further harm. The mechanism involved in acute pain is better understood. Whereas in the case of chronic pain, the onset is delayed and pain persists long after the healing process is completed; does not serve any perceived usefulness. It is different in character, mechanism and therapeutic options.
Andreas undergoes amputation under general anaesthesia and he continues on his PCA and other medication. Luckily, he does not experience the dreaded phantom limb pain which you attribute to your effective preoperative pain relief.
Was this article helpful?
Do You Suffer From Chronic Pain? Do You Feel Like You Might Be Addicted to Pain Killers For Life? Are You Trapped on a Merry-Go-Round of Escalating Pain Tolerance That Might Eventually Mean That No Pain Killer Treats Your Condition Anymore? Have you been prescribed pain killers with dangerous side effects?