Chronic pain is sustained irrespective of the original cause, and blocking or severing neural pathways merely shifts pain stimuli more centrally. Nurmikko and colleagues review the management of chronic pain, including developments in our understanding of the patho-physiology and pharmacology of chronic pain and selection management strategies.
Chronic neuropathic pain is believed to result from altered neuronal excitability resulting in abnormal discharges and central sensitization. Anticonvulsant drugs such as lamotrigine and gabapentin may be useful in the management of this type of pain. Studies validate the effectiveness of lamotrigine in allodynia and hyperalgesia, as well as trigeminal neuralgia (when used in combination with carbamazepine). Central neuropathic pain may respond to high dosages of lamotrigine (up to 600 mg per day), but the risk of side effects increases at high dosages. Gabapentin has been effective in studies of diabetic neuropathy and postherpetic neuralgia and is favored by many clinicians because it has few interactions with other drugs. The most common side effect is sedation.
The most frequently used external agents for chronic pain are capsaicin and transdermal fentanyl. Concentrations of capsaicin in ointment vary from 0.025 to 0.075 percent. The ointment causes local erythema but is reported to be effective in cases of diabetic neuropathy, postherpetic pain and other types of neuropathic pain. Transdermal fentanyl patches are suitable for patients with cancer pain who have difficulty swallowing. The patches release 25 to 100 μg per hour and are applied every 72 hours. Although their effectiveness is well-established in cancer therapy, the use of these medications for other types of chronic pain remains controversial. Side effects include constipation and hypoventilation.
Controlled studies of spinal cord stimulation are difficult to conduct. The literature suggests that spinal cord stimulation will relieve pain in approximately one half of cases of peripheral neuropathic pain and back pain persisting following surgery, in 80 percent of patients with intractable anginal pain, in 50 to 80 percent of patients with peripheral limb ischemia and in selected patients with diabetic neuropathy. The spinal cord stimulation procedure is expensive, with implant costs exceeding $10,000, and further expenses for prolonged monitoring., However, there is some evidence that the expense is cost effective in the long term.
Several analgesic agents are in development or undergoing clinical trial. Tramadol, a centrally acting opiate, has a low potential for addiction and does not interact with anticoagulants or irritate gastric mucosa. Selective and preferential inhibitors of cyclo-oxygenase 2 are also being developed to provide analgesia without gastrointestinal damage.
Despite use of optimal regimens and combinations of analgesics and pain-relieving techniques, many patients continue to have psychologic and physical disability because of chronic pain. Comprehensive pain management programs that incorporate relaxation therapy, education, cognitive therapy and a variety of other techniques have generated varied results, as measured by patients' ability to return to normal activities.
The authors conclude that the management of chronic pain continues to be challenging despite many promising developments. Changes in attitudes and in the organization and accessibility of health care services, as well as advances in therapy, will be needed to provide effective relief for patients with chronic pain.