Patients with pain can be divided into two groups:
those with diseases that limit life expectancy, such as malignant tumors (malignant or cancer pain);
those with normal life expectancy (chronic benign pain).
For practical purposes, patients expected to live 2 years or less are put into the malignant pain group. The medical and surgical management of these two groups is very different.
In general, the approach to treating pain begins with an attempt to diagnose the cause. The history of the underlying problem must also be considered. For example, pain in a region recently operated on is expected to resolve with wound healing. When possible, the source of the pain should be treated directly (eg, removal of a herniated disk, treatment of a primary tumor). If the pain cannot be treated directly or if the source of the pain is unknown, efforts are made to treat the pain as such.
The main goal in treating patients with malignant pain is to reduce suffering. Therapies that might be inappropriate in patients with normal life expectancies can be considered. Opioids, for example, can be used in high doses unless excessive sedation, respiratory depression, severe constipation, or other significant adverse effects occur. Any dose that relieves pain and maximizes useful function can be used. Surgical management of malignant pain involves two approaches:
epidural or intrathecally administered morphine
ablation to interrupt pain transmission.
The spinal cord contains opioid receptors which, when activated by locally administered morphine, produce rather profound pain relief. Temporary percutaneous catheters can be used to test this therapy; if significant pain relief is demonstrated, prolonged spinal administration of morphine is indicated. This can be accomplished by implanting a subcutaneous reservoir attached to an epidural or intrathecal catheter; morphine can be injected into the reservoir by the patient or caregiver. Although this technique provides useful pain relief, it is impractical because repeated skin punctures are required and there is a risk of infection. If the patient’s life expectancy is more than a few months, implantation of an infusion pump is recommended. Spinal morphine is most effective for midline and multifocal pain problems, such as sacral pain associated with prostate tumors, pelvic pain from carcinoma of the cervix, and multifocal bone pain from widely metastatic disease.
Some pain is experienced by nearly every patient after invasive surgical procedures. The amount and duration of pain varies with the type of procedure, with the individual patient, and with many other factors, including the rapidity of healing and associated complications such as infection. The fact that pain is expected after surgical procedures does not mean it should not be treated aggressively. Reducing postoperative pain not only lessens the most unpleasant aspect of the procedure for the patient, it also reduces the incidence of other types of morbidity. For these reasons, appropriate doses of opioids and other pain-reducing drugs such as anti-inflammatory agents are an essential part of postoperative care.
The treatment of postoperative pain has improved substantially in recent years. These improvements include the use of epidural opioids or intrathecal opioids, the use of long-acting nerve blocks at the end of a procedure. In addition, new delivery techniques such as patient-controlled analgesia reduce overall postoperative pain and actually decrease the amount of analgesic required during the postoperative period.
Chronic Benign Pain
Patients with a life expectancy longer than 2 years who have pain that cannot be eliminated by treating the underlying cause are considered to have chronic benign pain. Such patients should be evaluated by a team of individuals experienced in pain treatment (pain clinic), including specialists in anesthesiology, psychology, psychiatry and neurology or neurosurgery, physical therapy, and pharmacology. This multidisciplinary approach takes into account the multifactorial cause of long-standing pain.
In general, drugs with little or no potential for addiction or significant dependence should be used. When opioids are used, long-acting drugs on a time-contingent rather than pain-contingent schedule are preferred, and short-acting opioids should be avoided. Anticonvulsant and tricyclic antidepressant drugs may be effective and are best administered by physicians with experience in their use.
When medical management fails to reduce pain adequately, surgical treatment can be considered. Ablative techniques such as rhizotomy (cutting of nerve roots), neurectomy, and cordotomy are inappropriate in these cases except as noted below. Techniques of neuromodulation that do not cause neural injury should be used instead.
Neuromodulation techniques take advantage of the capacity of the nervous system to reduce the access of painful stimuli to higher central nervous system centers. Transcutaneous electrical nerve stimulation, the least invasive of these techniques, uses skin electrodes to activate the large fibers in peripheral nerves. This selective activation reduces the ability of nociceptive fibers (A and C) to activate spinal neurons, which transmit pain signals to higher centers. This technique is limited by the inability to stimulate large painful areas and by the inconvenience of wearing electrodes for long periods.
A more invasive approach to neuromodulation involves direct or percutaneous implantation of electrodes into the spinal canal to electrically stimulate the dorsal columns.
Specific Pain Syndromes
This is an episodic lancinating facial pain that conforms to one—or perhaps two—divisions of the trigeminal nerve. The mainstay of treatment is the anticonvulsant drug carbamazepine and, more recently, gabapentin. Patients who do not respond to medical management may be treated with a percutaneous, open, or radiosurgical procedure. The trigeminal nerve is then partially damaged with glycerol, a mildly toxic alcohol, a radiofrequency-induced heat lesion. The percutaneous approaches are minimally invasive and require very short hospitalization, but the recurrence rate is higher than with an open approach, and patients are left with some loss of facial sensation, perhaps including protective corneal sensation.
Many cases of trigeminal neuralgia are caused by irritation of the trigeminal nerve by blood vessels, such as the superior cerebellar and anterior inferior cerebellar arteries. This syndrome can often be cured by moving the offending vessel away from the nerve. When the trigeminal nerve is exposed and such a neurovascular relationship cannot be demonstrated, partial rhizotomy (nerve sectioning) can provide excellent pain relief. A recent addition to the surgical treatment of neuralgia is radiosurgery, a stereotaxic technique used to deliver high levels of radiation to precise brain regions.
Pain after Amputation
A variety of pain syndromes may follow limb amputation. Most patients experience sensory phenomena in the amputated limb. In a minority of cases, these so-called phantom sensations are painful. Phantom limb pain is a continuous pain, as if fingernails were digging into the palm or the limb were being twisted into painful postures. Some patients have painful neuromas at the cut ends of peripheral nerves; the pain is usually electrical and occurs each time the stump is pressed. Injury to the cutaneous nerves may cause painful skin sensitivity in the stump. Medications such as tricyclic antidepressants and anticonvulsants effective in these cases, but opioids usually are not. Surgical options include revision and burying of painful neuromas and, in refractory cases, spinal cord or brain stimulation.
Spinal Nerve Root Avulsion Pain
Injuries that radically displace the head and shoulder can avulse the spinal nerves from the spinal cord. This commonly occurs after motorcycle accidents in which the head and shoulder are rapidly and severely distracted. This type of brachial plexus injury is not surgically reparable and usually is not painful. In some cases the pain is severe; typically, it is a burning pain and may include a phantom-like pain sensation. When such pain is refractory to medical management, it can be alleviated in 80% of cases by a dorsal root entry zone lesion. In this procedure, the spinal cord is exposed, and the region where dorsal roots formerly entered the spinal cord is identified. Lesions are made either with small needles (radiofrequency) or with a laser. Care must be taken to avoid damaging nearby spinal tracts.
Most patients with severe spinal cord injury experience some pain. In some cases, it is severe and requires specific medical treatment. Pain after spinal cord injury may occur at the border zone between normal and abnormal sensation or may affect large areas of the body with little sensory function. The former may be treated with the dorsal root entry zone procedure described above; lesions are created at the region above and below the spinal cord injury. More diffuse spinal cord injury pain is very difficult to treat and does not respond to any surgical procedures. Such patients should be referred to multidisciplinary pain clinics.