PAIN: PATHOPHYSIOLOGY AND MANAGEMENT 2

CHRONIC PAIN

PATIENT EVALUATION

Managing patients with chronic pain is intellectually and emotionally challenging. The patient's problem is often difficult to diagnose: such patients are demanding of the physician's time and often appear emotionally distraught. The traditional medical approach of seeking an obscure organic pathology is usually unhelpful. On the other hand, psychological evaluation and behaviorally based treatment paradigms are frequently helpful, particularly in the setting of a multidisciplinary pain-management center.

There are several factors that can cause, perpetuate, or exacerbate chronic pain. First, of course, the patient may simply have a disease that is characteristically painful for which there is presently no cure. Arthritis, cancer, migraine headaches, fibromyalgia, and diabetic neuropathy are examples of this. Second, there may be secondary perpetuating factors that are initiated by disease and persist after that disease has resolved. Examples include damaged sensory nerves, sympathetic efferent activity, and painful reflex muscle contraction. Finally, a variety of psychological conditions can exacerbate or even cause pain.

There are certain areas to which special attention should be paid in the medical history. Because depression is the most common emotional disturbance in patients with chronic pain, patients should be questioned about their mood, appetite, sleep patterns, and daily activity. A simple standardized questionnaire, such as the Beck Depression Inventory, can be a useful screening device. It is important to remember that major depression is a common, treatable, and potentially fatal illness.

Other clues that a significant emotional disturbance is contributing to a patient's chronic pain complaint include: pain that occurs in multiple unrelated sites; a pattern of recurrent, but separate, pain problems beginning in childhood or adolescence; pain beginning at a time of emotional trauma, such as the loss of a parent or spouse; a history of physical or sexual abuse; and past or present substance abuse.

On examination, special attention should be paid to whether the patient guards the painful area and whether certain movements or postures are avoided because of pain. Discovering a mechanical component to the pain can be useful both diagnostically and therapeutically. Painful areas should be examined for deep tenderness, noting whether this is localized to muscle, ligamentous structures, or joints. Chronic myofascial pain is very common, and in these patients deep palpation may reveal highly localized trigger points that are firm bands or knots in muscle. If injection of local anesthetic into these trigger points relieves the pain, it supports the diagnosis. A neuropathic component to the pain is indicated by evidence of nerve damage, such as sensory impairment, exquisitely sensitive skin, weakness and muscle atrophy, or loss of deep tendon reflexes. Evidence suggesting sympathetic nervous system involvement includes the presence of diffuse swelling, changes in skin color and temperature, and hypersensitive skin and joint tenderness compared with the normal side. Relief of the pain with a sympathetic block is diagnostic.

A guiding principle in evaluating patients with chronic pain is to assess both emotional and organic factors before initiating therapy. Addressing these issues together, rather than waiting to "rule out" organic causes of the pain, improves compliance in part because it assures patients that a psychological evaluation does not mean that the physician is questioning the validity of their complaint. Even when an organic cause for a patient's pain can be found, it is still wise to look for other factors. For example, cancer patients with painful bony metastases may also have pain due to nerve damage and significant depression. Optimal therapy requires that each of these factors be looked for and treated.

TREATMENT

Once the evaluation process has been completed and the likely causative and exacerbating factors identified, an explicit treatment plan should be developed. An important part of this process is to identify specific and realistic functional goals for therapy, such as getting a good night's sleep, being able to go shopping, or returning to work. A multidisciplinary approach that utilizes medications, counseling, physical therapy, nerve blocks, and even surgery may be required to improve the patient's quality of life. This may require referral to a pain clinic; however, this is not necessary for all chronic pain patients. For some, pharmacologic management alone can provide significant help.

Antidepressant Medications The tricyclic antidepressants are extremely useful for the management of patients with chronic pain. Although developed for the treatment of depression, the tricyclics have a spectrum of dose-related biologic activities that include the production of analgesia in a variety of clinical conditions. Although the mechanism is unknown, the analgesic effect of tricyclics has a more rapid onset and occurs at a lower dose than is typically required for the treatment of depression. Furthermore, patients with chronic pain who are not depressed obtain pain relief with antidepressants. There is evidence that tricyclic drugs potentiate opioid analgesia, so they are useful adjuncts for the treatment of severe persistent pain such as occurs with malignant tumors. lists some of the painful conditions that respond to tricyclics. Tricyclics are of particular value in the management of neuropathic pain such as occurs in diabetic neuropathy and postherpetic neuralgia, for which there are few other therapeutic options.

The tricyclics that have been shown to relieve pain have significant side effects . Unfortunately, some of the serotonin-selective reuptake inhibitors such as fluoxetine (Prozac) that have fewer and less serious side effects have not been shown to provide pain relief. On the other hand, venlafaxine (Effexor), a nontricyclic antidepressant that blocks both serotonin and norepinephrine reuptake, appears to be useful in patients who cannot tolerate tricyclics.

Anticonvulsants and Antiarrhythmics These drugs are useful primarily for patients with neuropathic pain. Phenytoin (Dilantin) and carbamazepine (Tegretol) were first shown to relieve the pain of trigeminal neuralgia. This pain has a characteristic brief, shooting, electric shock-like quality. In fact, anticonvulsants seem to be helpful largely for pains that have such a lancinating quality. A new-generation anticonvulsant, gabapentin (Neurontin), which increases brain g-aminobutyric acid levels, is effective for a broad range of neuropathic pains.

Antiarrhythmic drugs such as low-dose lidocaine and mexiletine (Mexitil) are also effective for neuropathic pains. These drugs block the spontaneous activity of primary afferent nociceptors that appears when they are damaged.

Chronic Opioid Medication The long-term use of opioids is accepted for patients with pain due to malignant disease. Although its use for chronic pain of nonmalignant origin is controversial, it is clear that for many such patients opioid analgesics are the only option available for obtaining effective relief. This is understandable since opioids are the most potent and have the broadest range of efficacy of any analgesic medications. Although addiction is rare in patients who first use opioids for pain relief, some degree of tolerance and physical dependence are likely to occur with long-term use. Therefore, before embarking on opioid therapy, other options should be explored, and the limitations and risks of opioids should be explained to the patient. It is also important to point out that some opioid analgesic medications have mixed agonist-antagonist properties (e.g., pentazocine and butorphanol). From a practical standpoint, this means that they may worsen pain by inducing an abstinence syndrome in patients who are physically dependent on other opioid analgesics.

With long-term outpatient use of orally administered opioids it is desirable to use long-acting compounds such as levorphanol, methadone, or sustained-release morphine. The pharmacokinetic profile of these drugs enables prolonged pain relief, minimizes side effects such as sedation that are associated with high peak plasma levels, and, perhaps, reduces the likelihood of rebound pain associated with a rapid fall in plasma opioid concentration. Constipation is a virtually universal side effect of opioid use and should be treated expectantly.

It is worth emphasizing, in conclusion, that many patients, especially those with chronic pain, seek medical attention primarily because they are suffering and because only physicians can provide the medications required for their relief. A primary responsibility of all physicians is to minimize the physical and emotional discomfort of their patients. Familiarity with pain mechanisms and analgesic medications is an important step toward accomplishing this aim.