78: Chronic Pain Management

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CHAPTER 78 Chronic Pain Management

Ronald Valdivieso, MD

1 What is the definition of pain?

The International Association for the Study of Pain defines pain as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.” Acute pain is associated with an identifiable cause (e.g., surgery, trauma, acute illness) and usually resolves with healing. Chronic pain is more difficult to define. It is believed that pain becomes chronic if it persists beyond 1 month of the usual course of an acute problem. However, other authors do not believe that duration should be identified as one of the distinguishing features of chronic pain and point to cognitive-behavioral aspects as essential criteria in any chronic pain syndrome.

2 How does normal pain perception occur?

Nociceptors are structures located at the end of axons that are depolarized by noxious thermal, mechanical, or chemical stimuli. The distal ends of most A-δ and C fibers are nociceptors. These axons are the ones that carry nociceptive information to the dorsal root and trigeminal ganglion and enter the spinal cord via the posterior root. Once in the spinal cord these afferent fibers synapse with cells in Rexed’s laminae I and V preferentially but also in laminae II and X. Axons from the previously mentioned neurons that travel to the contralateral side of the spinal cord will eventually form the spinothalamic tract, spinoreticular tract, and spinomesencephalic tract. Axons that enter the dorsal column will form the postsynaptic dorsal column tract. These ascending nociceptive tracts will eventually synapse with superior structures such as the periaqueductal gray, hypothalamus, and thalamus. The ventroposterolateral, ventroposteromedial, and ventroposteroinferior nuclei of the thalamus then send projections to the somatosensory cortex and cingulated cortices.

3 What is the classification of pain based on neurophysiologic mechanisms?

Pain can be classified according to its neurophysiologic mechanism:

image Nociceptive pain: Occurs when nociceptors are stimulated by noxious stimuli. This category is divided into somatic and visceral pain. Somatic pain sensation originates from trauma, burns, and ischemia, to name the most common ones. It is transmitted via somatic nerves. Visceral pain, as its name implies, originates from visceral structures. It is transmitted preferentially via the sympathetic fibers and is usually produced by distention, ischemia, or spasm of hollow viscera.
image Neuropathic pain: Is pain produced by an alteration in structure or function of the nervous system. Neuropathic pain can be divided into peripheral and central. Examples of peripheral neuropathic pain include CRPS II (causalgia), postherpetic neuralgia, diabetic neuropathy, and radicular pain from mechanical compression. Central neuropathic pain syndromes include poststroke pain, postparaplegic pain, and pain syndromes from multiple sclerosis.
image Psychogenic pain: Is quite controversial in the sense that it is very difficult to define. One popular definition is that this pain is better described and understood in psychologic rather than physical language and is pain for which an adequate physical explanation cannot be found.

4 Name the most commonly used groups of medications for the treatment of chronic pain

See Table 78-1.

TABLE 78-1 Commonly used Groups of Medications for Treatment of Chronic Pain

image

5 How are nerve blocks helpful in the treatment of chronic pain?

image Diagnosis: Nerve blocks can help identify the nerve site responsible for the symptoms.
image Therapy: Nerve blocks temporarily reduce pain and therefore facilitate physical therapy. Based on the response to a diagnostic block, it might be possible to determine if a neuroablation procedure is appropriate to treat a given condition.

6 Are psychosocial factors important in the diagnosis and treatment of pain?

Painful disorders can present with associated psychologic or psychiatric symptoms. These can vary from mild depression to obvious substance abuse problems to suicidal thoughts. It is necessary to address such issues to improve the patient’s chance for recovery.

7 How is pain of malignant origin treated?

Pain of malignant origin should be aggressively treated with a multiple therapeutic approach. This approach should initially be pharmacologic with introduction of short- and long-acting opioid preparations and some adjuvants. Adjuvants should be chosen according to the symptomatology and their side-effect profile. For example, nonsteroidal antiinflammatory drugs (NSAIDs) and steroids are very useful in the treatment of bone pain from primary or metastatic disease; anticonvulsants and tricyclic antidepressants (TCAs) can be used in the treatment of neuropathic pain from compression or from previous interventions such as chemotherapy or radiation therapy.

Diagnostic nerve blocks can be followed by either chemical or radiofrequency ablation procedures. For malignancies located in the abdomen, celiac plexus chemical ablation can be attempted; for malignancies located in the pelvis, a superior hypogastric plexus block can be beneficial, and perineal pain can be treated with a ganglion impar ablation. With the introduction of sophisticated intrathecal delivery systems, neuraxial ablative procedures are becoming less popular but are still very useful. As mentioned, intrathecal delivery systems and long-term epidural catheters can be used to deliver opioids, local anesthetics, and other potentially beneficial drugs to the neuraxis and therefore improve the patient’s condition and decrease the side effects from other medications. Finally, radiation therapy and chemotherapy can also improve pain symptoms by reducing the extent of the disease.

8 Define CRPS I and II. What nerve blocks are commonly used to treat these conditions?

CRPS stands for complex regional pain syndrome. It is a painful condition usually centered in an extremity in which different degrees of sympathetic dysfunction can be identified. CRPS usually presents with spontaneous pain, hyperalgesia, hyperpathia, and allodynia that is not restricted to the territory of a single nerve. Sympathetic dysfunction presents as variations in regional blood flow that can cause edema and cyanosis. Localized sweating and trophic changes in the skin and nails of the affected part of the body can be seen as the disease progresses. CRPS I (formerly known as RSD) can follow minor trauma, venipuncture, or carpal tunnel surgery; sometimes no identifiable cause can be found. CRPS II (formerly causalgia) follows damage to a peripheral nerve. Sympathetic blocks are very useful since they can facilitate physical therapy and help the patient regain some function in the affected extremity. Upper-extremity sympathetic denervation is accomplished by blocking the stellate ganglion; for lower-extremity sympathetic block a lumbar sympathetic block is performed.

9 How is neuropathic pain treated?

Medical management that includes anticonvulsants and TCAs has been shown to improve symptomatology. Methadone, due to its N-methyl-d-aspartate receptor antagonism, is probably the most useful opioid in the treatment of neuropathic pain. Other agents such as clonidine and mexiletine have also been used successfully. Injection of local anesthetics and steroids has a role in the treatment of isolated peripheral neuropathies. The introduction of peripheral and spinal cord stimulators is now an accepted therapy for very complex problems such as postlaminectomy pain syndromes and CRPS types I and II.

10 Define myofascial pain syndrome

Myofascial pain syndrome is a group of muscle disorders characterized by hypersensitive areas called trigger points that can occur in more than one muscle group. When trigger points are mechanically stimulated, they are painful and refer pain to an area called the reference zone. This reference zone does not correlate with any dermatome or peripheral nerve innervation area.

11 Define fibromyalgia

Fibromyalgia is a chronic pain condition characterized by widespread musculoskeletal pain, aches, and stiffness, soft tissue tenderness, general fatigue, and sleep disturbances. The most common sites of pain include the neck, back, shoulders, pelvic girdle, and hands, but any body part can be involved. Fibromyalgia patients experience a range of symptoms of varying intensities that wax and wane over time.

12 How is fibromyalgia managed?

New research shows that fibromyalgia very likely has a central nervous system component. This theory helps explain the widespread nature of the patients’ symptoms and its association with sleep disturbances. Pregabalin is the only Food and Drug Administration–approved medication for the management of fibromyalgia. In a randomized, double-blinded, placebo-controlled study, monotherapy with pregabalin showed statistically significant benefit in mean pain scores and Patient Global Impression of Change (PGIC). Aerobic conditioning and management of underlying depression are also very important in the management of these patients.

13 List possible etiologies of low back pain

Low back pain can arise from multiple anatomic structures located in the lower back; these include the following:

image Muscle strain in the paravertebral muscles or quadratus lumborum
image Posterior elements of the vertebral column such as the facet joints and ligamentous structures
image Anterior elements such as vertebral compression fractures and damage to the annulus fibrosis of the intervertebral disk
image Vertebral canal and foraminal stenosis, which can cause myelopathy or radiculopathy, respectively
image Sacroiliac joint dysfunction

14 What is the rationale behind the use of epidural steroids in the treatment of radicular symptoms associated with a herniated disk?

Radiculopathy is pain that is present as a result of either mechanical or chemical (most of the time both) irritation of a nerve root and can lead to pain and edema of the same nerve. The site where this usually occurs is the neural foramen. Local injection of steroids will decrease the amount of time needed to recover from an acute episode of sciatica by three mechanisms:

1. Decreasing the inflammation of the nerve root due to the antiinflammatory properties of the steroids
2. Dilution of chemical irritants coming from a ruptured disc
3. Nerve membrane stabilizers

15 Explain the gate theory of pain

In 1965 Melzack and Wall proposed that the substantia gelatinosa in the spinal cord was the primary gate in the transmission of noxious and nonnoxious stimulus to the central nervous system. The pain gate is opened by information coming from slow unmyelinated C fibers and closed by the impulses from faster myelinated fibers such as A-β. Since pain is transmitted by slow A-δ and C fibers, they reason that, by activating faster fibers such as the ones that transmit proprioception, the gate will be closed, and the pain symptoms will improve. A practical application is the use of transcutaneous electrical nerve stimulation units and spinal and peripheral nerve stimulators for the treatment of pain.

16 Name some indications for the use of spinal cord stimulators

The most common indication for use of spinal cord stimulation in the United States is in the treatment of postlaminectomy pain syndromes. In Europe the most common indication is in the treatment of peripheral vascular disease. Among other uses are CRPS I and II, arachnoiditis, and intractable angina pectoris.

17 What are the most common medications used for intrathecal delivery via implantable delivery systems?

The most common medications used in the implantable delivery systems are opioids in general and morphine and hydromorphone in particular. Local anesthetics are used, usually in combination with an opioid for the treatment of malignant pain. Baclofen is used for the treatment of spasticity and painful muscle contractions. Clonidine is used for the treatment of neuropathic pain. Recent studies have explored the potential for the use of ketamine, neostigmine, and some calcium channel blockers.

Website image

International Association for the Study of Pain

http://www.iasp-pain.org

Key Points: Chronic Pain Management image

1. Chronic pain is best treated using multiple therapeutic modalities. These include physical therapy, psychologic support, pharmacologic management, and the rational use of more invasive procedures such as nerve blocks and implantable technologies.
2. Patients suffering from cancer pain often exhibit complex symptomatology that includes various forms of nociceptive and neuropathic pain.
3. In patients suffering from chronic pain, underlying psychologic/psychiatric conditions should be addressed if any meaningful recovery is to be achieved.
4. Neuropathic pain is usually less responsive to opioids than pain originating from nociceptors.

SUGGESTED READINGS

1. Cameron T. Safety and efficacy of spinal cord stimulation for the treatment of chronic pain: a 20-year literature review. J Neurosurg. 2004;100(3 Suppl):254-267.

2. Mease P.J., Russell I.J., Arnold L.M., et al. A randomized, double-blind, placebo-controlled, phase III trial of pregabalin in the treatment of patients with fibromyalgia. J Rheumatol. 2008;35:502-514.

3. Melzack R., Wall P. Pain Mechanism: a new theory. Science. 1965;150:971.

4. Vranken J.H., van der Vegt M.H., Kal J.E., et al. Treatment of neuropathic cancer pain with continuous intrathecal administration of S+ ketamine. Acta Anesthesiol Scand. 2004;48:249-252.

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