Somatic Pain

Somatic pain is the most common type of pain in patients with cancer and bone metastases are the most prevalent cause. Somatic pain is characterized as well localized, intermittent, or constant and described as aching, gnawing, throbbing, or cramping. Such metastases are characterized by bone destruction with concurrent new bone formation. Both myelinated and unmyelinated afferent fibers are present in bone, and their density is greatest in the periosteum. Prostaglandins and osteoclast-activating factors sensitize nociceptors and produce hyperalgesia and pain as osteolysis and osteoclast activity occurs.²⁷ Drugs that interfere with prostaglandin synthesis and osteoclast activity inhibit bone pain by inhibiting this sensitization and may also inhibit tumor growth. The widespread use of bisphosphonates has resulted in improved analgesia and a significant reduction of skeletal complications in patients with malignant bone pain (see the sections Adjuvants for Bone Pain and Other Adjuvants).

Visceral Pain

Visceral pain is mediated by discrete nociceptors in the cardiovascular, respiratory, gastrointestinal, and genitourinary systems and is usually described as deep, squeezing, or colicky, and is commonly referred to cutaneous sites, which may be tender. Somatic and visceral structures have dual innervation by common afferent fibers which converge in the dorsal horn in the spinal cord. The pain of a visceral site may therefore be misrepresented as a cutaneous one. Shoulder pain, resulting from diaphragmatic irritation from a pleural disease, is an example of a cutaneous referral of a visceral pain. Visceral pain results from mechanical or chemical activation of nociceptors by tumor compression, or visceral distention, or obstruction, and responds to a wide variety of pain management approaches including decompression and pharmacologic, anesthetic, and neurosurgical procedures.^28,29 Recent experimental data in animals suggests that kappa-opioid receptor agonists are uniquely efficacious in the treatment of visceral pain.^30,31 The role of such agents in the management of human cancer pain requires further investigation.

Neuropathic Pain

The third category of pain that is common in cancer patients is neuropathic pain, which results from injury to the peripheral receptor, afferent fiber, or central nervous system. Such injury is associated with spontaneous and ectopic firing in the peripheral nerve as well as at the level of the dorsal horn. Reorganization of the nervous system occurs, and spontaneous neural activity can be measured at the level of the thalamus. Neuropathic pain is clinically described as a burning, dysesthetic, squeezing sensation with paroxysms of shock-like pain.

Tumor infiltration of the brachial and lumbar plexus are the most common causes of neuropathic pain.^32,33 Such pain also results from injury to the peripheral nerve as occurs in postmastectomy and postthoracotomy pain. Tricyclic antidepressants, selective serotonin reuptake inhibitors, anticonvulsants, opioids, local anesthetics, NMDA antagonists, and some neurostimulatory procedures have all been used successfully in the management of neuropathic pain. New anticonvulsants and new data on opioid-responsive neuropathic pain have significantly improved the outcome for these patients (see below).

Common Pain Syndromes in Patients with Cancer

Over the last 15 years, a series of well-defined pain syndromes have been described in patients with cancer and pain. Many of these pain syndromes are unique to cancer and are often misdiagnosed because healthcare professionals are unfamiliar with their clinical presentation.^1,2,28 In each of these syndromes, pain is the overriding symptom that prompts medical attention. Knowledge of the common pain syndromes facilitates assessment and treatment and supports the need for training in cancer pain management as an integral part of the oncologist's training. Table 78-2 lists the three major categories of pain syndromes.

Table 78-2

Cancer Pain Syndromes.