Pain

September 20, 2011

Persons perceive pain when their internal environment is threatened. Threats may occur from a variety of sources. Pain receptors are present in the skin, connective tissue, blood vessels, bony surface, and most organs. A major exception is the brain where pain is sensed rather than originated.

Stimuli exiting these pain receptors travel from distant surfaces and tissues by nerves first to the spinal cord and up to the brain. Once signals enter the brain, they stop in a central relay station, the thalamus. Here, the quality and severity of the pain is realized. Within the thalamus, pain is recognized but not located. Localizing pain requires transmission from the thalamus to the map of the body surface located in the cerebral cortex. Surface pain is referred to the region or its origin. Organ pain is referred to various areas on the body surface sometimes creating confusion in finding the source of the pain. Some examples of referred organ pain include left arm and jaw pain with heart attack and right shoulder pain from gallbladder disease.

Pain also occurs when the brain loses some or all sensation to portions of the body. Injury to peripheral nerves from trauma, infection, or toxins often results in permanent sensory loss and pain. A common example occurs in diabetes where nerve injury in the feet results in incessant pain. Even after removal of an arm or leg, phantom limb pain may persist.

Pain resulting from nerve injury, neuropathic pain, may assume several qualities. Descriptions of incessant burning and tingling, parathesias, commonly are associated with nerve injury or removal. Intermittent stabbing pains, neuralgias, may occur in the face, trigeminal neuralgia, or back of head, greater occipital neuralgia. Nerve compression in the back commonly caused by degeneration of the spine causes intermittent stabbing pain that radiates from the thigh to foot, sciatica.

Injury to nerves in the arms and legs may result in persistent pain syndromes that clearly are in excess of the degree of nerve involvement. Heightened sensitivity of the sympathetic pathways (a part of the nervous system called the autonomic nervous system) results in a regional pain syndrome. Multiple classifications of these pain disorders exist.

Complex regional pain syndromes commonly involve the hand more than the foot. Burning, overly sensitive, and spreading pain throughout the limb are common characteristics. The dysfunctional sympathetic nervous system also causes swelling in the affected limb, skin changes with loss of hair and nail changes, overactive sweating, thinning of the underlying bone, coolness to touch, and bluish discoloration.

Pain that does not result from either an environmental threat or disruption of nerve pathways is currently designated idiopathic, cause unknown. Migraine headaches until recently dwelled within this group. Painful diagnoses still classified as idiopathic include tension headache and fibromyalgia. Depression is uncommonly the solitary cause for regional pain. Alternative diagnoses must be excluded prior to assigning a diagnosis of psychogenic pain. Even when a strong psychogenic component is evident, an exhaustive search for the root cause of the pain must be completed.

Understanding the cause of a person’s pain should be the first step in care. An attempt to find and eliminate or lessen the pain’s origin is essential. Patients with pain often have associated depression. This component must be addressed separately from the root source of the pain. Lack of clear separation of the emotional from the physical component may interfere in both communication and treatment.

Treatment of pain can be divided into several arenas: (a) medical, (b) surgical, and (c) behavioral. Medical management stresses pharmacologic intervention.

Surgical therapy includes both peripheral and central nervous system procedures along with implantation of analgesic pumps. Behavioral therapies include modalities such as biofeedback, exercise, and meditation.

Pain-reducing medications abound including both prescription and over-the-counter analgesics. Acetaminophen, aspirin, and nonsteroidal anti-inflammatory drugs (NSAIDs) are the most commonly used pain relievers. These over-the-counter (available without prescription) remedies are appropriate for pain caused by irritations from the environment. These medications interfere in the inflammatory cascade that is commonly engaged at sites of tissue injury.

Analgesic medications unfortunately can cause toxicity. Acetaminophen, the active ingredient in Tylenol, is the commonest cause of accidental liver failure. Daily doses of 3-4 g and solitary doses of more than 200 mg/kg can proceed to death from liver injury. Aspirin predisposes to hearing loss, kidney failure, and significant upper intestinal irritation and ulceration. NSAIDs, similar to aspirin, inhibit the normal protective coating of the stomach and upper small intestine. The stomach’s acid irritates and destroys the lining predisposing to ulcer formation. Kidney failure from NSAIDs is also a concern.

Opiates, one type of prescription analgesics, have been used for both recreational and medicinal purposes for several milleniums. These medications produce a sense of euphoria relieving pain of all types. Morphine, codeine, oxycodone, meperidine, fentanyl, and hydromorphone are some of the commonly available forms. Administration can occur by most methods imaginable including by mouth (oral), across skin (transdermal), through injection into veins (intravenous) or muscle (intramuscular), by inhalation, or through placement in the rectum by suppository. Electronic pumps are available that can be implanted under the skin for direct administration into the fluid surrounding (intrathecal) or space directly surrounding (epidural) the central nervous system.

Adverse effects abound with opioid use. Common side effects include nausea, constipation, and a feeling of uneasiness, dysphoria. Escalating doses lead to confusion, sleepiness, and depressed breathing. Tachyphylaxis, failing effectiveness over time, and dependence commonly develop with use in excess of several weeks. These effects are caused by changes within the central nervous system with increasing number of pain receptors being produced in response to chronic stimulation. If a person is thought to be dependent on narcotic analgesics, the patient’s pain management program should be reviewed.

Additional agents may complement opiates, acetaminophen, and NSAIDs. Anticonvulsants including carbamazepine, oxcarbazepine, and gabapentin may be used predominately when pain stems from a nerve injury. Conditions with predominant neuropathic pain (pain related to nerve damage) include trigeminal neuralgia (a disease of a facial nerve), reflex sympathetic dystrophy (disease of the autonomic nervous system), painful polyneuropathies (damage to multiple nerves), and all forms of nerve root irritation including postherpetic pain from shingles (nerve damage that sometime occurs after herpesvirus infection).

Tricyclic antidepressant medications including amitriptyline and nortriptyline act as enhancers or adjuvants to opiates for pain relief. These antidepressant compounds act by blocking the reuptake after release of the neurotransmitters serotonin and norepinephrine in the brain. Newer antidepressants classified as selective serotonin reuptake inhibitors (SSRIs) do not seem as effective as their predecessors. The older agents also cause drowsiness that may be desirable when sleep is disturbed by pain.

Corticosteroids such as prednisone affect pain related to local tissue injury similar to the NSAIDs. Additional benefits include increased energy and euphoria. However, significant side effects exist especially with prolonged use. Diabetes mellitus, thinning of bone (osteoporosis), central obesity, high blood pressure, and gastrointestinal irritation are among the most common and serious treatment-related complications. Careful surveillance for ill effects related to corticosteroid use is essential when prolonged use is necessary.

Several ointments also relieve pain. Liniments have been used for generations to alleviate joint and muscular pain. Topical capsaicin ointment was developed to relieve superficial burning pain. This ointment derived from hot peppers releases and eventually depletes the stores of the pain-producing neurotransmitter, substance P, from nerve terminals. Initial use, unfortunately, temporarily increases pain. Premature abandonment of this effective therapy is common. Lidocaine ointment and sprays are available providing local relief of painful skin and mucosal surfaces.

Surgical procedures to alleviate pain are available. Regional blockade through injection of local anesthetics along with corticosteroids into joints and the epidural space can reduce pain significantly. Cutting peripheral nerves or the sensory root involved in pain may be used in refractory cases. However, the pain that is relieved may be replaced by additional regional pain after normal sensation from that region is reduced or eliminated. Central nervous system operations for pain are usually reserved until all other mechanisms for pain relief have been eliminated. Cutting specific pain tracts within the spinal cord or destroying pain regions in the thalamus have been used successfully.

Other pain-modifying procedures also may be used. Electrical impulses applied to the skin surface may alleviate regional pain. The mechanism of action is believed to be mediated through pain gates that are blocked when the impulses occur. Acupuncture, cool and warm compresses, massage, and vibration are thought to act by a similar mechanism.

Vigorous exercise relieves pain through central mechanisms. Internal neurotransmitters, encephalins and endorphins, released by exercise are believed to lessen pain by binding to opiate receptors. Psychic distraction in the form of meditation, biorhythm training, or prayer may also lessen pain both during and following the activity.

SEE ALSO: Arthritis, Cancer, Depression, Headache, Pregnancy

Suggested Reading

  • Battista, E. M. (2002). The assessment and management of chronic pain in the elderly. A guide for practice. Advance Nurse Practitioner, 10(11), 28-32.
  • Bernstein, R. M. (2001). Injections and surgical therapy in chronic pain. Clinical Journal of Pain, 17(S4), S94-S104.
  • Frank, A. O., & DeSouza, L. H. (2001). Conservative management of low back pain. International Journal of Clinical Practice, 55(1), 21-31.
  • Hoffert, M. J. (1989). The neurophysiology of pain. Neurology Clinics, 7(2),183-203.
  • Leland, J. Y. (1999). Chronic pain: Primary care treatment of the older patient. Geriatrics, 54(1), 23-28, 33-34, 37.
  • Reid, M. C., Engles-Horton, L. L., Weber, M. B., Kerns, R. D., & Rogers, E. L. (2002). Use of opioid medications for chronic noncancer pain syndromes in primary care. Journal of General Internal Medicine, 17(3), 173-179.
  • Salerno, S. M., Browning, R., & Jackson, J. L. (2002). The effect of antidepressant treatment on chronic back pain: A meta-analysis. Archives of Internal Medicine, 162(1), 19-24.

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