OVERVIEW OF PAIN
Pain, considered the fifth vital sign, is one of the more common reasons that people seek treatment from a health care provider. Pain is a personal, individual, subjective experience: It is whatever the patient says it is and exists whenever the patient says it does. Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage.
Pain is classified as acute or chronic. Acute pain is a sudden but temporary, self-limiting response to some type of bodily injury; it generally lasts less than six months. Chronic pain is often associated with prolonged diseases such as cancer, fibromyalgia, and osteoarthritis; it persists for six months or longer.
Pain can be separated into two categories: nociceptive and neuropathic. Nociceptive pain originates from peripheral or visceral nociceptors as a result of injury and comprises somatic and visceral pain. Somatic pain is caused by injury to soft tissue, connective tissue, and bone; the classic description is a sharp, well-localized discomfort. Visceral pain originates from an organ or deeper structure; it is commonly described as dull, poorly localized, and sensitive to stretch, ischemia, and inflammation.
Neuropathic pain is an abnormal processing of pain stimuli by the peripheral nervous system or central nervous system (CNS) and can result from injury or inflammation to a nerve. Neuropathic pain is usually described by patients as electric, burning, and/or shooting. Examples include pain associated with cancer, diabetic neuropathy, and phantom-limb sensation (following amputation).
The physiologic experience of pain follows a defined set of phases. First is transduction, which occurs at the moment of injury or trauma; sensory nerve endings convert the noxious stimulus into a nerve impulse. Second is transmission of the pain impulse to the spinal column by means of chemical messengers known as neurotransmitters.
After the pain impulse reaches the spinal tract, it continues to the brain, at which point there is perception, the third step in the process. This leads to modulation (also known as anti-nociception). In this fourth step, neurons that originate in the brainstem are activated, releasing neurotransmitters that inhibit transmission of pain. Modulation occurs in several areas of the CNS and involves the neurotransmitters serotonin, norepinephrine, and endogenous opioids (eg, ß-endorphin).6 During modulation, the limbic nervous system provokes a response to the painful stimulus, triggering endogenous opioids to bind to opioid receptors.7
Continue to: ROLE OF OPIOIDS IN PAIN MANAGEMENT