​   ​PAIN
Pain may be acute or chronic. Acute pain begins suddenly and usually does not last long (up to about 3 months). Chronic pain lasts for months or years. When severe, acute pain may cause anxiety, a rapid heart rate, an increased breathing rate, elevated blood pressure, sweating, and dilated pupils. Usually, chronic pain does not have these effects, but it may result in other problems, such as depression, disturbed sleep, decreased energy, a poor appetite, weight loss, decreased sex drive, and loss of interest in activities.
Nociceptive pain is caused by an injury to body tissues. The injury may be a cut, bruise, bone fracture, crush injury, burn, or anything that damages tissues. Most pain is nociceptive pain. It results from stimulation of pain receptors for tissue injury (nociceptors), which are located mostly in the skin or in internal organs. This type of pain is typically aching, sharp, or throbbing, but it may be dull.

A blockage in an internal organ usually causes deep, cramping pain, and the pain's location may be hard to pinpoint. But when connective tissues in internal organs are damaged, the pain is sharp and easy to locate.
The pain almost universally experienced after surgery is nociceptive pain. The pain may be constant or intermittent, often worsening when a person moves, coughs, laughs, or breathes deeply or when the dressings over the surgical wound are changed.
Most of the pain due to cancer is nociceptive. When a tumor invades bones and organs, it may cause mild discomfort or severe, unrelenting pain. Some cancer treatments, such as surgery and radiation therapy, can also cause nociceptive pain.
Pain relievers (analgesics), including opioids, are usually effective.

Neuropathic pain is caused by damage to or dysfunction of the nerves, spinal cord, or brain.
Neuropathic pain may result from:

     - Compression of a nerve—for example, by a tumor, by a ruptured intervertebral disk, or as occurs in carpal tunnel syndrome

     - Nerve damage—as occurs in disorders that affect the whole body (such as diabetes mellitus) or only one or a few parts (such as shingles)

     - Abnormal or disrupted processing of pain signals by the brain and spinal cord

Processing of pain is abnormal in phantom limb pain, postherpetic neuralgia, and complex regional pain syndrome. Neuropathic pain can contribute to anxiety and/or depression. Anxiety and depression can also worsen pain. Neuropathic pain may also develop after surgery, such as removal of a breast (mastectomy) or lung surgery (thoracotomy). Neuropathic pain may be felt as burning or tingling or as hypersensitivity to touch or cold. Sometimes the pain is deep and aching. People may become very sensitive to touch. A light touch may cause pain.

If movement is painful, people may be reluctant to move the painful part of their body. In such cases, muscles that control the painful part may waste away, and movement may become limited. People continue to feel pain long after the cause resolves because structures in the nervous system have been changed, making the structures more sensitive to pain.

Phantom limb pain is pain that seems to be felt in an amputated part of the body, usually a limb. It differs from phantom limb sensation—the feeling that the amputated part is still there—which is much more common. Phantom limb pain cannot be caused by a problem in the limb. Rather, it must be caused by a change in the nervous system above the site where the limb was amputated. But the brain misinterprets the nerve signals as coming from the amputated limb. Usually, the pain seems to be in the toes, ankle, and foot of an amputated leg or in the fingers and hand of an amputated arm. The pain may resemble squeezing, burning, or crushing sensations, but it often differs from any sensation previously experienced. For some people, phantom limb pain occurs less frequently as time passes, but for others, it persists.

Postherpetic neuralgia is pain that results from shingles (herpes zoster, which causes inflammation of nerve tissue) but occurs only after shingles resolves.
Shingles is caused by reactivation of the varicella-zoster virus, the virus that causes chickenpox.
What causes postherpetic neuralgia is unknown. But it is more likely to develop if the shingles rash is severe or if people are older when shingles develops.
The pain is felt as a constant deep aching or burning, as a sharp and intermittent pain, or as hypersensitivity to touch or cold. The pain may be debilitating.
Vaccination with the shingles (herpes zoster) vaccine can help reduce the risk of getting shingles and postherpetic neuralgia. If people get shingles and postherpetic neuralgia, vaccination can help reduce the severity of symptoms.
Pain relievers and other drugs may be required, but no treatment is routinely effective.

Pain fibers enter the spinal cord at the dorsal root ganglia and synapse in the dorsal horn. From there, fibers cross to the other side and travel up the lateral columns to the thalamus and then to the cerebral cortex.
Repetitive stimulation (eg, from a prolonged painful condition) can sensitize neurons in the dorsal horn of the spinal cord so that a lesser peripheral stimulus causes pain (wind-up phenomenon). Peripheral nerves and nerves at other levels of the CNS may also be sensitized, producing long-term synaptic changes in cortical receptive fields (remodeling) that maintain exaggerated pain perception.
Substances released when tissue is injured, including those involved in the inflammatory cascade, can sensitize peripheral nociceptors. These substances include vasoactive peptides (eg, calcitonin gene-related protein, substance P, neurokinin A) and other mediators (eg, prostaglandin E2, serotonin, bradykinin, epinephrine).

The pain signal is modulated at multiple points in both segmental and descending pathways by many neurochemical mediators, including endorphins (eg, enkephalin) and monoamines (eg, serotonin, norepinephrine). These mediators interact in poorly understood ways to increase, sustain, shorten, or reduce the perception of and response to pain. They mediate the potential benefit of CNS-active drugs (eg, opioids, antidepressants, anticonvulsants, membrane stabilizers) that interact with specific receptors and neurochemicals in the treatment of chronic pain.
Psychologic factors are important modulators. They not only affect how patients speak about pain (eg, in a stoic, irritable, or complaining way) and how they behave in response to it (eg, whether they grimace), but they also generate neural output that modulates neurotransmission along pain pathways. Psychologic reaction to protracted pain interacts with other CNS factors to induce long-term changes in pain perception.

There are a variety of options for the treatment of chronic pain. Under the general category of medications, there are both oral and topical therapies for the treatment of chronic pain. Oral medications include those that can be taken by mouth, such as nonsteroidal anti-inflammatory drugs, acetaminophen, and opioids. Also available are medications that can be applied to the skin, whether as an ointment or cream or by a patch that is applied to the skin. Some of these patches work by being placed directly on top of the painful area where the active drug, such as lidocaine, is released. Others, such as fentanyl patches, may be placed at a location far from the painful area. Some medications are available over the-counter (OTC) while others may require a prescription.

There are many things that may help with your pain which do not involve medications. These things may help relieve some pain and reduce the medications required to control your pain. Examples include exercises, best performed under the direction of a physical therapist. There are also alternative modalities, such as acupuncture. Transcutaneous Electro-Nerve Stimulator (TENS) units use pads that are placed on your skin to provide stimulation around the area of pain and may help to reduce some types of pain symptoms.

Finally, there are interventional techniques that involve injections into or around various levels of the spinal region. These can involve relatively superficial injections into the painful muscles, called trigger point injections, or may involve more invasive procedures. There are multiple procedures that range from epidural injections for pain involving the neck and arm or the back and leg, facet injections into the joints that allow movement of the neck and back to injections for burning pain of the arms or legs due to a syndrome called Complex Regional Pain Syndrome or Reflex Sympathetic Dystrophy (CRPS).