While acute pain is a normal sensation triggered in the nervous system to alert you to possible injury and the need to take care of yourself, chronic pain persists and the pain signals keep firing in the nervous system for weeks, months, even years.
There may have been an initial mishap — sprained back, serious infection, or there may be an ongoing cause of pain — arthritis, cancer, ear infection, but some people suffer chronic pain in the absence of any past injury or evidence of body damage and many chronic pain conditions affect older adults.
Common chronic pain complaints include headache, low back pain, cancer pain, arthritis pain, neurogenic pain (pain resulting from damage to the peripheral nerves or to the central nervous system itself), psychogenic pain (pain not due to past disease or injury or any visible sign of damage inside or outside the nervous system). A person may have two or more co-existing chronic pain conditions. Such conditions can include chronic fatigue syndrome, endometriosis, fibromyalgia, inflammatory bowel disease, interstitial cystitis, temporomandibular joint dysfunction, and vulvodynia. It is not known whether these disorders share a common cause.
The pain experience can be functionally divided into acute and chronic types. Acute and chronic pain are due to different physiological mechanisms and thus require different treatments. In this document, we review theories of pain and examine the physiology of pain, with emphasis on the types and their manifestations.
Pathways of Pain
Nociceptors, or pain receptors, are free nerve endings that respond to painful stimuli. Nociceptors are found throughout all tissues except the brain, and they transmit information to the brain. They are stimulated by biological, electrical, thermal, mechanical, and chemical stimuli.
Pain perception occurs when these stimuli are transmitted to the spinal cord and then to the central areas of the brain. Pain impulses travel to the dorsal horn of the spine, where they synapse with dorsal horn neurons in the substantia gelatinosa and then ascend to the brain.
The basic sensation of pain occurs at the thalamus, it continues to the limbic system (emotional center) and the cerebral cortex, where pain is perceived and interpreted (Figure 1).
Two types of fibers are involved in pain transmission. The large A delta fibers produce sharp well-defined pain, called “fast pain” or “first pain,” typically stimulated by a cut, an electrical shock, or a physical blow. Transmission through the A fibers is so fast that the body’s reflexes can actually respond faster than the pain stimulus, resulting in retraction of the affected body part even before the person perceives the pain.
After this first pain, the smaller C fibers transmit dull burning or aching sensations, known as “second pain.” The C fibers transmit pain more slowly than the A fibers do because the C fibers are smaller and lack a myelin sheath. The C fibers are the ones that produce constant pain.
According to the gate control theory, stimulation of the fibers that transmit nonpainful stimuli can block pain impulses at the gate in the dorsal horn and this is the basis for neurostimualtion treatment.
For example, if touch receptors (A beta fibers) are stimulated, they dominate and close the gate. This ability to block pain impulses is the reason a person is prone to immediately grab and massage the foot when he or she stubs a toe.
The touch blocks the transmission and duration of pain impulses, and this capacity has implications for the use of touch and massage for some patients in pain.
Chronic pain is prolonged pain, persisting beyond the expected normal healing time.
This characterization was previously the official definition of chronic pain according to the International Association for the Study of Pain. The term chronic is still widely used, although many pain experts now think that all forms of chronic pain are variations of the same phenomenon and should be labeled specifically, such as neuropathic pain.
Chronic pain can be continuous (eg, arthritis) or intermittent (eg, migraines). Chronic pain is poorly understood and is more complex and difficult to manage than is acute pain. Understanding chronic pain requires recognizing that the nervous system is not hardwired. If it were hardwired, each noxious stimulus, such as a needle stick, would elicit exactly the same nervous system response at the same intensity every time, but pain is much more complex, involving affective and cognitive traits of the person who experiences it.
Melzack and Wall showed that repeated stimulation of C fibers results in progressive buildup of electrical response in the CNS, a phenomenon called windup, somewhat analogous to the effect of winding up a child’s windup toy. The more the toy is wound up, the faster and longer the toy will run.
This persistent stimulation of peripheral nerves winds up the CNS, leading to intensified stimulation of nerve fibers that is referred to as non nociceptive pain.
The concept of windup is crucial to understanding chronic pain. Windup is the reason pain can continue long after the expected recovery time for an injury or a pain-initiating event.
Patients with chronic pain may not have the behaviors associated with acute pain. Additionally, autonomic nervous system responses (eg, nausea, vomiting, pallor, sweating) decrease with prolonged pain.
The body’s fight-or-flight reaction, which normally occurs with acute pain, does not occur because the sympathetic nervous system has adapted to persistent pain impulses.
Understanding chronic pain, therefore, requires listening to the person’s description of it, because expected physical symptoms may not be present. Unfortunately, because of the lack of objective evidence of pain, many patients who report chronic pain are viewed as hypochondriacs and malingerers by health care professionals.
Some evidence indicates that chronic pain and depression share the same physiological pathway.
Chronic, often intractable pain due to injury to the peripheral nerves is known as neuropathic pain. According to Devor and Seltzer, this pain is a paradox.
Injury to peripheral nerves should deaden sensation, much as cutting a telephone wire leaves the phone line dead, but the opposite occurs in neuropathic pain.
Injury to the peripheral nerves can cause spontaneous paresthesias, numbness, pain with movement, tenderness of a partly denervated body part, and pain that is electric shock–like, burning, shooting, or tingling.
Abnormally amplified signals in the CNS due to windup result in central sensitization, which is an increased sensitivity of spinal neurons. Central sensitization causes allodynia (pain from a stimulus that does not normally produce pain, such as touch) and hyperalgesia (a heightened pain response to a stimulus that is painful).
Is there any treatment?
Medications, acupuncture, local electrical stimulation, and brain stimulation, as well as surgery, are some treatments for chronic pain. Psychotherapy, relaxation and medication therapies, biofeedback, and behavior modification may also be employed to treat chronic pain.
Spinal cord stimulation (neurostimulation) can be an effective alternative or adjunct treatment to other therapies to manage chronic back and/or leg pain and in our hospital we use Medtronic devices because offers a portfolio of spinal cord neurostimulators that deliver targeted chronic pain management.
Medtronic has been innovating in neurostimulator technology for more than 30 years. In that time, over 250,000 patients have benefited from spinal cord stimulation and provide clinicians and patients with reliable, best-in-class pain management systems and service.
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